volume 33 number 4 april 2018 pages 511–672 jaasenvsun.chem.chuo-u.ac.jp/paper/fukushima.pdf ·...

JAASJournal of Analytical Atomic Spectrometryrsc.li/jaas

ISSN 0267-9477

CRITICAL REVIEWJian Zheng et al.The role of mass spectrometry in radioactive contamination assessment after the Fukushima nuclear accident

Volume 33 Number 4 April 2018 Pages 511–672

JAAS

CRITICAL REVIEW

The role of mass

WP(niaIS2cNCP

are nuclear forensics, radioanalmental radioactivity.

aInstitute of Nuclear Physics and Chemistry,

Mianyang 621900, China. E-mail: wtbu@cabFukushima Project Headquarters, Nation

National Institutes for Quantum and R

Anagawa 4-9-1, Inage, Chiba 263-8555, JapcState Key Laboratory of Nuclear Physics an

University, Beijing 100871, China

Cite this: J. Anal. At. Spectrom., 2018,33, 519

Received 7th December 2017Accepted 19th February 2018

DOI: 10.1039/c7ja00401j

rsc.li/jaas

This journal is © The Royal Society of C

spectrometry in radioactivecontamination assessment after the Fukushimanuclear accident

Wenting Bu, a Youyi Ni,bc Georg Steinhauser,d Wang Zheng,e Jian Zheng *b

and Naoki Furutaf

The Fukushima nuclear accident caused the release of large amounts of radionuclides into the

environment. After the accident, radioactive contamination assessment in environmental samples is

essential for radiation dose estimation and radioactive remediation. Mass spectrometry characterized by

high sensitivity, low detection limit, short measuring time, high sample throughput, and the capability to

obtain atomic ratios is a promising technique for the analysis of the accident released long-lived

radionuclides. This review describes the developed analytical methods based on mass spectrometric

techniques for the determination of radionuclides (Pu isotopes, U isotopes, radiocesium, radioiodine,

radiostrontium, etc.) with regards to Fukushima samples. The real applications of mass spectrometric

techniques for radioactive source identification, radiation protection and geochemical tracing are

discussed to highlight the importance of mass spectrometric techniques in radioactive contamination

assessment after the accident. Future research prospects of mass spectrometric techniques for the

analysis of radionuclides with application to Fukushima samples are briefly outlined.

enting Bu received his B.S. inhysics from Peking UniversityChina) in 2010 and his Ph.D inuclear physics and technologyn 2015. He participated injoint study at the National

nstitute of Radiologicalciences, Japan from 2012 to014. He is currently an asso-iate professor at the Institute ofuclear Physics and Chemistry,hina Academy of Engineeringhysics. His research interestsytical chemistry and environ-

Youyi Ni is a fourth-year Ph.D.student from the School ofPhysics, Peking University inChina. His study is focused onradiation protection and envi-ronmental radioactivity.Currently he is participating ina joint study program supportedby the China ScholarshipCouncil (CSC) and studying atthe National Institute of Radio-logical Sciences (NIRS), Japanunder the guidance of Dr Jian

Zheng. At the NIRS he is devoted to the development of analyticalmethods for actinides in various environmental samples with ICP-MS as well as the transfer and migration behavior of actinides inthe biosphere.

China Academy of Engineering Physics,

ep.cn

al Institute of Radiological Sciences,

adiological Science and Technology,

an. E-mail: [email protected]

d Technology, School of Physics, Peking

dInstitute of Radioecology and Radiation Protection, Leibniz Universitat Hannover,

30419 Hannover, GermanyeSchool of Earth and Space Exploration, Arizona State University, Tempe, AZ 85201,

USAfFaculty of Science and Engineering, Department of Applied Chemistry, Chuo

University, 1-13-27 Kasuga, Bunkyo, Tokyo, 112-8551, Japan

hemistry 2018 J. Anal. At. Spectrom., 2018, 33, 519–546 | 519

http://crossmark.crossref.org/dialog/?doi=10.1039/c7ja00401j&domain=pdf&date_stamp=2018-03-30

http://orcid.org/0000-0003-0055-3097

http://orcid.org/0000-0003-0616-5570

JAAS Critical Review

1. Introduction

One of the most severe nuclear accidents occurred at theFukushima Daiichi Nuclear Power Plant (FDNPP) on March 11,2011, following the largest earthquake in Japanese history.Consequently, large amounts of radionuclides were releasedinto the environment. The radionuclides with high radiologicalrisk, thus raising public concerns and attracting scienticinterest, include 90Sr, 129I, 131I, 133Xe, 134Cs, 135Cs, 137Cs, Uisotopes, Pu isotopes, etc.Worldwide research efforts have beenmade for the investigation of the concentration and character-ization of these radionuclides in the environment in support ofdose assessment and radioactive decontamination. The prop-erties of the interesting radionuclides released from the FDNPPaccident and their typical measuring methods are summarizedin Table 1.

Georg Steinhauser received hisMSc in chemistry from theUniversity of Vienna (Austria) in2003 and his PhD in radio-chemistry in 2005 from theVienna University of Tech-nology. From 2013 to 2015, heworked for Colorado StateUniversity as an assistantprofessor, before assuminga position as a professor ofphysical radioecology at LeibnizUniversitat Hannover, Germany.

He authored/co-authored more than 90 papers and 6 bookcontributions. His main research focus is on environmentalforensics in context with the Fukushima nuclear accident.

Wang Zheng received his B.S.from the University of Scienceand Technology of China in2004 and obtained his PhD inenvironmental and life sciencesfrom Trent University, Canadain 2010. He is currentlya research specialist principle atArizona State University, USA.His research focuses on thedevelopment and application ofmass spectrometry for studyingthe biogeochemical cycle of trace

elements and their isotope fractionation in the environment. He iscurrently studying the stable isotope fractionations of heavy metalsand radioisotopes such as mercury and uranium, and theirapplications in environmental and earth sciences.

520 | J. Anal. At. Spectrom., 2018, 33, 519–546

Radionuclides are typically measured in two ways: conven-tional radiometric methods and mass spectrometric methods.Radiometric methods are widely used for the measurement ofradionuclides especially with short half-lives depending ontheir radiation characteristics. For relatively long-lived radio-nuclides, mass spectrometric methods are preferred because oftheir high sensitivity, low detection limit and short measuringtime.1 In addition, mass spectrometric methods can determineatomic ratios that are important for radioactive source identi-cation (for example 240Pu/239Pu and 236U/238U atomic ratios),whereas these atomic ratios typically cannot be determined byradiometric methods due to spectral interference such as closeenergies of the emitted a particles, extremely low isotopicabundances, or long half-lives of the nuclides in question.However, mass spectrometric methods can alternatively over-come these problems. In the past few decades, mass spec-trometry has become more and more important in the elds of

Jian Zheng graduated fromFudan University (China) in1987 and obtained his PhD inenvironmental analytical chem-istry from Karl-FranzensUniversity, Austria in 1998. Hecurrently works as a seniorprincipal researcher at theNational Institute of Radiolog-ical Sciences, Japan. He haspublished >130 research articlesin international journals. Hisresearch interests focus on the

development and application of atomic spectrometric techniquesfor trace element/radionuclide speciation and isotope ratiomeasurement, and the environmental behavior of radionuclides,especially actinides. He received a NIRS Research Award in 2009and the Society Award from the Japan Society of Nuclear andRadiochemical Sciences in 2015.

Naoki Furuta has beena professor in the Department ofApplied Chemistry at ChuoUniversity since 1994. Before hemoved to Chuo University, heworked at the National Institutefor Environmental Studies(1975–1994). He gained hisDoctor of Science from theUniversity of Tokyo in 1979. Hisresearch interests focus on theidentication of sources of toxicelements in environmental

samples, elucidation of the role of trace elements in biologicalsamples, and the determination of trace elements in semiconductormaterials. He has published 90 refereed papers, 7 reviews and 16books. He received Fellow of Royal Society of Chemistry in 2005.

This journal is © The Royal Society of Chemistry 2018

Table 1 Main radionuclides released from the FDNPP accident

Radionuclide Half-life DecayTypical measurementmethods

3H 12.32 y b� LSC14C 5730 y b� AMS89Sr 50.6 d b�, (g) LSC, ICP-MS, AMS90Sr 28.8 y b� LSC, ICP-MS, AMS99Tc 2.1 � 105 y b�, (g) ICP-MS, TIMS129I 1.57 � 107 y b�, (g) AMS131I 8.02 d b�, g g spectrometry132Te 3.2 d b�, g g spectrometry133Xe 8.24 d b�, g, e� g spectrometry134Cs 2.06 y b�, g g spectrometry135Cs 2.3 � 106 y b� ICP-MS, TIMS137Cs 30.2 y b�, g g spectrometry235U 7.04 � 108 y a (g) ICP-MS, TIMS236U 2.34 � 107 y a AMS, TIMS, ICP-MS238U 4.47 � 109 y a (b�) ICP-MS, TIMS237Np 2.14 � 106 y a (g) ICP-MS238Pu 87.74 y a a spectrometry239Pu 2.41 � 104 y a a spectrometry, ICP-MS,

TIMS, AMS240Pu 6537 y a a spectrometry, ICP-MS,

TIMS, AMS241Pu 14.4 y b� (a) LSC, ICP-MS, TIMS, AMS241Am 432.7 y a, g a spectrometry, ICP-MS242Cm 162.9 d a a spectrometry

Critical Review JAAS

nuclear safeguards, radiation protection, environmentalscience, geochemistry, etc.2–8 The updating of mass spectrom-etry equipment in recent years further expanded their applica-tions for the determination of hard-to-detect radionuclides. Forinstance, a type of inductively coupled plasma tandem massspectrometry (ICP-MS/MS) was commercially introduced byAgilent™ in 2012. An additional quadrupole was equipped infront of the collision/reaction cell compared with traditionalICP-QMS. This double mass lter design has been reportedmuch more previously.9,10 Benecially, the reactions takingplace in the cell can be better controlled resulting in betterelimination of isobaric and polyatomic interference.11,12 It hasbeen successfully used for the analysis of 135Cs, 137Cs, 129I and236U in Fukushima samples.13–15

There is a strong demand for the determination of radio-nuclides in environmental samples for radioactive contamina-tion assessment. A remarkable number of analytical methodsbased on mass spectrometry were developed for radionuclideanalysis. Some studies focused on the analysis of radionuclidesat ultra-trace levels.16–22 Others aimed to develop rapid separa-tion and detection procedures to meet the requirements fora quick response aer a nuclear accident.23–30 Thanks to thesedevelopments, the distributions and behaviors of the FDNPPaccident derived radionuclides in the terrestrial and marineenvironments around or far away from the FDNPP site havebeen extensively studied. The results are helpful not only forradiological considerations but also for geochemical studies asmany radionuclides are suitable geochemical tracers.

In this review, we focus on the developed mass spectrometricmethods for the detection of the FDNPP accident-releasedradionuclides and their applications in monitoring


Fukushima contaminants to highlight the importance of massspectrometry in radioactive contamination assessment aer theaccident. Future research directions are recommended as well.

2. Radionuclide emission from theFukushima accident

The FDNPP was equipped with six boiling water reactors (Units1–6), four of which (Units 1–4) were destroyed during the acci-dent. The reactors of Units 1–3 were in operation when theearthquake occurred. The reactor of Unit 4 had been shut downsince 30 November 2010, and the nuclear fuel was moved fromthe core to its spent nuclear fuel pool. Most of the nuclear fuelin the reactors was UO2. In the Unit 3 reactor, however, 32 of thefuel assemblies in the reactor core were mixed-oxide (UO2 +PuO2) MOX fuel with low Pu enrichment.31 The total amount ofnuclear fuel contained in the reactor cores of Units 1–3 wasabout 256 t. There was even more nuclear fuel (461 t) in thespent nuclear fuel pools in the four damaged reactor buildings.Therefore, two potential sources for the emitted radionuclidesfrom the FDNPP accident existed: the damaged nuclear reactorcores (Units 1–3) and the spent nuclear fuel pools.

The tsunamis caused by the earthquake destroyed the cool-ing systems of the reactors of Units 1–3. The temperatures andpressures in the reactor vessels raised beyond the designedspecications. At the initial stage, gases in the vessels werevented to the atmosphere by the Tokyo Electric Power Company(TEPCO) in an attempt to preserve the structural integrity.32

However, hydrogen gas released from the primary vesselsaccumulated within the superstructures above the core and ledto a series of explosions in Unit 1 and 3 reactors. Besides,another hydrogen explosion took place at the spent nuclear poolof the Unit 4 reactor as well. During those processes, signicantamounts of noble gases and volatile ssion products weredirectly released into the atmosphere. For example, it has beenevaluated that nearly 100% of 133Xe and other noble gases in thereactor cores of Units 1–3 were released into the atmosphereuntil 15 March 2011.33 For radiocesium, which has been studiedintensively, the atmospheric released fraction was estimated tobe about 4%.34 A considerable part of these atmosphericreleased radionuclides deposited on the Japanese land, espe-cially the areas around the FDNPP site, resulting in severeterrestrial radioactive contamination (Fig. 1). However, morethan 80% of the atmospheric releases were blown offshore inthe direction of the Pacic Ocean.35,37

In order to prevent an aggravation of the situation, largequantities of cooling water were injected into the reactor cores.The injected water became highly contaminated through thecontact with the nuclear fuel. As the coolant circulation wasdamaged, the radioactively contaminated waste water had to bestored separately. About 9000 m3 waste radioactive water wasdirectly discharged into the sea by the TEPCO because oflimited storage capacities.38 Although the coolant system wasrecovered later, the leakage of the radioactive liquid continuedeven two and a half years aer the accident.39 For the volatilession products (131I and 137Cs), the estimated amounts

J. Anal. At. Spectrom., 2018, 33, 519–546 | 521

Fig. 1 Deposition and distribution of 134Cs and 137Cs obtained byairborne monitoring and concurrent ground-based observations fromApril to November 2011 (redrawn from ref. 35, and data from theMEXT36).


released through direct discharge into the sea were comparableto that through atmospheric release.34,40–42 Recently, under-ground leakage of radionuclides has been identied as anunexpected source of radionuclides leaking into the PacicOcean.43 Moreover, more than 70% of the atmospheric releasedradionuclides eventually deposited over the North PacicOcean.35 Therefore, monitoring these radionuclides in themarine environment is important in support of seafood safety.

Source term estimation is essential to assess the environ-mental impact of the FDNPP accident. Radiometric and massspectrometric methods were widely used for the analysis ofvarious environmental samples contaminated by the FDNPPaccident-released radionuclides. The results for the estimatedamounts of radionuclides released from the accident collectedfrom the literature are summarized in Table 2. The overallatmospheric release of radionuclides (excluding noble gases)from the FDNPP accident was about 5.2� 1017 Bq, about 10% ofthe Chernobyl releases (5.3 � 1018 Bq).58 Besides, almost all theinventory of radioactive gas 133Xe in the Units 1–3 reactor cores,that is about 1017 Bq, has been released.33 For the two mostintensively studied ssion products 131I and 137Cs, the totalrelease amounts exceeded 1016 Bq.53,54 Although the releases of


non-volatile radionuclides such as 90Sr and actinides weremuch less than that of the volatile radionuclides, they stillpresent high radiation risk to human health due to their long-term radiotoxicity.52 For instance, the release amounts of239+240Pu and 241Pu from the FDNPP accident were estimated tobe about 109 Bq and 1011 Bq, respectively.60 These extra inputsof Pu isotopes to the environment could lead to internal radi-ation exposure via ingestion of contaminated agriculture cropsand seafood, particularly for the relatively short half-life (t1/2 ¼14.4 y) 241Pu. The ingrowth of 241Am due to the decay of 241Pumay present a new radiation risk.60 Therefore, aer the acci-dent, much attention has been paid to the contamination of Puisotopes.

3. Comparison of radiometricmethods and mass spectrometrictechniques employed for radionuclidemonitoring3.1. Traditional radiometric methods

In radionuclide monitoring, alpha spectrometry, beta countingand gamma spectrometry are the most common radiometricmethods. Beneted from their low cost and relatively easyoperation, they have been traditionally employed in the eld ofradiation protection and environmental radioactivity.

Gamma spectrometry measurement requires almost nosample pretreatment due to the high penetrating capacity ofgamma rays as well as good energy resolution, which eliminatesthe risk of contamination and allows the rapid detection ofcontaminants. Radionuclides such as 132Te, 131I, 134Cs, and137Cs are normally measured by gamma spectrometry. Rightaer the FDNPP accident, gamma spectrometry was intensivelyemployed to provide a rst-hand gamma topographic mapillustrating the distribution and dynamic variation of the largeamounts of released 134Cs, 137Cs, 131I, 129mTe, etc.63 This rapidresponsemeets the requirement for the primary investigation ofheavily contaminated areas. However, the detection limit ofgamma spectrometry (ca. 50 mBq) is typically higher than thoseof beta counting and alpha spectrometry resulting from its lowcounting efficiency (less than 10%) and the ubiquitous back-ground of gamma spectrometers.

Beta counting is a sensitive methodology for the determi-nation of beta emitters over a wide range of energies. Whilevarious kinds of gas ionization detectors are applicable, LSC ismost commonly used.64 Generally, beta counting givesa compromised detection limit (>4 mBq) compared withgamma spectrometry and alpha spectrometry. To obtain highcounting efficiency, samples are required to be prepared as thinsolid sources for most gas ionization detectors, just as thesample preparation for alpha spectrometry, by electroplate and/or micro-coprecipitation. Another drawback of beta counting isthat its lack of energy resolution due to continuous energydistribution of emitted electrons during the beta decay process.

Alpha spectrometry shows an improved detection limitcompared with the other two methods of beta counting. Limitsof detection at the level of 0.1 mBq can be realized routinely.65 It


Table 2 Estimated release amounts and percentages of radionuclides compared to the total core inventory in the FDNPP accident

Radionuclide

Total inventoryof cores 1–3at the time of theaccidenta (Bq)

Atmosphericreleaseb (Bq)

Percentage ofatmosphericrelease (%)

Liquiddischargeb (Bq)

Percentageof liquiddischarge (%) Referencesc

14C 1.0–1.6 � 1012 4489Sr 5.9 � 1018 1.96 � 1015 3.3 � 10�2 4590Sr 5.2 � 1017 1.4 � 1014 2.7 � 10�2 1 � 1015 1.9 � 10�1 46 and 47129I 2.1 � 1011 8.06 � 109 3.9 2.35–7 � 109 1.1–3.4 41, 48 and 49131I 6.0 � 1018 1–2 � 1017 1.7–3.3 6.2–12 � 1016 1–2 34, 40, 41 and 46132Te 8.7 � 1018 8.8 � 1016 1 50133Xe 1.2 � 1019 1.2–1.5 � 1019 1 � 102 33 and 51134Cs 7.2 � 1017 1.18–1.75 � 1016 1.6–2.4 3.5–16 � 1015 0.5–2.2 52–54135Cs 3.3 � 1012 6.74 � 1010 2 7.01 � 1010 2.1 13137Cs 7.0 � 1017 1.2–1.5 � 1016 1.7–2.1 3.5–27 � 1015 0.5–3.9 40, 53, 55 and 56Total U 5.7 � 1012 3.9 � 106 6.9 � 10�5 57236U 2.0 � 1012 1.2 � 106 6.1 � 10�5 57238Pu 1.5 � 1016 2–6.9 � 109 1.4–4.7 � 10�5 58 and 59239+240Pu 5.9 � 1015 1.0–2.4 � 109 1.7–4.1 � 10�5 57, 60 and 61241Pu 8.2 � 1017 1.1–2.6 � 1011 1.3–3.2 � 10�5 60241Am 1.6 � 1015 0.5–1.3 � 109 3.2–8.4 � 10�5 61242Cm 2.8 � 1017 2.7–6.5 � 109 9.6–23 � 10�7 61

a Total inventory of 14C (in fuel and fuel cladding) was cited from ref. 44 and total inventory of other radionuclides was calculated from ref. 62.b Decay corrected to 11 March 2011. c References for the released amounts of radionuclides.


is suitable for the determination of radionuclides with highspecic activities and is widely adopted for routine analysis ofalpha emitters because of its relatively high sensitivity, selec-tivity and low operational/maintenance cost. However, whenanalyzing long-lived radionuclides in environmental samples,a long counting time (e.g. 1 to 30 days) is usually necessary toachieve a low detection limit, which makes it less attractive fora rapid response in emergency situations. The rapid measure-ment of radionuclides is required as a basis for decisionmakingon the countermeasures in the immediate aermath of anaccident.66 Shortening the analytical time has always beena requisite for emergency monitoring of environmental samplesnear nuclear facilities.24 Besides, in nuclear accidents, timelyand accurate information on the level of possible internalcontamination of workers is imperative for proper medicalintervention; for such demands for rapid bioassay, alpha spec-trometry is less competitive than mass spectrometric methodsby which analysis could be nished in several hours with ordersof magnitude lower detection limits.67,68 Moreover, because ofits relatively poor energy resolution, it is difficult to resolveradionuclides with similar alpha energy peaks such as 233U and234U, and 230Th, 239Pu and 240Pu. This further hampers itsapplication in source identication as the atomic ratio (e.g.240Pu/239Pu) is a characteristic indicator.

In summary, radiometric methods are basic methods todirectly determine radiation emitters. As the respective radia-tion detection apparatuses are part of the fundamental equip-ment of environmental laboratories, radiometric methods playimportant roles in the evaluation of radioactive accidentsregarding short-lived radionuclides and/or radionuclides withsignicantly high activities. However, to obtain comprehensiveinformation of accidents and achieve quick response to


emergency situations, mass spectrometry is robust and irre-placeable especially in areas such as measurement of long-livedradionuclides, source identication with atomic ratios, fast andhigh throughput sample analysis, etc.

3.2. ICP-MS

ICP-MS was commercially introduced to researchers in the1980s and was soon applied in nuclear science, geochemistry,radiochemistry, environmental science and biologicalstudies.1,69,70 Samples containing elements of interest areintroduced into the plasma operating at high temperatures(typically >6000 K) to form ions by decomposition and ioniza-tion. Usually a sample is prepared in the form of solution andintroduced into ICP-MS either via nebulization or other sampleintroduction systems (membrane desolvation, laser ablation,online separations by liquid chromatography, etc.). Accordingto specic requirements, solid samples can also be directlyanalyzed with the use of a laser ablation system (LA-ICP-MS).Ions generated by the plasma are injected into either quadru-pole or magnetic and electrostatic sector eldmass lters whichscreen ions with a specic mass to charge ratio and nally aredetected by the detector.

Beneted from the suitable price, easy operation, highsensitivity and ability for multi-element determination, ICP-MShas been the most frequently used mass spectrometry in theanalysis of trace/ultra-trace elements and radionuclides.2,5,69,71,72

Detection limits from 10�15–10�18 g can be achieved dependingon interference and instrument sensitivity.73–76 For example,Zheng75 combined a SF-ICP-MS with a high efficiency intro-duction system APEX-Q and/or Aridus II for the determinationof Pu. A detection limit down to attogram (10�18 g) was reached,which was comparable to or even better than that of AMS.



Challenges for ICP-MS are isobaric and polyatomic interfer-ence, matrix effects and low abundance sensitivity. Isobaricinterference is a major problem for the ICP-MS analysis. Forexample, 129Xe interferes with 129I, and 238U interferes with238Pu. Some polyatomic interference can be resolved underhigher mass resolution mode with the sacrice of instrumentalsensitivity whereas other interference still cannot be distin-guished from the main analyte's signals when required reso-lution is beyond the ability of the instrument such asinterference of 239Pu from 238UH+. For the latter situation, theuse of a collision/reaction cell (DRC) in the quadrupole ICP-MScan signicantly suppress interfering signals by specic ion–molecular reaction.76

Recently, a triple quadrupole ICP-MS (ICP-MS/MS, Agilent8800) has become commercially available in 2012. The mostimportant design feature compared with other ICP-MS is thatquadrupole mass lters are set both before and aer thereaction/collision cell, which realizes “double screening” oftarget nuclides and facilitates the thorough removal of inter-ferents. Additionally, the combination of these two quadrupolestheoretically results in overall instrumental abundance sensi-tivity as the product of Q1 abundance sensitivity multiplied byQ2 abundance sensitivity, which is theoretically square of thatachieved by traditional single quadrupole ICP-MS (ICP-QMS).77

The excellent abundance sensitivity facilitates the reduction ofthe interference frommain isotopes on minor isotopes (e.g. 88Srinterference on 90Sr, 133Cs interference on 135Cs, 238U interfer-ence on 236U, etc.). Tutorial reviews on the operational princi-ples and specic considerations for the interference-freedetermination of elements with ICP-MS/MS have been made byBalcaen et al. and Bolea-Fernandez et al.11,12 In 2016, the Agilent8800 was replaced by a second-generation triple quadrupoleICP-MS Agilent 8900 with a higher sensitivity, lower instru-mental background and faster dwell time.

Since the FDNPP accident, ICP-MS/MS has been widelyemployed in the determination of 90Sr, 129I, 135Cs, 137Cs, and236U in various types of samples relating to the accident.13–15,78–80

One of the most important considerations for researchers whenmeasuring radionuclides with a collision/reaction cell coupledICP-MS is the choice of reaction gas for a specic radionuclide.Both on-mass mode and mass-shimode can be used to reduceinterference.11,76,81,82 When using ICP-MS/MS, generally, underthe on-mass mode, interfering nuclides are pre-ltered by theQ1 quadrupole and react with the reaction gas in the reaction/collision cell, followed by being further screened by the Q2mass lter. The target nuclide is less reactive, so it can be nallymeasured by the detector at the “original”m/z of the nuclide, forexample the determination of 90Sr, 129I and 135Cs with ICP-MS/MS.14,78,83,84 In contrast, under mass-shi mode, the targetnuclide is more reactive and is converted to a product ionthrough reaction with the reaction gas and nally detected ata different m/z than that of the original target ion m/z. As anexample Tanimizu et al.80 used O2 as the reaction gas to convert236U+ to 236UO+, and Q1 and Q2 mass lters were set at m/z 236and 252, respectively. The choices of reaction gases as well asthe mass lter models for the measurement of 90Sr, 129I, 135Csand 236U are illustrated in Fig. 2. By choosing a suitable reaction


gas and optimizing the instrument accordingly, signals frominterference ions could be signicantly reduced with either on-mass or mass-shi mode.

In spite of the instrumental advantages of different types ofICP-MS, another imperative step to achieve accurate determi-nation of radionuclides is to remove the interfering nuclidesand matrix elements as much as possible at sample pretreat-ment stages. For example, a high decontamination factor of238U is necessary for 239Pu measurement;20,85,86 the removal ofrare earth elements (REEs) is necessary to reduce matrix effectsfor 241Am determination,21,87,88 and the removal of high contentof matrix elements, such as K and Na, is required before 135Csdetermination.78,89 The related pretreatment methods will bediscussed in a later section of this review.

3.3. AMS

AMS rst emerged from nuclear physics laboratories in the late1970s and soon found its application in the measurement ofelements and radionuclides.90–92 Most AMS facilities consist oftwo mass spectrometry units: an injector unit and an analyzerunit, linked by a tandem accelerator. In principle, during theanalysis process, radionuclides are rst subjected to ion sput-tering to form polyatomic negative ion sources. Multiplecharged ions can be produced and the choosing of the appro-priate charge state species is one of the key issues for methoddevelopment. The sputter source utilizes usually Cs+ ions. Thenegative analyte ions are then accelerated and stripped in a lowpressure gas, disassociating the molecular ions from the targetradionuclide ions. Thereaer, the stripped positive ions areaccelerated again, then, selected by a magnetic spectrometerand nally detected in the terminal. It was developed to over-come the fundamental limitations of both radiometric spec-trometry and mass spectrometry. The same as other massspectrometry, it counts the analyzed isotopes through the mass-to-charge ratio, resulting in huge advantages for long-livedradionuclide determination compared to radiometricmethods. In addition, it shows excellent discrimination againstpolyatomic interference compared with conventional massspectrometers e.g. ICP-MS. AMS is versatile in the determina-tion of ultra-trace long-lived radionuclides such as 236U, 237Np,239Pu, 240Pu, etc. It is an extremely efficient method in destroy-ing and avoiding polyatomic isobaric interference in the courseof the analytical process.

However, there are still several disadvantages of AMS. Thehigh price of AMS restricts its accessibility for researchers. Todate there are only about 110 AMS facilities around the world,limiting its wide application.93 Moreover, most of them areemployed in 14C analysis for dating purposes and they oenlack exibility to switch from one element of interest toanother.65 Besides, it is also a bottleneck for AMS to distinguishbetween isotopes with the same mass to charge ratio. Forexample, 238Pu cannot be measured by conventional AMSbecause of the interference from 238U; the latter is generallyseveral orders of magnitude higher than 238Pu and is ubiquitousin the whole sample preparation procedure despite the besteffort of chemists. Other, lighter, radionuclides are extremely


Fig. 2 Representative schematic diagram of ICP-MS/MS for the measurement of 90Sr, 129I, 135Cs and 236U (ref. 30, 80, 83 and 84).


challenging for AMS, for example, long-lived 60Fe (T1/2 ¼ 2.6 �106 y) with its isobaric interference with 60Ni. Currently, onlytwo AMS facilities (at Technische Universitat Munchen,Germany, and Australian National University) are capable ofanalyzing 60Fe.94,95

3.4. TIMS

During the past 30 years, TIMS has been a work-horse in thedetermination of radionuclides such as uranium, plutonium,neptunium and radium.1,4 In view of its outstanding highsensitivity and high precision, TIMS is considered as a denitiveanalytical methodology and has been used as a reference tech-nique to calibrate other methods for atomic ratio measure-ment.4 Samples to be analyzed by TIMS are required to bedeposited onto a clean lament surface beforehand. Usually twoseparate laments are used in favor of decoupling the


evaporation and ionization processes. Samples are evaporatedby heating one of the laments followed by further ionization ofthe sample vapor on the other. This lament assembly alsofacilitates multi-element analysis for analysts by adjusting thelament temperatures. Low-energy spread of the ions formed inthe thermal ionization source makes it possible to separate ionswith a single magnetic sector-eld mass spectrometer. Aerseparation, isotopes pass through an exit slit and are detectedby a multi-Faraday cup detector or by a single secondary elec-tron multiplier (SEM) or an array of SEM detectors.96 To furtherimprove instrumental abundance sensitivity for minor isotopeanalysis (e.g. 236U and 230Th), the ion beam passed through themagnetic analyzer is screened by another energy lter, i.e. theretarding potential quadrupole (RPQ; on a TRITON PLUS ofThermo Fisher Scientic) or wide aperture retarding potential(WARP; on a PHOENIX of Isotopx), which typically increases theabundance sensitivity from 10�6 to 10�8 (measured at m/z 237



with respect to m/z 238). Precision better than 0.1% was ach-ieved for the atomic ratio measurement of long-lived radionu-clides with TIMS.97 Internal normalization and interferingelement correction can be applied to correct for mass bias aswell as isobaric interference, respectively. In this way, highprecision (0.001% or better) is achieved in atomic ratio anal-ysis.65 Owing to these advantages, TIMS has been employed asa standard method especially in atomic ratio measurement. Forexample Beasley et al.98 and Kelley et al.99 used TIMS to deter-mine the atomic ratios of 237Np/239Pu, 240Pu/239Pu, 241Pu/239Pu,and 242Pu/239Pu in soils collected around the world to charac-terize these anthropogenic radionuclides of global fallout. Sofar these ratios have been golden baselines for related envi-ronmental radioactivity research. Regarding the FDNPP acci-dent, TIMS has also been employed in the analysis of releasedradionuclides in environmental samples. One of the mostrepresentative examples of the application of TIMS is thesuccessive work reported by the team of Shibahara et al.22,100–103

They have comprehensively studied the possible contaminationof cesium, strontium, uranium and plutonium in variousenvironmental samples. The high precision of atomic ratiosmeasured with TIMS contributed much to the source identi-cation. For example, in their preliminary study of using134Cs/137Cs and 135Cs/137Cs atomic ratios for source analysis,plant samples containing 5 Bq of 137Cs were analyzed and thetypical analytical error was as low as 0.5% for 134Cs/137Cs and0.1% for 135Cs/137Cs atomic ratios.22 A comparable precision ofthe 135Cs/137Cs atomic ratio was also obtained later foranalyzing soil samples contaminated by the FDNPP.100

The radionuclides in the nal samples prepared for TIMSdetermination are supposed to be chemically pure. Complexmatrix or impurity elements in the ultimate samples cansuppress the ion formation process and thus deteriorate theoutput signal.104 For this reason, robust chemical separationand purication of samples before instrumental analysis is ofvital importance. Nevertheless, the sample preparation proce-dure required is usually complicated and laborious in mostcases. Besides, the measuring time for TIMS is relatively long,e.g. 2–3 hours per sample, making it less attractive comparedwith other fast and high throughput methodologies such asICP-MS. The widest application of TIMS is for the accurate andhigh precision measurement of U and Pu atomic ratios inresponse to radioactive contamination assessment.

4. Development of analyticalmethods for the determination ofradionuclides released from theFukushima nuclear accident

As mentioned above, there is a strong demand for the moni-toring of radionuclides in environmental samples contami-nated by the FDNPP accident. Laboratories related to nuclearphysics, radiochemistry, geochemistry, etc., from all over theworld carried out research on the distribution, characterizationand migration behavior of radionuclides in the environment.To some extent, this nuclear accident advanced the


development of the related research elds. In the past six anda half years, the development of analytical methods based onmass spectrometry has been made for the determination ofvarious radionuclides. These methods either improved thechemical separation procedures for sample preparation oroptimized multiple instrumental setups to achieve betteranalytical performance. It should be noted that we are notaiming to make an exhaustive review about all the methods forradionuclide monitoring developed aer 2011. Only theanalytical methods pertinent to the environmental samplescontaminated by the Fukushima accident are discussed below(Table 3).

4.1. Analytical methods for the determination of actinides

Uranium isotopes are the main components of nuclear fuels.This non-natural, enriched uranium is characterized by a shif-ted isotopic composition compared with natural uranium. Thedetermination of (non-natural) U in environmental samples isimportant not only for the radioactivity assessment purpose butalso for understanding the nuclear reactor core meltdown.Takagai et al.116 established a rapid method using ICP-MS inconjunction with microwave digestion for the analysis of 235Uand 238U in Fukushima soil samples. A mixture of nitric acidand hydrogen peroxide was used to reduce the dissolution ofnatural U in silicates. The digestion solution was diluted andanalyzed directly without chemical separation. The detectionlimit for U by this method was 0.01 ng g�1. Rapid analyticalmethods for the monitoring of U in plant samples are alsoneeded for a quick food safety test aer the accident. Zhenget al.28 developed a method for the analysis of U in vegetablesusing a SF-ICP-MS coupled with an APEX-Q high-efficiencysample introduction system. The contaminated vegetablesamples were acid-digested rst. Aer heating to dryness, theresiduals were dissolved in 10 mL nitric acid, and then thesample solutions were diluted for U analysis. This method iscapable of determination of 235U and 238U at several tens of pgg�1 concentration levels in fresh vegetables. The entire analyt-ical procedure took less than 3 h. More recently, Shibaharaet al.101 established a method for the analysis of 235U/238Uatomic ratios in plant samples from Fukushima by TIMS. Ion-exchange and extraction chromatographic methods wereemployed for U separation. The U atomic ratios were deter-mined with high precision and accuracy. Based on this method,the contamination of U in plant samples near the FDNPP sitewas investigated.

A relevant anthropogenic uranium isotope is 236U. It ismainly produced in nuclear reactors via the reaction of235U(n,g)236U. It is of scientic importance as it allows radio-active source identication via the 236U/238U atomic ratio. Forthe measurement of 236U in environmental samples by massspectrometry, the main challenges are the extremely lowconcentration of 236U (1010 atom per g or below) and the pres-ence of large excess of 235U and 238U. Normally, mass spec-trometry with abundance sensitivity for the 236U/238U atomicratio below 10�8 is required for resolving the peak tailing effectof 238U during the analysis of 236U. Furthermore, the polyatomic


Table 3 Summary of analytical methods based onmass spectrometry for the determination of radionuclides with respect to Fukushima samples

Analyticaltechniques Sample matrix LOD Remarks Reference

Pu isotopesSF-ICP-MS Marine sediment 0.14 fg mL�1 >10 g sample amount; two-stage anion-exchange chromatography; 241Pu

analysis capability20

SF-ICP-MS Seawater 0.11 fg mL�1

239Pu, 0.08 fgmL�1 240Pu

20–60 L sample amount; Fe(OH)3 co-precipitation; two-stage anion-exchange chromatography; 0.3–1 � 108 U decontamination factor

105

SF-ICP-MS Soil andsediment

0.24 fg g�1 239Pu,0.14 fg g�1 240Pu,0.09 fg g�1 241Pu

CaF2/LaF3 co-precipitation; three-stage extraction chromatography 106

AMS Seawater — 20 L sample amount; extraction chromatography 107TIMS Soil Several fg g�1 Extraction chromatography and anion-exchange chromatography 102

U isotopesSF-ICP-MS Vegetable — 10 g sample amount; acid digestion; analysis nished within 3 h 28ICP-MS/MS Soil — Extraction chromatography 93AMS Black substances 0.01 fg 236U 3–5 g sample amount; anion-exchange chromatography 57TIMS Soil 2.5 ng g�1 238U Extraction chromatography 102

237NpAMS Seawater — Fe(OH)3 co-precipitation; extraction chromatography; Pu, Np and Am

were separated simultaneously108

241AmSF-ICP-MS Soil 0.1 fg g�1 2–20 g sample amount; CaC2O4 co-precipitation; three-stage extraction

chromatography; Pu decontamination factor 7 � 10521 and 106

90SrICP-QMS Soil 0.77 pg kg�1 Microwave digestion; Sr specic resin; on-line chemical separation 29ICP-DRC-MS Snow muddy

water and soil0.6 ng L�1 Extraction chromatography; automated SPE separation 109

ICP-MS/MS Air 0.68 pg m�3 Combination of a gas-exchange device (GED) with ICP-MS/MS; real-timeanalysis of 90Sr in atmospheric particulate matter; short analytical runtime (10 min)

30

129IICP-DRC-MS Radioactive

water andprocessed water

— 0.1 mL sample; solid phase extraction; 10�6 g 127I carrier; 94% chemicalrecovery

110

ICP-ORS-MS Soil — Pyrohydrolysis and solvent extraction 79ICP-MS/MS Soil — Pyrohydrolysis and solvent extraction; O2 as the reaction gas for 129Xe

signal suppressing84

ICP-SF-MS Snow 0.7 pg g�1 Ion chromatography for isobaric interference elimination; 129Xeinterference is stable to be treated as background correction

111

AMS Aerosol 1.5 fg Sequential extraction, iodine speciation analysis 112

135Cs–137CsICP-MS/MS Rainwater 0.04 pg mL�1

134Cs, 0.12 pgmL�1 135Cs, 0.28pg mL�1 137Cs

Concentrated 100–200 times; without chemical separation 14

ICP-MS/MS Litter, lichen andsoil

2 fg mL�1 135Cs,2 fg mL�1 137Cs

AMP sorption; anion-exchange and cation-exchange chromatography;104–107 decontamination factors for major matrix and interferingelements

13 and 83

ICP-MS/MS River suspendedparticle

10 fg mL�1 135Cs,7 fg mL�1 137Cs

AMP sorption; anion-exchange and cation-exchange chromatography;radiocesium and Pu isotopes were determined simultaneously

113

ICP-MS/MS Soil — AMP sorption; single anion-exchange chromatography 15TIMS Soil — AMP sorption; anion-exchange and cation-exchange chromatography;

TaO as the Cs ionization activator on the Re lament22 and100–103

TIMS Soil andvegetation

— AMP-PAN, anion-exchange chromatography and microcation exchangechromatography resins for Cs separation; Re lament

104

This journal is © The Royal Society of Chemistry 2018 J. Anal. At. Spectrom., 2018, 33, 519–546 | 527


Table 3 (Contd. )

Analyticaltechniques Sample matrix LOD Remarks Reference

TIMS Soil <12 fg g�1,137Cs/133Cs< 10�8

AMP-PAN, anion-exchange chromatography and Sr-spec resins for Srseparation; Rb decontamination factor > 600; glucose as the Csionization activator on the Re lament

114

132TeSF-ICP-MS Soil and plant 0.12 pg mL�1

125Te; 0.17 pgmL�1 126Te

Acid digestion; analysis without chemical separation 115


interference of 235UH+ needs to be carefully checked as it cannotbe distinguished from the 236U signals. AMS is the most widelyused technique for the analysis of 236U in environmentalsamples with a very low 236U content (236U/238U � 10�6).117 Theabundance sensitivity for the 236U/238U atomic ratio of AMScould be as low as 10�14–10�13.118,119 A detailed comparison ofdifferent instrumental setups of AMS for the analysis of 236Uwas recently summarized by Bu et al. (ref. 119 and referencestherein). Aer the FDNPP accident, AMS has been used for theanalysis of 236U in various environmental samples.57,120–124 Forexample, in order to identify the contamination sources of theblack materials (mixture of road dust and organic matter)collected from the roadside in Fukushima Prefecture, Saka-guchi et al.57 used AMS for the measurement of 236U. The236U/238U atomic ratios in these samples were successfully ob-tained and the released amount of 236U from the FDNPP acci-dent was estimated. ICP-MS/MS also showed the potential forthe determination of ultra-trace level 236U as its theoreticalabundance sensitivity could be as low as 10�14.11 Tanimizuet al.80 rst employed ICP-MS/MS for the analysis of 236U.Oxygen was introduced as the reaction gas into the collision/reaction cell. Ion-molecule reactions with O2 between U+ andUH+ sufficiently reduced the effect of 235UH+ on 236U+ analysis.The signals of UO+ instead of U+ were detected for 236Umeasurement. Based on the excellent abundance sensitivity ofthis instrument, the peak tailing effect of 238U could also beensolved. The detection limit for 236U was about 1 fg and themeasurement of the 236U/238U atomic ratio down to the level of10�10 was achieved. Although this detection limit is higher thanthe typical natural background of the 236U/238U ratio (10�14–

10�13),117 it was competent to resolve global-fallout contami-nated environmental samples with a typically 236U/238U of 10�8–

10�9.122,125,126 Based on the instrumental setups, Yang et al.93

further explored this method for the determination of 236U inreal environmental samples. A single extraction chromato-graphic procedure using DGA resin was employed for theseparation of U. The concentrations and 236U/238U atomic ratiosin several reference materials (soil and sediment) and Fukush-ima soil samples were investigated. A very recent studysuccessfully employed ICP-MS/MS for the isotopic analysis of236U/238U in soil samples from Fukushima; meanwhile theactivity concentration of 238Pu, 239+240Pu and 240Pu/239Pu atomicratios of these samples were also determined by alpha spec-trometry and SF-ICP-MS.127


More attention has been paid to Pu isotopes since theconrmation of their atmospheric releases into the terrestrialenvironment from the FNDPP accident.60 Pu isotopes wereproduced continuously in the Fukushima nuclear reactor unitssince the beginning of their operation, especially in the Unit 3reactor, where MOX fuel assemblies containing about 6% Puwere initially loaded. Analytical methods based on mass spec-trometric techniques for the determination of 239Pu and 240Puin environmental samples have been well established andreviewed previously.6,128 However, few studies focused on theanalysis of 241Pu. The concentration of 241Pu and the atomicratio of 241Pu/239Pu in environmental samples can serve asuseful indicators for radioactive source identication at theearly stage of a nuclear accident because the background of241Pu from the deposition of global fallout in the environment iscurrently very low (<0.5 fg g�1 in surface soil) due to its relativelyshort half-life (14.4 y).

For a more comprehensive assessment of Pu contaminationin the environment aer the accident, the distribution andcharacterization of Pu isotopes in sediment and seawatersamples collected around the FDNPP site in the Pacic need tobe analyzed. Marine sediment samples with large samplequantities (>10 g) are required for 241Pu analysis by ICP-MS.When analyzing Pu isotopes by mass spectrometry, the forma-tion of uranium hydride species 238UH+ and 238UH2

+ and thepeak tailing effect of 238U+ are the main problems as theconcentration of U is usually several orders of magnitude higherthan that of Pu in environmental samples. Therefore, highsensitivity of the mass spectrometry instrument and a high Udecontamination factor for sufficient separation of Pu from Uare needed for the method applied for the Fukushima sedimentsamples. A new SF-ICP-MS (Element XR) with a jet interface isone of the most sensitive mass spectrometric instruments.Coupled with high efficiency sample introduction systems, itsdetection limit could be as low as the attogram level (10�18 g),showing great potential for the ultra-trace determination of Puisotopes in environmental samples.75 Bu et al.20 establisheda two stage anion-exchange chromatographic separationmethod for the analysis of Pu isotopes in high U content marinesediments. A U decontamination factor higher than 106 wasachieved and the concentrations of U in the nal sample solu-tions for SF-ICP-MS analysis were below 4 pg mL�1 (the samelevel as the operational blank). The typical mass spectra of theoperational blank and Fukushima marine sediment sample



obtained by this method are shown in Fig. 3. Considering thatthe 238UH+/238U+ ratio for the SF-ICP-MS system was about 10�5,the contributions of uranium hydrides to Pu signals was lessthan 1 cps. More recently, Wang et al.106 systematically investi-gated the behaviors of Pu and interfering elements (U, Pb, Bi, Tl,Hg, Pt, etc.) on extraction chromatography resins. They devel-oped a TEVA + UTEVA + DGA procedure for the separation of Pufrom sediment and soil samples prior to SF-ICP-MS analysis.Very low detection limits (0.24 fg g�1 239Pu, 0.14 fg g�1 240Pu,and 0.09 fg g�1 241Pu) for Pu isotopes were reported. Thismethod could be used for the long-term monitoring of Pucontamination in environmental samples around the FDNPPsite.

The determination of Pu isotopes in seawater is challengingdue to the low concentration (0.2–0.7 pg m�3 in the surfaceseawater of the Pacic) of Pu and the large sample volume todeal with.129,130 When using the conventional radiometricmethod, ca. 200 L seawater is needed and the counting time canbe as long as several weeks. Aer the FDNPP accident, seawatersamples from the Pacic were routinely collected for the anal-ysis of radionuclides derived from the accident. Normally, theseawater sample volume from each marine station is limited to20–60 L due to the difficulty of seawater sampling and trans-portation. Therefore, a rapid method for the accurate determi-nation of Pu isotopes in such a small volume of Fukushimaseawater samples is required. Bu et al.105 developed a newmethod for the determination of Pu isotopes in Fukushimaseawater samples by SF-ICP-MS. Pu isotopes were rst co-precipitated with Fe(OH)3 and then a two stage anion-exchange chromatographic method using Dowex 1X8 resinwas employed for Pu separation. This method exhibited

Fig. 3 Typical mass spectra of operational blank and Fukushimasediment samples detected by SF-ICP-MS (242Pu was used as the yieldtracer) (reprinted with permission from {ref. 20, Bu et al. et al., Environ.Sci. Technol. 2014, 48, 9070}. Copyright {2014} American ChemicalSociety).


sufficient U removal ability and the Pu chemical recovery wasabout 67%. The detection limits for 239Pu and 240Pu were 4.4 fgm�3 and 3.5 fg m�3, respectively, when 20 L seawater was usedfor measurement. This method was further modied by addinganother TEVA resin column for Pu purication prior to SF-ICP-MS analysis.131 Up to 100 L seawater could be dealt with andcomparable Pu detection limits were achieved. Hain et al.108 alsomade efforts to determine Pu isotopes in Fukushima seawatersamples using AMS. Aer the Fe(OH)3 co-precipitation, a singleextraction chromatography column (TEVA resin) was used forthe separation of Pu. This method was capable of analyzing239Pu, 240Pu and 241Pu in 20 L Fukushima seawater samplesalthough the 241Pu data showed large uncertainties.107

Using the method reported by Hain et al.108 237Np can bedetermined with Pu isotopes in seawater samples by AMSsimultaneously. The different adsorption behaviors of Np andPu on the TEVA resin ensured the sequential elution of thesetwo elements. 239Np was added as a yield monitor and thechemical recovery of Np was more than 80%. The concentrationof 237Np in a seawater reference material (IAEA-443) wasmeasured and the result was comparable with the literaturevalue. The lack of a suitable commercial tracer is a key limita-tion for the accurate determination of 237Np by mass spec-trometry. The short-lived (t1/2 ¼ 2.3 d) tracer 239Np used by Hainet al.108 which can be produced by neutron irradiation of 238U orseparated from 243Am solution is difficult to obtain and cannotbe stored for a long time. Another Np isotope 236Np (t1/2 ¼ 1.5 �105 y) could be amore appropriate yield tracer for the analysis of237Np. However, it is currently not commercially available insuitable purity.

Besides U, Pu and Np, the contamination of the FDNPP-released 241Am also attracted scientic interest. Traditionally,241Am has been measured by a spectrometry. However, thedeveloped mass spectrometry techniques achieved a similar oreven better detection limit for 241Am than that obtained bya spectrometry with a much shorter measuring time.132 Wanget al.21 developed a new method for the determination of 241Amin large soil samples by SF-ICP-MS. A CaC2O4 co-precipitationmethod was used for major metal removal, followed bya UTEVA + DGA-N + TEVA procedure for Am/interfering elementseparation. An excellent Pu decontamination factor (7 � 105)and 241Am detection limit (0.024 fg mL�1) were obtained. Amore comprehensive investigation about the extraction behav-iors of 241Am and the interfering elements (Pu, Bi, Hf, Hg, Pb,etc.) was carried out later by the same research group.133

Chemical separation methods using extraction resins (TRU,DGA-N and DGA-B) were proposed. These methods will be usedfor the determination of 241Am in Fukushima soil samples andfor the evaluation of the soil–plant transfer factor of 241Am fordose assessment.

4.2. Analytical methods for the determination of ssionproducts

In this section, we will focus on the developed analyticalmethods for the determination of ssion products, mainly 129I,135Cs, 137Cs and 90Sr. During the FDNPP accident, large



amounts of long-lived 129I were released into the environmenttogether with 131I and other I isotopes. It was transportedthrough atmospheric dispersion and seawater movement andcaused worldwide radioactive contamination.45,49,134 Before theFDNPP accident, the distribution and behavior of 129I in theenvironment had already been intensively studied, as 129I isregarded as a useful tracer for geochemistry and oceanography.Mass spectrometry is the most widely employed method for theanalysis of 129I in environmental samples although othermethods such as LSC and NAA have also been used.135,136

Comprehensive reviews about methods for the determination of129I in environmental samples have been published previ-ously.137–140 There were a few studies aimed at developinganalytical methods based on mass spectrometry for the deter-mination of 129I with respect to Fukushima samples.79,84,141

Zhang et al.112 developed a new analytical method for thespeciation analysis of 129I in aerosol samples. The species(iodide, iodate, NaOH soluble iodine, and insoluble iodine)were sequentially extracted and chromatographically separatedfor mass spectrometry measurement. ICP-MS was used for theanalysis of the concentration of stable 127I and AMS was used forthe detection of the atomic ratio of 129I/127I. The detection limitfor 129I was reported to be 1.5 fg. This method was lateremployed for the speciation analysis of 129I in European aerosolsamples to identify the possible contamination from the FDNPPaccident.141 In order to determine 129I in snow samples, Ezer-inskis et al.111 established a novel analytical approach using SF-ICP-MS. Ion chromatography was used for the separation of 129Ifrom the sample matrix and interfering elements. When usingICP-MS for the analysis of 129I, the biggest problem is theisobaric interference of 129Xe, which exists as a trace impurity inthe Ar carrier gas. The authors monitored Xe isotopes for a longanalysis period and found that the Xe signals were stable.Therefore, the 129Xe interference for 129I analysis could be cor-rected as a stable background. The LOD for 129I was 0.7 pg g�1.As for the elimination of the 129Xe isobaric interference, an ICP-QMS equipped with a collision/reaction cell is favored overother types of ICP-MS instruments for the analysis of 129I.142–145

Ohno et al.79 applied ICP-QMS (Agilent 7700x, Agilent Tech-nologies Inc) for the determination of 129I in Fukushima soilsamples. The interfering 129Xe+ signal was suppressed by theintroduction of O2 as a collision gas using the following reac-tion: Xe+ + O2 / Xe + O2

+. A twenty-fold improvement of the127I+/129Xe+ ratio was achieved when the oxygen ow rate wasoptimized to be 0.8 mL min�1. The result was comparable withthat obtained by Izmer et al.143 using another type of ICP-QMS(Platform ICP, Micromass Ltd.). In a previous study, Eidenet al.142 reported a 104 fold faster reaction of Xe with O2

compared with I in a plasma source ion trap mass spectrometer(VG Element PlasmaQuad I), showing great ability for reducingthe Xe background although the experimental data were notprovided. However, when using O2 as the reaction gas in ICP-QMS the 127I intensity decreased (almost 100 times) due tothe positive voltage gap between the cell and the quadrupole forthe energy discrimination.79 Ohno et al.84 further tested theability of the newly updated ICP-MS/MS instrument and usedthe negative voltage gap between the cell and the quadrupole for


the analysis of 129I. A 1000-fold improvement of the signal/noiseratio was achieved without depression of the 127I intensity. Thebackground signal for 129I detection was reduced to about 0.2cps. In a recent study, Shikamori et al.146 used a 2nd generationICP-MS/MS, Agilent 8900, to analyze 129I. By applying an axialacceleration voltage to the octopole rods, higher instrumentalsensitivity and better discrimination of interferents were real-ized. Based on the instrumental conditions, they investigatedthe effect of IH2 and ID interference on 129I measurement indetail using MS/MS mode and axial acceleration.146

134Cs and 137Cs are the most widely studied radionuclidesreleased from the FDNPP accident. Their distributions andbehaviors in various environmental samples have been inves-tigated by radiometric methods (gamma spectrometry). Inrecent years, much scientic attention has been paid also to135Cs as the 135Cs/137Cs atomic ratio has proven to be animportant long-term tracer for radioactive source identica-tion.13 Efforts were made to measure the 135Cs/137Cs atomicratio by mass spectrometry especially aer the FDNPP accident.The state-of-the-art for the measurements of 135Cs and 137Cs bymass spectrometric techniques and their applications to envi-ronmental samples has been reviewed by Russell et al.147 and inour previous work.148 As for the monitoring of 135Cs and 137Cs inFukushima samples, methods based on TIMS and ICP-MS havebeen established and will be briey discussed here.

Ohno et al.14 rst investigated the potential applicability ofICP-MS/MS for the determination of the 135Cs/137Cs atomic ratioin Fukushima accident contaminated rainwater samples. N2Ogas was introduced in the reaction cell to suppress the signals ofthe isobaric interference of 135Ba and 137Ba. Moreover, theadditional quadrupole mass lter located in front of the reac-tion cell allowed only the analyte mass to enter the cell and thuslimited the formation of the polyatomic interferents 119Sn16Oand 121Sb16O. The representative schematic diagram of the ICP-MS/MS for the analysis of 135Cs is shown in Fig. 2c. The detec-tion limits for 135Cs and 137Cs in samples containing 100 ppb Bawere 0.10 pg mL�1 and 0.27 pg mL�1, respectively andFukushima rainwater samples could be concentrated andmeasured directly without any chemical separation. For thedetermination of 135Cs and 137Cs in other environmentalsamples with more complicated sample matrices, however,chemical separation is needed prior to ICP-MS/MS analysis. Inorder to determine the 135Cs/137Cs atomic ratios in highlycontaminated Fukushima soil and litter samples, Zheng et al.83

established a chemical procedure combined with AMP sorption,anion-exchange chromatography and cation-exchange chro-matography for the separation of Cs. High decontaminationfactors were achieved and ranged from 104 to 105 for theinterfering elements Ba, Mo, Sb, and Sn. Aer appropriateoptimization of the ow rate of N2O gas to eliminate polyatomicinterferents such as 40Ar95Mo+, 119Sn16O+, 121Sb16O+, and40Ar97Mo+, the detection limits for 135Cs and 137Cs wereimproved to the level of 0.01 pg mL�1. This method was furthermodied to separate Cs from sample matrices (alkali andalkaline earth elements), and demonstrated its capability forthe determination of 135Cs and 137Cs in soil samples with largesample quantities (10–40 g) for the accurate measurement of



the 135Cs/137Cs atomic ratios in environmental samplescontaminated by global fallout.78

Compared to ICP-MS, TIMS is a more powerful technique forthe measurement of the 135Cs/137Cs atomic ratio due to itshigher precision and accuracy for atomic ratio analysis. Cs canbe ionized from the lament and detected in TIMS with a rela-tively low lament current (�1 A) while the ionization of Baoccurs to a lower extent due to the differences of the ionizationpotentials of Cs and Ba. Shibahara et al.22 developed ananalytical method for the determination of the 135Cs/137Csatomic ratios in Fukushima accident contaminated environ-mental samples. This method separates Cs from the samplematrix using procedures that are similar to those reported byZheng et al.83 involving AMP sorption, anion-exchange chro-matography and cation-exchange chromatography. The ulti-mate samples were loaded onto single Re laments for themeasurement and a TaO activator was used to assist in theformation of strong and stable ion beams. When using TIMS forthe measurement of radionuclides in environmental samples,the sample solutions need to be prepared in the size of severalmL for sample loading. Therefore, chemical separation methodswith high decontamination factors are required not only formass spectral interferents but also for alkali metals. Snowet al.104 established a novel method for the separation of Csprior to TIMS analysis. An AMP-PAN column replacing AMPpowder was used for the sorption of Cs from sample solutionand the initial separation of Cs from other alkali metals. Aerthe anion-exchange and cation-exchange chromatographyseparation procedure as reported in other studies, a micro-cation exchange column was employed for the further cleanupof the sample solution. This method improved the Cs ionintensities by several orders of magnitude. By using thismethod, the 135Cs/137Cs atomic ratios in environmentalsamples in which the concentrations of 137Cs were even below0.03 pg could be determined with high precisions.149,150 Morerecently, Dunne et al.114 conducted a systematic investigationusing TIMS for the determination of the 135Cs/137Cs atomicratio in environmental samples. A chemical separation proce-dure modied from that reported by Snow et al.104 usinga combination of double AMP-PAN separation and Sr-spec resincolumn purication was developed. This method showedexcellent separation of Cs from Rb (decontamination factors >600), which suppressed the ionization of Cs. The sample solu-tions were loaded onto single Re laments. When employingglucose as a Cs emission activator, the ionization efficiency ofCs was improved by more than two orders of magnitude withoutintroducing extra polyatomic interference. The effects of scat-tered 133Cs+ on the detection of 135Cs and 137Cs were investi-gated and instrumental optimizations for 133Cs+ signalcollection with a Faraday-cup and RPQ lens voltages for ionltering were suggested. This method was capable of quanti-fying trace amounts (12 fg g�1) of radiocesium at extremely low135,137Cs/133Cs levels (<1 � 10�8).

Another ssion product that attracted broad scienticinterest is 90Sr. The Fukushima nuclear accident is not typicallyassociated with the release of 90Sr, as the releases were lower bya factor of about 103 compared with 137Cs.151 The measurement


of 90Sr has been conventionally achieved by liquid scintillation.Aer chemical separation and isolation, the 90Sr fraction can bemeasured promptly; however, detection limits are improvedaer the ingrowth of its beta emitting daughter nuclide 90Y. Ittakes about 3 weeks for the complete ingrowth of 90Y intoradioactive equilibrium with 90Sr and as much as 24 h for theanalysis.148 Faster measurement methods for a rapid responseaer a nuclear accident by mass spectrometry methods are thusdesirable. Developments have been made using ICP-MS for theanalysis of 90Sr at ultra-trace levels in various samples.142,152–155

Aer the Fukushima accident, Sakama et al.109 appliedsequential automated SPE separation equipment coupled toICP-MS for the analysis of 90Sr in Fukushima samples. Theautomated SPE separation equipment incorporated an Insert-Sep ME-1 column which could reduce the chemical separationtime. Nevertheless, the detection limit for 90Sr was as high asseveral tens of pg kg�1 due to the poor decontamination of theisobaric interference from 90Zr. Takagai et al.29 also used an on-line chelate column for 90Sr pre-concentration prior to ICP-QMSanalysis. Oxygen was introduced at an appropriate ow rate intothe reaction cell to achieve the highest Sr signal to interferencesignal ratio. They suppressed the signals of the interfering Zrand Y signicantly (six to seven orders of magnitude) while theintensity of the Sr signal remained almost stable. The detectionlimit for 90Sr was reported to be 0.46 fg mL�1 (2.3 Bq L�1),making it possible for the analysis of 90Sr in Fukushimasamples. This method could measure more than 80 soilsamples per day, showing good ability for nuclear emergencyresponse. Recently, Suzuki et al.30 developed a new system forrapid real-time analysis of 90Sr in atmospheric particulatematter. Air mass was rstly introduced into an impactor toselect PM2.5 particulate matter; then part of the sample wastransferred to a gas-exchange device to replace air with Ar; aerthat, this portion of the sample was introduced into the ICP-MS/MS by means of a micro-diaphragm gas-sampling pump.Systematic investigation has been made to choose appropriatereaction gases and optimize the ow rates in order to minimizethe interference from 90Zr+, 89YH+ and 90Y+. With the optimizedmixed reaction gases: 1.0 mL min�1 O2, 10.0 mL min�1 H2 and1.0 mL min�1 NH3, the detection limit of this system was esti-mated to be 6.7 � 10�4 fg cm�3 (3.4 � 10�6 Bq cm�3) for real-time monitoring of PM2.5. This detection limit was sufficientto satisfy the regulation requirement in Japan as the lowestregulation concentrations in the atmosphere and exhaust are 7� 10�4 and 5 � 10�6 Bq cm�3, respectively.156 Other researchgroups also devoted themselves to developing analyticalmethods for the determination of 90Sr in low-level contami-nated environmental samples using AMS and ICP-MS/MS.157–159

Currently, the detection limit for 90Sr is as low as 1.3 pg g�1 forsoil samples.159

4.3. Analytical methods for the determination of otherrelated nuclides

Radioactive tellurium isotopes, including 127mTe (T1/2 ¼ 109 d),129mTe (T1/2 ¼ 33.6 d), 131mTe (T1/2 ¼ 30 h) and 132Te (T1/2 ¼3.204 d), represent important contributors to the inhalation



dose during the early stage of the FDNPP accident because thetotal released amounts were as high as 1017 Bq. Soil-to-planttransfer factors (the ratio of concentration of the nuclide inthe plant to that in the soil) of Te is essential for the accurateestimation and reconstruction of the internal radiation dose. Asthe half-lives of radioactive tellurium isotopes are relativelyshort, the direct acquisition of their transfer factors is difficult.An alternative option is to determine the soil-to-plant transferfactor of stable Te. Yang et al.115 reported a rapid and sensitiveanalytical method for the determination of Te in soil and plantsamples using SF-ICP-MS. The samples were digested with aquaregia and diluted to an appropriate volume. Aer adding Rh asthe internal standard and ltering through a 0.45 mm syringelter, the sample solution was directly analyzed by SF-ICP-MSwithout any further chemical separation. The isobaric andpolyatomic interference for Te isotopes was investigatedsystematically and 126Te was nally chosen for the analysis. Thedetection limits for Te are 0.17 ng g�1 and 0.02 ng g�1 for soiland plant samples, respectively. Based on this method, the soil-to-plant transfer factors of Te in large amounts of agriculturalsoil and associated crop samples in Japan have beendetermined.115,160

The long-lived activation product 60Fe (T1/2 ¼ 2.6 � 106 y) isof interest for the interpretation of early results showing therelease of 59Fe (T1/2 ¼ 44.5 d) into the environment. It wasunclear if this radioiron was an activation product of the reactorvessel material or if it originated from iron-containing scales onthe surface or the nuclear fuel elements. Iron-60 is the unusualresult of double neutron capture in iron (or steel components),following the reactions 58Fe(n,g)59Fe(n,g)60Fe, which requiresextremely high neutron ux densities to allow for its formation,especially since 59Fe is a relatively short-lived nuclide. Thedetermination of iron-60 is extremely challenging due to itsisobaric interference by ubiquitous 60Ni, which cannot beresolved by chemical pre-treatment (as even minute traces ofresidual 60Ni aer chemical separation will largely exceed theabundance of the rare radionuclide 60Fe). When accelerated to150 MeV, the nuclides collide with gas at low atmosphericpressure in a magnetic eld. Both elements, Ni and Fe, will thenget a different charge distribution, depending on their differentchemical behaviors. Upon detection, different braking behav-iors will allow 60Fe to be distinguished from 60Ni. This can onlybe done at large AMS facilities, i.e. Technische UniverstitatMunchen, Germany and at Australian National University.

5. Monitoring of radionuclides invarious environmental samples

By using the developed mass spectrometry based analyticalmethods, the FDNPP-accident-released radionuclides in variousenvironmental samples have been determined. In this section,we will focus on the applications of mass spectrometry for themonitoring of real Fukushima environmental samples. Thesestudies contribute a lot to radioactive contamination assess-ment aer the accident and have great implications for futureradiological research.


5.1. 14C

Radiocarbon is either produced naturally in the stratosphere bycosmic rays or articially by human nuclear activities. The mostprominent nuclear reaction for the production of 14C is14N(n,p)14C, which also takes place in the high atmosphere. Inaddition, it is also produced by neutron capture in 13C(13C(n,g)14C) and by nuclear ssion as a ternary ssion product(ssion yields are 1.6 � 10�4 for 235U and 3 � 10�4 for 239Pu).Relevant anthropogenic nuclear activities responsible for therelease of 14C include primarily nuclear weapon tests, and toa minor extent nuclear fuel reprocessing plant discharges, andnuclear power plant accidents. The assessment of the release of14C from a damaged nuclear reactor can provide importantinformation for tracer studies for the application of 14C inenvironmental and climate change research. AMS is the mostpowerful method for the analysis of 14C in environmentalsamples and high precision for 14C/12C atomic ratio analysis canbe achieved in a short analysis time with a small sample amount(1 mg).161

Xu et al.44,162 investigated the distributions of 14C in the ringsof two >30 year old Japanese cedar trees in the southwestdirection (1–50 km) of the FDNPP site. The annual records of14C from the pre-accident to the post-accident time were ob-tained. Small but visible 14C pulses were observed in the 2011tree rings, suggesting the release of extra 14C into the environ-ment during the FDNPP accident. Recently, the same groupconducted a more comprehensive study about the distributionsof 14C in the rings of Japanese cedar trees from six sites rangingfrom 2.5–38 km northwest and north of the FDNPP site.163 Therelease of 14C during the accident was further conrmed by thehigh-resolution 14C analysis of the 2011 ring. Another study164

found elevated 14C levels in tree rings in the area southwest ofthe FDNPP; however, the exact source of this radiocarbon hasnot been fully claried. Although these studies suggest thatelevated 14C levels can be observed frequently, it should not beforgotten that 14C is a quite exotic Fukushima-nuclide andmany studies have failed to observe a statistically signicantimpact from the accident.165,166 Given the complications in 14Cmeasurements, it may be useful to know that a study founda clear correlation between 14C and 137Cs (which is easily andrapidly measured) in soil.167 Due to the long-range trans-portation of 14C in the atmosphere, global impact of the FDNPPreleased 14C could be observed. For example, Park et al.168,169

analyzed the concentrations of 14C in tree leaves collected fromKorea aer the accident by AMS and they found that thedecrease rate of D14C between 2010 and 2011 was smaller thanthose of other years. It has been reported that the additionalannual effective dose caused by the excess 14C was less than 2mSv via the food ingestion pathway in the location �1 kmsouthwest of the FDNPP site.170 As 14C is treated as a tracer tounderstand ocean and ocean-atmospheric processes, thebehavior of 14C in the marine environment aer the FDNPPaccident is also of scientic interest. Povinec et al.171 collectedseawater samples in the Pacic from 30 km to about 600 kmeast of the Japanese coast aer the FDNPP accident andmeasured D14C in these samples. When compared with the pre-



accident results, they concluded that 6% to 9% of 14C in thesurface and 100–200 m depth seawater originated from theFDNPP accident. The impact of the accident on the 14C level inthe near coastal areas of Fukushima could be even stronger.

5.2. 90Sr

As the amount of 90Sr activity released was among the highestfrom the FDNPP accident as shown in Table 2, studies on boththe determination methodology and the environmentalbehavior of 90Sr derived from the FDNPP could be very neces-sary. On the one hand, the deposition of 90Sr could occur withrainfall or other precipitation processes and thereaer stron-tium is very accessible to plants through the soil–plant pathwaysince transfer factors of 90Sr were generally at the level of 10�1

for various plants and/or crops.172,173 In addition, in the aquaticenvironment, strontium would be quickly concentrated byaquatic biota, for example the concentration ratio (the ratio ofactivity concentration of the radionuclide in the biota on a perunit tissue fresh weight basis to that in water) of strontium wastypically 1.9� 102 L kg�1 for freshwater sh (for the whole body)as recommended by the IAEA.173 Once ingested or inhaled intothe human body, 90Sr would behave similarly to calcium andabout 20–30% of the total ingested 90Sr would accumulate inbones or teeth.174 Due to its short half-life (28.8 y) and b� decayproperties, it would pose the risk of cancer especially bonecancer and leukemia and the damage might increase when 90Sris in equilibrium with its short-lived b-emitter daughter 90Y.174

Although efforts have been made to develop rapid analyticalmethods based on mass spectrometry for the determination of90Sr in environmental samples, the real applications of thesemethods for Fukushima sample monitoring are currentlylimited. To the best of our knowledge, the results for 90Srdistribution in Fukushima samples obtained by using a massspectrometric technique were only given by Takagai et al.29 Theyanalyzed several soil samples from two sites 10 km and 20 kmnorthwest of the FDNPP site by ICP-QMS. The 90Sr activitiesranged from 10.3 pg kg�1 (52.1 Bq kg�1) to 14.5 pg kg�1 (73.5 Bqkg�1), comparable with those determined by traditional radio-metric methods.151,175 In the study of Odashima et al.176 theydeveloped an analytical method coupling online solid phaseextraction and ICP-MS detection of 90Sr in radioactive waste-water samples targeting the future decommissioning process ofthe FDNPP. One actual waste water sample from the FDNPP wasanalyzed, and the quantitative result was 5.0 pg (25.2 Bq), whichwas comparable to that (23.6 Bq) obtained by traditionalradiometric measurement.176

As discussed before, the mass spectrometry-based methodsfor 90Sr analysis showed a high sample throughput andacceptable detection limit, and more applications of the massspectrometry techniques for the routine monitoring of 90Sr inFukushima accident contaminated environmental samplescould be expected.

5.3. 129I

During the FDNPP accident, a large quantity of 131I (T1/2 ¼ 8.01d) was released into the environment (Table 2) and it was


regarded as one of the most important contributors to theradiation dose. Intensive investigations about the distributionsof 131I in the environment were conducted at the early stageaer the accident. However, the measurement of 131I becameimpossible months aer the accident due to its relatively shorthalf-life. Another iodine isotope 129I (T1/2 ¼ 1.57 � 107 y) wasstudied to reconstruct the distributions of 131I and the doses itcontributed to the population years aer the accident. Theatomic ratios of 129I/131I for the FDNPP accident contaminatedenvironmental samples were reported to be 20–30.177,178 More-over, 129I has also been shown to be a useful tracer forgeochemical studies.137,140 We summarized the related investi-gations about 129I distributions in the environment aer theFDNPP accident in Table 4.

Xu et al.163,179 investigated the 129I concentrations and 129I/127Iatomic ratios in long-term rainwater samples collected fromFukushima before and aer the FDNPP accident. In the samplecollected in March 2011, the 129I concentration was found to be7.6� 1011 atom L�1,�4 orders of magnitude higher than that ofthe samples collected before the FDNPP accident. A high 129I/127Iatomic ratio up to 7 � 10�5 was also observed for the samplescollected right aer the accident. In the recent work of Yanget al.,185 they analyzed 60 soil samples collected from April toJune 2011 which were contaminated by the FDNPP accident.They found that the maximum values of both 129I concentrationand 129I/127I atomic ratios were �3 orders of magnitude higherthan those before the accident.185 Those results strongly sug-gested that the FDNPP accident caused 129I contamination in theenvironment. During the three years aer the accident, theconcentrations of 129I in rainwater samples showed a decliningtrend. In the samples collected in 2013 and 2014, the 129I/127Iatomic ratios decreased down to 10�8, comparable with thatobserved in atmospheric fallout collected in Japan before theFDNPP accident.190,191 The speciation analysis of the FDNPPreleased radiocesium and 129I in aerosol samples revealed thatchemical forms of radiocesium and radioiodine varied signi-cantly and different dispersion and deposition patterns couldoccur aer the atmospheric release of these two radionuclidesfrom the FDNPP accident.180 Long-range atmospheric dispersionand contamination of 131I from the FDNPP accident wereconrmed in Europe aer the accident although the contami-nation level was quite low compared to the inuence of theChernobyl accident.192 Recently, Gomez-Guzman et al.134 alsoinvestigated the distributions of 129I in aerosol and gaseoussamples collected in Spain aer the FDNPP accident by AMS. Theconcentrations (105 atom per m3 in gas) of 129I were similar tothose observed globally before the FDNPP accident, suggestingthat the impact of the accident on these areas was very limited.

In order to reconstruct the distributions of 131I in theterrestrial environment, investigations about 129I in soilsamples collected around the FDNPP site have been conducted.Miyake et al.177 and Muramatsu et al.178 analyzed 131I and 129I insurface soils collected within 100 km around the FDNPP site.Fujiwara181 determined 131I and 129I in deposit samplescollected in Tsukuba (�180 km from the FDNPP site). Theresults for the deposition densities of 129I and 131I weresummarized and are plotted in Fig. 4. We can see that there was


Table 4 Summary of investigations about 129I distributions in environmental samples following the FDNPP accident

Location Sample type Sampling time

Distance fromthe FDNPPsite (km)

Analyticaltechnique 129I concentration

129I/127I atomicratio Reference

Fukushima Rainwater March 2011 toDecember 2012

�60 AMS 0.13–98.6 mBq m�3 0.19–68.9 � 10�6 179

Tsukuba Aerosol March 2011 �170 AMS 2.74–6.74 �10�4 mBq m�3

1.75–8.27 � 10�6 180

Fukushima Rainwater March 2011 toOctober 2014

�60 AMS 0.13–981 mBq m�3 0.03–6.89 � 10�6 163

Fukushima Soil — 5–30 ICP-QMS 37–150 mBq kg�1 5.1–18 � 10�7 79Fukushima Soil — 5–60 ICP-MS/MS 2.01–154 mBq kg�1 1.04–20.3 � 10�7 84Fukushima Soil June 2011 0–100 AMS 1.04–39.5 mBq kg�1 0.22–44.3 � 10�7 178Tsukuba Soil March 2011 to

May 2011180 AMS 0.275–0.724 mBq kg�1 181

Fukushima Soil April 2011 3–60 AMS 0.19–84.4 mBq kg�1 177Fukushima Soil April 2011 to

March 201220 AMS 0.036–2.413 mBq kg�1 182

Fukushima Soil November 2012 <10 AMS 10.3–37.9 mBq kg�1 4.18–23.3 � 10�7 183and 184

Fukushima Soil April 2011 toJune 2011

�20–80 ICP-MS/MS 0.926–275 mBq kg�1 0.215–79.3 � 10�7 185

Kuji River River water April to July 2011 �100 AMS 3.3–8.4 � 10�9 186Ibaraki Pond water April to July 2011 �100 AMS 3.7–6.5 � 10�8 186Pacic Seawater June 2011 40–530 AMS 0.01–0.8 mBq m�3 0.26–22 � 10�10 45Pacic Seawater May to June 2011 30–600 AMS 0.002–0.46 mBq m�3 0.045–12 � 10�10 49Pacic Seawater April to

October 2011— AMS 0.01–1.06 mBq m�3 4.47–362 � 10�11 187

Pacic Seawater June 2011 40–530 AMS 0.01–0.81 mBq m�3 0.36–21.95 � 10�10 41Pacic Seawater April to July 2013 8770 AMS 0.01–2.2 mBq m�3 2.2–610 � 10�11 188Pacic Seawater March 2012 6350 AMS 1.1 � 10�10 189Fukushima Radioactive

water andprocessedwater

— DRC-ICP-MS 0.021–1.3 Bq g�1 — 110

Fig. 4 Relationship between 129I and 131I deposition densities inFukushima atmospheric deposit and soil samples (data from ref. 177,178, and 181, decay corrected to 11 March 2011).


a strong correlation between the two radionuclides, suggestingthat the concentrations of 131I in environmental samples at theearly stage aer the accident could be reconstructed throughthe analysis of 129I. Once deposited on the ground, 129I couldmigrate to deeper soil layers. Therefore, the vertical proles of129I in soils need to be investigated to calculate the depositiondensities. Matsunaka et al.183,184 determined the vertical distri-butions of 129I in soil core samples collected at several placesnear the FDNPP site before (2008 and 2009) and aer(November 2012) the accident. The accumulative inventories of129I in the post-accident soil cores were more than 10 timeshigher than those of the pre-accident soil cores, suggesting theextra input of 129I from the accident. The 129I concentrationsand 129I/127I atomic ratios decreased exponentially with depthand �90% of the accident derived 129I still remained in theupper soil layers (0–5 cm) although more than one and a halfyears had passed since the accident. Mobility studies of 129I insoil depth proles found that 129I mainly combined with thesoil's organic content.193 Xu et al.165 found elevated 129I andradiocesium levels in Japanese cedar leaves and observedinsignicant isotopic or elemental fractionation during theinternal translocation from older parts of the tree, but a signif-icant fractionation between 129I and 134,137Cs during atmo-spheric transport from the source.


Aer release into the atmosphere, the majority of 129I wouldnally deposit in the ocean and cause radioactive contamina-tion in the marine environment. Moreover, 129I was directly



discharged into the Pacic Ocean through radioactive liquidwastes. The concentrations of 129I in the Fukushima radioactiveand processed water were reported to be as high as 1 Bq g�1 (ref.110) and the total amount of 129I discharged into the PacicOcean was estimated to be 2.35–7 � 109 Bq.41,45,49 More recentstudies found only minute 129I contamination levels in pro-cessed primary coolant water from the reactor of Unit 5.189 Late(from June 2011 forward) releases of 129I aer the immediateaermath of the accident were estimated to be 110 g or 7 � 108

Bq.194 Suzuki et al.187 analyzed 129I in seawater samples collectednear the Fukushima coast before and aer the FDNPP accident.The concentrations of 129I ranged from 0.01–1.06 mBq m�3 andthe average concentration was approximately 8 times higherthan those before the accident for the same sea area. Theeffective dose of 129I from seafood ingestion was estimated to be6.7–550 � 10�11 Sv y�1, much smaller than the annual doselimit. A wide range of investigations were conducted by variousresearch groups.41,45,49,188,189,195 The highest concentration of 129Iin the Pacic seawater was nearly three orders of magnitudehigher than the pre-Fukushima background level. It has beenshown that the Fukushima accident released 129I reached theUSA West Pacic Coast two years aer the accident, resulting inthe 129I/127I atomic ratio in seawater to be two orders ofmagnitude higher than before.188 The 129I results also providenew evidence for the understanding of seawater movement. Forexample, based on the routine analysis of 129I in seawatersamples from the Pacic Ocean aer the accident, the averagetransport speed of the radioactive plume was estimated to beabout 12 cm s�1, in accordance with the zonal current speed inthe Pacic Ocean.188

5.4. 135Cs and 137Cs

The isotopic compositions of radiocesium are important forrelease source identication. Aer the FDNPP accident, thedistributions of 134Cs and 137Cs in a large number of environ-mental samples had been immediately investigated by g spec-trometry. The observed high activities of these two nuclides anda 134Cs/137Cs activity ratio of �1 suggested the atmosphericrelease and liquid discharge of radiocesium from the acci-dent.50,196,197 However, the relatively short half-life (2.06 y) of134Cs makes this 134Cs/137Cs tracer unavailable to be deter-mined in the near future. The radiocesium nuclide 135Cs andthe atomic ratio of 135Cs/137Cs are believed to be alternativeoptions for long-term radioactive source identication. Theaccumulated ssion yields of 135Cs and 137Cs from thermalneutron ssion of 235U are similar (6.58% and 6.22% for 135Csand 137Cs, respectively). Therefore, the productions of thesenuclides from nuclear weapon tests are comparable anda 135Cs/137Cs atomic ratio of around 1 can be expected for theglobal fallout-sourced contamination in the 1960s.198,199 Theprecursor of 135Cs in nuclear reactors, however, is 135Xe (T1/2 ¼ 9h), a ssion product with an extremely high cross section forneutron capture (so-called “reactor poison”). When exposed tothe neutron ux of an operating reactor, it is likely to formstable 136Xe through neutron capture, instead of decaying to135Cs while the production of 137Cs is virtually unaffected. Upon


shutdown of a reactor, however, 135Xe has the chance to decay to135Cs, and thereby build up 135Cs. This opens a new dimensionfor the distinctness of an atomic ratio. While both 134Cs and137Cs are accumulated in the nuclear fuel with increasing burn-up, 135Cs is mainly produced with each shut-down event ofa reactor. The 135Cs/137Cs ratio thus reects two differentaspects of the reactor history instead of just one, like the134Cs/137Cs ratio. Both aspects make the 135Cs/137Cs atomicratios in nuclear reactors vary signicantly depending on theneutron uxes and the reactor operation history.

Ohno and Muramatsu14 rst determined 135Cs and 137Cs inrainwater samples from four locations 40–200 km from theFDNPP site several days aer the accident using ICP-MS/MS.The atomic ratios of 135Cs/137Cs ranged from 0.32 to 0.41 withan average of 0.37, revealing the contamination from theFDNPP accident. Zheng et al.13,83 analyzed the distributions of135Cs and 137Cs in litter and soil samples collected 30 km fromthe FDNPP site. The observed atomic ratios of 135Cs/137Cs werecomparable with those found by Ohno and Muramatsu.14 Sys-tematical investigation of 135Cs/137Cs atomic ratios in environ-mental samples (mainly soil samples) has been reported byYang et al.200 and Shibahara et al.100 and in total near 100samples were analyzed in their studies, providing sufficientinformation for radioactive contamination assessment andsource identication from the FDNPP accident. Besides, in thestudy of Yang et al.200 dozens of soil samples collected before theaccident were also analyzed in comparison with those collectedaer the accident, further illustrating the distinctive signatureof cesium released from the accident. Additionally, the char-acterization of 135Cs and 137Cs in various Fukushima environ-mental samples such as litter and various plants (moss,soybean, grass, etc.) has also been reported intermittently inother studies.22,100,114,201 Based on the method of Zheng et al.,13,83

Cao et al.113 further developed a method for sequential deter-mination of Pu and Cs in river suspended particles. Generally,the collected amount of suspended particles is oen small dueto the difficulty of sampling; this method thus showed a bigadvantage since Pu and Cs could be separated sequentially.Long-term monitoring of the radionuclides (135Cs, 137Cs, 239Puand 240Pu) in the river suspended particles would be informa-tive for studying their fate and transfer in the river–oceansystem.113 The published data about the 135Cs/137Cs atomicratios in the Fukushima samples were collected and are plottedin Fig. 5. The overall mean ratio was about 0.368, slightly lowerthan that (�0.45) of the Chernobyl accident.199

In the FDNPP accident, not only the damaged nuclear reac-tors but also the spent fuel pools, in particular the spent fuelpool in the Unit 4 reactor building where a hydrogen explosionoccurred and re had happened, were believed to contribute tothe environmental radioactive contamination.33 The isotopiccompositions of radionuclides in the Fukushima nuclear reac-tors and spent fuel pools were calculated by the JAEA using theORIGEN code, which provided useful information for sourceterm attribution.62 Zheng et al.13,83 determined the isotopiccompositions of both radiocesium and Pu isotopes in severalheavily contaminated (137Cs activity > 104 Bq kg�1) litter and soilsamples, and found that the 240Pu/239Pu and 135Cs/137Cs ratios


Fig. 5 135Cs/137Cs atomic ratios determined in Fukushima environmental samples (data cited from ref. 13, 22, 83, 100, 113, 114, 149, 200 and 201).


in these environmental samples were similar to those of theFukushima reactor cores, and were distinguishable from thoseof the spent fuel pools (Fig. 6). They suggested that the mainsources for radiocesium emission were the nuclear reactors,especially the Unit 2 reactor, and the release of radiocesiumfrom the spent nuclear fuel pools was very limited. Thisconrmed early studies on Unit 4 nding a negligible contri-bution of this reactor to the environmental contaminationlevels in Japan.197 The total amount of the atmosphericallyreleased 135Cs was estimated to be �7 � 1010 Bq (1.64 kg).13

In the past six and a half years, a large number of studiesabout the fate and behavior of the Fukushima accident derivedradiocesium in the environment have been conducted.However, a lot of important issues with respect to downwardmigration in the terrestrial environment, long range trans-portation in the ocean and internal dose estimation from

Fig. 6 Comparison of Pu and Cs isotopic compositions amongFukushima environmental samples and nuclear fuels in the nuclearreactors and in the spent fuel pools (decay corrected to 11 March 2011)(reprinted with permission from {ref. 13, Zheng et al. et al., Environ. Sci.Technol. 2014, 48, 5433}. Copyright {2014} American ChemicalSociety).


ingestion of contaminated vegetation and seafood still needfurther investigations that may last for decades or even longer.For example, it has been predicted that the Fukushima released137Cs in the Pacic Ocean which has been transported into theocean interior would come back to the ocean surface off theFukushima coast through its transport by ocean currents in 30years.202,203 Therefore, the 135Cs/137Cs atomic ratio can beregarded as a new tracer for long-term radioactive sourceidentication. As a technique for ultra-trace measurement of135Cs and 137Cs has recently become available, more studiesfocusing on the measurement of these two nuclides in marineenvironmental samples by mass spectrometry could be ex-pected in the future.204

5.5. 60Fe

So far, only one study has been conducted searching for 60Fe inthe environment as a result of the Fukushima nuclear acci-dent.166 The search for 60Fe has been inspired by early ndingsof short-lived, gamma-emitting 59Fe in the environment in theearly aermath of the nuclear accident.205 It had to be shownwhether the radioiron emitted from the reactors has beenformed by exposure to neutrons at extremely high neutron uxdensities (i.e., directly on scales formed on the outside of thenuclear fuel). In such a scenario, the double neutron activationproduct 60Fe would have been formed. The absence of 60Fewould have indicated a production of the released 59Fe in a low-ux eld, i.e., in the steel components of the pressure vessel.Several soil and plant samples were analyzed by AMS at thefacility of Technische Universitat Munchen, Germany.166 Notraces of 60Fe were found, indicating that the released quantitiesof 59Fe originated from a low-neutron-ux environment insidethe reactors.

5.6. U isotopes

Uranium has three naturally occurring isotopes (234U: av.0.0054%, 235U: av. 0.7200% and 238U: av. 99.2745%) in theenvironment. The isotopic compositions of U from nuclear


Fig. 7 Comparison of 238Pu and 236U concentrations in “blanksubstances” contaminated by the FDNPP (reprinted with permissionfrom {ref. 57, Sakaguchi et al., Environ. Sci. Technol. 2014, 48, 3691}.Copyright {2014} American Chemical Society).


power plant accidents differ signicantly from uranium withnatural composition.62 Therefore, the extra input of anthropo-genic U in the environment can be distinguished. In addition,the development of mass spectrometric techniques makes itpossible to accurately determine another low abundance Uisotope, 236U, which has recently proved to be a more sensitivengerprint for radioactive source identication.

Several months aer the accident, more than 100 soilsamples were collected in the areas 7–90 km from the FDNPPsite for the analysis of 235U and 238U by ICP-MS.116 The resultsshowed that although the concentrations of U varied signi-cantly among different sampling locations, the 235U/238U atomicratios were similar to the natural abundance, suggesting thelimited release of U from the FDNPP accident into the envi-ronment. Shibahara et al.102 determined the isotopic composi-tions of U and Pu in soil samples collected closer to the FDNPPsite (1.2–39.6 km) by TIMS. The release of Pu isotopes from theFDNPP accident had been observed in these samples while theextra contamination of U could not be distinguished. Theconcentrations of U in plant and vegetable samples aer theaccident are also of scientic concern due to the potentialinternal radiation risk. The investigation results also revealedthat the U contamination from the FDNPP accident in theenvironment was indistinguishable from the background.28,101

Some researchers also focused on the ultra-trace level of 236Uin the environment. The atomic ratio of 236U/238U is moresensitive than that of 235U/238U for radioactive source identi-cation. Sakaguchi et al.120 rst analyzed the distributions of 236Uin several river water and seawater samples collected near theFDNPP site aer the accident using AMS. The 236U concentra-tions and 236U/238U atomic ratios were (0.35–4.48) � 105 atomsper kg and (0.57–8.20) � 10�9, respectively. By comparing withthe distributions of 236U in pre-Fukushima accident river waterand seawater samples obtained from global fallout, theyconcluded that the input of 236U in these samples was negligiblysmall. Eigl et al.122 determined the concentrations and 236U/238Uatomic ratios in seawater samples collected in the North PacicOcean. The 236U/238U atomic ratios were even lower than theresults reported by Sakaguchi et al.120 No impact of the FDNPPaccident has been observed. Similar results (neither elevated236U nor Fukushima-derived Pu) were reported by Casacubertaet al.194 In the terrestrial environment, Yang et al.93 analyzed the236U/238U atomic ratios in Fukushima surface soil samples andfound that the results were consistent with the global falloutvalues. More recently, Schneider et al.123 investigated thevertical distributions of 236U in soil cores collected 0.95–32.84km from the FDNPP site. In several samples, 236U/238U atomicratios up to 10�7 were observed, suggesting a possible inuenceof the FDNPP accident. As the concentrations of natural U inenvironmental samples are normally at the level of several mgg�1 and the release amount of 236U from global fallout could bemuch bigger than that from the FDNPP accident, the identi-cation of trace U released from the accident is difficult.However, for the samples that were heavily contaminated by theaccident and received relatively low contamination from othersources, the existence of anthropogenic 236U can provideevidence for FDNPP-released U. Shinonaga et al.121 investigated


Pu and U isotopes in aerosol samples collected 120 km from theFDNPP site aer the accident. High 236U/238U atomic ratios(10�7–10�6) were observed in the samples collected in the weeksimmediately aer the reactor hydrogen explosions. In thesesamples, 240Pu/239Pu and 241Pu/239Pu atomic ratios higher than0.3 and 0.05, respectively, were also observed. These resultssuggested that the FDNPP accident caused other actinidecontamination in the environment through atmosphericdeposition as well as Pu contamination although the contami-nation level was limited. A comprehensive study about thedistribution of radionuclides (137Cs, Pu isotopes and Uisotopes) in black substances from the roadside in Fukushimawas conducted by Sakaguchi et al.57 Most of the activities of137Cs in these samples were beyond 2000 Bq g�1, revealing thatthese samples were heavily contaminated by the FDNPP acci-dent. The activities of 236U and 236U/238U atomic ratios were atthe levels of 10�4 Bq kg�1 and 10�7, respectively. Based on theactivity ratio of U/137Cs, the total amount of U from the FDNPPaccident was estimated to be 150 g (including 0.5 g 236U). Forcomparison, the 236U amount released from global fallout wasreported to be �1000 kg.124,125,206 Besides, a good linear corre-lation between the concentrations of the fuel-burning products238Pu and 236U in the blank substances is presented in Fig. 7,indicating the similar deposition behavior of these two refrac-tory radionuclides in the transportation process. From furtherstudies on the relationship between 238Pu/239+240Pu and236U/239+240Pu activity ratios, Sakaguchi et al.57 concluded thattrace amounts of non-natural U were released from the fuelcores of the FDNPP together with Pu, however without largefractionation.



5.7. Pu isotopes

In order to evaluate the possible contamination of Pu isotopesin the environment from the FDNPP accident, the backgrounddistribution levels of Pu have to be known. Yang et al.207 deter-mined 239+240Pu activities and 240Pu/239Pu atomic ratios in 80surface soil samples from central-eastern Japan collectedaround the 1970s. The 239+240Pu activities ranged from 0.004–1.46 mBq g�1 and 240Pu/239Pu atomic ratios were 0.148–0.229with an average of 0.186. Besides, based on the 241Pu/239+240Puratios in atmospheric fallout samples in Japan,208 they recon-structed the deposition density of 241Pu in surface soils of Japanwith a range from 3.9 to 394 Bq m�2 in the 1970s. These resultscan provide a baseline for the extra Pu input contamination.The MEXT (Ministry of Education, Culture, Sports, Science andTechnology) conducted investigations about Pu distribution inFukushima surface soils by a spectrometry.156 As a spectrometrycannot resolve 239Pu and 240Pu due to the close energies of theemitted a particles, only the sum activities of these two isotopes(239,240Pu) were given. Although there was no signicantincrease of 239+240Pu activities, 238Pu/239+240Pu activity ratios of0.33–2.2, higher than that of global fallout (0.026), were detec-ted in ve sites, indicating possible Pu contamination from theFDNPP accident. Zheng et al.60 for the rst time obtainedisotopic evidence of the release of Pu from the FDNPP accidentby measurements of Pu isotopic composition in litter andsurface soil samples collected 20–30 km northwest of theFDNPP site using SF-ICP-MS. The results of 137Cs activitiesshowed that these samples received radioactive depositionsfrom the accident. In three samples, high 241Pu activities(4.5–34.8 mBq g�1) were observed. The 240Pu/239Pu and241Pu/239Pu atomic ratios were beyond 0.3 and 0.1, respectively,both signicantly higher than the global fallout values. Theresults further demonstrated that the FDNPP accident releasedPu isotopes in addition to volatile ssion products into theatmosphere. Schneider et al.209 conrmed Zheng's ndings byidentifying two vegetation samples with a high 240Pu/239Puratio. A comprehensive investigation about the isotopic signa-tures of actinides in Fukushima environmental samples wasconducted by Yamamoto et al.61 In total, more than 100 samples(including black substances, soils and litters) were collectedand analyzed. The averaged 240Pu/239Pu atomic ratio and238Pu/239+240Pu activity ratio were 0.33 and 1.73, respectively,comparable with those reported by the MEXT156 and Zhenget al.60 Following these studies, more studies have been pub-lished focusing on the distributions of Pu isotopes in aerosolsamples121,192 and terrestrial environmentalsamples.57,102,175,210–214 The Pu atomic ratios observed in theinvestigated environmental samples clearly showed a mixingbetween global fallout sources and the FDNPP accident source.

The inventories of Pu isotopes in each Fukushima reactorcore and spent nuclear fuel pool have been theoretically calcu-lated and given by Nishihara et al.62 Zheng et al.59 compared thePu isotopic compositions (240Pu/239Pu, 241Pu/239Pu and238Pu/239+240Pu) of environmental samples with the results ofthe reactor core fuel and spent nuclear fuel inside the storagepools. The comparison results revealed that Pu isotopes were


released from the damaged nuclear reactors, not from the spentfuel pools. This conclusion was consistent with that obtainedfrom the investigation of the 135Cs/137Cs atomic ratios in envi-ronmental samples.13 The total release amount of 137Cs hasbeen well addressed soon aer the accident (Table 2). Assumingthat 137Cs and Pu isotopes followed a similar depositionmechanism and there was no signicant variation in the activityratio of 137Cs/239+240Pu during the release and deposition, thereleased amounts of 239+240Pu, 241Pu and 238Pu from the FDNPPaccident were estimated to be (1.0–2.4) � 109 Bq, (1.1–2.6) �1011 Bq and (2.9–6.9) � 109 Bq, respectively. The releasedpercentage of the core inventory of Pu was �10�5%.59

Most of the atmospheric released radionuclides from theFDNPP accident ultimately deposited in the ocean. In addition,large amounts of highly contaminated radioactive liquid weredirectly released or discharged into the Pacic Ocean. There-fore, the distributions of Pu isotopes in themarine environmentneed to be investigated for a better assessment of the Pucontamination from the FDNPP accident. Due to the lowmobility of Pu in the marine environment, the possible impactof the accident derived Pu contamination would likely remainin the vicinity of Fukushima.215 Before the accident, there weretwo sources for Pu contamination in the sea areas aroundJapan: global fallout and the Pacic Proving Ground (PPG)close-in fallout, which was transported by oceanic currents fromthe nuclear weapon test sites in the Marshall Islands to thewestern North Pacic.216–219 The information about the distri-butions of Pu isotopes in marine sediments and seawatersamples in the western North Pacic and its marginal seasbefore the FDNPP accident has been summarized by severalstudies to establish a baseline for the assessment of the possibleaccident released Pu.220–223

Aer the accident, investigations about the distributions ofPu in marine sediment and seawater samples in the westernNorth Pacic were carried out mainly by using mass spectro-metric analysis.120,220–226 The 239+240Pu activities showed nosignicant increase compared with the background level beforethe accident. The 240Pu/239Pu atomic ratios were typicallybetween 0.18 and 0.30, higher than the global fallout value.However, as mentioned above, the PPG close-in falloutcontributed to Pu contamination in the marine environment aswell. The PPG close-in fallout Pu was characterized by high240Pu/239Pu atomic ratios (>0.3),217,227 which were similar to thatof the FDNPP accident derived Pu. Therefore, it is difficult toidentify the FDNPP accident-derived Pu in the marine envi-ronment only by the characterization of 239Pu and 240Pu. Due tothe improved chemical separation procedures and the highsensitivity of mass spectrometric techniques, another Puisotope, 241Pu, in the marine environmental samples could alsobe measured.20,108 The 241Pu/239Pu atomic ratio for the newlyreleased Pu from the FDNPP accident was �0.1, almost twoorders of magnitude higher than both the global fallout (0.001)and the PPG source (0.002).99,220,228,229 Bu et al.225 investigated thedistributions of 239�241Pu in marine sediments collected withinthe 30 km zone around the FDNPP site. The Pu isotopiccompositions for the sediment samples are plotted in Fig. 8 incomparison with that of other sources. It can be seen that global


Fig. 8 Pu atomic ratios in Fukushima marine sediments for sourceidentification (data cited from ref. 20, 99, 121, 220, 225, 228 and 229,decay corrected to 11 March 2011).


fallout and the PPG close-in fallout were still the two mainsources for Pu contamination in the marine environment aerthe FDNPP accident, and the release of Pu isotopes from theaccident to the marine environment cannot be identied.225

Recently, Hain et al.107 determined 239�241Pu in seawatersamples collected in the Pacic Ocean aer the accident byAMS. The results further support that there was no signicantamount of Pu derived from the FDNPP accident compared withthe background level before the accident.

5.8. Minor actinides

The investigations of minor actinides such as Np, Am and Cmin the environment aer the FDNPP accident are very limited.Early gamma spectrometric indications for a release of short-lived 239Np205 proved to be, most likely, a miss-attribution dueto the spectral interference with the prominent ssion product129mTe in the gamma spectrum.166 This illustrates a possibleproblem of radiometric measurements. Yamamoto et al.61,211

measured the activity ratios of 241Am/239+240Pu, 242Cm/239+240Puand 243+244Cm/239+240Pu in black substances and litter samples

Fig. 9 SEMmapping image of a radioactive Cs-bearing particle released fwith permission).


by a spectrometry. The results were comparable to the ratios ofcore inventory, conrming the release of Am and Cm into theenvironment. Oikawa et al.230 investigated 241Am distributionsin surface sediments collected off the Japanese coast before andaer the accident. No temporal variation in the 241Am concen-trations was observed. Although mass spectrometry-basedanalytical methods have been developed for the determina-tion of minor actinides in environmental samples aer theaccident, the applications for real Fukushima samples have notbeen reported.21,106

5.9. The analysis of radioactive Cs-bearing particles

During nuclear accidents, a major fraction of radionuclides canbe released as radioactive particles. It has been estimated that 3–4 tons of U fuels with variable burn-up were released into theenvironment as pure oxide U and fuel construction particles.231

Investigating the radioactive particles can give important infor-mation on the emission source term. Moreover, as these particlesare extremely small and characterized by high specic activities,missing them during the sampling or the sample preparationprocedures would lead to underestimation of the radioactivecontamination level in the environment. For example, Yamamotoet al.232 analyzed Pu distributions in the soils collected around theChernobyl nuclear power plant site and found that in somesamples more than 90% Pu isotopes were combined in the “hotparticles”, which could not be dissolved with nitric acid.

Adachi et al.233 for the rst time found spherical Cs-bearingparticles which fell onto the ground through dry deposition atthe early stage of the accident in Tsukuba about 170 km fromthe FDNPP site. The particle sizes were at the level of several mmand, in stark contrast to Chernobyl's hot (fuel) particles, theycontained elements such as Fe and Zn besides Cs. A typical SEMmapping image of a spherical radioactive Cs-bearing particlereleased from the FDNPP accident and its X-ray spectrum areshown in Fig. 9. Since then, much scientic attention has beenpaid to Cs-bearing particles with regards to the size distribu-tion, internal structure, elemental composition and radiog-raphy.234–239 Currently, the presence of Cs-bearing particles inthe terrestrial environment around the FDNPP site has beenwell conrmed and the chemical forms of radiocesium in the

rom the FDNPP accident and its X-ray spectrum (redrawn from ref. 233



particles have been investigated. Abe et al.240 characterizedmorethan 10 heavy elements in three Cs-bearing particles collectedon March 14th and March 15th, 2011, in Tsukuba. Theseelements were present in a high oxidation state in the glassmatrix. The glassy state of the radioactive materials suggestedthat they may remain longer in the terrestrial environment thanthe water-soluble radioactive Cs aerosol particles, thus causingmore problems for remediation. In addition, the U fuel wasfound in two of these particles, revealing that U and its ssionproducts were contained in the particles along with radio-cesium. In a more recent study, U-containing particles were alsofound in sediments collected from Iitate Village about 30 kmfrom the FDNPP site in May 2014.241 A method based on SEMwas established to isolate and remove individual particlesbefore radiation analysis.

These investigations highlight the importance of radioactiveparticles emitted from the FDNPP accident for radioactiveassessment purposes. However, to date, no pure spent fuelparticles have been identied in the environment around theFDNPP site; the studies remained on the analysis of stablerefractory metals using techniques such as X-ray spectrometry.Measurements of transuranium atomic ratios as well as thelong half-life ssion or activation products are needed to furtheridentify nuclear fuel particles.242 More work focusing on quan-titative compositional analysis of these particles using massspectrometric techniques can be expected.

6. Conclusions and future researchprospects

Mass spectrometric techniques have been widely used for theanalysis of radionuclides in support of radiation protection,radioactive waste characterization and nuclear forensics sincetheir introduction. Aer the FDNPP accident, there was a strongdemand for the determination of radionuclides in variousFukushima samples for environmental radioactive contamina-tion assessment. During the last six and a half years, a remark-able number of analytical methods based on mass spectrometryfor radionuclide analysis were developed with applications forFukushima samples. These methods further expanded theapplication of mass spectrometry for environmental researchand contributed to the studies about the contamination situa-tions and migration behaviors of the accident released Pu, U,radioiodine, radiocesium, radiostrontium, etc., in the environ-ment. Due to the excellent analytical characteristics such ashigh sensitivity, good accuracy, short measuring time andcapabilities for atomic ratio measurement and multi-radionuclide detection, mass spectrometry will play a moreimportant role in the long term monitoring of Fukushimasamples. For short-lived radionuclides (especially gammaemitters), radiometric methods will remain relevant and therst choice for identication and quantication.

Several issues should be addressed for the environmentalradioactive assessment by using mass spectrometric techniquesfor the next step. First, on-line separation coupled with massspectrometry detection for the analysis of radionuclides is


highly desirable for a rapid response aer a nuclear accident.Data need to be collected in a short time for assessing theaccident situation and making policies. Second, long-livedssion products and minor actinides such as 99Tc, 237Np,241Am and Cm isotopes were released into the environmentfrom the FDNPP accident. However, the information about theirdistributions in the Fukushima samples is currently limited.Mass spectrometry is the most promising technique for theanalysis of these radionuclides. More studies focusing on thedevelopment of analytical methods for these radionuclides andapplications for Fukushima samples are expected. Third,radioactive particles emitted from the FDNPP accident havebeen conrmed. The internal structure and stable metalcomposition of these particles have been investigated. Moreinformation about the distribution and isotopic composition ofactinides, ssion products and activation products will help forunderstanding the damaged reactors and the emission mech-anism of these radionuclides. Finally, there are currently morethan 400 nuclear reactors under operation all over the world.However, in many cases, the radionuclides in the environmentaround these nuclear facilities have not been thoroughly char-acterized. More studies focusing on the concentration ofradionuclides in the related environmental samples (soil, water,plant, etc.) and the isotopic information are needed. An estab-lished background database will serve as a baseline for theassessment of possible radioactive contamination like theFDNPP accident in the future.

Conflicts of interest

There are no conicts to declare.

Abbreviations

AMS
Accelerator mass spectrometry AMP Ammonium phosphomolybdate AMP-PAN Amp based on polyacrylonitrile DRC-ICP-MS
Inductively coupled plasma mass spectrometry witha dynamic reaction cell

FDNPP
Fukushima Daiichi Nuclear Power Plant ICP-MS Inductively coupled plasma mass spectrometry ICP-QMS Inductively coupled plasma quadrupole mass
spectrometry
ICP-MS/MS
Inductively coupled plasma mass spectrometry/mass spectrometry

LSC
Liquid scintillation counting MEXT Ministry of Education, Culture, Sports, Science and
Technology
NAA Neutron activation analysis RPQ Retarding potential quadrupole TIMS Thermal ionization mass spectrometry
Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (No. 11605172), the Grant of Fukushima



Prefecture related to Research and Development in RadiologicalSciences, the Science Challenge Project (No. TZ2016004), andthe Grand-in-Aid for Scientic Research by the Ministry ofEducation, Culture, Sports, Science and Technology, Japan(grant number JP17k00537, 17H01874). We thank the editorand two anonymous reviewers for their numerous commentswhich helped to improve the manuscript.

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