French expert group gives backing for proposed deep nuclear waste repository plans

The French nuclear regulator's permanent waste working group of experts has given a broadly positive evaluation of radioactive waste management agency Andra's plans for the post-closure phase of its proposed deep underground radioactive waste repository project, Cigeo.

The group, however, expressed, in its report published July 8, some reservations and called for further work and studies of certain aspects of the post-closure plan, such as the safety case for demonstrating that a fission chain reaction will not occur from spent fuel stored in the deep underground galleries.

"This [safety case] could prove difficult to demonstrate given the time durations under consideration," the report cautioned.

The latest expert working group report is the last of three demanded by the Autorite de Surete Nucleaire et de Radioprotection as part of its wider evaluation of Andra's request for authorization to build the repository. The authorization request was submitted Jan. 16, 2023. The experts have already evaluated Andra's overall safety case and plans for the repository's around 100-year operational phase before final closure, expected around 2170. Both these reports also delivered broadly favorable evaluations.

The planned repository, sited in northeastern France, will comprise around 140 miles of galleries around 500 meters (1,640 feet) underground in a layer of clay rock. Construction of the repository is expected to begin around 2030, if all authorizations are given, with a 10-15 year pilot industrial stage from the beginning of construction expected to test some of the waste storage assumptions. The repository is expected to take around 10,000 cubic meters (353,150 cubic feet) of highly radioactive waste and 73,000 cubic meters of medium-activity, long long-duration radioactive waste. At present, only a reference inventory and not the final waste inventory has been prepared.

The latest expert report described Andra's post-closure plans as "satisfactory for this stage of the project. Overall, it presents a good capacity for the confinement [of radioactive waste] and a robust basis regarding the overall risks and uncertainties considered after closure."

Solubility issue remains

Some "important challenges" still require further consideration, the report said. These include the solubility of the radioactive isotope selenium and its ability to move through the clay rock layer. Selenium is present as a long-lived fission product from spent fuel and reprocessed fuel.

Andra also needs to demonstrate improved safety around the gallery seals designed to be a key part of the repository's radioactive containment and protection measures. "The justification for the number of gallery seals, their positioning and performance following closure still has to be presented," the report said.

Certain scenarios, such as that of an undetected tunnel collapse, and damaged waste containers, should also be investigated further and more work is required on the hydraulic properties of the clay rock formation, the report said. Andra has not fully responded to questions about what impact the length of underground galleries and positioning of different radioactive waste types along them will have on safety, the report added.

The safety case for the corrosion resistance of metallic equipment and components used in the galleries also requires further elaboration, as well as a continued search for other alternative materials with a lower risk of corrosion, the report said.

Questions around bitumen

Further work also needs to be carried out on the long-term behaviors of high- and medium-level radioactive waste stored in bitumen emulsions in containers, the report concluded. France began storing radioactive waste in bitumen from 1966 as a temporary measure, with more than 44,000 containers of such medium-level, long-life waste destined for the Cigeo repository.

The report stressed that it expects Andra to test and substantiate many of its radiation safety scenarios and expectations in the initial repository galleries during the pilot phase of the project, with changes made if needed.

The ASNR is now expected to carry out its own overall analysis of the Cigeo repository construction application, with its conclusions expected to be published in mid-November. These will then be the basis for a public inquiry into the project, scheduled to take place in 2026. The regulator is expected to decide whether to give a construction permit for Cigeo by the end of 2026 or in 2027.