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Distribution of radioactive nuclides of boring coresamples extracted from concrete structures ofreactor buildings in the Fukushima Daiichi NuclearPower PlantKoji Maedaa, Shinji Sasakia, Misaki Kumaia, Isamu Satoa, Mitsuo Sutoa, Masahiko Ohsakaa,Tetsuo Gotob, Hitoshi Sakaib, Takayuki Chigirac & Hirotoshi Muratac

a Japan Atomic Energy Agency, 4002 Narita, Oarai, Higashiibaraki, Ibaraki 311-1393,Japanb Toshiba Corporation, 8, Shinsugita, Isogo-ku, Yokohama 235-8523, Japanc Tokyo Electric Power Company, 1-1-3 Uchisaiwai, Chiyoda-ku, Tokyo, 100-8560, JapanPublished online: 15 May 2014.

To cite this article: Koji Maeda, Shinji Sasaki, Misaki Kumai, Isamu Sato, Mitsuo Suto, Masahiko Ohsaka, Tetsuo Goto,Hitoshi Sakai, Takayuki Chigira & Hirotoshi Murata (2014) Distribution of radioactive nuclides of boring core samplesextracted from concrete structures of reactor buildings in the Fukushima Daiichi Nuclear Power Plant, Journal of NuclearScience and Technology, 51:7-8, 1006-1023, DOI: 10.1080/00223131.2014.915769

To link to this article: http://dx.doi.org/10.1080/00223131.2014.915769

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Journal of Nuclear Science and Technology, 2014Vol. 51, Nos. 7–8, 1006–1023, http://dx.doi.org/10.1080/00223131.2014.915769

ARTICLE

Distribution of radioactive nuclides of boring core samples extracted from concrete structuresof reactor buildings in the Fukushima Daiichi Nuclear Power Plant

Koji Maedaa∗, Shinji Sasakia, Misaki Kumaia, Isamu Satoa, Mitsuo Sutoa, Masahiko Ohsakaa, Tetsuo Gotob,Hitoshi Sakaib, Takayuki Chigirac and Hirotoshi Muratac

aJapan Atomic Energy Agency, 4002 Narita, Oarai, Higashiibaraki, Ibaraki 311-1393, Japan; bToshiba Corporation, 8, Shinsugita,Isogo-ku, Yokohama 235-8523, Japan; cTokyo Electric Power Company, 1-1-3 Uchisaiwai, Chiyoda-ku, Tokyo, 100-8560, Japan

(Received 19 November 2013; accepted final version for publication 11 April 2014)

Since the start of the severe accidents at the Fukushima Daiichi Nuclear Power Plant in March 2011, con-crete surfaces within the reactor buildings (RBs) have been exposed to radioactive contaminants. Releasedradionuclides still remain too high to permit entry into some areas of the RBs to allow the damage tobe assessed and to allow carrying out the restoration of lost safety functions, decommissioning activities,etc. In order to clarify the situation of this contamination in the RBs of Units 1, 2 and 3, samples of con-taminants were collected and subjected to analyses to determine the surface radionuclide concentrationsand to characterize the radionuclide distributions in the samples. Especially, decontamination tests on theboring core sample of Unit 2 were conducted to quantitatively determine the effectiveness of several basicdecontamination techniques. As a result of the tests, the level of radioactivity of this sample was reducedwith the removal of ∼97% of the contamination present near the sample surface, and it was confirmed forthe boring core sample that the contamination mainly had the characteristics of fixed contamination of thesurface.

Keywords: cesium 137; decommissioning; decontamination; Fukushima Daiichi Nuclear Power Plant; reactorbuilding; boring core

1. Introduction

Since the massive earthquake and tsunami on 11March 2011, and the following severe accidents atthe Fukushima Daiichi Nuclear Power Plant, con-crete surfaces within the reactor buildings (RBs) havebeen exposed to radioactive contaminants. Therefore,decontamination of operation areas is necessary toallow adequate restoration of lost safety conditionsand to allow decommissioning work. The R&D projectfor decontamination of the RBs as a large nationalprogram [1] was started in 2011 soon after the accident;this project evaluates various potential methods fordecontamination and determines the most effectiveones. In order to begin to clarify the situation of con-tamination mainly in the first floors of the RBs of Units1, 2 and 3, an on-site investigation [2] was performedin 2012. Soon after the on-site investigation, selectedsamples including four boring cores were transportedto the Fuels Monitoring Facility (FMF) in the OaraiEngineering Center of JAEA. These samples were sub-jected to a series of analyses to determine their surface

∗Corresponding author. Email: maeda.koji@jaea.go.jp

radionuclide concentrations and to characterize theirradionuclide distributions. In particular, penetrationof radiocesium into the surface epoxy coating and intothe sub-surface concrete was investigated to identifythe characteristics of radionuclide penetration into theboring core samples. Furthermore, decontaminationtests on the retained contamination near the surfaceof the boring core sample of Unit 2 were conducted byseveral decontamination techniques. Simultaneously,by reduction in the level of radioactivity present nearthe surface of the epoxy coating layer of the boring coresample, the effectiveness of several decontaminationtechniques was quantitatively evaluated.

2. Sample preparation, measurement methodsand decontamination tests

2.1. Sampling and selection of samplesThe first systematic sampling of surfaces in each

RB took place in May and June 2012. Samples were

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

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collected at 14 individual locations from each RB asshown in Figure 1(a)–1(c). The locations were chosen tobe representative of the major concrete surfaces, whichwere undamaged by the recovery work after the acci-dents. In order to evaluate the major contaminationstate and contamination level, three types of contamina-tion samples were separately collected at the same placefor each location: they were flannel cloth for loose con-tamination, strippable paint (a water-borne vinyl pee-lable decontamination coating) for fixed contaminationand boring core for penetrated contamination. Photosof the three kinds of samples are shown in Figure 2.

The sampling procedure from a concrete structurewas as follows. Before extraction of a boring core fromthe concrete structure, loose contamination (dust andstructural fragments of building materials) was initiallywiped away, without rubbing, from the sampling placesusing dry flannel cloths and then the fixed contamina-tion on the surface of the epoxy-based coating was col-lected by using the strippable paint. The strippable paint(ALARA SD manufactured by Japan Environment Re-search Co. Ltd.) was placed on the surface of the epoxycoating of the concrete structure, and the paint was driedand peeled off after 48 hours. Finally, the boring coresample was extracted using a boring core machine at thesame place. The boring core sampling was conductedonly from the first floor of each RB due to high radi-ation doses in the RB upper floors. At other locationsof upper floors of each RB, pairs of flannel cloth andstrippable paint samples were collected.

Sampling locations and dose rates of samples ob-tained from RBs of Units 1, 2 and 3 are summarized inTable 1. Three large boring core samples: two samples(floor and wall) in Unit 1 and one sample (floor) in Unit2, each approximately 10 cm in diameter by 10 cm inlength, were extracted. Additionally, one small boringcore sample (floor), approximately 3 cm in diameter by5 cm in length, was extracted from the RB concrete floorof Unit 3 using a portable core boring machine, becauseof the extremely high radiation dose present there. Thesampling surfaces of concrete structures at the individ-ual locations had been protected by coatings of epoxy-based, nuclear grade paints. Unfortunately, the twoboring cores ofUnit 1were damaged during core boring;that is, they had separation of epoxy coating (floor sam-ple No. 1) and significant cracking (wall sample No. 6).

Soon after sampling and the on-site investigation,18 samples, including all four boring core samples, wereselected in order to clarify the situation of contami-nation in the RBs of Units 1, 2 and 3, as shown inTable 1. These samples were transported to the FMFwhere they were subjected to a series of analyses to de-termine the surface radionuclide concentrations and tocharacterize the radionuclide distributions. In particu-lar, penetration of radiocesium into the surface epoxycoating layer and into the sub-surface concrete was an-alyzed using scanning electron microscopy (SEM) andwavelength-dispersive X-ray spectroscopy (WDS); these

analyses also gave information on the integrity of theepoxy coating of the boring core samples. These analy-sis methods were chosen based on three mile island unit-2 (TMI-2) investigations [3,4]. Before detailed examina-tions, cross-contamination of radionuclides in the twodamaged boring core samples ofUnit 1 was identified byimaging plate (IP) measurements [5]. As a result, loosecontamination of concrete dust particles between coat-ing layer and concrete was not negligible in floor sam-ple No. 1. It was due to contaminated drilling powderduring boring. Then using adhesive (AralditeR© manufac-tured by Huntsman Advanced Materials) the separatedcoating layer of floor sample No. 1 was glued to the con-crete, and the cracked concrete pieces of wall sample No.6 were glued to each other.

2.2. Analysis and measurements methodsAnalysis and sectioning flow of samples is shown in

Figure 3. Prior to any destructive analyses, IP measure-ments were made on each sample to clarify the radioac-tivity distribution of the samples. The sample surfaceradioactivity was measured by placing the samples onIPs for times ranging from several tens of seconds to20 hours. The range of beta particles of each sample wasmeasured by an ionization chamber type survey meterand using aluminum shield plates of various thicknesses.As a result the major source of beta particles was iden-tified as radiocesium (134Cs and 137Cs). Detailed betaspectrometry on the core samples is planned at TokaiResearch and Development Center of JAEA.

Figure 4 shows sectioning plans and analysis itemsof boring core samples. In the case of the boring coresamples, the radioactivity of the epoxy coating surface ofeach boring core sample was initially measured with theIP. Then the sample was longitudinally sectioned witha rotary blade and the thicknesses of coating layers ofeach boring core sample were obtained by a visual ex-amination. The thicknesses of coating layers of each bor-ing core sample were obtained as approximately 5.2 mm(floor, Unit 1), 0.5mm (wall, Unit 1), 2.4mm (floor, Unit2) and 6.5 mm (floor, Unit 3).

Moreover, to determine the distribution of radioac-tivity that had migrated through the coating layer andinto the sub-surface concrete, the four boring core sec-tioned samples were longitudinally gamma-scanned us-ing a collimated intrinsic lithium-drifted germanium[Ge(Li)] spectrometer, and the inventory of gamma-ray-emitting radionuclides in each was measured. Thegamma scanning technique for axially sectioned samplesinvolved making repeated count rate measurements ofthe photo peaks of the radionuclides while moving thesample incrementally across the field of view of the spec-trometer. In addition, the distributions of radioactivityon the longitudinal section surface of each boring coresample were measured by placing the cut surface ontothe IP.

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Figure 1. (a) Sampling locations in the reactor building of Unit 1. (b) Sampling locations in the reactor building of Unit 2. (c)Sampling locations in the reactor building of Unit 3.

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Figure 2. Photos of three kinds of samples obtained in the on-site investigation.

Flannel cloth pieces of the stained area were individ-ually cut for the spectrometry measurements and piecesof the strippable paint samples were cut from near thecenter and periphery of the paint areas for the samepurpose. Sampling places of spectroscopies (flannelcloth and strippable paint) are shown in Figure 5. Eachsampling place of flannel cloth and strippable paintwas confirmed by IP measurement. Dust and structuralfragments were not completely collected at the peripheryof the paint by flannel cloth. Figure 6 shows the samplepreparation procedure of the boring core sample forspectrometry. One longitudinal section of each boringcore sample was axially milled with emery papers anda diamond wheel from the surface of the epoxy coatingand dozens of ground powder samples were collected atregular intervals (0.5 mm intervals for the surface epoxycoating and 1.0 mm intervals for the concrete boring

core). After each milling, the surface of the boring coresample was placed on an IP and measurements madeto investigate distribution and penetration depth ofradionuclides. The collected ground powder sampleswere put into individual clean polyethylene bags andplaced inside a second polyethylene bag to provideadditional protection against cross-contamination. Toprevent sample cross-contamination, the rotary bladeand diamond wheel were changed after each sectioningor milling process. Each ground powder samples wasplaced in a large thin-layer miler or vial as shown inFigure 6 for spectrometry analyses.

Cut pieces of flannel cloth and strippable paint sam-ples, and each sampled powder of boring core sampleswere individually analyzed for radioactivity content bygamma-ray spectrometry and total alpha ray spectrome-try (long-timemeasurement, 10,000 s) to obtain detailed

Table 1. Summaries of 14 sampling locations and dose rates of 32 samples obtained from RBs of Units 1, 2 and 3. Eighteensamples were selected for detailed analysis in the Oarai Engineering Center of JAEA.

Dose rate (mSv/h, βγ )

Unit Floor level Location ID Sampling place Flannel cloth Strippable paint Boring core Detailed analysis

1 1 No. 1 Floor 3.1 3.5 0.33 Selected1 No. 2 Wall 0.063 0.05 –1 No. 5 Floor 1.1 0.72 –1 No. 6 Wall 0.16 0.13 0.009 Selected1 No. 8 Surface of

hardware0.1 0.7 –

1 No. 9 Surface ofhardware

Back ground 0.065 –

2 No. 10 Floor 4.0 0.26 – Selected3 No. 11 Floor 2.85 0.45 –

2 1 No. 1 Floor 2.5 3.7 3.8 Selected1 No. 2 Floor 10 6.9 –2 No. 3 Floor 5.7 1.9 – Selected3 No. 4 Floor 3.4 3.4 – Selected

3 1 No. 1 Floor 63.2 8.7 1.8 Selected1 No. 2 Floor 2.2 3.3 –

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Figure 3. The flow sheet of the detailed analysis items on the18 selected samples.

distributions of radioactivities in each boring core sam-ple. The spectrometer was calibrated using point andvolume standard sources and the calibration transferfrom the calibration source to the sample source geome-try (a large thin-layer miler or vial as shown in Figure 6)was done using theMonte Carlo computer code [6]. Theuncertainty of the spectrometry was calculated by er-ror propagation equation from uncertainties of radioac-tivity measurement, mass measurement and dimensionmeasurement, which was less than 10% of analysis value.The high-resolution alpha particle spectra were first cor-rected for gain shift. The detection limit of the alpha en-ergy peaks of samples is <0.4 Bq.

In order to qualitatively evaluate the distribution ofcontamination on samples using IPs, a relationship be-tween surface contamination densities and exposure ra-diation dose of IP measurements was required. This wasdrawn using both the IP measurement and gamma spec-trometry results. By using this relation, IP datawere con-verted into values of the surface contamination density.

Figure 7 shows pieces from flannel cloth, strippablepaint and boring core samples in sample holders for ob-servation and elemental analysis using field emission-scanning electron microscopy (FE-SEM). The flannelcloth piece was cut from the stained area of the sample.Pieces of strippable paint samples were cut from nearthe center and periphery of each paint area. Surface andcross-sectional pieces of boring core samples were ob-tained from one of the longitudinal sections of each bor-ing core sample.

2.3. Decontamination tests on the boring coresample of Unit 2

From the results of the on-site investigation, it wasknown that the circumstances of sample locations inUnit 2 were significantly different from those in Units1 and 3 in terms of the presence of structural concretedust particles and fragments of building materials thatresulted mainly from the hydrogen explosion. The epoxycoating of the core sample of Unit 2 had been directlyexposed to radioactive contaminants without being cov-ered by concrete dust particles and fragments. Hence,preliminary decontamination tests on the boring coresectioned pieces from Unit 2 as shown in Figure 4 wereconducted in order to characterize the contaminationnear the epoxy coating surface. Figure 8 shows sectionedpieces of the boring core sample of Unit 2 used for de-contamination tests. Three roughly rectangular shapedsamples (sample piece Nos. 2-1, 2-2 and 2-3) were ob-tained after further sectioning one of the longitudinalsections of this boring core sample.

The flow sheet of decontamination techniques forthe tests on these sample pieces is shown in Figure 9.Wiping with wet cotton, soaking in immersion fluid,removing by strippable paint and steam-cleaning wereadopted for the tests as basic decontamination tech-niques. These decontamination techniques were used inturn to remove the contamination near the epoxy coat-ing surface of each sample. The procedure flow was de-termined in order to avoid physically attacking the epoxycoating as much as possible before conducting the nextdecontamination technique.

Sectioned sample piece No. 2-1 underwent all decon-tamination techniques except the removal by strippablepaint technique, and deionized water was used as thedecontamination fluid. Sectioned sample piece No. 2-2underwent all the decontamination techniques, and twoaqueous solutions of citric acid (acid concentrationsof 0.1 and 1.0 wt%) were used as the decontaminationfluids for the wiping with wet cotton technique and thesoaking in immersion fluid technique. Kanayama et al.[7] conducted a few simulated decontamination tests onepoxy coating of concrete test pieces to identify the con-tamination characteristics by non-radioactive cesium.The decontamination technique using 1 wt% citric acidaqueous solution has been reported to give the bestdecontamination performance among tests using citricacid concentrations of 1, 5 and 10 wt% [7]. Althoughthis report [7] showed that the decontamination perfor-mance clearly decreased with increasing concentrationof citric acid, concentrations of citric acid less than1 wt%were not investigated. For this reason, the presenttests used two different concentrations of citric acid(0.1 and 1 wt%). Using aqueous solutions of citric acidis desirable because the solutions are environmentallycompliant. Althoughmechanical scrubbing is also effec-tive in decreasing radioactivity, it was not employed hereto avoiding physical damage to the epoxy coatings. Sec-tioned sample piece No. 2-3 was kept in its initial state

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Figure 4. Sectioning plans and analysis items of boring core samples (Units 2 and 3).

without decontamination, to be used as a reference in or-der to evaluate the effectiveness of the decontaminationtechniques. In order to obtain quantitative radioactivitydata, IP measurements were made on the sample piecesbefore and after the decontamination tests. Althoughthe numbers of iterations of each decontamination tech-nique were different, they were enough to confirm the

Figure 5. Sampling places of spectroscopies (flannel clothand strippable paint). Dust and structural fragments were notsufficiently collected at periphery of the paint by flannel cloth.

initial significant change and the final nearly unchangedradioactivity after repeated decontaminations.

Each decontamination test was performed as fol-lows. (1) In the case of the wiping with wet cottontechnique, the epoxy coating surface of a sectioned sam-ple was wiped lightly with wet cotton by hand. Then, thesurface of the epoxy coating was dried in air and con-taminants remaining on the epoxy coating surface weremeasured by IP. This sequence was repeated 10 times.(2) In the case of the soaking in immersion fluid tech-nique, the epoxy coating surface was statically soakedin immersion fluid for 10 min; this time was determinedfrom favorable reported results of a simulated decon-tamination test [7]. Then, the epoxy coating surface wasdried in air and contaminants remaining on this surfacewere measured by IP. This sequence was repeated threetimes using deionized water, three times using 0.1 wt%citric acid and five times using 1.0 wt% citric acid. (3) Inthe case of the removal by strippable paint technique,strippable paint was applied on the epoxy coating sur-face and gauze was placed over the paint. DeconGelR©

(1101 GEL) manufactured by Cellular BioengineeringInc. was selected as a peeling-off agent. The paint wasallowed to dry in air for 48 hours, and then the gauzeand paint were peeled off together, which prevented

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Figure 6. Sample preparation for spectrometry analyses (boring core sample).

cross-contamination. Also, contaminants remaining onthe epoxy coating surface were measured by IP. Thissequence was repeated three times. (4) In the case of thesteam-cleaning technique, a commercial steam-cleaner(KALCHERR© K1100), manufactured for generalhousehold use, was used to wash the epoxy coatingsurface of each sample. Measured outlet temperatureof steam at the nozzle of the cleaner was approximately75 ◦C. Generally, epoxy coatings show reasonabletemperature resistance to 100 ◦C. After steam-cleaninga sample, the epoxy coating surface was dried in air andcontaminants remaining on this surface were measuredby IP. This sequence was repeated six times.

3. Results and discussion

3.1. Surface radioactivity concentrationsThe surface radioactivity concentrations were mea-

sured by gamma-ray spectroscopy prior to undertak-ing any destructive analyses of the boring core samples.The counts by IP measurements were converted to ra-

dioactivity per unit area. The calibration formula wasderived by plotting counts versus gamma spectrometryvalues for the samples. Typical distributions of contam-ination on the samples as measured by the IP techniqueare shown in Figure 10. For samples taken inUnits 1 and3, highly contaminated dust particles and coarse con-crete particles were present on the flannel cloth samples,but contamination was low in the central regions of thestrippable paint samples. This suggested that loose con-taminated dust particles are the major contaminants inUnits 1 and 3. Therefore, the activity on the surface ofthe boring core samples was as low as that in the centralregion of the samples of strippable paints. Dust particleswere thought to be derived from hydrogen explosions inthe RBs of Units 1 and 3. Three kinds of samples col-lected from the wall of the RB of Unit 1 showed a simi-lar distribution of contaminants to that of the floor sam-ples of Unit 1 but contamination activities were lower inthan those of the floor samples. By contrast, for samplesof Unit 2, contaminated dust particles were found onthe flannel cloth samples and activities on the strippable

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Figure 7. Appearance of pieces of samples placed in sampleholder. These samples were supplied for FE-SEM observationand elemental analysis.

paint samples were relatively uniformly distributed. Theactivity on the surface of the boring core sample wasextensive and similar to those of the strippable paintsamples. It is considered that fixed or penetrated con-tamination is the major form on the floor of Unit 2.During the early stage of the accident, high-temperaturesteam and condensed water were thought to be releasedfrom the primary containment vessel into the atmo-sphere of each RB. But dust and fragments of concretestructures covered the surface of the floors of the Units1 and 3 RBs, soon after the hydrogen explosions. In thecase of Unit 2, since a hydrogen explosion did not oc-cur in the RB, the RB floor surface was directly exposedlonger to high-temperature steam and condensed water.For this reason, contamination seen in the samples ofUnit 2 could be different from that of the other two units.

High-resolution gamma-ray-spectroscopy analysisof each sample was done and Figures 11–13 showgamma energy spectra of typical samples for each unit asmeasured with the [Ge(Li)] spectrometer. The spectrumpeaks of the fission products 137Cs, 134Cs and 110mAgwere detected in samples of Units 1, 2 and 3. In addition,

the spectral peak of 125Sb was detected only in the sam-ples of Unit 2. The activity ratio of 134Cs to 137Cs of thesamples was almost 2/3 at the time of measurement, Au-gust 2012. The release and transport of elemental formsof antimony and silver are influenced by the extent ofcladding oxidation by steam [8]. This result is also usefulto consider the reason why the situation of contamina-tion in Unit 2 is different from the situation in the othertwo units. Activities of 110mAg and 125Sb were 1/10 and1/100 of the activity of 137Cs, respectively.

Figure 14 shows the alpha energy spectrum of a typ-ical sample of Unit 2. Intensities of alpha energy peaksseen in all samples from all units were background leveland were not significant. Hence, it indicated the alphaenergy peaks of samples from all three units were lessthan the detection limit (<0.4 Bq).

Pieces for gamma spectrometry measurement werelocally sectioned from samples as described in Sec-tion 2.2. These pieces were also measured by IP beforegamma spectrometry. From the results, a relation be-tween surface contamination densities and exposure ra-diation dose of IP measurements was drawn using boththe IP measurement and gamma spectrometry results.By using this relation, IP data were converted into val-ues of the surface contamination density. In the caseof strippable paint samples, since extra contamination(dust particles and fragments of concrete structures)was found in the gauze and the periphery of the paintarea (already shown in Figure 10), these portions wereexcluded in evaluation of contamination density. Inte-grated 137Cs radioactivity was calculated using samplesat each location. Integrated values of 137Cs radioactivi-ties of three kinds of samples obtained from the selectedlocations in Units 1, 2 and 3 RBs and of flannel clothand strippable paint samples obtained at locations No.10 of Unit 1 and No. 3 and No. 4 of Unit 2 are shown inFigure 15. The integrated 137Cs value of radioactivity ofeach of the three kinds of samples fromUnit 3 wasmuchhigher than the corresponding values of sample loca-tions No. 1 and No. 6 of Unit 1 and No. 1 of Unit 2. Ra-dioactivities of loose contamination measured for flan-nel cloth samples obtained by wiping the floors of Units1 and 3 were high and 137Cs accounted for the largestportion of the integrated value of radioactivities fromthe three kinds of samples. Radioactivities of fixed con-tamination measured for strippable paint samples weresignificantly lower than radioactivities from loose con-tamination seen in Units 1 and 3. These samples havinga large radioactivity of loose contamination seemed tohave relatively lower radioactivity of fixed contamina-tion. In the case of sample locations No. 1 and No. 4of Unit 2, there were higher radioactivities of fixed con-tamination than loose contamination. Radioactivities ofboring core samples from Unit 1 were significantly low.On the other hand, radioactivities of boring core sam-ples from Units 2 and 3 were not sufficiently low afterthe sampling of flannel cloth and strippable paint. Al-though the major contamination types (loose or fixed)

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Figure 8. Appearance of sectioned pieces from longitudinally sectioned boring core sample fromUnit 2 used for decontaminationtests.

Figure 9. The flow sheet of the decontamination tests on sectioned pieces from core of samples of Unit 2.

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Figure 10. Typical results of IP measurements for three kinds of samples from RBs of Units 1, 2 and 3.

were different in these samples, it seemed that contami-nations remained at a certain low level.

3.2. Depth of radionuclide penetration intocoating and concrete

The first examination made on the longitudinallysectioned concrete boring core samples was gamma-ray-spectroscopy analysis to determine the rough distribu-

tion of gamma-ray-emitting radionuclides (e.g. 137Cs) inthem. A plot of count rate versus scan position pro-vides information about the rough distributions of 137Cs,134Cs and other gamma-ray emitters as a function ofdepth within a boring core sample in advance of thespectrometry measurement. This analysis indicated thatmuch of the radionuclide content of the samples was de-posited near the surface (i.e. on and in the epoxy coating)and that the epoxy coating substantially restricted the

Figure 11. Gamma energy spectrum of the flannel cloth sample from location No. 10 of Unit 1.

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Figure 12. Gamma energy spectrum of the flannel cloth sample from location No. 1 of Unit 2.

uptake of fission products by the concrete. In the caseof the boring core sample (location No. 1) of Unit 1,separation between epoxy coating and sub-surface con-crete occurred during boring core sampling. For thisreason cross contamination between the epoxy coatingand sub-surface concrete was recognized in the analy-sis but no radioactivity was detected in the epoxy coat-ing layer. Thus, it was thought that the epoxy coatingalso prevented the penetration of radioactivity into thefloor boring core sample (location No. 1) of Unit 1. Theabove-mentioned result was also confirmed by the IPmeasurements of longitudinal sections of all four bor-ing core samples.

In particular the high-resolution gamma-ray-spectroscopy analysis of each ground powder fromthe longitudinally sectioned boring core samples wasperformed in order to obtain activity distributions of

gamma emitters as a function of depth within the boringcore samples. Typical results of profiles of Units 2 and 3are shown in Figures 16 and 17, respectively. The activityof radiocesium (134Cs and 137Cs) decreased with depthin the boring core samples, and a significant activitylevel was seen in the epoxy coating region less than1 mm from the surface. In Figure 16, a slight increaseof 137Cs seen at 3.5 mm from the surface was due tocontaminant present in the periphery of the core sampleduring its extraction in the RB. In addition, presence ofgamma emitters in milled boring core samples was alsoconfirmed by gamma-ray spectrometry for 137Cs, 134Csand 110mAg (Units 1, 2 and 3) and 125Sb (only in Unit 2).

Furthermore, the penetration depth and dispersionof gamma emitters in the boring core samples werealso examined by the IP measurement technique. Thelongitudinally sectioned boring core samples were each

Figure 13. Gamma energy spectrum of the flannel cloth sample from location No. 1 of Unit 3.

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Figure 14. Alpha energy spectrum of the strippable paintsample from location No. 1 of Unit 2.

slightly ground and IP measurements of each groundsurface were made, then each sample was ground againand the measurements repeated; this sequence was con-tinued until radioactivity was undetectable. Typical re-sults for the longitudinally sectioned boring core sam-ple of Unit 2 are shown in Figure 18. Details about pen-etration depths of each boring core sample were con-firmed by this technique. Gamma emitters were foundto be confined within a millimeter of the top surface ofthe coating layer (less than 0.5 mm in the wall and floorof Unit 1, 1 mm in the floor of Unit 2 and 0.5 mm inthe floor of Unit 3). Significantly, gamma emitters seenin the ground surface of the boring core samples werelocalized and not uniformly penetrated into the coatinglayer. This indicated existence of local penetration pathsfor the contaminant and also suggested the possibilitythat the slight local penetration of gamma emitters intothe coating layer was not based on typical infiltrationbehavior, a simple diffusionmechanism of radionuclidesinto the coating matrix.

Figure 15. Integrated 137Cs radioactivities of samples ob-tained from the selected locations in RBs of Units 1, 2 and3.

Figure 16. The axial profile of Cs radioactivity in the boringcore sample from location No. 1 of Unit 2.

3.3. FE-SEM and WDS analyses on the samplesAll samples of flannel cloths and strippable paints in

addition to the longitudinally sectioned samples of thefour boring core samples were analyzed by wavelength-dispersive x-ray spectroscopy (WDS) to identify the el-ements present in the samples. Constituent elements ofconcrete and epoxy coating such as Ca, Fe, Zn, Si, Cl, Ti,Na, Al, K and Mg were present in the samples but themajor fission products Cs and Sr were not detected. Thisresult indicated that the major fission products had notbeen locally deposited on the concrete floor at high con-centrations of over 100 ppm. In the longitudinally sec-tioned samples, it was clearly seen that the epoxy coatingactually consisted of many thin layers of epoxy coating.

Additionally, accident-related characteristic changesin the coating such as significant breakage and layerseparation were not recognized by SEM observationsof the longitudinally sectioned samples. Furthermore,it was confirmed by elemental analysis with WDS that

Figure 17. The axial profile of Cs radioactivity in the boringcore sample from location No. 1 of Unit 3.

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Figure 18. Typical results of IP measurement of the longitudinally sectioned boring core sample from the RB of Unit 2.

constituent elements of epoxy coating were uniformlydistributed from the surface to the interior of the epoxycoating of the longitudinally sectioned samples. A localshallow scar was seen in the surface layer of the coatingon the boring core sample from Unit 3 as shown inFigure 19. Such scars could slightly extend towards theinterior of the boring core sample and would be ableto hold contaminants. In addition, many dust particleswere seen deposited on the epoxy coating, especiallyin the boring core sample from Unit 3. They couldbe conceived as the possible source of contaminantsremaining on the boring core sample.

The FE-SEM observations of the epoxy coating sur-faces of the boring core sample of each unit indicatedthat there were only shallow scars due to age deterio-ration. Thus, it was confirmed that the integrity of the

Figure 19. A typical SEMphotograph from the coating layerof the longitudinally sectioned boring core sample from theRBof Unit 3.

coating as protection against contamination to the sub-surface concrete was functionally maintained. The re-sults indicated that the slight local penetration of gammaemitters into the boring core samples probably only oc-curred through these aged scars of epoxy coating.

3.4. Removal of radionuclides from the boringcore sectioned samples of Unit 2

Results of decontamination tests are summarized inTable 2. Decontamination factor is obtained by dividingthe initial radioactivity before a decontamination test bythe radioactivity measured after each decontaminationtest. Changes in radioactivity and percentage reductionin radioactivity after each decontamination test of thesamples (Nos. 2-1 and 2-2) are shown in Figure 20(a)and 20(b), respectively. Furthermore, representative re-sults of IP measurements on samples after decontami-nation tests are shown in Figure 21.

The results of decontamination tests on sample No.2-1 are as follows. Radioactivities after the test of thewiping with wet cotton technique and after the test ofthe soaking in immersion fluid technique with deionizedwater decreased from 81.4 to 70.1 and 56.0 kBq/cm2, re-spectively. Then the test of the steam-cleaning techniquewas additionally conducted on the sample, which de-creased radioactivity further from 56.0 to 49.3 kBq/cm2,which was equivalent to 60.5% of the initial value.

The results of decontamination tests on sample No.2-2 are as follows. Radioactivities after the tests of thewiping with wet cotton technique and after the tests ofthe soaking in immersion fluid technique with 0.1 wt%citric acid aqueous solution decreased from 88.0 to 67.4

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Table 2. Summaries of decontamination tests on the sectioned pieces of boring core sample from the RB in Unit 2.

Test Round Decontamination Decontamination Radioactivity Reduction in DecontaminationNo. No. technique fluid (kBq/cm2) radioactivity (%) factor (–)

No. 2-1 No. 2-2 No. 2-1 No. 2-2 No. 2-1 No. 2-2 No. 2-1 No. 2-2

0 − – – 81.4 88.0 – – – –

1 1 Wiping with Deionized 0.1 wt% citric acid 75.8 81.4 6.9 7.5 1.1 1.1wet cotton water aqueous solution

2 2 75.5 75.6 7.2 14.2 1.1 1.23 3 73.4 77.1 9.9 12.4 1.1 1.14 4 74.4 73.9 8.6 16.1 1.1 1.25 5 73.8 76.4 9.4 13.2 1.1 1.26 6 72.3 73.7 11.2 16.3 1.1 1.27 7 72.1 68.7 11.4 21.9 1.1 1.38 8 70.5 70.5 13.4 19.9 1.2 1.29 9 73.5 72.0 9.7 18.2 1.1 1.210 10 70.1 67.4 13.9 23.4 1.2 1.3

11 1 Soaking in Deionized 0.1 wt% citric acid 64.8 66.0 20.4 25.0 1.3 1.3immersion fluid water aqueous solution

12 2 61.8 65.0 24.0 26.2 1.3 1.413 3 56.0 59.2 31.3 32.7 1.5 1.5

14 1 Wiping with – 1.0 wt% citric acid − 62.9 − 28.5 − 1.4wet cotton aqueous solution

15 2 66.1 24.9 1.316 3 60.0 31.9 1.517 4 65.0 26.1 1.418 5 62.7 28.8 1.419 6 61.5 30.1 1.420 7 64.2 27.1 1.421 8 59.9 32.0 1.522 9 61.8 29.8 1.423 10 62.0 29.6 1.4

24 1 Soaking in – 1.0 wt% citric acid − 61.9 − 29.7 − 1.4immersion fluid aqueous solution

25 2 63.3 28.1 1.426 3 64.9 26.3 1.427 4 60.9 30.9 1.428 5 60.2 31.6 1.5

29 1 Removal by − − − 7.1 − 92.0 − 12.5strippable paint

30 2 5.1 94.2 17.131 3 4.7 94.7 18.7

32 1 Steam- Deionized Deionized 53.4 2.8 34.4 96.8 1.5 31.1cleaning water water

33 2 51.9 3.3 36.3 96.2 1.6 26.634 3 54.2 3.5 33.4 96.0 1.5 25.035 4 49.6 2.6 39.1 97.0 1.6 33.436 5 49.3 2.4 39.5 97.3 1.7 36.937 6 51.2 3.1 37.1 96.5 1.6 28.4

and 59.2 kBq/cm2, respectively. Both techniques of thewiping with wet cotton and the soaking in immersionfluid did not show any additional marked decrease in ra-dioactivity when decontaminating with 1 wt% citric acidaqueous solution.

The results of the tests of the removal by strippablepaint technique on sample No. 2-2 are as follows. In

the tests, the first removal of strippable paint showed ahigher fixing strength between the paint and the con-taminants on the epoxy coating, and radioactivity wasdrastically decreased from 60.2 to 7.1 kBq/cm2. IP mea-surements on samples and peeled strippable paint afterthis first removal are shown in Figure 22. Contaminantsseen in the peeled strippable paint were similar to those

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Figure 20. Changes in (a) radioactivity and (b) percentage reduction in radioactivity of samples from the RB of Unit 2 in decon-tamination tests.

seen in the epoxy coating of the No. 2-2 sample surfacebefore the test of removal by strippable paint techniquewas carried out. The subsequent removal test by strip-pable paint did not show any further effectiveness. It wasthought that the decrease of contaminants on the epoxycoating surface lowered the fixing strength of strippablepaint against the contaminants. Radioactivity after thetest was 4.7 kBq/cm2, which was equivalent to 5.3% ofthe initial value. As shown in Figure 22, local spots ofcontamination in tiny pits in the epoxy coating were seenafter the strippable paint removal test, which could notbe completely removed even by steam-cleaning. Final ra-dioactivity after the steam-cleaning test decreased from4.7 to 2.4 kBq/cm2, which was equivalent to 2.7% of theinitial value.

From the results of the above series of decontami-nation tests, it was concluded that the removal by strip-pable paint technique showed the best effectiveness, andthe remaining fixed contamination on the epoxy coatingof the boring core sample fromUnit 2 could be removedto a low contamination level by a series of decontami-nation techniques. It was reported [2] that it was diffi-cult to dry strippable paint in air due to the moist envi-ronment in the RB of Unit 2 where the present sampleswere collected. It seemed that sufficient performance ofstrippable paint was prevented by such a moist condi-tion. Therefore, it would be reasonable to consider thatstrongly fixed contamination on epoxy coating surfaceswas not completely removed by strippable paint in theRB of Unit 2.

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Figure 21. Representative results of IP measurements on sectioned pieces from boring core sample of Unit 2 in decontaminationtests.

Contamination level in the boring core sectionedsample No. 2-2 dropped to almost nothing as a resultof the decontamination tests as shown in Figure 20(a).However, except for cross-contamination present at theperiphery of the sample, low radioactive contaminationwas still found locally in shallow scars and tiny pitson the epoxy coating. This indicated that more aggres-sive techniques within a few millimeters from the sur-face of the epoxy coating are required for complete re-moval of local contamination in shallow scars and tinypits, which will vary depending on each situation. Fromthe results of the fundamental non-destructive decon-tamination tests, significant reduction of radioactivityproved that contamination of the boring core sample ofUnit 2 was without the occurrence of actual infiltrationinto the matrix of the epoxy coating.

The results of the decontamination tests on theepoxy coating of the boring core sectioned samples ofUnit 2 especially regarding local penetration of con-tamination would be sufficiently explained according

to investigation results for TMI-2 [9–11]. Collins et al.[9] used samples of contaminated concrete from theTMI-2 RB for a leaching test. Their leaching data of aconcrete sample without epoxy coating indicated thatcomplete decontamination by matrix diffusion requiredseveral years, and sorption of 137Cs into the concretestructures was directly and linearly proportional to thepermeability of all materials (concrete, epoxy coating,etc.). Especially, permeability of epoxy-coated concretewas reported to be about four orders of magnitude lowerthan that for uncoated concrete. As a result, removalof 137Cs from the epoxy-coated sample was slower thanfrom non-epoxy-coated sample. They concluded thatthe epoxy coating layers were effective in preventingcontamination to some extent, but once contaminatedthrough damage to the coating layer or other means, thecoating then served to retard decontamination. Datafrom epoxy-coated samples were fitted to an equationdescribing diffusion from the coating, and comparisonof the calculated fraction of 137Cs leached with fractions

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Figure 22. Local spots of contamination that remained in epoxy coating of the sectioned piece from core sample of Unit 2 afterusing the removal by strippable paint technique.

measured in the leach tests were in good agreement forall of the samples. Lazo [10] conducted an RB grossdecontamination experiment at TMI-2 in March 1982.Several decontamination techniques were tested toqualitatively determine the effectiveness and to actuallyreduce the level of contamination in accessible areasof the RB. The exposure period between the accidentand the decontamination experiment of the coatedconcrete surface was in excess of four years. Althoughthe coating did indeed absorb a significant quantity ofradionuclides, the strippable paint removal techniqueproduced some of the best decontamination factors ofthe techniques tested. The effectiveness of the strippablepaint removal technique was fairly similar to that ofthe high-pressure flushing. Davis [11] evaluated theradionuclide penetration to structural concrete surfacesof the TMI-2 RB. The concrete had not been penetratedmore than 1 mm in depth except at locations wherethe coating had been damaged prior to exposure toaccident-generated liquid contamination. The coatingson the boring core sample were carefully collected in upto four stages. Especially, the initial sampling treatmentwas conducted by hand using ultrafine emery cloth to

collect the uppermost layer of coating, which resultedin an 85% reduction of total activity.

According to the above studies [9–11], where epoxycoatings were intact, penetration past the matrix of thecoating was insignificant (∼1 mm). Furthermore, thesurface scars of pre-accident origin in the epoxy coatingcould directly act as a contamination path into the coat-ing. Due to low permeability of the epoxy coating, theepoxy coating matrix would be nearly impervious andprevent penetration into the concrete core, but once con-taminated through damage of the coating, the coatingwould possibly retard decontamination. Although thelocal contaminants seen in the epoxy coating of severalmillimeters in depth would be subsequently infiltratedby matrix diffusion, these local contaminants in air at-mosphere would not penetrate deeper through epoxycoating even after several years.

4. Summary

Selected samples collected from the FukushimaDaiichi Nuclear Power Plant RBs of Units 1, 2 and 3were analyzed to determine the surface radionuclide

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concentrations and to characterize the radionuclidedistributions. In addition, decontamination tests on thesectioned boring core samples obtained from the Unit 2RB were done. This was the first time that examinationsof samples obtained from the RBs have been attempted.

The analysis and decontamination results are sum-marized as follows.

(1) Radioactive contamination (radiocesium andradiosilver) in the RBs of Units 1 and 3, in mostcases, consisted of dust particles and structuralfragments produced in the hydrogen explosions,and had characteristics of loose contamination,which would be easy to remove by decontamina-tion work.

(2) For the RB of Unit 2, which did not undergoa hydrogen explosion, radiocesium and slightamounts of radiosilver and radioantimony werefound on the samples and they had characteris-tics of fixed contamination; this difference mightresult from a different contamination mecha-nism having occurred in Unit 2. However, al-most all of the contamination of the boring coresample was strongly fixed contamination thatremained on the epoxy coating surface, and itcould be removed by a series of fundamental de-contamination techniques.

(3) From the measurements of core samples, pene-tration of radiocesium was found to be confinedwithin one millimeter of the top surface of thecoating layer. From the results of FE-SEM ob-servations of the boring core sectioned samplesand IP measurements of gradually ground sur-faces of the boring core sectioned sample, it wasseen that contaminants had locally penetratedshallow scars of the protective coatings on theconcrete. After the decontamination tests, lowradioactive contamination was still present lo-cally in shallow scars and tiny pits on the epoxycoating. Significant reduction of radioactivity inthe tests proved that contamination of the bor-ing core sample of Unit 2 was without the oc-currence of actual infiltration into the matrix ofthe epoxy coating.

From the quantitative data obtained in this study,decontamination techniques were evaluated. In terms ofloose surface contamination seen in samples obtainedfrom Units 1 and 3, removal by wiping with wet cottondecreased contamination to a low level. In terms ofstrongly fixed contamination on the epoxy coating of theboring core sample obtained in Unit 2, removal bythe strippable paint technique was effective to de-crease the contamination level. Both the analysis anddecontamination results will be used in selecting anddeveloping the methods applicable to decontaminationof the structural materials of the RBs.

A further detailed investigation for high-activity ar-eas of the concrete upper floors of each unit is scheduledin 2014. The data obtained in the present study will becompiled with these future data to determine effectivedecontamination methods.

AcknowledgementsThe authors gratefully acknowledge Messrs H. Fukasaku,

R. Ogiya, S. Misawa, H. Haga, Y. Kurosawa and S. Sakurai ofthe fuelmonitoring section of JAEA for their technical supportin the examinations and for helpful discussions on the results.

References[1] Tokyo Electric Power Company. Intensive reform im-

plementation action plan. [cited 2012]. Available fromhttp://www.tepco.co.jp/en/press/corp-com/release/betu12e/images/121107e0201.pdf.

[2] Goto T, Sakai H, Onizuka H, Chigira T. Basic dataacquisitions for development of remote decontamina-tion techniques (3) sampling and onsite analysis re-sults. Annual Meeting of the Atomic Energy Society ofJapan; 2013Mar 26–29; Kinki University, Osaka, Japan.Japanese.

[3] McIsaac CV, Davis CM, Horan JT, Keefer DG. Resultsof analyses performed on concrete cores removed fromfloors and D-ring walls of the TMI-2 reactor building.IdahoFalls (ID): EG andG Idaho, Inc.; 1984. (Technicalreport GEND-INF-054).

[4] Akers DW, Roybal GS, Examination of concrete sam-ples from the TMI-2 reactor building basement. IdahoFalls (ID): EG and G Idaho, Inc.; 1987. (Technical re-port GEND-INF-081).

[5] Konica Minolta Medical & Graphic Inc. REGIUSModel 190 technical explanatory and imaging adjust-ment manual. DD-941. Japan: Konica Minolta Inc.;2006. Japanese.

[6] X-5MonteCarlo Team.MCNP– a generalMonteCarloN-particle transport code, version 5 – vol. I: overviewand theory. Los Alamos National Laboratory Re-port LA-UR-03-1987 [file MCNP5˙manual˙VOL˙I.pdf][2003 April, revised 2008 February 1]. Los Alamos, NewMexico: Los Alamos National Laboratory.

[7] Kanayama F, Hayashi T, Kawatsuma S. Decontamina-tion experiment for floor of Fukushima Daiichi reactorbuilding. In: Proceedings of American Nuclear SocietyEmbedded Topical onDecommissioning, Decontamina-tion and Reutilization and Technology Expo (DD&R2012); 2012 Jun 24–28; Chicago, IL. p. 14–15. On DVD-ROM.

[8] Osborne MF, Collins JL, Lorenz RA, Norwood KS,Strain RV. Measurement and characterization of fis-sion products released from LWR fuel. CONF-840914-28. International Meeting on Thermal Nuclear ReactorSafety; 1984 Sept. 10; Karlsruhe, Germany.

[9] Collins ED, Box WD, Godbee HW. Analysis of datafrom leaching concrete samples taken from the ThreeMile Island Unit 2 reactor building. Nucl Technol.1989;87:786–796.

[10] Lazo EN. The ThreeMile Island Unit 2 reactor buildinggross decontamination experiment: effects on loose sur-face contamination levels. Nucl Technol. 1989;87:407–420.

[11] Davis C. The evaluation of radionuclide penetration ofstructural concrete surfaces in the Three Mile IslandUnit 2 reactor building. Nucl Technol. 1989;87:778–785.

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