Thermal Effect on Long-Term Shale Behavior in Nuclear Waste Storage

Abstract

ABSTRACT Shale-like formations can serve as barriers for geologic disposal of high-level nuclear waste. Adopting these tight materials has an advantage that any radionuclides emitted from the canisters would stay with hardly mobile pore water within the rock. However, there are a few issues associated with the sealing capacity of shales as high temperatures experienced in nuclear waste disposal can strongly affect their geomechanical and flow properties. In this study, we investigate how the continuous heating and hydromechanical loading are affecting the flow properties of a shale with the clay content above 50% and the dominant pore size on the order of tens of nanometers. Both intact and fractured specimens are considered, and parameters associated with a coupled hydromechanical model are measured in high-pressure laboratory experiments, including the time-dependent deformation introduced to the model through bulk viscosity. It appears that even a small increase in temperature from 24°C to 40°C significantly impacts long-term (viscoelastic) response of shale, while the effect on the short-term (poroelastic) behavior is less pronounced. At the same time, the thermal effect on the fluid flow is ambiguous: the permeability increases with temperature but is predicted to eventually decrease due to the accelerated rock compaction at elevated temperatures. INTRODUCTION Argillite geological formations have been considered worldwide as potential host rocks for geological disposal of high-level radioactive waste (HLW) because of their low permeability, high retention capacity for radionuclides, and capability to self-seal fractures (Sassani et al., 2021). Other favorable characteristics of argillites (often times represented by shales) are the strong sorptive behavior for many radionuclides reducing conditions because of the lack of oxygen transport from the surface, and chemical buffering of the effects caused by materials introduced during repository construction and operation (Tournassat et al., 2015; Urpi et al., 2019). One unfavorable feature of argillites compared to other potential host rock types is a relatively low thermal conductivity that would lead to higher host rock temperatures (potentially above 100 °C). This means that it will be crucial to gain reliable data for being able to confidently predict long-term, high-temperature responses of shaly host rock. This is exacerbated by the recent interest of disposal in large-sized canister of high decay heat that may lead to higher host rock temperature (Rutqvist, 2020). Moreover, with almost a hundred nuclear power plants across the US waiting for the permanent storage solutions, it is necessary to establish a procedure for obtaining properties of a wider range of typical argillites, from the more ductile materials with high clay content to more brittle rock that could respond vastly different when exposed to high temperature.