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Home arrow Environment arrow Reflections on the Fukushima Daiichi Nuclear Accident

Comparison Between Approaches

Evaluation results based on these approaches are compared in Table 3.5. All of them exceeded the criteria of INES accident level 7 (>1016Bq). For comparison, results for the Chernobyl accident are also listed. These rough evaluations, calculations practically done by hand, were able to obtain approximated release amounts of radioactive materials.

It is worthwhile to mention that the total amounts of radionuclides of the Chernobyl accident were 1,800 PBq for 131I, and 85 PBq for 137Cs, yielding the radiological equivalence of 131I of 5,200 PBq. This is considerably larger than that of the Fukushima Daiichi accident. The reason for this fact is not only because the PCV covers the RPV in the Fukushima case, but also because in the Chernobyl case, a massive amount of Cs, I, Sr, and Pu was released to the environment by steam explosion of the melted fuel.

The accident level assessment based on INES (see Appendix C) has been performed by the radiological equivalence to 131I for 131I and 137Cs release. The release of 134Cs has not been included. Indeed, the effect of 134Cs is small if one

Table 3.5 Comparison between different approaches to evaluate the total release in PBq

Method

131I

equivalent

131I

134Cs

137Cs

x 1

x 3

x 40

Radionuclide release analysis

Model 1 (this study)

ORIGEN CORSOR-O

chemical analysis

490

60

7.4

7.6

Model 2 by NISA

MAAP/MELCOR

370 (770)

130

(160)

6.1 (15)

Radiation monitor

Model 3 by NSC

SPEEDI dust sampling

630 (570)

150

(130)

12 (11)

Model 4 (this study)

Radiation map (ground shine)

337–782

59–199

6.6–13.8

6.5–13.6

Chernobyl

Core inventory analysis code

5,200

1,800

85

Some data have been updated from the initial publication, indicated in parentheses. We kept 131I equivalent multiplication factor for 134Cs to be 3 in spite of the fact that the correct one is 20 (see Appendix C)

uses the wrong radiological equivalence multiplication factor of 3. If one applies, however, the correct multiplication factor of 20 instead, the effect of the 134Cs release contributes 50 % of that of 137Cs and contributes more than 131I itself. Therefore, the (second) author would like to recommend re-evaluating the past accident including 134Cs.

Contamination and Environmental Cleanup

The ambient dose rate, surface dose rate, and radioactivity in the soil have been measured around the site after the accident. The detailed distribution maps of the radiation dose are made with a smaller mesh on June 6–14, and June 27–July 8, 2011 [22]. Figure 3.14 indicates that massive amounts of radiation fell to the surface of the ground by snowfall after the plume, which contained radioactive materials released by the rupture at 1F2 (suspected), and had been transported to the northwest by wind from the southeast. Although 131I with a half-life of 8 days was predominant in the early stage, 134Cs (2.06 y), 137Cs (30 y), and 129mTe (33.6 d) are now the main reactive materials. 137Cs with a half-life of 30 years will be a major target for cleanup in the future.

The radioactive materials absorbed by particles in the air are detected by dust sampling at various points in and out of the site. 89Sr (50.5 d) and 90Sr (29 y) are detected in the soil within a range of 20 km from the site. These elements have a value range between 1/10 and 1/10,000 that of Cs. These elements may be evidence of the fact that they were released in this accident because the half-life of 89Sr is as short as 50.5 days. Moreover, 140La, 95Nb, and 110mAg have been detected slightly in the soil toward the northwest at a distance of 30 km from the site. A small amount of

Fig. 3.14 Map of deposition of radioactive cesium (sum of 134Cs and 137Cs) for the land area within 80 km of the Fukushima Daiichi plant, reported by MEXT [22]

Pu isotopes was detected and the evaluation of the isotope ratio 238Pu/(239Pu 240Pu)

indicated that some samples contained released Pu by this accident. MEXT reports that the difference in behavior of these elements might account for the wide range of detected values and suggests that more detailed investigation is needed.

The amount of released Sr and Pu is estimated to be much less than that of the Chernobyl accident in which contamination by Sr and Pu was a severe problem.

Fig. 3.15 Becquerel Ratio of 90Sr/137Cs [%] as a function of the distance from 1F. Color of dots corresponds to the latitude, but no clear tendency has been observed. Data from [23]

Specifically, the highest value of 90Sr/137Cs was 8.2 % for the sample obtained at Sōma City, but on average, it was about 0.37 % regardless of the location [23], as shown in Fig. 3.15. Simple scaling of the radiological equivalence of averaged 90Sr release yields about 0.7 PBq from analysis in both Sects. 3.1 and 3.2.2 (=10 PBq × 0.37 % × 20), that is much smaller than that for the Chernobyl accident of 160 PBq (131I-eq.), where 90Sr/137Cs was 1/10.

Monitoring of radioactivity under the sea has been executed. Although concentration of radioactivity at a sampling point within the harbor of the nuclear power plant was high because highly concentrated radioactive water was released from the concrete near the sluice gate of 1F2 (i.e., 131I 2.8 × 1015Bq, 134Cs

9.4 × 1014Bq, 137Cs 9.4 × 1014Bq), concentrations outside the plant, especially in the area at a distance of more than 30 km from the plant, was low [11].

The mechanism of soil contamination by Cs depends on the fraction of it absorbed on the outer surface of minerals or on the layer structure of clay. While an effective method for desorption of Cs from clay has not yet been found, it is expected in the future. Various ways of cleanup of paddy soils should be taken depending on the level of contamination: stripping surface soil, elimination of clay particles by plowing, and removal of vegetation. Effective decontamination methods for soil, which includes methods for disposal of secondary waste, are essential for allowing the return of evacuated residents to their homes if the evacuation zone is to be reopened. For secondary waste from decontamination, temporary keeping, interim storage, and final disposal are required depending on the radiation level. Communication among stakeholders, residents, local governments, and the central government is crucial in the process of determining sites to locate this waste. Academic societies should play an important role by supplying scientific information on RI behavior and safety evaluation for storage and disposal.

Summary and Conclusion

We have seen that the release of radionuclides is subject to the physical and chemical properties and composition of the fuel core, which is highly dependent on its temperature. The source term can be evaluated by the fraction of the release to the environment. The integrated source term can also be evaluated alternatively based on radiation monitoring by assuming the fraction of the land deposition, or by making use of atmospheric simulation. Although the exact value of the radioactive release has considerable ambiguity, the amount of the release derived from these methods is roughly consistent, and is considerably less than that released by the Chernobyl accident.

The atmospheric diffusion/transport mechanism of each nuclide has not yet been fully understood. However, in the present situation, Cs is considered to be the most serious radionuclide while the other nuclides may have minor effects on the environment. The environmental behavior of each species must still be investigated from both scientific and political points of view to find a better roadmap for decontamination procedures.

Acknowledgments The authors wish to acknowledge Dr. Takuji Oda for providing data based on the ORIGEN code, and Ms. Ayumi Ito for support in compiling data.

 
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