Numerical analysis of safety of a borehole repository for vitrified high-level nuclear waste

Highlights: • Modification of governing equations can be necessary for simulation of radionuclides migration. • Borehole repositories are effective for disposal of vitrified radioactive waste. • Substantially irregular network calls for modification of successive overrelaxation. We developed a model and carried out transient 3-D computer simulation of radionuclides migration from an underground repository for vitrified high-level radioactive waste (HLW) with long-lived actinides. The repository represents an array of deep boreholes. Canisters with vitrified HLW are disposed one above another in lower parts of the boreholes. The upper parts of the boreholes are sealed. Radionuclides leached from the disposed waste are carried away by thermal convection of groundwater caused by heat generation in HLW. The computer simulation was carried out with use of a refined technique taking into account significant difference between radionuclides concentration in the groundwater in the loaded parts of the boreholes and at the earth surface, dependence of the groundwater properties on pT-parameters, composition of HLW. Values of leaching rate of the waste forms by groundwater depending on time and temperature, as well as role of colloid facilitated transport of radionuclides were determined from previous experiments with Na–Al–P-glass which was used in Russian Federation for HLW vitrification. The results of the calculations allowed us to conclude that distance between containers with HLW and the ground surface of 240 m and more is sufficient for safe disposal of the considered HLW.