Gas Transport Properties of Pumice Tuff for Performance Assessment of LLW Disposal Facility

Abstract

In Japan, some of the radioactive waste with a relatively higher radioactivity concentration from nuclear facilities is to be packaged in rectangle steel containers and disposed of in sub-surface disposal facilities, where normal human intrusion is unlikely to occur. If dissolved oxygen in pore water is consumed by steel corrosion after the closure of the facility, hydrogen gas will be generated from the metallic waste, steel containers and concrete reinforcing bars largely 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 fracturing of surrounding rock and cementitious materials or plastic deformation of the bentonite buffer. In this study laboratory experiments for gas and water transport properties of the rock were performed to evaluate gas flow through the rock mass (pumice tuff) around a facility for low level waste disposal. Based on the experimental results two-phase flow properties were evaluated by means of an inverse analysis method. The pumice tuff was subjected to hydraulic conductivity tests, water retention (moisture characteristic) tests, and gas injection tests. Non-linear properties such as relative permeability and water retention curve and hydraulic conductivity as a function of confined stress obtained from these tests are discussed. It was possible to estimate the intrinsic permeability, the relative permeability for gas and water and the water retention curve by applying an inverse analysis method using the multi-phase flow analysis code GETFLOWS and universal sensitivity analysis code UCODE_2005 to the gas injection tests. It was found from this study that gas flow in the pumice tuff is reasonably well described by classical two-phase flow concepts and that the two-phase flow properties can be applied to performance assessment of the facility with regard to the influence of gas generation and migration.