Effect of Alkaline Environment on the Swell Pressures of Compacted Bentonite Under Thermal History

Abstract

Bentonite is used as a buffer material in the deep geological repository for the safe disposal of high-level nuclear waste (HLW). The repository consists of a natural barrier system and an engineered barried system. Natural barrier system consists of a host rock and its surroundings and engineered barrier system consists of a buffer material as well as a waste canister. Bentonite is compacted around the waste canister to isolate it from the atmosphere as well as to provide long-term stability to the barrier. The high temperature (150–250 °C) of the waste canister reduces over thousands of years till the radioactivity of the waste degrades as well as while the long-term operation of the repository, the concrete components will deteriorate and produce alkaline fluids having pH > 12. This long-term influence of high temperature may create a thermal history on the compacted bentonite, and the highly alkaline solution may alter the bentonite near the concrete affecting the physical and chemical properties of compacted bentonite. Hence, it is necessary to investigate the influence of alkaline environment along with the induced thermal history on the swelling pressure of compacted bentonite. The paper discusses an assessment of swell pressures of compacted bentonite [Barmer 1 (B1)] from Barmer district of Rajasthan, India, with an initial dry density of 1.5 Mg/m3 and subjected to 110 °C, hydrated with cement solution and distilled water. The swelling load is recorded with the help of digital load frame and compared with non-heated samples of the same density. The swelling time data of compacted bentonite is presented and compared. Experimental evidence indicated that the swelling pressure of compacted bentonite was less when hydrated with cement water as compared to distilled water. But the time taken for saturation was more when hydrated with cement water as compared to distilled water. However, further investigation is required to understand the swelling mechanism under highly alkaline solutions.