A review on the impact of thermal history on compacted bentonite in the context of nuclear waste management

Abstract

A deep geological repository is the most sustainable way to dispose of high-level radioactive waste. It consists of a multi-barrier system with waste containing canister, sealing material, and geology that avails a facility to provide the long-term isolation of these harmful wastes. The bentonite or sand bentonite blocks serve as a sealing material in the DGR around the waste canisters to isolate them from the surrounding atmosphere. Buffer surrounding the waste canister experiences variation in the temperature for a long duration due to the continuous reduction in the radioactivity and the temperature emitted by the element with time. The thermal flux is induced on the compacted bentonite when a high-temperature waste canister comes in contact with the bentonite buffer and due to seasonal temperature change resulting from a heat source for a long time. As the temperature inside the deep geological repository varies with a long duration of time, it creates a thermal history on the bentonite buffer and changes its physicochemical and hydromechanical properties affecting its overall performance to serve as a sealant. Past reported studies had mentioned the influence of temperature on the various bentonites, but the effect of thermal history on bentonite efficiency is rarely reported. This study presents a review of existing knowledge about the high-temperature impact on bentonite buffer behavior. It has critically investigated the impact of temperature on the physicochemical, hydromechanical, and mineralogical characteristics of bentonite. Further, the analyses are made on the influence and importance of thermal history to be considered while designing the buffer material from few recently reported studies. The physicochemical and hydromechanical properties such as Atterberg’s limits or swell pressure reduced when subjected to heat. It further reduced when time also came into the picture along with the temperature, i.e., thermal history. Such research will be very helpful and will help better understand the different physicochemical and hydromechanical reactions in a bentonite buffer before its designed values are established as part of the engineering barrier systems in the deep-geological repository.