A conservative estimation method for radionuclide concentrations in radioactive waste during Fukushima Daiichi Decommissioning

The Fukushima Daiichi accident produced large and diverse radioactive waste, and further increases are expected as decommissioning progresses. Sample collection and laboratory exposure; therefore, the amount of data that can be obtained is limited. Accordingly, radionuclide concentrations need to be estimated precisely from a small number of samples. With regard to statistical estimation methods for small samples, while best-estimate and model-based analyses have advanced by incorporating various assumptions, few methods independently corroborate these results from a conservative perspective that critically examining the risk that the underlying assumptions may not hold. Therefore, in this study, we developed a method aimed at minimizing potential risks by taking a critical stance toward assumptions. To explain the specific process, We extended a frequentist method and derived an estimation formula that provides an upper estimation bound at the 95% confidence level while retaining only assumptions that can be rigorously justified. In this paper, we present the derivation process and its results, together with an illustrative application to actual data. As a result, although we were able to establish a derived formula with enhanced reliability, some part of the evaluation was overly conservative, leaving room for improvement. Future work should continue to improve the method and enhance its precision to a level that is useful at the scale of the concentration criteria for radioactive waste disposal.