Thermal Pressurization Process in Nuclear Waste Disposal in Argillaceous Claystone

Abstract:

The Callovo-Oxfordian Claystone (COx) is a potential host rock for radioactive waste disposal in France. The heat emanating from the decaying waste will induce pore pressure increases in the near field of the emplacement tunnels, due to the low permeability and relative rigidity of the host rock. As part of the international DECOVALEX-2019 project, the TOUGH-FLAC simulator is applied for modeling of these coupled Thermo-Hydro-Mechanical (THM) processes in a generic geologic repository for nuclear waste in the COx at Bure, France. A series of in-situ experiments conducted by the French national radioactive waste management agency (Andra) provide the geological information, rock properties, and validation examples for modeling.

A thermoporoelastical model for argillaceous claystone is applied in this work. The model is validated against two in-situ heating experiments both at Bure, and is utilized to investigate the COx response at the repository scale, focusing on the THM behavior of the COx under thermal loads, such as pore pressure build-up due to thermal expansion of the fluid and solid skeleton, stress evolution around a repository, fluid flow in the rock matrix, and the potential for such changes to induce hydro-fracturing. To reach this goal, several scenarios and model configurations are conducted. The model simulations show that: 1) the highest potential for hydro-fracturing occurs in the middle between parallel emplacement micro-tunnels (cells) at the center of repository rather than at the edge of the repository; 2) a geometric simplification with a 2D model is reasonably accurate for modeling coupled THM processes at the center of the repository, though it would tend to overestimate the likelihood for hydro-fracturing and would substantially overestimate ground surface uplift; 3) the study confirms the importance of the center (or average) repository host-rock temperature evolution that can be the limiting temperature in the thermal management and repository design.