ABSTRACT:

Compacted bentonite is often considered as a backfill material for the engineered barrier system in a geological repository of high-level radioactive waste. After emplacement of waste canisters and backfill materials, bentonite will be simultaneously heated from the decaying radioactive waste and hydrated from the surrounding host rock. These perturbations trigger complex coupled THMC (thermal-hydrological-mechanical-chemical) processes, which need to be well understood to allow forecasting of long-term performance. Data at high-temperature conditions however is limited. We conducted bench-scale laboratory experiments, creating a radial temperature distribution with temperatures up to 200 °C in a bentonite column to obtain a comprehensive set of characterization data and monitoring measurements. Two test columns were used, a control column undergoing only hydration, and an experiment column experiencing both heating and hydration. Hydration was delivered to the bentonite from a sand layer surrounding a column. During the experiment, we took frequent X-ray CT images to provide insight into the spatio-temporal evolution of (1) hydration, (2) clay swelling, (3) heating dehydration, (4) swelling- and heating-induced large deformation, and (5) mineral precipitation. Results from this study will improve our understanding of bentonite THMC processes, help to develop a prototype of an experimentation platform, and inform repository design.

1. Introduction

The barrier system surrounding the waste containers is key to provide sealing for nuclear waste storage in addition to the natural repository rock (Villar and Lloret, 2004). Engineered barrier system (EBS) with bentonite buffer has been proposed in the repository design for spent nuclear fuel and waste because of its low permeability, large swelling capacity and radionuclide retention, as well as thermal stability among other desired characteristics (Meunier et al., 1998; Bourg et al., 2003; Sellin and Leupin, 2013). After emplacement of waste canisters and backfill materials, bentonite will be simultaneously heated from the decaying radioactive waste and hydrated from the surrounding host rock (Rutqvist et al., 2014; Zheng et al., 2017). These perturbations trigger complex coupled THMC (thermalhydrological- mechanical-chemical) processes, involving (1) moisture transport controlled by multiphase flow and large thermal gradient controlled processes near the heat source; (2) swelling and shrinkage due to bentonite hydration or de-hydration; (3) dilution/concentration, evaporation, migration and exchange of ions impacted by moisture/thermal interactions, (4) dissolution, precipitation and mineral phase transformation, etc. (Zheng et al., 2010). While bentonite behavior for temperature <100 °C has been well-documented based on laboratory (LIoret and Villar, 2007; Fernández and Villar, 2010) and field experiments (Samper et al., 2008; Ye et al., 2010; Bossart et al., 2017), data is very limited for high temperature (>100 °C) conditions (Zheng et al., 2015, 2017). It is important to understand the behavior of bentonite at high temperature for the storage of highlevel nuclear waste with large heat-generating capacity because of potential physico-chemical changes of bentonite, such as cementation and illitization, which may detrimentally affect the long-term performance of bentonite barrier systems.

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