Quantification of the radioactive waste inventory remaining inside the reactors at Fukushima Daiichi NPS is necessary to effectively plan their recovery, treatment, and disposal. Analysis of radionuclide concentrations and secondary wastes in the contaminated water treatment system can provide a means to estimate the radioactive waste inventory, which is not possible by more direct methods due to problems of accessibility and high levels of radiation. A predictive model has therefore been developed to estimate the radioactive waste inventory from the radionuclide concentrations and throughputs in the contaminated water. Model fitting has enabled the estimation of the key parameters, such as the initial radionuclide concentration C 0 , the continuous release rate F, and inventory of source of continuous release I S0 . An estimated one-third of the total 137 Cs inventory has already made its way into the water treatment system as secondary wastes, whereas half still remains inside the damaged reactors as of 13 March 2014.
In order to enhance the safety of geological disposal and the reliability of the safety assessment carried out for each stage of the geological disposal project, an assessment methodology focused on a sensitivity analysis and an evaluation of importance, which incorporates “system understanding” and “information feedback” into the existing assessment approach, has been developed in this study. In this paper, the assessment methodology and the assessment results as applied for the transuranic (TRU) waste disposal system in Japan will be described. In the sensitivity analysis, an approximate analytical solution was used in order to understand the response characteristics of the engineered barrier system (EBS). In the importance evaluation, important factors relating to the robustness of system safety were identified based on the response characteristics. Furthermore, important features, events, and processes (FEPs) related to such factors and high-impact scenarios were identified based on the information accumulated through “system understanding” and “information feedback”. Based on this approach, the robustness of the TRU waste disposal system was assessed and measures for improving the robustness were identified.
A comprehensive sensitivity analysis method has been developed with the aim of providing quantitative information in an efficient manner. This methodology is composed of the following two components: (1) a statistical method with random sampling of independent parameters, which identifies important parameters and extracts threshold values of parameters and/or combinations yielding a 'successful condition' where maximum dose does not exceed a target value, (2) A nuclide migration model that as far as possible incorporates a comprehensive set of phenomena occurring within the repository. This approach was applied as part of a safety assessment of the geological disposal of transuranic (TRU) waste in Japan (TRU-2). It was shown that the concept of TRU waste disposal is robust from the point of view of safety.Mater. Res. Soc. Symp. Proc. Vol. 985
To perform a safety assessment for the geological disposal of radioactive waste, it is important to understand the response characteristics of the disposal system. In this study, approximate analytical solutions for steady-state nuclide releases from the engineered barrier system (EBS) of a repository were derived for an orthogonal one-dimensional diffusion model. In these approximate analytical solutions, inventory depletion, decay during migration and the influence of groundwater flow in the excavation damaged zone (EDZ) were considered. These solutions were simplified by the Taylor theorem in order to clearly represent the response characteristics of the EBS. The validity of these solutions was shown by comparison with numerical solutions. The response characteristics of the EBS are useful for identifying target values for important parameters that would have the effect of improving the robustness of system safety. The robustness of the geological disposal system and the reliability of the safety assessment can thus potentially be improved using the approximate analytical solutions.
The comprehensive sensitivity analysis method developed by JAEA can be applied to extraction of the threshold values of parameters yielding a ''successful condition'' where the maximum dose does not exceed a target value with regard to the geological disposal concept of high-level radioactive waste (HLW) and transuranic (TRU) waste in Japan. In this study, as a part of the joint research between JAEA and KAERI, the method was applied to a Korean HLW disposal concept that spent fuel will be directly disposed in a suitable host rock. The purpose of this study was to confirm whether a successful condition can be extracted with regard to the direct disposal concept of spent fuel. As a result, the successful conditions for near-field parameters for a target value were illustrated under a conservative condition of the far field. The comprehensive sensitivity analysis method can be applied to extract a successful condition of the near-field parameters for the direct disposal concept of spent fuel. Furthermore, it was illustrated that dissolution from waste is the potential key issue to complement uncertainty of flow rate. Such an approach will offer insights into construction of a robust system for the direct disposal concept of spent fuel and provide useful information for scenario analysis.
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