System performance assessment of final repository for the radioactive wastes using first-order reliability method

Abstract

When a final option for radioactive waste is determined, it is necessary to demonstrate compliance of disposal system chosen with relevant regulations. Considering the large number of physical and chemical factors involved, the complexity of their inter-relationships, and long time periods, a system approach is required. And a stochastic analysis is needed to provide that these regulatory criteria can be met with confidence. Among a variety of models developed to treat the effect of uncertainty on the system performance, the first-order reliability method is suggested as an attractive tool to stochastic problems incorporating any amount of probabilistic informations, which is particularly useful when statistical information is incomplete, as is common for problems occurring in the subsurface environment. The method is based on using the first-order Taylor series expansion at a specific linearization point to calculate a measure of reliability. Results from a firstorder reliability analysis include an estimate of the probability of exceeding a specified performance criteria and measure of the sensitivity of the stochastic solution to the changes in input random variables and their statistical moments. In this dissertation, for reliable prediction of the performance of repository for the radioactive wastes, first-order reliability method is applied in treating the parameter uncertainties of predictive models. First, a thickness of canister corrosion and breach time of canister is calculated using uniform corrosion model, which are combined with the diffusional transport modeling of radionuclides in the backfill material. And, a fractional release rate for each radionuclide is derived from the evaluation of the performance of waste package which is consisted of canister and backfill material. Second, a cumulative release at the accessible environment is obtained by geosphere transport modeling with the source term given as fractional release rate. Third, consideri...