Above: key construction milestones of the field test with two sectors that are separated by retaining walls.
Around the world in the many nations where nuclear power is used, isolating nuclear waste deep underground is deemed as the viable permanent solution. Geological repository for high-level nuclear waste, the type of nuclear waste created by the reprocessing of spent nuclear fuel, is usually a multi-barriers system that includes waste packages, engineered barrier system (EBS), and host rocks. One of the most important design variables for a repository is the temperature limit up to which the EBS and the natural geologic environment can be exposed.
Up to now, almost all design concepts that involve bentonite-backfilled emplacement tunnels have chosen a maximum allowable temperature of about 100 °C. However, whether bentonite can sustain higher than 100 °C is still an open question and raising maximum allowable temperature will significantly lower the cost of repository. In new research inspired by a 2015 study from Berkeley Lab geoscientists, an international collaboration has taken a big step towards studying how bentonite will perform under the influence of long-term high temperature heating.
Represented by these Energy Geosciences Division experts, the Department of Energy is one of five major partners in the “HotBENT” project led by Nagra, the centerpiece of which is long-term field test of a bentonite buffer heated to 175-200 °C at the Grimsel Test Site in Switzerland. After focusing more than a year on constructing the field test at this underground rock laboratory, this month the team began heating the bentonite buffer and will reach 200 °C in about two months, much higher than that of previous tests at any field test site anywhere in the world.
Staff scientist LianGe Zheng who leads Berkeley Lab’s participation in the collaboration said, “The concern is that when nuclear waste emits a lot of heat it will change both the geophysical and geochemical properties of bentonite buffer and host rock. For this long-term series of experiments, the objective is to evaluate the thermal, hydrological, chemical, and mechanical (THMC) changes in the EBS bentonite and how that affects the bentonite barrier’s safety function over time.”
For five years the team will monitor how the bentonite buffer in one of the two sectors behaves in response to simulated heating to shed light on whether bentonite can withstand heating at temperatures higher than 100 °C. Then they will dismantle this sector to take samples for geochemical measurements and final check of THM properties . The collaborators will continue monitoring the other sectors for probably an additional 15 years to evaluate even longer term THMC over time.
Zheng and his colleagues at Berkeley Lab had been actively participating in the project since the very beginning, supported the design of the test with numerical models, and will contribute to the project using the coupled THMC model to predict bentonite evolution and interpret the data collected during the monitoring period.