Repository heat experiment comes to an end

A test container that has been stored in a tunnel at the Grimsel underground laboratory in Switzerland for 18 years is being removed as the experiment is dismantled. The full-scale experiment set out to determine the effects of heat from high-level waste on the surrounding rock and backfill material.

A test container that has been stored in a tunnel at the Grimsel underground laboratory in Switzerland for 18 years is being removed as the experiment is dismantled. The full-scale experiment set out to determine the effects of heat from high-level waste on the surrounding rock and backfill material.

Workers set about removing the container from the rock and backfill (Image: Nagra)

The FEBEX (Full-scale Engineered Barriers Experiment) began at the underground research laboratory for a high-level waste repository at Grimsel in 1997. The experiment involved emplacing two full-scale radioactive waste containers in a tunnel in the bedrock and backfilling the tunnel with a bentonite backfill material. The waste containers incorporated heating elements to simulate the heat generated by high-level waste.

FEBEX has the multiple objective of demonstrating the feasibility of manufacturing, handling and constructing the engineered barriers and of developing codes for the thermo-hydro-mechanical and thermo-hydro-geochemical performance assessment of a deep geological repository for high-level wastes.

The first of the two containers was excavated from the tunnel in 2002. It was found that of more than 500 sensors installed to monitor the experiment, more than 70% were still functional.

Florian Kober, project manager with Switzerland's national radioactive waste disposal cooperative Nagra, said, "This cannot be taken for granted as the conditions in the tunnel are harsh, with temperatures between 80°C and 100°C, high pressures and humidity."

A decision was made at that time to continue running the experiment using the remaining waste container.

Nagra has now announced that excavation of the second container has now started which, when completed, will bring the experiment to an end.

"This allows us to compare the results from 2002 with the new data that are now available after a further 13 years," Kober noted.

Nagra said, "During the excavation process, the scientists will proceed like archaeologists: layer by layer they will excavate through the test set-up and will collect samples following a defined pattern." The excavation work is expected to be completed at the end of June.

Kober said the data obtained is used to verify the results of computer modelling. "We use the models to make accurate predictions for new experiments - including experiments in the Opalinus Clay - and finally for a future geological repository."

Nagra said FEBEX "is the only experiment of its type to date that has run over such a long time period." In addition to determining the heat effects of the waste, it also "teaches us something about waste emplacement procedures", it said.

The FEBEX experiment is being coordinated by Spanish nuclear waste management company Enresa. Ten partner organizations have signalled their interest in being involved in dismantling the experiment, Nagra said. It noted that while some of the partners - such as Sweden and Finland - plan to construct their deep repositories in granite, Switzerland is focusing on Opalinus Clay as a host rock. The experiment, however, is providing important data on the heat effects on the bentonite backfill material, Nagra noted.

Researched and written
by World Nuclear News