Modelling and laboratory work at three US Department of Energy (DOE) national laboratories is being undertaken to support the HotBENT international field experiment to evaluate the behaviour of bentonite clay used as an impermeable backfill in deep geological repositories for high-level waste.
GTS's HotBENT 2021 operational overview video shows one of the heaters being installed inside the Grimsel tunnelheater installed in the Grimsel tunnel
Deep geological disposal concepts feature a system consisting of several engineered and natural safety barriers to isolate high-level radioactive waste from the environment for many thousands of years. Bentonite is a natural clay-based material with a very low permeability, which swells upon contact with moisture.
This makes it a suitable backfill material with which to surround waste containers or back-fill tunnels inside a geologic repository: it keeps flowing water away from the radioactive waste as well as helping to retain the radioactive materials. It forms a third barrier in a multiple barrier approach in which high-level radioactive waste is immobilised in an insoluble matrix (the first barrier) and placed inside sealed corrosion-resistant containers (the second barrier).
The HotBENT (High Temperature Effects on Bentonite Buffers) project, led by Switerland's national radioactive waste disposal cooperative Nagra, is investigating how well bentonite retains its safety features when exposed to long-term heating, to address concerns that heat emitted by underground nuclear waste will change the geophysical and geochemical properties of the buffer and the host rock.
A long-term field test to investigate the behaviour of bentonite under temperatures of up to 200°C - twice the current maximum allowable temperature for such repositories - began in September 2021 at Switzerland's Grimsel Test Site (GTS) underground waste disposal testing facility.
Geoscientists from the Lawrence Berkeley, Sandia and Los Alamos National Laboratories are carrying out modelling and laboratory work to supplement the field test. The Berkeley team is simulating the field test in a scaled-down laboratory set-up to look at changes in the materials over a year and a half, compared with five- and 15- to 20-year periods in the field, with a focus on understanding possible losses in bentonite's ability to swell.
Teams at Sandia will analyse bentonite samples sent from the Berkeley experiment and unheated samples from the Swiss test site to investigate heat-induced changes to the material's mineralogy and its ability to swell. At Los Alamos, studies are focusing on how a mixture of bentonite and other engineered barrier materials respond to heating, as well as testing the host rock the canisters may be buried within to see how the overall combination of materials responds at temperatures up to 300°C.
Verifying that the bentonite buffer is able to retain much of its protective function at much higher temperatures than previously considered would allow tighter spacing of canisters placed inside underground repositories, reducing the size of their overall footprint, according to Berkeley lead scientist LianGe Zheng.
"It's important to analyse a range of conditions such as possible host rock materials like the granite, clay rocks, and so on, to inform decisions about the most suitable underground location for these nuclear repositories," Zheng said.