Analyses of radionuclide migration in geologic media using compartment models.

摘要

In order to observe the effects of waste-canister array configurations on the performance of a radioactive-waste repository, compartment models for radionuclide migration in the repository and geologic media have been developed. The models can be applied to various types of repositories.; Analyses are made for a water-saturated repository and an unsaturated repository. For the unsaturated repository, instantaneous dissolution of radionuclide into the water, radionuclide migration in the unsaturated zones and the aquifer, and the effect of far-field barrier have been analyzed. For the saturated repository, congruent release and solubility-limited release of a radionuclide is considered. In the analyses for an unsaturated repository, migration of a radionuclide is characterized by the residence time in the unsaturated zones, the residence time in an aquifer compartment, and the lifetime of the radionuclide. Greater effect of vault-array configuration is observed for a nuclide with relatively long residence time in the aquifer.; In the analyses for a saturated repository, the peak concentration at the downstream end of the repository has been analytically formulated. A theoretical upper bound has been found for the concentration. The radionuclide concentration does not exceed this upper bound regardless of the canister array configuration or the number of waste canisters. In such a situation, increasing the number of waste canisters in the same water stream would not increase the release rate of the radionuclide from the repository. The peak concentrations are characterized by the time duration of radionuclide release from waste forms and the migration time across the length of the repository. To account for multidimensional heterogeneous medium at a greater scale, compartment models based on stochastic processes are developed. The Markov chain model is developed for migration in multi-dimensional media with heterogeneity.; Thus, the present study has shown that the compartment-model approach can be applied for various types of geologic repositories.