Safety assessment methodology focused on response characteristics of the disposal system and safety assessment for TRU waste in Japan

Abstract

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 new assessment method for demonstrating the sufficiency of the safety assessment and the safety margins of the geological disposal system

“…In this demonstration, the disposal concept, the safety assessment scenario, the analytical case and the data used in NUMO's safety report on TRU waste disposal in Japan [18,19], which is based on the TRU-2 report [8], were used.…”

“…Previously, approximate analytical solutions considering exhaustion of the inventory [16][17][18][19] and representing the release rate F N from the EBS as a function of the hydraulic conductivity k of the host rock were derived. Therefore, based on analytical solutions for g and F N , it becomes possible to systematically extract successful conditions at the level of factors or parameters with respect to variables g and F N .…”

“…On the other hand, in the case of the existing assessment [8,18,19], in order to estimate the release rate from the EBS conservatively, the volume of the EDZ (V m ) was set relatively small and Rd m was set as 1. Then, e m Rd m V m was set negligibly small as follows:…”

“…In this study, the analytical solution for fractured media is used for consistency with the existing assessment [8,18,19]. A governing equation for nuclide migration considering advection-dispersion, decay, sorption on fracture surfaces, matrix diffusion and sorption on the rock matrix in an orthogonal two-dimensional model is described as follows [13][14][15]:…”

“…Waste Group 1 is a Japanese TRU waste type, which consists of emission filters including spent silver absorbent. I-129 is regarded as an important nuclide for the safety assessment as it dominates dose rate for TRU waste disposal in the existing assessments [8,18,19].…”