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Radiochemical analysis of rubble and trees collectedfrom Fukushima Daiichi Nuclear Power StationKiwamu Tanakaa, Asako Shimadaa, Akiko Hoshia, Mari Yasudaa, Mayumi Ozawaa & YutakaKameoa

a Japan Atomic Energy Agency, 2-4 Shirane, Shirakata, Tokai-mura, Naka-gun, Ibaraki319-1195, JapanPublished online: 29 May 2014.

To cite this article: Kiwamu Tanaka, Asako Shimada, Akiko Hoshi, Mari Yasuda, Mayumi Ozawa & Yutaka Kameo (2014)Radiochemical analysis of rubble and trees collected from Fukushima Daiichi Nuclear Power Station, Journal of NuclearScience and Technology, 51:7-8, 1032-1043, DOI: 10.1080/00223131.2014.921583

To link to this article: http://dx.doi.org/10.1080/00223131.2014.921583

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Journal of Nuclear Science and Technology, 2014Vol. 51, Nos. 7–8, 1032–1043, http://dx.doi.org/10.1080/00223131.2014.921583

ARTICLE

Radiochemical analysis of rubble and trees collected from Fukushima DaiichiNuclear Power Station

Kiwamu Tanaka∗, Asako Shimada, Akiko Hoshi, Mari Yasuda, Mayumi Ozawa and Yutaka Kameo

Japan Atomic Energy Agency, 2-4 Shirane, Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan

(Received 22 January 2014; accepted final version for publication 30 April 2014)

To characterize the rubble and trees contaminated by radionuclides released by the recent accident at theFukushimaDaiichi Nuclear Power Station, the radiochemical analysis protocols weremodified using thosedeveloped by the JapanAtomic Energy Agency for the waste generated by research, industrial, andmedicalfacilities. The radioactivity concentrations of gamma-ray-emitting nuclides 60Co, 94Nb, 152Eu, and 154Eu,and beta-particle-emitting nuclides 14C, 129I, 36Cl, 79Se, and 99Tc were successfully applied by the modi-fied analytical method. In contrast, the radioactivity concentrations of 3H, 90Sr, 137Cs, and alpha-particle-emitting nuclides were applied by the conventional method. Unfortunately, 36Cl, 94Nb, 129I, 152Eu, 154Eu,and alpha-particle-emitting nuclides were below the detection limit of the conventional method. The mea-sured radioactivity concentrations, except for that of 3H, were not uniform in the area but depended onthe reactor unit. Although the radioactivity concentrations were varied widely, this analysis successfullyclarified the characteristics of the radioactivity concentrations of the rubble and trees.

Keywords: FukushimaDaiichiNuclear Power Station; radioactive wastemanagement; radiochemical analysis;cesium-137; strontium-90; tritium; carbon-14; cobalt-60; selenium-79; technetium-99

1. Introduction

Fukushima Daiichi Nuclear Power Station (F1NPS,owned andmaintained by the Tokyo Electric Power Co.)was severely damaged by hydrogen explosions resultingfrom the Great East Japan Earthquake that occurredon 11March 2011. After the accident, radionuclides, in-cluding 137Cs and 131I, were released from the F1NPS tothe ocean and the land [1–3], therefore huge quantitiesof rubble and many trees contaminated by radionu-clides were generated at the site of the F1NPS. As of 30September 2013, 65,000 m3 of contaminated rubble atthe F1NPS site, generated mainly by the collapse anddismantling of reactor buildings, was stored; the volumeof trees that were cut down to install storage tanks forradioactively contaminated water was 51,000 m3 [4].To examine a strategy for treatment and subsequentdisposal of the rubble and trees, it was essential toclarify their radionuclide and radioactivity concentra-tions. The radioactivity inventory of 335 radionuclidespresent in units 1–3 at the F1NPS was estimated usingthe ORIGEN2 computer code [5]. However, not allof these calculated radionuclides can be analyzedbecause of limitations on the analytical resources,such as measurement apparatuses and technical person-

∗Corresponding author. Email: tanaka.kiwamu@jaea.go.jp

nel, and absence of appropriate method of analysis. Inaddition, the important nuclides in terms of treatmentand disposal have not yet been selected for the rubbleand trees at the F1NPS site [6].

The recent policy in Japan for disposal of radioactivewaste generated by the operation and decommissioningof nuclear power plants is predicated upon the safetyassessment of the disposal [7]. Important nuclides havebeen selected for disposal of waste generated by nuclearreprocessing plants [6]. The following are the importantnuclides for waste generated by the operation and de-commissioning of nuclear power plants and by repro-cessing:3H, 14C, 36Cl, 41Ca, 59Ni, 60Co, 63Ni, 79Se, 90Sr, 93Zr,93Mo, 94Nb, 99Tc, 107Pd, 126Sn, 129I, 135Cs, 137Cs, 151Sm,152Eu, 154Eu, 233U, 234U, 235U, 236U, 238U, 237Np, 238Pu,239Pu, 240Pu, 241Pu, 242Pu, 241Am, 242mAm, 243Am,244Cm, 245Cm, and 246Cm.

In this study, in order to clarify the main contamina-tion radionuclides and a correlation between them forthe rubble and trees of the F1NPS, the radionuclidesto be characterized by appropriate techniques were se-lected from the above list on the basis of the contents

C© 2014 Atomic Energy Society of Japan. All rights reserved.

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estimated by ORNL isotope generation and depletioncode 2 (ORIGEN2) [5] and the concentration of stag-nant water [8]:

• nuclides measurable by gamma-ray spectrometry:60Co, 94Nb, 137Cs, 152Eu, and 154Eu;

• highly volatile fission products (FPs) or neutron-activated nuclides: 3H, 14C, 36Cl, 79Se, 90Sr, 99Tc,and 129I;

• alpha-particle-emitting nuclides (transuranic ele-ments) of which the disposal is important: 238Pu,239Pu, 241Am, and 244Cm.

Analytical methods for the radionuclides describedabove have already been developed by the Japan AtomicEnergy Agency (JAEA) and applied to various typesof radioactive waste generated by research, industrial,and medical facilities [9]. However, the FP concentra-tion in the rubble and trees at the F1NPS was esti-mated to be significantly higher (owing to the nature ofthe catastrophic, uncontrolled reactor failure) than that

in the waste generated by the research laboratories un-der tightly controlled conditions. Therefore, it was nec-essary to modify the existing analytical methods to de-termine the low content of important nuclides coexistingwith the high content of FPs such as 137Cs.

In this study, the modified analytical proceduresand their application to the collected samples generatedby the F1NPS accident are described in detail. Subse-quently, the radioactivity concentrations of the samplescollected from the F1NPS obtained by the modified an-alytical methods and the conventional method are re-ported. From the radioactivity concentrations of thesamples, the characteristics of the contaminated samplesgenerated by the F1NPS accident are discussed.

2. Samples

To determine the radionuclides in/on the rubble andtrees, samples were collected at the F1NPS on 25 June2012 and 26–27 July 2012. Figure 1 shows the samplinglocations of the rubble and trees at the F1NPS. Ten

Figure 1. Sampling locations of rubble and trees at the F1NPS. Rubble around units 1, 3, and 4 was collected on 25 June and 27July 2012. Trees were collected on 26 July 2012.

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Table 1. Description of samples analyzed in this study.

One centimeter doseNo. Sample equivalent rate (μSv/h) Weight (g) Form

1 Rubble aroundunit 1 (1U)

1U-06 63.4 165.4 Massive (fist size), including aqua andbeige-colored coating material

2 1U-07 2.4 131.2 Massive (fist size), includingbeige-colored coating material

3 1U-08 15.4 155.7 Massive (fist size), including ash-coloredcoating material

4 1U-09 16.4 92.6 Massive (fist size), including aqua-coloredcoating material

5 Rubble aroundunit 3 (3U)

3U-02 95.4 85.1 Massive (fist size), including aqua-coloredcoating material

6 3U-07 22.4 122.3 Massive (fist size), includingbeige-colored coating material

7 3U-09 1000 115.6 Massive (fist size), including aqua-coloredcoating material

8 3U-10 113 142.6 Massive (fist size), including green andbeige-colored coating material

9 Rubble aroundunit 4 (4U)

4U-01 2.4 40.0 Massive (fist size), includingbeige-colored coating material

10 4U-02 B.G. 152.9 Massive (fist size), includingbeige-colored coating material

11 4U-05 B.G. 177.4 Grain geometry12 4U-08 B.G. 116.0 Grain geometry13 Cut tree T-01 6.4 103.0 Branch14 T-02 4.6 45.1 Leaf15 T-04 2.1 128.8 Branch16 T-05 2.2 101.4 Leaf17 Living tree T-07 3.4 204.8 Branch and leaf located at a height of 2 m

rubble samples scattered by the hydrogen explosion werecollected around the reactor buildings of units 1, 3, and4. Cut tree samples were collected from the stock yardin the northern area of the F1NPS (A and B areas).Because the precise collection locations for the cut treesamples were not confirmed, the branches and leaves ofa living tree were also collected from the area aroundunit 3 (C area). Three samples were collected from Aarea in Figure 1, and two samples each were obtainedfromBandCareas. The collected samples wereweighed,and put in plastic bags. Each of the sealed samples wasstored in a plastic container. One centimeter dose equiv-alent rates of their samples were measured using an ionchamber (Aloka Co., Ltd., ICS-321).

Because of limitations on our analytical capabilities,the samples analyzed in this study were selected accord-ing to the following criteria:

• the samples should weigh more than 30 g, which isthe minimum amount required for radiochemicalanalysis;

• the rubble samples should exhibit maximum andminimum dose rate values;

• the branches and leaves were selected from the cuttree samples of A and B areas;

• for the living tree collected fromC area, the higherdose rate sample was selected.

Information such as the dose rate and weight of theselected rubble and trees is summarized in Table 1. Thesamples were transported from the F1NPS to the Nu-clear Science Research Institute of the JAEA on 26 Oc-tober 2012 for measurement.

3. Experimental

In this study, radiochemical analysis of the collectedsamples was conducted according to the analytical pro-cedure shown in Figure 2. Beta-particle-emitting nu-clides 90Sr and 3H and alpha-particle-emitting nuclides238Pu, 239Pu, 241Am, and 244Cm were analyzed usingthe conventional method described in previous reports[9–16]. For the gamma-ray-emitting nuclides 60Co, 94Nb,152Eu, and 154Eu and beta-particle-emitting nuclides14C, 129I, 36Cl, 79Se, and 99Tc, the procedure was mod-ified from the conventional method. The modified ana-lytical procedure for the collected samples is describedin detail in the following section.

3.1. Homogenization and subdivisionAbout 50 g of the rubble was ground in a stainless

steel mortar until the particle diameter was less than0.5 mm. The powdered rubble was spread out on a sheet,and 0.5 g or 1 g samples were collected from more than10 places. For the trees, scissors were used to cut the trees

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Figure 2. Flowchart for analysis of 3H, 14C, 36Cl, 60Co, 79Se, 90Sr, 94Nb, 99Tc, 129I, 137Cs, 152Eu, 154Eu, and alpha-particle-emittingnuclides in rubble and tree samples collected at the F1NPS.

into small pieces, which were then collected in the samemanner as that used for the rubble. To confirm that therewas no significant deviation in the radioactivity concen-trations of the subdivided rubble and trees, the 137Csconcentration was measured using a high-purity Ge de-tector (HPGe, Canberra Industries, Inc., GC3519). Af-ter ensuring that the variation in the 137Cs concentrationof each subdivided sample was within 10%, the collectedsample was subjected to a variety of radiochemicalanalyses.

3.2. Analysis of gamma-ray-emitting nuclides3.2.1. Pretreatment

One gram of a rubble sample was dissolved in a mix-ture of 12 ml of 11 M HCl and 4 ml of 13 M HNO3

(aqua regia) using amicrowave oven (Milestone S. R. L.,ETO900). Each sample was heated continuously for (1)5 min at 250 W, (2) 5 min at 450 W, (3) 10 min at 650 W,and (4) 10 min at 400 W. Most of the metal elements inthe collected sample were dissolved in the solution. Thesolution was filtered, and the residue was measured withthe HPGe to check the recovery; after ensuring that the137Cs in the residue was less than 5% of that in the sam-ple before dissolution, the sample solution was used inthe subsequent analysis. In the case of tree sample, 1 gof the sample was burned by an electric furnace, and the

resulting ash was dissolved by the same method of rub-ble sample.

3.2.2. Removal of radiocesium

For the gamma-ray-emitting nuclides, such as 60Co,94Nb, 152Eu, and 154Eu, 1 g of ammonium phospho-molybdate (AMP) was added and stirred into 20 ml ofthe pretreatment solution. After settling for longer thanone night, the solution was filtered through a 0.45 μmmembrane filter to remove the Cs-adsorbed AMP. Thefiltrate was dried in a glass vial and measured with theHPGe.

3.3. Analysis of 36Cl, 79Se, and 99Tc3.3.1. Pretreatment

An alkaline fusion method was applied for the pre-treatment of the rubble sample, and an acid extractionmethod was applied for the tree sample. The rubble sam-ple was fused with 10 g of NaOH at 550 ◦C for 30 minin an electric furnace, and the melted sample was dis-solved in water. The tree sample for the 36Cl analysiswas extracted with 100 ml of 1 M HNO3, and those forthe 79Se and 99Tc analyses were extracted with 100 ml of10 M HNO3. To determine the recovery ratios, carriers

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(Cl [Cl−] 5 mg, Se [HSeO3−] 20 mg, Re [ReO4

−] 1 mg)were added to the sample before the pretreatment.

3.3.2. Analysis of 36Cl

To remove radioactive Cs, the solution obtainedfrom alkaline fusion or acid extraction was filtered af-ter theAMP treatment. Subsequently, NaOHwas addedto the sample solution until it became neutral, and ma-trix elements such as Si and Al were removed as hy-droxide precipitates. Sodium iodide and AgNO3 wereadded to the filtrate, and the resulting precipitate ofAgCl and AgI was recovered. Ammonium hydroxidewas added to the precipitate and heated to separate Clfrom AgI. This treatment was repeated until no pre-cipitate of AgCl was formed. The final AgCl precipi-tate mounted on a filter paper was measured by a betaray spectrometer (Pico Beta, Fuji Electric Systems Co.,Ltd.) to check the shape of beta-ray spectrum and thepresence of other beta-particle-emitting nuclides, suchas 90Sr. Subsequently, 36Cl was measured using a gas-flow detector (LBC, Aloka Co., Ltd., LBC-4312). TheCl recovery was determined by the gravimetric determi-nation of the precipitate.

3.3.3. Analysis of 79Se and 99Tc

To remove the radioactive Cs, the sample solutionwas filtered through a 0.45 μm membrane filter afterthe AMP treatment. After ammonium hydroxide andCaCl2 were added to the filtrate to remove Al and Siby the precipitation of Al(OH)3 and CaSiO3, the addi-tion of Na2CO3 to the filtrate removed Ca by the pre-cipitation of CaCO3. Hydrogen peroxide and HNO3

were added to the filtrate to adjust the pH to 1, andthe filtrate was passed through a solid-phase extrac-tion resin (Eichrom Technologies, Inc., TEVA Resin).Technetium-99 was retained on the resin, and 79Se waseluted. After the resin was washed with 30 ml of 2 MHNO3, Tc was eluted from the resin with 15 ml of 8 MHNO3 and separated from the other radionuclides [15].The Re in the Tc-eluted solution was measured using in-

ductively coupled plasma atomic emission spectroscopy(Shimadzu Co., ICPS-7510) to determine the chemicalyield of Tc. Because there is no stable Tc isotope, the re-covery was determined using Re, which behaves in thesamemanner as Tc [15]. After the Tc-eluted solution wasmade an addition toAquasol-2 (PerkinElmer, Inc.), 99Tcwas measured using a liquid scintillation counter (LSC,PerkinElmer, Inc., TriCarb 2910LL).

After HBr was added to the eluted solution from theTEVA Resin, hydroxylamine was added to reduce theSe. This solution was filtered resulting in a precipitate ofSe. The Se precipitate mounted on the filter paper wasmeasured using the Pico Beta to check for the shape ofthe beta-ray spectrum and the existence of interferingradionuclides. The precipitate was dissolved in HNO3,and Aquasol-2 was added to the solution. The contentsof 79Se was measured using the LSC. The Se recoverywas determined by the gravimetric determination of theprecipitate.

3.4. Analysis of 14CThis analysis used the same combustion apparatus

as the conventional method for 3H and 14C [13], butthe 14C trap solution containing 1 M NaOH was in-creased to 80ml from 20ml. This is because the tree sam-ple contained a large amount of organic compounds.A schematic of combustion apparatus is shown inFigure 3.

3.4.1. Rubble samples

An alumina boat charged with 1 g of the rubble sam-ple was inserted into a quartz tube and placed in the elec-tric furnace I. The central zone of the electric furnaceII was filled with 4.1 g of Hopcalite(I) as an oxidationcatalyst. The furnaces I and II were heated to 100 and500 ◦C, respectively, to dry the sample and Hopcalite(I).After the drying, the furnace I was heated at 500 ◦C for1 h and then heated at 900 ◦C for 1 h under an oxygengas flow of 150 ml min−1.

Figure 3. Schematic of combustion apparatus for analysis of 3H and 14C.

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After combustion, the 14C trap solution from thecombustion process was poured into a three-neck flask.This was connected to two impingers containing 5 mlof Carbo-Sorb E (PerkinElmer, Inc.). Nitric acid wasadded gradually to acidify the solution, which wasstirred for 3 h under nitrogen gas at a flow rate of 50 mlmin−1. The 14C-trapped Carbo-Sorb E was transferredinto a vial with 10 ml of Permafluor E+ (PerkinElmer,Inc.). Themixture was thoroughly shaken andmeasuredusing the LSC.

3.4.2. Tree samples

The combustion apparatus for the tree sample wasthe same as for the rubble samples. Since the tree samplescontained higher organic compounds than in the rub-ble samples, the amount of tree sample was decreasedto 0.5 g and the weight of Hopcalite(I) was increased to8.2 g.

3.5. Analysis of 129IThe conventional method [14, 16] was used for the

129I analysis of the rubble sample. Since the tree samplescontained a large quantity of organic material, impuri-ties such as tar were decomposed using an oxidation cat-alyst. Here, Ag-free Hopcalite(II) was used, as the Ag inHopcalite(I) adsorbs 129I. The decomposition generateda large quantity of carbon dioxide, which wasmixed intothe trap solution. Since the solid-phase extraction of 129Iis affected by the presence of large amounts of carbondioxide, 129I was trapped with 2% of tetramethylammo-nium hydroxide. The heating condition was the same asthat used for the 3H and 14C analyses [13].

Iodine-129 was measured using an inductivelycoupled plasma mass spectrometer equipped with adynamic reaction cell (ICP-MS) (PerkinElmer, Inc.,ELAN DRC-e) [16].

4. Results

4.1. Applicability of analytical method tocollected samples

The gamma-ray spectrum of the rubble sample col-lected at the F1NPSmeasured by themodified analyticalmethod proposed in this study is shown in Figure 4(a).The rubble sample after the AMP treatment was placednear the HPGe and measured for 70,000 s. A distinct60Co peak (arrow labeled a) was confirmed in the spec-trum, whereas the 94Nb (arrow labeled b), 152Eu (arrowlabeled c), and 154Eu (arrow labeled d) peaks were notdistinguished even though their detection limit (DL)wasimproved to 5 × 10−1 Bq/g. The intensity of the ra-diocesium peaks was not strong because of the elimi-nation by AMP in the modified analytical method. Incontrast, as shown in Figure 4(b), the gamma-ray spec-trum of powdered samples without the elimination by

Figure 4. Gamma-ray spectrum of rubble sample (3U-09) (a)after and (b) before treatment by modified analytical method.Arrows labeled a, b, c, and d indicate peaks of 60Co, 94Nb,152Eu, and 154Eu, respectively.

the AMP showed an extremely high intensity of radio-cesiumwhen the sample wasmeasured for only 500 s at adistance of 50 cm from theHPGe. The peaks of the otherradionuclides, including 60Co, were not distinguishedin the spectra. These results indicated that the accumula-tion of gamma-ray counts should be avoided in the de-termination of 137Cs by omitting the AMP treatment,and the determination of 60Co, 94Nb, 152Eu, and 154Eurequired the AMP treatment. In addition, from a con-firmation test using known amounts of 60Co, 94Nb, and152Eu, recoveries ofmore than 99%were obtained by thisprocedure.

In the conventional method for beta-particle-emitting nuclides, the measured samples still containedsome radiocesium. The AMP treatment decreased theradiocesium content of the measured samples, indicat-ing that it was also effective for accurate determinationof beta-particle-emitting nuclides. The modified analyt-ical method had a lower DL for beta-particle-emittingnuclides than the conventional method. The DL, shownin Table 3, was 50–100 times lower than that of theconventional method.

In addition, the modified analytical method im-proved the recovery of the beta-particle-emitting

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Table 2. Concentrations of gamma-ray-emitting nuclides on 26 October 2012 (Bq/g).

No. Sample 60Co 94Nb 137Cs 152Eu 154Eu

1 1U-06 (1.1 ± 0.4)× 10−1 <5× 10−1 (3.8 ± 0.1)× 103 <5× 10−1 <5× 10−1

2 1U-07 <1× 10−1 <5× 10−1 (5.9 ± 0.1)× 102 <5× 10−1 <5× 10−1

3 1U-08 <1× 10−1 <5× 10−1 (1.8 ± 0.1) × 103 <5× 10−1 <5× 10−1

4 1U-09 (1.1 ± 0.4)× 10−1 <5× 10−1 (2.2 ± 0.1)× 103 <5× 10−1 <5× 10−1

5 3U-02 (4.3 ± 0.4)× 10−1 <5× 10−1 (1.9 ± 0.1)× 104 <5× 10−1 <5× 10−1

6 3U-07 <1× 10−1 <5× 10−1 (2.3 ± 0.1)× 103 <5× 10−1 <5× 10−1

7 3U-09 (5.6 ± 0.1)× 100 <5× 10−1 (1.9 ± 0.1)× 105 <5× 10−1 <5× 10−1

8 3U-10 (5.0 ± 0.4)× 10−1 <5× 10−1 (1.4 ± 0.1)× 104 <5× 10−1 <5× 10−1

9 4U-01 <1× 10−1 <5× 10−1 (1.5 ± 0.1)× 103 <5× 10−1 <5× 10−1

10 4U-02 <1× 10−1 <5× 10−1 (3.2 ± 0.1)× 100 <5× 10−1 <5× 10−1

11 4U-05 <1× 10−1 <5× 10−1 (6.1 ± 0.1)× 101 <5× 10−1 <5× 10−1

12 4U-08 (9.4 ± 0.4)× 10−1 <5× 10−1 (1.5 ±0.1)× 102 <5× 10−1 <5× 10−1

13 T-01 <1× 10−1 <5× 10−1 (9.3 ± 0.1)× 102 <5× 10−1 <5× 10−1

14 T-02 <1× 10−1 <5× 10−1 (1.5 ± 0.1)× 103 <5× 10−1 <5× 10−1

15 T-04 <1× 10−1 <5× 10−1 (3.7 ± 0.1)× 102 <5× 10−1 <5× 10−1

16 T-05 <1× 10−1 <5× 10−1 (7.5 ± 0.1)× 102 <5× 10−1 <5× 10−1

17 T-07 <1× 10−1 <5× 10−1 (4.7 ± 0.1)× 102 <5× 10−1 <5× 10−1

nuclides. When alkaline fusion, which is a conventionalpretreatment method, was used for the analysis of 36Cl,79Se, and 99Tc in the tree samples, less than 50% of thesenuclides were recovered after pretreatment. In contrast,the loss of these nuclides during the pretreatmentwas improved to less than 15% by switching to acidextraction in the modified method.

In the conventional analysis of 36Cl in the rubble andtree samples, the recoveries were extremely low (<20%).Cumulative Cl loss was found to occur in the Cl separa-tion steps of the conventional method. Thus, consider-ing the presumed radionuclide compositions of the rub-ble and trees, the 36Cl analytical flow was dramaticallysimplified; radiocesium is selectively removed by AMP,and other radionuclides are separated from 36Cl by theformation of AgCl precipitates. As a result, a recovery ofmore than 70% could certainly be obtained, and the pre-cipitate could be formed at high radiochemical purity bythe modified method. In the conventional 79Se analysis,the recoveries were also less than 20%, because of theloss of Se in the removal of interference radionuclidessuch as 60Co and 63Ni by coprecipitation with iron hy-droxide. Therefore, the coprecipitation step was omittedto improve the Se recovery in the modified method. Inaddition, because the analytical flow of 79Se had manysteps in common with that of 99Tc, these flows weremerged and simplified to improve the labor efficiency ofthe analysis. The Se precipitates and Tc solutions ulti-mately recovered by the modified method had high ra-diochemical purity.

The DLs of 14C for the conventional and modifiedmethods were about 5× 101 and 2× 10−1 Bq/g, respec-tively. In the latter, a decontamination factor (DF) ofmore than 107 was obtained by the double purificationof the 14CO2 gas generated by combustion. Further-more, the loss of 14C in the purification step in the modi-fiedmethodwas negligible (<1%). In a confirmation test

using a known amount of 14C, more than 98% of the 14Ccould be recovered by this procedure.

When the conventional 129I analysis method was ap-plied to the tree samples, a 129I trap solution was con-taminated with tar consisting of the residual undecom-posed components of the trees, and the Anion-SR wasclogged with the tar. To decompose the tar, an oxida-tion catalyst including an Ag-free type was added to thiscombustion system. As a result of the modification, a129I recovery of more than 99%was obtained for the treesamples.

4.2. Radioactivity concentrations of collectedsamples

The radioactivity concentrations of the gamma-ray-emitting, beta-particle-emitting, and alpha-particle-emitting nuclides in the rubble and tree samples are tab-ulated in Tables 2, 3, and 4, respectively. Table 2 showsthat 60Co was detected in six rubble samples between1.1× 10−1 and 5.6× 100 Bq/g, and that six rubble sam-ples contained less than the DL of 0.1 Bq/g. The 60Cocontent of the tree samples was below the DL. The 94Nbcontent was below the DL of 0.5 Bq/g in all the tree andrubble samples. Cesium-137 was detected in all the sam-ples and was distributed between 3.2× 100 and 1.9× 105

Bq/g in the rubble samples. In the tree samples, 137Cs wasdetected between 3.7× 102 and 1.5× 103 Bq/g. The ra-dioeuropium content was below theDLof 5× 10−1 Bq/gin all the rubble and tree samples.

Of the beta-particle-emitting nuclides, 3H was de-tected in all the rubble samples between 1.7× 10−1 and1.8× 100 Bq/g. Further, 3Hwas detected in the tree sam-ples from 2.2× 10−1 to 4.6× 10−1 Bq/g, and in one sam-ple below the DL of 2× 10−1 Bq/g. Carbon-14 was de-tected in the rubble samples collected at units 3 and4 between 1.3× 10−1 and 2.7× 100 Bq/g, but was not

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Con

centration

sof

beta-particle-em

itting

nuclides

on26

Octob

er20

12(B

q/g).

No.

Sample

3H

14C

36Cl

79Se

90Sr

99Tc

129I

11U

-06

(4.0

±0.2)

×10

−1<5

×10

−2<5

×10

−2<5

×10

−2(5.2

±0.1)

×10

0<5

×10

−2<5

×10

−2

21U

-07

(3.0

±0.2)

×10

−1<5

×10

−2<5

×10

−2<5

×10

−2(3.3

±0.1)

×10

0<5

×10

−2<5

×10

−2

31U

-08

(2.8

±0.2)

×10

−1<5

×10

−2<5

×10

−2<5

×10

−2(1.0

±0.1)

×10

1<5

×10

−2<5

×10

−2

41U

-09

(3.1

±0.2)

×10

−1<5

×10

−2<5

×10

−2<5

×10

−2(8.0

±0.1)

×10

0<5

×10

−2<5

×10

−2

53U

-02

(1.7

±0.2)

×10

−1(3.1

±0.1)

×10

−1<5

×10

−2<5

×10

−2(5.3

±0.1)

×10

0<5

×10

−2<5

×10

−2

63U

-07

(2.7

±0.2)

×10

−1<5

×10

−2<5

×10

−2<5

×10

−2(1.3

±0.1)

×10

−1<5

×10

−2<5

×10

−2

73U

-09

(3.5

±0.2)

×10

−1(6.1

±0.1)

×10

−1<5

×10

−2<5

×10

−2(3.9

±0.1)

×10

0<5

×10

−2<5

×10

−2

83U

-10

(1.5

±0.1)

×10

0(4.1

±0.1)

×10

−1<5

×10

−2<5

×10

−2(1.2

±0.1)

×10

0<5

×10

−2<5

×10

−2

94U

-01

(5.2

±0.2)

×10

−1(1.3

±0.1)

×10

−1<5

×10

−2<5

×10

−2(2.1

±0.1)

×10

−1<5

×10

−2<5

×10

−2

104U

-02

(1.8

±0.1)

×10

0(2.7

±0.1)

×10

0<5

×10

−2<5

×10

−2<5

×10

−2<5

×10

−2<5

×10

−2

114U

-05

(3.1

±0.2)

×10

−1(4.9

±0.1)

×10

−1<5

×10

−2<5

×10

−2<5

×10

−2<5

×10

−2<5

×10

−2

124U

-08

(1.2

±0.1)

×10

0<5

×10

−2<5

×10

−2<5

×10

−2(2.7

±0.1)

×10

−1<5

×10

−2<5

×10

−2

13T-01

(3.0

±0.5)

×10

−1<2

×10

−1<5

×10

−2(1.7

±0.2)

×10

−1(3.5

±0.1)

×10

0<5

×10

−2<5

×10

−2

14T-02

(3.9

±0.4)

×10

−1<2

×10

−1<5

×10

−2(2.0

±0.2)

×10

−1(9.1

±0.1)

×10

−1(8.9

±1.2)

×10

−2<5

×10

−2

15T-04

<2

×10

−1<2

×10

−1<5

×10

−2(2.1

±0.1)

×10

−1(1.5

±0.1)

×10

−1(6.2

±0.9)

×10

−2<5

×10

−2

16T-05

(2.2

±0.4)

×10

−1<2

×10

−1<5

×10

−2<5

×10

−2(2.6

±0.1)

×10

−1<5

×10

−2<5

×10

−2

17T-07

(4.6

±0.4)

×10

−1<2

×10

−1<5

×10

−2(1.5

±0.1)

×10

−1(2.7

±0.1)

×10

−1<5

×10

−2<5

×10

−2 detected in the tree samples. Selenium-79was detected inthe tree samples from 1.5× 10−1 to 1.0× 101 Bq/g. Fur-ther, the 79Se content of the rubble samples was belowthe DL of 5× 10−2 Bq/g. Four tree samples containedapproximately 1.8× 10−1 Bq/g of 79Se, which is threetimes the DL of 5× 10−2 Bq/g. For 90Sr, 10 rubble sam-ples contained between 1.3× 10−1 and 2.1× 101 Bq/g,which is higher than the DL of 5× 10−2 Bq/g. In thetree samples, amounts between 1.5× 10−1 and 3.5× 100

Bq/g of 90Sr were identified. The 99Tc content of all therubble samples was below the DL of 5× 10−2 Bq/g. Intwo tree samples, 99Tc was detected at about 7.6× 10−2

Bq/g. The 36Cl and 129I contents of all the rubble andtree samples were below the DL.

Table 4 shows that all the alpha-particle-emitting nu-clides, 238Pu, 239Pu, 241Am, and 244Cm, were below theDL (1× 10−2 Bq/g).

We summarize the contamination of the rubble andtree samples as follows. In both types of collected sam-ples, 3H, 90Sr, and 137Cs were detected. The radioactiv-ity concentrations of 3H at each sampling location werealmost comparable; the average value for all the sam-ples was 5.3× 10−1 Bq/g. Strontium-90 and 137Cs weredetected in both the rubble and tree samples. Carbon-14 and 60Co were detected only in the rubble. In partic-ular, 14C was detected only in the rubble from aroundunits 3 and 4. Cobalt-60 was detected in the rubble sam-ples from around units 1, 3, and 4. In contrast, 79Se and99Tc were detected in the tree samples. Note that the de-termined values for 79Se and 99Tc were close to the DL(5× 10−2 Bq/g), but the maximum beta-ray energy ob-served in the LSC spectra agreedwell with the data in theliterature [17]. The radioactivity concentrations of 36Cl,94Nb, 129I, 152Eu, 154Eu, 238Pu, 239Pu, 241Am, and 244Cmwere less than the DL.

Because 137Cs is the main radionuclide of FP andeasily determined by nondestructivemeasurement, 137Cswould be useful as a key nuclide when a simple rapid ra-dioactivity estimation is needed. In that case, it is alsopossible to combine the data calculated using a com-puter code and the values determined by the analysis.For this reason, the radioactivity ratios of 3H, 14C, 60Co,and 90Sr to 137Cs determined in this study are plotted inFigure 5.

As shown in Figure 5, 3H seems to be uniformly dis-tributed among all the rubble samples and its concentra-tion did not depend on the 137Cs concentration or sam-pling location. In contrast, the 14C/137Cs radioactivityratio varied with the sampling location; the average ra-tios for units 1, 3, and 4 were 2.9× 10−5, 1.4× 10−5, and3.7× 10−3, respectively. In particular, the ratio for unit 4varied widely, ranging from 8.7× 10−5 to 8.4× 10−1.

For 60Co in the rubble samples, a weak correlationwas observed between the radioactivity concentrationsof 60Co and 137Cs, and the radioactivity ratios in therubble samples collected around units 3 and 4 were dis-tributed around the linear lines for ratios of 0.01% and1%, respectively.

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1040 K. Tanaka et al.

Table 4. Concentrations of alpha-particle-emitting nuclides on 26 October 2012 (Bq/g).

No. Sample 238Pu 239Pu 241Am 244Cm

1 1U-06 <1× 10−2 <1× 10−2 <1× 10−2 <1× 10−2

2 1U-07 <1× 10−2 <1× 10−2 <1× 10−2 <1× 10−2

3 1U-08 <1× 10−2 <1× 10−2 <1× 10−2 <1× 10−2

4 1U-09 <1× 10−2 <1× 10−2 <1× 10−2 <1× 10−2

5 3U-02 <1× 10−2 <1× 10−2 <1× 10−2 <1× 10−2

6 3U-07 <1× 10−2 <1× 10−2 <1× 10−2 <1× 10−2

7 3U-09 <1× 10−2 <1× 10−2 <1× 10−2 <1× 10−2

8 3U-10 <1× 10−2 <1× 10−2 <1× 10−2 <1× 10−2

9 4U-01 <1× 10−2 <1× 10−2 <1× 10−2 <1× 10−2

10 4U-02 <1× 10−2 <1× 10−2 <1× 10−2 <1× 10−2

11 4U-05 <1× 10−2 <1× 10−2 <1× 10−2 <1× 10−2

12 4U-08 <1× 10−2 <1× 10−2 <1× 10−2 <1× 10−2

13 T-01 <1× 10−2 <1× 10−2 <1× 10−2 <1× 10−2

14 T-02 <1× 10−2 <1× 10−2 <1× 10−2 <1× 10−2

15 T-04 <1× 10−2 <1× 10−2 <1× 10−2 <1× 10−2

16 T-05 <1× 10−2 <1× 10−2 <1× 10−2 <1× 10−2

17 T-07 <1× 10−2 <1× 10−2 <1× 10−2 <1× 10−2

Figure 5. Concentrations of detected radionuclides (a) 3H, (b) 14C, (c) 60Co, and (d) 90Sr as a function of those of 137Cs.

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Journal of Nuclear Science and Technology, Volume 51, Nos. 7–8, July–August 2014 1041

The radioactivity ratio of 90Sr to 137Cs clearly de-pended on the sampling location. The ratios in the rub-ble samples around units 1 and 4 and in the tree sampleswere distributed between the lines of 1% and 0.1%. Fur-ther, the ratios around unit 3 were distributed around theline of 0.01%.

5. Discussion

5.1. Effectiveness of modified analytical methodIn this analysis, a pretreatment method based on

acid leaching for radioactive waste samples such as ce-ment solidified product and ash was applied to the col-lected samples. Because more than 95% of the 137Cs wasquantitatively extracted from the samples to a leachingsolution, the adherent involatile radionuclides such as60Co, 90Sr, and Pu were also considered to be extractedsuccessfully by this pretreatment [9]. In the gamma-raymeasurement after the pretreatment, theDL for gamma-ray-emitting nuclides, except for 137Cs, decreased sig-nificantly compared with that of ordinary gamma-raymeasurement. This is because the background countsin the gamma-ray spectra were increased by scatteredand bremsstrahlung radiation from the high concentra-tion of radiocesium. The removal of radiocesium withAMP was very effective for the measurement of minorradionuclides. Consequently, the DLs for 60Co, 94Nb,152Eu, and 154Eu were improved to 1/50 to 1/100 of thoseunder the maximum 137Cs concentration.

In the conventional 14C analysis method for radioac-tive waste from research institutes, 14C is combusted to14CO2 and trapped in an adsorbent; this is followed bydirect LSC measurement. Unfortunately, the samplescollected here contained large amounts of FPs, and thetrap solution was contaminated with a small amountof 137Cs. Therefore, a further purification step consist-ing of the distillation of 14CO3

2− from the trap solu-tion was indispensable for an accurate 14C measurementwith the LSC. It is well known that 36Cl, 79Se, and 99Tcare vaporized by acid digestion using a microwave oven.Therefore, those analyses required an appropriate pre-treatment method for the recovery of these volatile ra-dionuclides. Using the trial-and-error method, it wasfound that the conventional alkaline fusion method us-ing NaOH was applicable to the rubble samples butcould not be applied to the tree samples because of ex-cess loss of these nuclides. Instead of the conventional al-kaline fusion or acid digestion, an acid leaching methodusing HNO3 as a pretreatment was successfully appliedto the tree samples. After the pretreatment, 36Cl was ef-fectively separated frommost of the FP andmatrix com-ponents by the formation of AgCl. To recover the highlyrefined AgCl precipitate, it was important to remove Siand Al, which were major chemical components of therubble, by precipitating them as silicate and hydroxide,respectively, near the neutral region.

The recovery of 129I using the modified method wasalmost 100%, indicating that the combustion methodprovides sufficient recovery of 129I for both the rubbleand tree samples. In the conventional method, an al-kaline fusion method was applied as a pretreatment inthe 129I analysis, but the recovery of 129I decreased asthe amount of the I carrier decreased. This tendencywas especially remarkable when the amount of the I car-rier was less than 1 mg per sample. In our system, theamount of the I carrier should bemaintained at less than1μg because problems such as amemory effect occurredin the ICP-MSmeasurement. These results indicate thatthe modified method is more effective than the conven-tional method for measuring 129I in both the rubble andtree samples.

5.2. Characteristics of rubble at each samplinglocation

As shown in Figure 5(a), 3H seems to be uniformlydistributed in all the rubble samples, and its concentra-tion did not depend on the 137Cs concentration or sam-pling location. In contrast, 14C was detected in the rub-ble around units 3 and 4 but not in that from aroundunit 1. Furthermore, the 14C/137Cs radioactivity ratiovaried with the sampling location; the average ratiosfor units 1, 3, and 4 were 2.9× 10−5, 1.4× 10−5, and3.7× 10−3, respectively. The ratio for unit 4 in particu-lar varied widely, ranging from 8.7× 10−5 to 8.4× 10−1.These results imply that the distribution of 14C contam-ination differs from that of 137Cs contamination and isnot uniform, unlike that of 3H. The 14C concentrationsin the rubble ranged within about two orders of magni-tude; thus, the 14C contamination of the rubble may beregarded as being as uniform as that of 3H. As describedin the foregoing section, a further investigation of the re-lease of 14C from the reactor buildings and its adhesionand migration behaviors is needed to estimate the 14Cinventory of rubble generated by the accident.

The radioactivity concentrations of 60Co in the rub-ble were correlated with those of 137Cs. The radioactiv-ity ratios for unit 3 were distributed around the line of0.01%, which differs from that of unit 4, where the ra-tios were distributed around the line of 1%. In general,60Co is produced mainly by neutron activation of 59Co,which is present as an impurity in carbon and stainlesssteels. In normal nuclear power plant operation, 60Co ispresent in the surface cladding of fuels that are loadedinto a reactor or stored in a spent fuel pool. This 60Co isa major source of pre-accident contamination in nuclearpower plants. This seems to be related to the reasons thatthe 60Co/137Cs ratio of unit 4, in which the fuel was notdamaged, is higher than that of unit 3. It is thought thatthe rubble from unit 4 was contaminated with water con-taining 60Co in the fuel pool. In contrast, the correlationbetween 60Co and 137Cs for unit 3 implies that 60Co wasreleased, together with 137Cs, into a gas phase from the

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1042 K. Tanaka et al.

reactor core. To clarify the process of 60Co contamina-tion for each unit in detail, the radionuclides that weregenerated by neutron activation, such as 54Mn and 63Ni,should be analyzed in a further investigation.

The radioactivity ratio of 90Sr to 137Cs probably de-pended on the sampling location. The ratios for units1, 3, and 4 were 3.5× 10−3, 7.3× 10−5, and 1.3× 10−3,respectively. According to the calculation of the radioac-tivity released from the reactor buildings, the 90Sr/137Csradioactivity ratios for units 1, 2, and 3 were 1.0× 10−2,3.4× 10−3, and 1.2× 10−1, respectively [18]. Because thereactor building of unit 2 at the F1NPS did not undergoa hydrogen explosion, the rubble from unit 2 could notbe collected in this analysis. In addition, the radioactiv-ity released from unit 4 has not been calculated becauseunit 4 was not in operation when the accident occurred.Therefore, although the calculated and analyzed data forunits 2 and 4 could not be compared, the 90Sr/137Cs ratioof the rubble around unit 3 was found to differ from thatof the rubble around unit 1. It is also suggested that thecontamination of the rubble around units 1 and 3 wasinfluenced by the radioactivity released from unit 2. Infact, the amount of radioactivity released from the re-actor building in unit 2 was estimated to be more than10 times that from unit 1 or unit 3. A comparison ofthe calculated and analyzed data implies that the rub-ble collected from one unit contained a mixture of ra-dionuclides released from several units; the analysis ofthe rubble and tree samples should continue and the re-sults should be comparedwith the calculated data to fur-ther refine the figures determined in this work.

5.3. Comparison of rubble and treesAs shown in Figure 5(a), there is little difference

between the 3H concentrations of the rubble and thetrees. The average 3H concentrations of the rubble andtrees are 6.2× 10−1 and 3.1× 10−1 Bq/g, respectively.The 90Sr/137Cs ratios of the trees ranged within those ofthe rubble, shown in Figure 5(d). The average 90Sr/137Csratio of the trees is calculated to be 7.1× 10−4, and thatof the rubble is 7.0× 10−4. With regard to the 90Sr/137Csratios, there is little difference between those of the rub-ble and the trees. Carbon-14 was not detected in thetrees but was detected in the rubble. Although the DL of14C for the tree samples was four times that for the rub-ble samples, the 14C concentrations of the rubble werehigher than the DL of the tree samples. Similarly, 60Cowas also detected only in the rubble. These results implythat large amounts of 14C and 60Co were not scatteredaway from the reactor buildings.

In contrast, 79Se and 99Tc were detected only in thetree samples. The detected 79Se and 99Tc might not becaused by simple adhesion. It is well known that varioustypes of plants adsorb Se and Tc through the roots [19].The change in the radioactivity ratios of 79Se and 99Tc to137Cs may be attributable to the plants’ species and ages.

This secondary transportation of 79Se and 99Tc from thedeposited area may be one of the reasons.

6. Conclusions

To characterize the radioactivity composition ofthe rubble and trees contaminated by the accident atthe F1NPS, modified radiochemical analysis procedureswere successfully applied to rubble and trees sampledin the areas around the reactor buildings and the stockyards (for cut trees). Our study presents the followingconclusions:

• Tritium was uniformly distributed in the collectedsamples in low radioactivity concentrations (<2Bq/g).

• Carbon-14 and 60Cowere detected only in the rub-ble, and the concentrations of 14C are not corre-lated with those of 137Cs. In the rubble around unit3, a weak correlation between 60Co and 137Cs wasobserved.

• Selenium-79 and 99Tc were detected only in thetrees in low radioactivity concentrations (<1Bq/g).

• The radioactivity concentrations of 90Sr were cor-related with those of 137Cs, and the 90Sr/137Cs ratiowas at almost the same level as that in environ-mental soil samples collected after the accident.The observed results for 90Sr also implied that the90Sr/137Cs ratio was different for each unit of theF1NPS.

AcknowledgementsThe authors are grateful to K. Sato, K. Kimiyama,

T. Unno, M. Kajio, N. Yonekawa, S. Seki, T. Niiyama, andK. Watanabe for their assistance in the experiment, and toT. Hiyama, A. Katayama, and K. Nishinoiri for their fruitfuldiscussions and assistance in the sampling of rubble and trees.This work was performed under a contract with the Ministryof Economy, Trade and Industry of Japan, to whom theauthors express their appreciation.

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