Concluding Remarks: Conflicting Values and Motives
This result from the waste generation analysis indicates the importance of waste volume reduction, for which basically two approaches can be considered. The fi is strategic selection of areas for decontamination. Decontamination has been found to effectively contribute to reduction of the air dose rate if it is applied in areas where natural dispersion is slow. The second is development of volume reduction technologies, which include incineration, physical and chemical partitioning, and compaction. Both approaches should be applied in a concerted manner.
Thanks to fast natural dispersion processes as observed in Fig. 4.6, the air dose rate due to surface soil contamination in the environment has been decreasing more rapidly than expected. To take advantage of this natural process, it is crucially important to strategically select areas for artificial decontamination, i.e., where natural dispersion occurs more slowly than in other areas, so that generation of unnecessary waste can be effectively avoided. This will accelerate decontamination, and consequently help return evacuees to their homes.
Unfortunately, sufficient information and knowledge that enable strategic prioritization of areas for decontamination are not currently available. From the analysis shown in this chapter, these are primarily related to in-depth understanding about natural dispersion phenomena represented by λS, including (1) the interaction of radionuclides with materials in the natural environment, (2) the transport and dispersion of radionuclides in the natural environment, and (3) the measurement of radiation and radionuclides in the environment. Furthermore, the value of the rate λR of artificial decontamination for the model used in this chapter should have been obtained through actual decontamination work. In the past 3 years, although decontamination has been carried out in more than 100 local municipalities, data, experience, and knowledge have not been made available in the public domain in forms that can be utilized for further analyses and feedback.
However, even with perfect knowledge and information about natural dispersion phenomena and decontamination effects, strategic prioritization cannot be actually implemented unless a broad range of stakeholders agrees on prioritization. On the contrary, what has actually occurred in the past 3 years indicates that the issue of decontamination has sensitized differences among people about what needs to be achieved by decontamination, resulting in belated decision making on various important matters, which has led to greater and prolonged hardship for the evacuees. We observe a vicious cycle consisting of a lack of integrated scientific knowledge base about environmental contamination and deterioration in trust among stakeholders in society. For trust building, a goal that can be shared by various stakeholders needs to be set, and exactly for that purpose, a solid scientific basis is crucially important. At the same time, without understanding the goal, the right set
of scientific bases cannot be defined.
To halt this vicious cycle, we need to establish a fundamental scientifi basis, both natural and social, for enabling in-depth analysis about what has been the most crucial damage resulting from the accident and why that occurred, and how radiological risk can or should be compared with other risks in society. Coupled with such scientifi efforts, advanced concepts and technologies should be developed and implemented to facilitate decision making by a broad range of stakeholders, which would signifi enhance the resilience of society (see more discussion in Chap. 24).
Acknowledgments The author would like to express his deepest gratitude to Dr. Shinichi Nakayama of Japan Atomic Energy Agency for his help in arranging the trip to Fukushima in September 2012, as well as his insightful comments to this chapter. The author also would like to extend his special thanks to Professor Gayle K. Sato of Meiji University for her excellent comments and advice to improve readability of this chapter. Needless to say, any inaccuracy or lack of readability in this chapter is laid to the author's responsibilities.
Appendix: Mathematical Formulations
For Decontamination
During Decontamination (0 ≤ t < t1)
For the radioactivity Mi [kBq] of nuclide i in contaminated area of A [m2]:
where M0 = βArikBq, where r137 + r134 = 1. The quantity ri is the mass fraction of nuclide i included in the contamination. The quantity β is the initial soil contamination [kBq/m2] for the area of interest. The constants λi, λR, and λS are the radioactive decay constant, the rate of artificial decontamination, and the rate of natural dispersion, respectively. The time t1 is the time when the air dose of the area becomes 1 mSv/year and the decontamination actions are stopped.
The solution for this is written as:
With the dose conversion factor Ci [(µSv/h)/(kBq/m2)], the air dose rate is written as CiMi/A [µSv/h]. Assume the person stays outside for 8 h a day and inside 16 h a day, and 40 % dose while inside, the annual dose is calculated to be FCiMi (t)/A [mSv/year], where F = (8 h + 16 × 0.4 h) × 365/1,000 = 5.26 [(mSv/µSv) • (hour/year)]. The annual dose Si(t) [mSv/year] due to nuclide i in this area is formulated as:
The cumulative dose due to nuclide i is obtained by integrating this with respect to time as:
Termination of Decontamination (t = t1)
The time t1 for terminating decontamination is when the total air dose rate becomes less than 1 mSv/year. The time t1 can be obtained by solving numerically
If the dose rate is already less than 1 mSv/year at t = 0, then no decontamination is necessary. For that, the initial soil contamination level is obtained as
With the values of F = 5.26, C137 = 2.1E−3, C134 = 5.6E−3, r137 = r134 = 0.5,
βthreshold = 49.4 kBq/m2.
After Termination of Decontamination (t > t1)
For the radioactivity Mi [kBq] of nuclide i in contaminated area of A [m2]:
The solution for this is written as:
Mi(t) = Mi(t1)exp(−(Jci + JcS )(t − t1)), t ≥ t1.
The annual dose Si(t) [mSv/year] due to nuclide i in this area is formulated as:
The cumulative dose due to nuclide i is obtained by integrating this with respect to time as:
For Waste Characterization
During Decontamination (0 ≤ t < t1)
For the radioactivity Wi [kBq] of nuclide i in waste:
where
The solution is
Assume that radionuclides are included in the waste materials removed from the area. The cumulative volume, WV(t) [m3], of the waste materials is formulated as:
The cumulative mass, WM(t) [kg], of the waste materials is formulated as:
The average radioactivity concentration of the waste is
After Termination of Decontamination (t > t1)
For the radioactivity Wi [kBq] of nuclide i in waste:
The solution is
After t1, no more waste is generated. Thus, the cumulative volume, WV(t) [m3], and the cumulative mass, WM(t) [kg], of the waste materials are constant at the value of t1:
The average radioactivity concentration of the waste is:
