Bench-Scale Column Tests on Coupled THMC Processes in Bentonite-Backfilled Engineer Barrier System for Spent Nuclear Fuel and Waste Disposal

A potential design for spent nuclear fuel and waste disposal involves emplacement of the waste packages in rock tunnels and refill the gap with compacted bentonite clay. The bentonite backfill would be heated from the waste packages and hydrated from the host rock. Understanding the THMC behavior of bentonite backfill is key to evaluate and predicate its long-term performance. While data on bentonite under low temperature (< 100 ºC) has been extensively collected, data at high temperature (100-200 ºC) conditions is limited. We have been conducting a long-term bench-scale laboratory experiment with the maximum temperature (at the heater) up to 200 ºC, along with high spatial and temporal resolution measurements of the THMC parameters, as an analogue of the large scale in situ experiment for bentonite backfill, HotBENT, at Grimsel, Switzerland. In order to better understand the effects of heating and hydration, two identical test columns were prepared, with the control column undergoing only hydration, and the experiment column experiencing both heating and hydration. In both columns, hydration initiated from the outer sand/clay boundary and propagated towards the interior portion where the heater is. X-ray CT scans were conducted frequently to obtain 3D image of the density distribution, and visualize the spatially and temporally dependent processes of (1) hydration, (2) clay swelling, (3) heating-induced dehydration, (4) swelling- and heating-induced large deformation/displacement, and (5) mineral precipitation. At the early stage, most part of bentonite was compressed inward by the hydration-induced swelling near the sand/clay boundary. However, this initial displacement was reversed as the hydration front propagated toward the center. Applying heating at the center resulted in larger compaction at early times, followed by larger and sustained swelling near the heater shaft. Potential mineral precipitation due to water vaporization was also observed in the heating zone, resulting in continuous transport of dissolved minerals towards the heater. This study will improve understanding of bentonite THMC processes under heating and hydration for model parameterization and benchmarking, and help develop an experimentation platform for future studies of bentonite under high temperatures.