HotBENT project investigates safety material for underground waste disposal sites

25 January 2022

The emplacement of the bentonite buffer by an auger-machine in the HotBENT field test. (Credit: Grimsel Test Site)Geoscientists from the US Lawrence Berkeley National Laboratory (Berkeley Lab) and from the Sandia and Los Alamos national labs, are collaborating on the HotBENT project. This is an international field experiment to evaluate how well the natural, clay-based material (bentonite) placed around canisters of buried, high-level waste (HLW) retains its safety functions when exposed to simulated long-term heating.  

“The concern is that heat emitted by underground nuclear waste will change the geophysical and geochemical properties of the bentonite buffer and the host rock,” said LianGe Zheng, Berkeley Lab’s lead scientist on HotBENT, whose previous computer simulation studies helped to initiate the HotBENT research. “For this long-term series of experiments, we will evaluate the thermal, hydrological, chemical, and mechanical changes in the bentonite and how that affects the material’s safety function over time.”

The HotBENT project led by Nagra (Switzerland’s National Cooperative for the Disposal of Radioactive Waste) involves the DOE and partners in Canada, Japan, the UK and elsewhere. A demonstrated higher temperature tolerance could allow for more radioactive waste to be safely packed within tunnels in the Earth’s subsurface. 

The US has not identified a new site for long-term disposal of highly radioactive by-products of nuclear power since Congress stopped funding the Yucca Mountain site in 2010. However, Finland has begun constructing a deep underground repository, and other countries are at various stages of considering repository locations for HLW. 

The HotBENT field test began in September with scientists deploying heaters within the research tunnel at Switzerlands’s Grimsel Test Site, a designated underground testing facility for radioactive waste disposal. The temperature of the four heaters in the HotBENT field test will be slowly raised over the next several months to eventually reach 200 degrees Celsius – twice the current maximum allowable temperature under consideration for similar repositories. The team has set up a large array of sensors to detect temperature and corresponding hydrologic and mechanical changes in the bentonite buffer and the surrounding rock. 

Previous research by Zheng suggested that, at twice the widely used maximum temperature, the soft, clay-based bentonite does not lose much of its protective properties. Bentonite is being studied partly because the natural material would swell when water reaches it within an underground repository (due to groundwater percolation at depth). The clay material’s swelling ability can help protect canister waste from escaping because of interactions with fluids that would be less able to travel through swollen bentonite. 

Zheng’s modelling study, published in Engineering Geology, indicated that about two-thirds of the key, swelling-related mineral in bentonite clay (smectite) had degraded at 200 degrees Celsius rather than 100 degrees Celsius. Yet the ability of the bentonite to swell as part of its protective function only decreased by 4% at most in the simulations. 

“If the Swiss test site can largely verify the modelling results, then the bentonite buffer may be able to retain much of its protective function at much higher temperatures than previously considered,” said Zheng, noting this would allow tighter spacing, and more waste canisters, to be placed inside underground repositories, reducing the size of their overall footprint. 

To supplement the HotBENT experiments, Zheng and his colleagues are simulating the field test setup in the laboratory using column tests about one-tenth the size of a single heating element at the Switzerland underground field test site. In the lab, the scientists will be looking at changes in the materials over a year and a half compared to five- and 15- to 20-year periods in the field. A particular focus will be understanding how much smectite degrades into another clay mineral (illite) that lacks the ability to swell. 

Meanwhile, colleagues in one of Sandia’s nuclear fuel cycle research departments will analyse geochemical changes in bentonite samples sent from Zheng’s scaled-down column experiment — and directly from unheated samples at the Swiss test site. 

“We want to know if high temperatures cause the minerology of the bentonite buffer to change, potentially affecting its properties like permeability and radionuclide adsorption,” said the department’s manager, Emily Stein. “It’s really important for developing the safety strategy for deep geologic repositories to understand these temperature-related processes.” 

Sandia scientists led by Carlos Jové Colón will be investigating whether the heating tests at Berkeley Lab alter bentonite’s ability to swell, and how the raw material at Grimsel responds to being heated to 60 and 90 degrees Celsius. In addition, they will analyse the bentonite sample’s chemical composition, such as for the presence of the breakdown material illite, and analyse structural changes in the clay. 

A research group led by Florie Caporuscio at Los Alamos National Laboratory has been studying how a mixture of bentonite and other engineered barrier materials respond to heating. They will also test types of host rock the canisters may be buried within, with the goal to see how the overall combination of materials responds to temperatures of up to 300 degrees Celsius.

“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. 

Photo: The emplacement of the bentonite buffer by an auger-machine in the HotBENT field test. (Credit: Grimsel Test Site)

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