A battle of forces in radioactive waste repositories

According to statistics by the World Nuclear Association, OECD countries produce about 81,000 cubic meters of conditioned nuclear waste each year. Radioactive over thousands, sometimes millions of years, countries are developing long-term strategies to safely store this waste underground. In 2006, the Swiss government finalized its nuclear waste disposal strategy for what will amount to 100,000 m3 of packaged radioactive waste, 7000 m3 of which will be high-level waste. With underground storage to commence in 2050, scientists are continuing to investigate and improve the long-term safety of the strategy. Publishing in the journal Acta Geotechnica, researchers from EPFL presented findings on the medium-term behavior of one the barriers involved in containing the waste, highlighting the need to focus on its design.

Switzerland’s radioactive waste disposal strategy is based on storing highly contaminated waste in a network of deep underground vaults, locked away behind three protective barriers. The waste would first be vitrified and poured into five-meter long, 25-ton, cylindrical metal canisters. These canisters would then be laid out single file, three meters apart, within the underground tunnel network. Resting on blocks of bentonite – a material that absorbs nearly five times its weight of water and when wet at full saturation it occupies a volume of six times its dry bulk weight – the canisters would be surrounded by bentonite pellets to fill the remaining space. Experts agree that implementing multiple engineered barriers half a kilometer underground in a stable Opalinus clay formation – the geological barrier –, is the safest known strategy to store nuclear waste in Switzerland.

The first 1000 years are critical
But what actually happens to the barriers holding back the radiation as time passes? Several forces act on them in unison, caused by the infiltration of water from the host rock, heat emanating from the canisters that contain the radioactive waste, increasing pressure due to the swelling of the bentonite, and the chemical degradation of the materials used. To study the combined effect of the thermal, mechanical, and hydraulic forces, researchers working with Lyesse Laloui from EPFL’s Soil Mechanics Laboratory developed a numerical model with which they can fast-forward into the future and see how this battle of forces plays out.

Understanding the first thousand years is paramount, as it takes several centuries for the heat caused by the radioactive decay to subside and the bentonite buffer to saturate with humidity. “These coupled phenomena would be impossible to account for without relying on either extremely long-term studies in the field or numerical simulations,” says Laloui. “Heat diffusion depends on water content, the diffusion of water indirectly depends on temperature, and mechanical responses depend on temperature and suction.” These interactions are compounded by mechanical forces due to the weight of the canister and stresses that arise from the swelling of the buffer material.

Uneven swelling
Years can pass from the moment the underground tunnels are excavated to the moment they are filled, during which their Opalinus clay walls are dried by air circulating through them. The thermal, mechanical, and hydraulic forces only begin to compete once the canisters bearing the radioactive waste are inserted into the tunnel and the bentonite buffer is added. Humidity from the host rock then begins to slowly creep into the bentonite buffer from the outside, while heat from the waste-containing canister is released from the center, in turn slowing the progression of the humidity.

Because the bentonite blocks that the canisters rest upon and the pellets that surround them respond differently to heat and humidity, the researchers were concerned that forces would act asymmetrically on the canisters. This, they predicted, could cause the canisters to sink as the humidity in the bentonite buffer increases, causing it to swell. And the simulations showed that the concerns were warranted. Over time, the swelling could reach up to a few centimeters in certain locations, potentially weakening the canisters, which will have be reinforced in the face of these new findings.

What if, under these circumstances, the canisters were to move? According to Laloui, the repository is designed to be sufficiently robust to withstand such a scenario. “With 500 meters of bedrock separating the surface from the repository, which is located at the heart of an impermeable and immobile geological formation, the movement of a canister would not be ideal – but the multiple barriers in place would be able to handle it,” he says.