E-Journal of Advanced Maintenance Vol.7-2 (2015) 138-144
Japan Society of Maintenology

Inventory estimation of 137Cs in radioactive wastes generated fromcontaminated water treatment system in Fukushima Daiichi Nuclear PowerStationJun KATO1,2,* and Yoshihiro MEGURO1,212

Japan Atomic Energy Agency, Tokai-mura, Naka-gun, Ibaraki 319-1194, Japan
International Research Institute for Nuclear Decommissioning, Minato-ku, Tokyo 105-0004, JapanABSTRACT
Based on the analysis of data from contaminated water, the concentration of 137Cs in radioactive waste such as
used cesium adsorption vessels and sludge generated from the Cesium Adsorption Device and Second Cesium
Adsorption Device and from the Decontamination Device, which were operating or suspended as a part of thecontaminated water treatment system at the Fukushima Daiichi nuclear power station was calculated. An
estimate is made of the total amount of 137Cs recovered by decontamination from 28 June, 2011 to 12 August,
2014.
KEYWORDS

ARTICLE INFORMATION

Fukushima Daiichi Nuclear Power Station, Contaminated water treatment,
Radioactive waste, 137Cs inventory estimation

Article history:
Received 31 October 2014
Accepted 3 June 2015

1. Introduction
In the process of decommissioning the Fukushima Daiichi nuclear power station[1], contaminatedwater accumulated in reactor buildings is decontaminated using water treatment systems such as aCesium Adsorption Device and Second Cesium Adsorption Device and a Decontamination Device. A
large amount of radioactive waste such as used cesium adsorption vessels and sludge was generated
by these systems. A quantitative inventory of the radioactive waste is needed to progress in the timely
processing and disposal of this waste. However, directly analyzing the waste is difficult because itsradioactivity is extremely significant for humans to handle, and the contaminated water treatment
devices are not equipped with sampling devices because of their urgent installation after the accident.
Therefore, the waste inventory is estimated by analyzing contaminated water upstream anddownstream of the water treatment devices. Because many analytical data regarding 137Cs have now
become available, the present study contributes to the effort of making a quantitative inventory of thewaste by estimating the concentration of 137Cs in the waste and the total amount of 137Cs removed
from contaminated water from 28 June, 2011 to 12 August, 2014.

2. Method to Determine 137Cs Concentration in Waste and Decontamination
Amount2.1 Data
The data used consisted of the results of weekly analyses of radioactivity of contaminated water
upstream and downstream of the water treatment devices [1,2] and weekly reports of the contaminatedwater treatment system [3]. These data detailed the amount of contaminated water that was treated
and the amount of waste generated. The waste took the form of used adsorption vessels and sludge.
The data were provided by the Tokyo electric power company.
*Corresponding author, E-mail: kato.jun94@jaea.go.jp

ISSN-1883-9894/10 © 2010 – JSM and the authors. All rights reserved.

138J.Kato, et al./ Inventory estimation of 137Cs in 1F water treatment wastes

2.2 Adsorption vessels
For a given time period, usually one week, we determined the amount Aves (Bq) of 137Cs removed
from the contaminated water by the Cesium Adsorption Device. The result for Aves is expressed as Aves
= (Cin − Cout)V, here Cin and Cout (Bq/m3) are the 137Cs concentrations in the contaminated waterupstream and downstream of the Cesium Adsorption Device, respectively, and V is the volume in m3of water treated during the given time period. The total amount ΣAves of 137Cs removed by the Cesium
Adsorption Device is obtained by summing over Aves. The concentration Cves (Bq/vessel) of 137Cs ineach vessel is calculated by summing Aves over the period that the vessel served in the device. The
average concentration Cvesav of 137Cs in a vessel over a given time period is calculated by dividing
ΣAves by the number n of the adsorption vessels used over the given time period. The values for ΣAves
and Cvesav for the Second Cesium Adsorption Device are calculated in the same way.
2.3 Sludge
A precipitate was generated by a Decontamination Device and was stored together with thecontaminated water in a concrete pit (the precipitate could not be separated from the water).
Hereinafter, the mixture of precipitate and contaminated water is called “sludge.” Because precipitateand solution were both present in the sludge, the amount Aslu of 137Cs equals the sum of the amount
Apre of 137Cs in the precipitate and that in the solution, Asol. The amount Apre generated for a given
water treatment period is calculated by Apre = (Cdin − Cdout)V, where Cdin and Cdout are the 137Csconcentration in the contaminated solution upstream and downstream of the Decontamination Device,respectively, and V is the volume in m3 of water treated. The amount Asol is calculated by Asol =CdoutVslu, where Vslu is the volume of sludge generated. The sum ΣAslu is obtained by summing over all
Apre and Asol. The concentration Cslu of 137Cs in the sludge is obtained by dividing ΣAslu by the total
volume ΣVslu of sludge. However, because Apre became much, much larger than Asol due to theefficient transfer of 137Cs from the contaminated water into the precipitate, Asol can be neglected forcalculating ΣAslu and Cslu.

3. Determination ofAmount137

Cs Concentration in Waste and Decontamination

3.1 Adsorption vessels from Cesium Adsorption Device
Figure 1 plots the concentrations Cin and Cout of 137Cs in the contaminated water upstream anddownstream, respectively, of the Cesium Adsorption Device. These results are obtained fromsampling data acquired from 17 June, 2011 to 12 August, 2014. The concentration of 137Cs in thecontaminated water decreased over this period; that is, the contaminated water was effectivelydecontaminated. The decontamination factor DF (=Cin/Cout) exceeded 104 over most of the operating
period, and the Cesium Adsorption Device worked well. Although the downstream concentration Coutwas reduced by six orders of magnitude with respect to the initial concentration after the accident, theupstream concentration Cin was only reduced by two orders of magnitude. Because the Cesium
Adsorption Device had a good decontamination factor and because water free of 137Cs was introducedin the reactor buildings, a lower concentration of cesium was expected in the accumulated water.
However, the concentration was not reduced below 1010 Bq/m3. This fact implies that fromsomewhere in the reactor buildings, 137Cs is being supplied to some degree to the accumulated water.
The concentrations Cin and Cout are used to calculate the amount Aves of 137Cs removed. The
concentration Cves from all adsorption vessels used by the Cesium Adsorption Device is calculated
from the weekly values for Aves and Cves and is plotted in Fig. 2 together with numerous adsorption
vessels used each week. When more than one vessel is used in a given week, the concentration Cves iscalculated by taking the mean value for that week. During the initial period after the accident, manyadsorption vessels were used per week, and the concentration was more than ten times its value forlater periods. The concentration Cves decreased with time due to the progress of decontamination, asshown in Fig. 1. During the period studied, 514 vessels were used.

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Figure 3 plots the sum ΣAves for the same period. Although ΣAves sharply increased in the initialperiod after the accident, the increase after October 2011 was much less. This result is also attributed
to the progression of 137Cs decontamination of contaminated water. Based on these results, the amountΣAves is calculated to be 1.7 × 1017 [Bq], which accounts for 99.6% of the total amount of 137Cs input
in the Cesium Adsorption Device. In addition, the concentration Cvesav is 3.3 × 1014 Bq/vessel during
this period.

Fig. 1. Evolution of 137Cs concentration in contaminated water upstream and downstream of Cesium
Adsorption Device. Red circles (○) are Cin and blue triangles (△) are Cout.

Fig. 2. 137Cs concentration in used adsorption vessel (points) and numerous vessels (bars) used in Cesium
Adsorption Device.

Fig. 3. Evolution of amount ΣAves in Cesium Adsorption Device.

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J.Kato, et al./ Inventory estimation of 137Cs in 1F water treatment wastes3.2 Adsorption vessel from Second Cesium Adsorption Device
Figure 4 shows the concentrations Cin and Cout in the Second Cesium Adsorption Device plottedfrom 19 August, 2011 to 12 August, 2014. This Cin is slightly different from that of Fig.1 because
water source of the Cesium Adsorption Device is different from that of the Second Cesium
Adsorption Device [1]. The concentrations Cin and Cout are used for calculating the amount Aves of137
Cs removed by the Second Cesium Adsorption Device. The Second Cesium Adsorption Device also
had a high decontamination factor (approximately 105). As is the case for the results shown in Fig. 1,
the concentration Cin did not decrease below 1010 Bq/m3.

Fig. 4. Evolution of 137Cs concentration in contaminated water upstream and downstream of Second
Cesium Adsorption Device. Red circles (○) are Cin and blue triangles (△) are Cout.

The concentration Cves for all adsorption vessels used in the Second Cesium Adsorption Device
were calculated and are plotted in Fig. 5 for each week together with numerous adsorption vessels
used each week. The trend of Cves is similar to that observed for the Cesium Adsorption Device.
However, 110 vessels were changed out during this period, which is less than that for the Cesium
Adsorption Device.

Fig. 5. 137Cs concentration in used adsorption vessels (points) and numerous vessels (bars) for SecondCesium Adsorption Device.

The amount ΣAves is plotted for the same period in Fig. 6. The trend is also similar to that observed
for the Cesium Adsorption Device; the rapid increase in ΣAves changes to a slow increase after January
2012. Based on these results, the amount ΣAves for the Second Cesium Adsorption Device is calculated
to be 9.0 × 1016 Bq, which accounts for over 99.9% of the total amount of 137Cs input amount into the141

J.Kato, et al./ Inventory estimation of 137Cs in 1F water treatment wastes

Second Cesium Adsorption Device. In addition, the concentration Cvesav was 8.1 × 1014 Bq/vessel
during this period.

Fig. 6. Evolution of amount ΣAves in Second Cesium Adsorption Device.

3.3 Sludge from Decontamination Device
The concentrations Cdin and Cdout in the Decontamination Device are plotted in Fig. 7 from 22 June,
2011 to 13 September, 2011. The concentration Cdin was rather less than Cin for the Cesium
Adsorption Device and Second Cesium Adsorption Device because the Decontamination Device wasintroduced downstream of the Cesium Adsorption Device[1]. The decontamination factor for theDecontamination Device is also high, i.e., approximately 104.

Fig. 7. Evolution of 137Cs concentration in contaminated water upstream and downstream of theDecontamination Device. Red circles (○) are Cdin and blue triangles (△) are Cdout.

The amount ΣAslu is plotted in Fig. 8 for the same period as for Fig. 7. The total is 8.0 × 1014 Bq.
This value accounts for more than 99.9% of the total amount of 137Cs input. The Decontamination
Device was not used since 16 September, 2011, and the amount ΣAslu was fixed. When in operation,
the Decontamination Device treated water downstream of the Cesium Adsorption Device.
Consequently, ΣAslu is two orders of magnitude less than ΣAves (Fig. 3). The treated volume ΣVslu was579 m3 during this period, which gives Cslu = 1.4 × 1012 Bq/m3.

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Fig. 8. Evolution of ΣAslu in Decontamination Device.

3.4 Total amount of 137Cs removed
Based on the results of the sums of 137Cs removed by the three devices, the total amount of 137Cs
removed by the overall water treatment system between 28 June, 2011 and 12 August, 2014 isestimated to be 2.6 × 1017 Bq, which corresponds to approximately 37% of the 137Cs contained in thefuels of reactors 1–3 of the Fukushima Daiichi nuclear power station, that is, 7.0×1017 Bq(100 %) [4].
However, the amount of 137Cs released in the atmosphere and ocean is estimated to be 1.0 × 1016 Bq(approximately 1%) [5] and 3.6 × 1015 Bq (approximately 1%) [6], respectively, and that remaining in
the accumulated water is estimated to be 9.1 × 1015 Bq (approximately 1%) [2]. Thus, 4.2 × 1017 Bq(approximately 60%) of the 137Cs remains somewhere in the reactor building is estimated.

4. Conclusion
In this study, as a part of the inventory estimation, we estimate the concentration of 137Cs inradioactive waste such as used cesium adsorption vessels and sludge and the total amount of 137Cs
removed by those devices between 28 June, 2011 and 12 August, 2014. In the future, an estimate of
the inventory of other radioactive nuclides in that waste will also be attempted.

Acknowledgment

This study includes the results of “Development of technology for disposal of accident waste”
entrusted to Japan Atomic Energy Agency and International Research Institute for Nuclear
Decommissioning by the Agency for Natural Resources and Energy.

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