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Radiation Measurements 60 (2014) 53e58

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Radiation Measurements

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Observation of fallout deposition in an outdoor swimming pool 50 kmaway from the Fukushima Daiichi nuclear power plant

Jun Saegusa a,*, Ryo Yasuda b, Hiroshi Kurikami a

a Fukushima Environmental Safety Center, Headquarters of Fukushima Partnership Operations, Japan Atomic Energy Agency, 7th Floor, NBF-Unix Building,6-6 Sakae-machi, Fukushima 960-8031, JapanbMaterials Science Research Division, Quantum Beam Science Directorate, Japan Atomic Energy Agency, 2-4 Shirakata-Shirane, Tokai-mura,Ibaraki 319-1195, Japan

h i g h l i g h t s

� Deposition density of radiocesium was estimated at a swimming pool in Fukushima.� The density was determined with a small standard uncertainty of approximately 10%.� Water balance was simulated for estimating radioactivity budget in the pool.� Detected gamma-emitting nuclide was 110mAg other than radiocesium.� Radiocesium was much dominant compared with 89Sr, 90Sr, 110mAg, 238Pu and 239þ240Pu.

a r t i c l e i n f o

Article history:Received 16 July 2013Received in revised form16 October 2013Accepted 2 December 2013

Keywords:Fukushima nuclear accidentFalloutRadioactivity measurementDecontaminationDeposition density

* Corresponding author. Tel.: þ81 24 524 1060; faxE-mail address: saegusa.jun@jaea.go.jp (J. Saegusa

1350-4487/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.radmeas.2013.12.001

a b s t r a c t

After the accident at the Fukushima Daiichi nuclear power plant (NPP), outdoor school swimming poolsat Fukushima were decontaminated to curb the redistribution of radioactivity into downstream farm-lands. In the process, the radioactivity concentrations of the pool water and sediment substances (res-idue) were measured to estimate the deposition density of the fallout. At a pool situated 50 km awayfrom the NPP, the average concentrations of radiocesium (134þ137Cs) for the water and residue werequantified as 170 Bq L�1 and 3.6 � 105 Bq kg�1, respectively. Taking account of the radioactivity con-centrations and of the water balance in and around the pool, the deposition density of radiocesium, as ofAugust 2011, was precisely determined to be 0.32 � 0.03 MBq m�2 (k ¼ 1). The density corroborated theprevious results obtained by other methods, i.e., airborne surveys, in-situ Ge surveys and soil samplingsat neighboring locations. Other than radiocesium, the only gamma-emitting nuclide detected was 110mAg,with a concentration of 560 Bq kg�1 in the residue. The radioactivity concentrations of 89Sr, 90Sr, 238Puand 239þ240Pu in the water were all less than the minimum detectable activities e 2, 0.1, 0.002 and0.002 Bq L�1, respectively.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

After the Fukushima Daiichi nuclear power plant (NPP) accidentin March 2011, many school swimming pools in Fukushima sus-pended water discharge, due to the concern that the falloutdeposited in a pool would otherwise flow into farmlands via wa-terways for agricultural use. Japan Atomic Energy Agency reviewedthe existing flocculation-coagulation method for decontaminatingthe water, and after improving the method through on-sitedemonstration tests, they established a practical method that

: þ81 24 524 1069.).

All rights reserved.

used zeolite powder and flocculant. The method focused on trap-ping radiocesium (134þ137Cs), which were dominant radionuclidesin view of the decontamination works in Fukushima area. Thesedetails were previously reported (Matsuhashi et al., 2011; Saegusaet al., 2013a).

While it is difficult to understand the existing forms of radio-cesium that are present in the water, the radioactivity of the waterand sediment (residue) generated by the flocculation-coagulationprocess provides insights into the forms and amounts of depos-ited fallout (deposition density) in and around a pool. Both of thesefactors are important in making effective decontamination plansand to examine the decontamination effectiveness at a site. Typi-cally, the deposition density is evaluated by sampling the fallout ina w0.5 m2 water-filled collector and then measuring the

J. Saegusa et al. / Radiation Measurements 60 (2014) 53e5854

radioactivity of the water (Saegusa et al., 2013b). Other approacheshave also been used for evaluating the deposition density. Saito andhis collaborators conducted radioactivity measurements of soilsampled at more than 3,000 locations that were up to 100 km fromthe NPP and prepared a detailed contamination map of the depo-sition densities (Saito, 2011), while Torii et al. (2012) evaluated thedistribution of radiocesium deposition based on airborne moni-toring surveys. These results are summarized in a database pro-vided by the Ministry of Education, Culture, Sports, Science andTechnology (MEXT) of Japan (MEXT, 2013).

When one considers an outdoor swimming pool as a largefallout collector, it allows a more precise overall estimation of thedeposition density, not only because the collection area is large butbecause the inflow and outflow of water in a pool are significantlyquantifiable compared to land surfaces, rivers or other naturalaquatic environments. Also, it can provide radioactivity informationless interfered by the global fallout deposited prior to the NPPaccident.

Herein, an estimation method and result of the depositiondensity at a pool is reported. The result is compared with thoseobtained using conventional methods. The radioactivity concen-trations of materials other than radiocesium, including pure beta-emitting nuclides and alpha-emitting ones, are also presented asa guide.

2. Decontamination of pool

Pool water was decontaminated if its radiocesium concentrationexceeded the target level. The reference value for this cutoff was setto 200 Bq L�1, which corresponded to the provisional regulatoryvalue for drinking water stated in the “Handling of food contami-nated by radioactivity” guide issued by the Ministry of Health, La-bour and Welfare of Japan (MHLW, 2011). If the radiocesiumconcentration was lower than this level, then the supernatantwater was pumped up and was discharged without water treat-ment. Otherwise, the flocculation-coagulation treatment wasemployed.

The treatment was performed using 1 m3 plastic tanks with thepumped-up water containing a solid suspension (Fig. 1). In thismethod, zeolite powder (100 g per 1 m3 water) and flocculant

1 m3 tank

PAC

Pool water

Supernatant

residue

pH neutralizer Sampling

Zeolite

(i) (ii)

(iii) (iv) (v)

Fig. 1. Flow diagram of the flocculation-coagulation method using a 1 m3 plastic tank.

(polyaluminum chloride (PAC), 100e200 mL per 1 m3 water) wereput into the tanks for trapping the radiocesium present in thewater. The compound solution was left at rest for 20e30 min toallow for sufficient precipitation. If the radiocesium concentrationof the supernatant was less than the targeted level, then it wasdischarged after neutralization (the water was acidified by theaddition of the PAC solution). The residue at the bottom of the tankwas then collected.

After pumping, a mixture of slurry and foreign substances, suchas leaves, remained at the bottom of the pool. These substanceswere naturally dehydrated and collected in jute bags before beingstored in a temporary storage space.

These works were carried out during 23e25 August 2011.

3. Observation of fallout radioactivity

3.1. Outline of pool

The deposition density was evaluated for an outdoor swimmingpool at the Date City Tsukidate Elementary School. The concretepool, as shown in Fig. 2(a), has a surface area of 25.0� 16.6m2 and aslanted bottomwith varying depths of 0.8e1.1 m (average: 0.95 m).The school (37�440N, 140�360E) is located approximately 50 kmnorthwest of the damaged NPP.

3.2. Radioactivity measurement

3.2.1. Gamma-ray emitting nuclidesFor gamma nuclides, the radioactivity concentration of the pool

water was measured on-site with a 2 in. � 2 in. NaI(Tl) detectorsystem (Hitachi Aloka Medical CAN-OSP-NAI), which wascommercially available as a foodstuff monitor. The system has aportable 38-mm lead shield with a sample chamber over the de-tector. Each water sample was sealed into a 900 cm3 container andplaced into the sample chamber for a measurement. The radioac-tivity concentrations of 131I, 134Cs, 137Cs and 40K were evaluatedbased on the pulse-height analysis with the dedicated softwaredeveloped by the manufacturer. The efficiency calibration and self-attenuation corrections were implemented on the basis of the ef-ficiency transfer algorithm included in the software. The minimumdetectable activity (MDA) at a 95% confidence level for 134Cs and137Cs was approximately 30 Bq L�1 each according to a 600 smeasurement.

The radioactivity of the residue was measured using a p-typeHPGe detector (ORTEC GEM30185) with a relative efficiency of 30%.For the pulse-height analysis, a multi-channel analyzer MCA7700was used in line with the spectrum analysis software GammaStudio (both provided by SEIKO EG & G). The efficiency calibrationwas carried out with a multiple gamma-ray emitting standardsource (including 10 nuclides) provided by QSA Global GmbH. Thesample was collected in a 300 cm3 plastic container on 25 August2011 and measured for 600 s on 23 February 2012.

3.2.2. 89Sr and 90SrThe radioactivity concentrations of the pure beta-emitting nu-

clides 89Sr and 90Sr in the supernatant pool water were quantifiedbased on radiochemical analysis. The detailed procedure of themethod is described in a handbook issued byMEXT (2003) while itsoutline can be found in other references (IAEA, 1989; Kusakabeet al., 2013; L’Annunziata, 2012).

First, strontium carrier was added into the sample water(248 mL; collected on 23 August 2011) to evaluate the strontiumcollection rate. Strontiumwas separated and purified with a cation-exchange resin column. After 90Y generated from 90Sr was scav-enged with ferric hydroxide, strontium was collected as a form of

Fig. 2. Decontamination of pool water at the Date City Tsukidate Elementary School. (a) View of the pool (before decontamination), (b) weighing of residue in jute bags.

J. Saegusa et al. / Radiation Measurements 60 (2014) 53e58 55

the strontium carbonate precipitation. It was allowed to stand for20 days. The radioactivity of the precipitationwas thenmeasured todetermine the 89Sr concentration, which was estimated by sub-tracting the gross beta-ray counts of 90Sr and 90Y from those of 90Sr,90Y and 89Sr. After the measurement for 89Sr, the precipitation wasdissolved and the 90Y generated was again co-precipitated withferric hydroxidee the so-calledmilking. The resultant precipitationwas used for measuring beta-rays from 90Y, which was in radio-active equilibrium with 90Sr, and the measurement results wereused for the 89Sr determination. For the beta-ray measurements,each sample was mounted on a filter and was measured for 3,600 swith a gas-flow type low background beta-ray spectrometer(Hitachi AlokaMedical LBC-471Q). The spectrometerwas calibratedusing 89Sr and 90Sr reference solution sources provided by Eckert &Ziegler Isotope Products. The measurements were carried outduring January and February 2012.

3.2.3. 238Pu and 239þ240PuFor alpha-emitting nuclides 238Pu and 239þ240Pu, the radioac-

tivity concentrations were also determined by radiochemicalanalysis. A volume of 247 mL supernatant water collected from thepool on 23 August 2011 was measured in January 2012.

After an addition of Fe3þ carrier into the sample water, the ferrichydroxide precipitated was collected and dissolved for the sepa-ration and refinement of plutonium with an anion-exchange resincolumn. The extracted plutonium was electrodeposited on astainless steel disk for the alpha-raymeasurement (80,000 s) with a

Overflowed(Vo, Cw)

Collected as residue(Mr, Cr)

Supernatant

water in pool

Supernatant

water in tanks

Dehydrated slurry

at pool bottom

Sediment in tanks

Water treatment in tanks

Discharged (Vs, Cs)

Discharged(Vw, Cw)

Pool

Fig. 3. Water and radioactivity budget in a workflow for the pool decontamination. Seetext (Sect. 3.3.1) for the definition of parameters in the parentheses.

silicon semiconductor detector (ORTEC BU-020-450-AS). Thechemical yield of plutoniumwas calculated using the collection rateof 242Pu, whichwas initially added into the samplewater as a tracer.The 242Pu tracer was obtained from the National Institute of Stan-dards and Technology.

The analysis was performed according to a handbook issued byMEXT (1990). Similar procedures are described in other references(IAEA, 1989; Qiao et al., 2009).

3.3. Estimation of deposition density of radiocesium

3.3.1. Model formulaThe balance of water and radioactivity in the overall decon-

tamination process can be summarized in a diagram shown inFig. 3. The deposition density P for a pool can be estimated from theradioactivity deposited in the pool water and residue:

P ¼ CwðVw þ VoÞ þ CsVs þ CrMr

A; (1)

where Cw is the radioactivity concentration of the supernatantwater in the pool; Vw is the volume of the supernatant water in thepool (excludes Vs); Vo is the integral quantity of the overflowedwater due to the addition of rain water; Cs is the radioactivityconcentration of the decontaminated water in the 1 m3 plastictanks; Vs is the volume of the decontaminatedwater in the tanks; Cris the average radioactivity concentration of the residue; Mr is thetotal weight of the residue collected from both the pool and tankbottoms and A is the surface area of the pool.1

3.3.2. Evaluation of parametersBefore beginning the decontamination process, the supernatant

water concentration was measured at different points in theswimming pool (Table 1). The water in the deeper part of the poolwas sampled carefully with a small capacity pump so as not todisturb the sediment on the bottom of the pool. The amount of thedischarged water Vw (without the flocculation-coagulation treat-ment) was 370m3. Its radioactivity concentrationwas measured bysampling the discharging water periodically. The measured resultsare summarized in Table 2. From these measurements, the averageradioactivity concentration of the pool water Cw was estimated tobe 170 Bq L�1 (134Cs: 81 Bq L�1, 137Cs: 85 Bq L�1).

The remaining 23 m3 water with slurry was purified in the 1 m3

tanks. In most cases, the supernatant water concentration after the

1 The radioactivity concentrations and deposition density referred in this paperare decay corrected to 25 August 2011 unless otherwise stated.

100

150

950

1000

ipita

tion

(mm

)

vel (

mm

)

Simulated water level Drainage level (950 mm)

Overflow

Precipitation

Table 1Radioactivity concentrations of pool water at different sampling points. The sam-pling points A and B locate at the diagonally opposite corners of the pool (1 m awayfrom the pool walls).

Run Depth Samplingpoint

Radioactivity concentration(Bq L�1)

134Cs 137Cs Total

No. 1 40 cm A 70 81 150No. 1 20 cm B 86 100 190No. 2 67 cm B 81 72 150No. 2 38 cm B 82 72 150No. 2 10 cm B 81 83 160

J. Saegusa et al. / Radiation Measurements 60 (2014) 53e5856

flocculation-coagulation treatment was less than the detectionlimit, which corresponded to approximately 60 Bq L�1. Therefore,the concentration Cs was set at 30 � 17 Bq L�1, assuming a uniformprobability distribution between 0 and 60 Bq L�1.

After completion of the decontamination, approximately 180 kgof residue (Mr) in a naturally dehydrated state was collected in 27jute bags (Fig. 2(b)). This amount includes both the zeolite and PAC,although the weight of these reagents was substantially less than5 kg. The average radiocesium concentration Cr was estimated byrandom samplings of residue from 27 jute bags and observed to be3.6� 105 Bq kg�1 (134Cs: 1.6� 105 Bq kg�1, 137Cs: 2.0� 105 Bq kg�1).

Vo was estimated based on the water balance in and around thepool during the period betweenMarch and August 2011. In order toestimate the daily evaporation, the daily and monthly climate dataprovided by the Fukushima Meteorological Observatory (JapanMeteorological Agency, 2011) was used. The daily potential evap-oration Ep (mm day�1) was simulated using the Penman equation(Penman, 1948), a semi-empirical method given below:

Ep ¼ DDþ g

$Slþ g

Dþ gf ðu2Þðesa � eaÞ; (2)

f ðu2Þ ¼ 0:26ð1þ 0:54u2Þ; (3)

where D is the slope of the saturation vapor pressure curve(mbar �C�1); g is the psychrometric constant (¼0.66 mbar �C�1); Sis the net radiation of evaporable water (MJ m�2 day�1); l is thelatent heat of vaporization (MJ 103 m�3); u2 is the wind speed atheight 2 m (m s�1); esa is the saturated vapor pressure (mbar) andea is the actual vapor pressure (mbar). The integrated Ep from 11March 2011 to 25 August 2011 was 626 mm. This value wascompared by another empirical formula by Thornthwaite (1948):

Ep;M ¼ 16L12

$N30

�10TaI

�a

; (4)

Table 2Radioactivity concentrations of the discharged water. The water discharge started at13:35 on 23 August 2011 and ended at 16:50 on the same day.

Time Radioactivity concentration (Bq L�1)

134Cs 137Cs Total

13:35 65 98 16014:05 77 90 17014:35 98 67 16015:05 90 76 17015:50 92 90 18016:20 69 110 18016:50 81 81 160

I ¼X12 Tai

5

1:514; (5)

i¼1

� �

a ¼�6:75� 10�7

�I3 �

�7:71� 10�5

�I2 þ

�1:792� 10�2

�I

þ 0:49239;

(6)

where Ep,M is the monthly potential evaporation (mm month�1); Lis the average day length of the month (hours); N is the number ofdays in the month and Ta is the average daily temperature of themonth (�C). Ep,M during the entire months from April to August2011 was 553 mm, whereas that by the Penman equation for thecorresponding period was 603 mm. Since two results agreedreasonably well with small discrepancy of 8%, more realistic (Kumaret al., 1987) daily data from the Penman equation was chosen fordetermining the actual evaporation trend E (mm day�1), using thefollowing relationship proposed by Kondo (1998):

E=Ep ¼ 0:7: (7)

Fig. 4 shows the simulated water level of the Tsukidate pool,together with the daily precipitation data. From the relationshipbetween the daily evaporations and rainfalls, Vo was estimated tobe 29 m3.

With this data, the deposition density P was determined as0.32 MBq m�2. Considering the surface area of the pool (A; 415 m2)and the total radioactivity in the supernatant water in the pool(¼Cw$Vw), the total deposition of radiocesium in the pool was130MBq and that in the supernatant water was 62MBq. This showsthat approximately 50% of radiocesium has been dissolved in thepool water either as a form of ions or with suspended solid parti-cles, i.e., fine clay and algae, and the remainder has existed in theresidue.

3.3.3. UncertaintyThe combined standard uncertainty of Pwas calculated through

the following equation (BIPM et al., 1995):

uðPÞ ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiXi

�vPvxi

�2u2ðxiÞ

vuut ; (8)

0

50

850

900

Dai

ly p

rec

11 Mar. 12 Aug.12 Jul.11 Jun.11 May

Wat

er le

Date10 Apr.

Fig. 4. Simulated transition of the water level of the pool (solid line, left axis) and dailyprecipitation data (right axis) from 11 March 2011 to 25 August 2011. The drainagelevel (dash dot line, left axis) is equivalent to the average water depth from the watersurface to the pool bottom.

Table 3Parameters used for estimating the deposition density P in accordance with the Eq. (1).

Parameter(unit)

Value (xi) Standard uncertainty(u(xi))

Relative standarduncertainty (%)

Sensitivity coefficient(vP/vxi)

UPC (%)

Cw (Bq m�3) 165,792 28,461 17 9.63 � 10�1 72Vw (m3) 371.25 12.57 3.4 3.99 � 102 2.4Vo (m3) 28.58 11.43 40 3.99 � 102 2.0Cs (Bq m�3) 30,000 17,321 58 5.54 � 10�2 0.089Vs (m3) 23 1.0 4.3 7.23 � 101 0.00050Cr (Bq kg�1) 356,000 23,140 6.5 4.46 � 10�1 10Mr (kg) 184.95 13.5 7.3 8.58 � 102 13A (m2) 415.00 0.908 0.22 �7.71 � 102 0.047

P (Bq m�2) 320,048 32,246 10Expanded uncertainty (k ¼ 2) 64,492 20

J. Saegusa et al. / Radiation Measurements 60 (2014) 53e58 57

where xi is represented by the value of each parameter in eq. (1),u(xi) is the standard uncertainty of xi and (vP/vxi) is the sensitivitycoefficient for xi. These parameters are listed in Table 3. Theexpanded uncertainty of P amounted to 20% at a 95% confidencelevel (k ¼ 2).

In Table 3, the extent of uncertainty contribution by eachcomponent is listed as the uncertainty percentage contribution(UPC) factor (Coleman and Steele, 1999) which is defined as:

UPCi ¼ðvP=vxiÞ2u2ðxiÞ

u2ðPÞ � 100: (9)

The comparison of the UPCs made it obvious that the uncer-tainty in Cw was the most dominant contribution to the total un-certainty, while that in Vo and other parameters were notinfluential.

3.3.4. Comparison of deposition density with other methodsThe determined deposition density is consistent with the values

estimated from the airborne surveys (0.1e0.3 MBq m�2 (Fig. 5)(Torii et al., 2012; MEXT, 2011)), the in-situ Ge surveys (0.2e0.7 MBq m�2 (MEXT, 2013)) and the soil samplings (0.3e

Fig. 5. Map of the radiation dose distribution by airborne monitoring (MEXT, 2

0.6 MBq m�2 (Saito, 2011; MEXT, 2013)) at neighboring locations inthe Tsukidate district.

3.4. Radioactivity concentration of other nuclides

Fig. 6 shows the measured pulse-height distribution by gamma-rays from the residue. In the spectrum, 23 gamma lines areobserved in the energy range between 50 and 2048 keV, all ofwhich are identified as either total absorption peaks or sum peaksof 134Cs, 137Cs and 110mAg. The 110mAg radioactivity concentrationand its standard uncertainty were 560 � 40 Bq kg�1.

The radioactivity concentrations of 89Sr, 90Sr, 238Pu and239þ240Pu were all below the MDAs of 2, 0.1, 0.002 and 0.002 Bq L�1,respectively. Each MDA corresponds to the radioactivity concen-tration which is calculated from the count rates of magnitude threetimes the background count rates of each measurement.

Although the effective dose coefficients for these nuclides are upto twenty times as high as that of radiocesium (ICRP, 2012), themeasurement results suggest that attention should be first focusedon radiocesium for the effective and efficient decontamination ofthe environment.

011; Deposition densities of 134Cs þ 137Cs (Bq m�2) as of 28 August 2011).

0 500 1000 1500 2000100

102

104

106

Cou

nts

per c

hann

el

Energy (keV)

Fig. 6. Pulse-height distribution for the residue measured by a p-type HPGe detectorfor 600 s.

J. Saegusa et al. / Radiation Measurements 60 (2014) 53e5858

4. Summary and conclusion

The radioactivity concentrations of the water and residue weremeasured to observe the fallout deposition in an outdoor pool50 km away from the Fukushima Daiichi NPP. From the observation,the following facts were elucidated.

- Average radiocesium concentrations of the supernatant waterand the collected residue were 170 Bq L�1 and 3.6� 105 Bq kg�1,respectively. The concentrations of other nuclides, including110mAg, radioactive strontium and plutonium, were insignificantwith respect to the radiocesium.

- The deposition density of the fallout was precisely determinedto be 0.32 � 0.03 MBq m�2 (k ¼ 1) in terms of radiocesium. Thisdata underpinned the previous results estimated from theairborne surveys, in-situ Ge surveys and soil samplings atneighboring locations.

- The uncertainty in the radioactivity concentration of the su-pernatant water dominated the total uncertainty in the depo-sition density. In contrast, the quantity of the overflowed water,estimated from the water balance simulation, was notinfluential.

- Approximately 50% of the fallout was present in the supernatantwater rather than the sediment in the pool.

In Fukushima, extensive decontamination works are underwayin various places including the aquatic environment as discussedhere. Acquisition of reliable radioactivity data and its analysis areimportant to better understand the radioactivity behavior and to

promote proper and efficient decontaminations in theenvironment.

Acknowledgments

The authors wish to thank the people who carried out thedecontamination of the pool. The authors are also indebted to theschool teachers of the Date City Tsukidate Elementary School.

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