Test of the Thermal-Hydro-mechanical Behaviors of Cherkasy Bentonite as Buffer Material of HLW Repository

Abstract

The principle of HLW disposal is based on the multiple-barrier system consisting of two basic components—a natural and an engineering barrier. The most common buffer material for engineering barrier systems (EBS) is compacted bentonite, which features low permeability and high retardation of radionuclide transport (Zlobenko et al. in Studies of clays from deposits in Ukraine as a barrier material for radioactive waste repositories, pp 857–861 [1]; International Atomic Energy Agency in Characterization of swelling clays as components of the engineered barrier system for geological repositories [2]; Zlobenko and Fedorenko in Ukrainian programme on characterization and evaluation of swelling clays for use in engi-neered barrier system of the geological repository [3]). The bentonite barrier should prevent the potential migration of radionuclides from the radioactive waste into the surrounding biosphere. Establishing the thermal limit for bentonite in a nuclear waste repository is potentially important, as the thermal limit plays on a major financial challenge requiring long-term strategic planning for used fuel management. Char-acterization of long-term mineralogical changes for EBS concerning the long-term geological evolution is needed for safety assessment purposes. To test the suitability and predicted functions of bentonite-based buffers under simulated repository condi-tions and to assess geochemical changes in minerals and porosity variations, thermal dehydration studies of bentonite were carried out at the temperature of 150 °C in “dry” and “wet” conditions. Commercial calcium bentonite (PBA-22 «Extra») was chosen as a clay component of the buffer materials as less sensitive to mineralized rock water. The connection between the structural peculiarities of bentonite and processes of heat treatment is considered. The montmorillonite shows changes induced by dehydration with temperature, there are changes and a decrease of the XRD profile intensity with heating to 150 °C. The predicting evolution of bentonite behaviour so as the degree of montmorillonite hydration is a very important parameter for cation behaviour as a function of the thermal load.