Study on mechanical influence of gas generation and migration on engineered barrier system in radioactive waste disposal facility

In Japan, some radioactive waste with a relatively higher radioactivity concentration from nuclear facilities is to be packaged in rectangle steel containers and disposed of in subsurface disposal facilities, where normal human intrusion rarely occurs. After the closure of a facility, its pore is saturated with groundwater. If the dissolved oxygen of the pore water is consumed by steel corrosion, hydrogen gas will be generated from the metallic waste, steel containers, and reinforcing bars of concrete mainly by anaerobic corrosion. If the generated gas accumulates and the gas pressure increases excessively in the facility, the facility's barrier performance might be degraded by mechanical influences such as crack formation in cementitious material or deformation of bentonite material. Firstly, in this study, we assessed the time evolution of the gas pressure and the water saturation in a sub-surface disposal facility by using a multi-phase flow numerical analysis code, GETFLOWS, in which a pathway dilation model is introduced and modified in order to reproduce the gas migration mechanism through the highly compacted bentonite. Next, we calculated the stress applied to the engineered barriers of the facility from the results of the time evolution of the pressure and the saturation. Then, we conducted a mechanical stability analysis of the engineered barriers by using a nonlinear finite element code, ABAQUS, in order to evaluate their performances after the closure of the facility. (author)