Two-phase reactive transport modelling of gas production and pressure build up over a gallery cross section in a low-level radioactive waste repository

A widely-proposed approach to dispose low- and intermediate-level radioactive waste is to store it in a deep underground repository with a multiple barrier system. The outer barrier consists of the combination of the host rock along with appropriate backfill materials. The inner barriers include steel drums and emplacement containers. Cement has been found to support a high pH environment that is favourable for radionuclide retention, as well as to suppress microbial activity and slow down metal corrosion. However, different gases, such as hydrogen, methane and carbon dioxide, can be produced during the process, with the latter being absorbed by a cement carbonation reaction. Degradation of organic waste and metal corrosion will consume water while the produced gasses lead to a local pressure build-up which in turn may leads to reduced water supply suppressing further gas generation. Understanding this feedback system and the geochemical evolution of the barriers, and assessing the maximum pressure build-up, is critical to the performance assessment of the repository.

 

In this case study, we use the coupled reactive transport model of component based two-phase flow in the OpenGeoSys framework to simulate a 2-dimensional cross-section of a disposal gallery, which is following the Swiss disposal strategy in a low permeable clay-type host rock. In this strategy, several concrete containers filled with metal waste or steel drums of organic waste are stacked into a gallery, which is then mostly backfilled with low capillarity, high porosity mortar with low initial water saturation. In this concept, the mortar should buffer gas production, but due to low capillarity, the full re-saturation of the gallery will take thousands of years. We have implemented a geochemical model that treats the degradation of different cement materials with a lookup-table approach. These tables store pre-calculated changes in porosity, the consumption and release of water and gases, and change in cement pore water pH.

 

In this presentation, we show simulation results covering the geochemical evolution of a gallery cross section over several thousand years. We quantify the local gas generation rates within the waste packages and show how gas will be distributed within the gallery cross section and how gas components will dissolve and dissipate within the host rock. We have performed simulations with different clay rock permeabilities and initial liquid saturation of the mortar and the organic waste packages to investigate maximum pressure build-up. We also show how gas generation rates change when metal waste containers are stored either at the bottom or at the top of the gallery. Our results show that simulations of the whole repository could be improved by using local gas generation rates instead of using estimates of gas generation rates based only on waste inventories and chemical reaction kinetics.