Nuclear Waste: First Technical Instruction Phase of Cigéo Completed

On the occasion of completing the first component of the technical instruction for the request for authorisation to build Cigéo, the Sfen met with Frédéric Plas, director of the Cigéo program at the National Agency for Radioactive Waste Management (Andra), to review this instruction.

The technical instruction phase of the Cigéo technological burial project, structured by the Nuclear Safety Authority (ASN), was scheduled over 30 months. The first phase, which concerned the “input data for design and safety assessment” of Cigéo (Industrial Center for Geological Storage), has just been completed. The conclusions of the reviews by the Institute for Radioprotection and Nuclear Safety (IRSN) and the standing group requested by the ASN emphasise that all the basic data reviewed were robust and sufficient to assess Cigéo’s safety [1]. The second phase of the instruction concerning operational safety has already begun. Its conclusions are expected by the end of this year, 2024.

There seem to be lingering uncertainties about the rock structure. What does this involve, and what has Andra planned?

The work on processing the high-resolution 3D seismic data carried out in 2009/2010 in the area earmarked for Cigéo installation highlighted the possible existence of flexure at the future waste storage zone for high-level radioactive waste (HLW). Well-known geologically, this flexure posed a perfectly normal question from the IRSN regarding a possible correlation with the properties and overall behaviour of the host rock at this level. Additional work and seismic corrections, which had not been done then, were carried out thanks to external expertise and led to a refinement of the zone’s geometry. This time, the results of these studies do not show the presence of this flexure.

Therefore, it is perfectly normal for the IRSN to question us again and ask us to ensure this evaluation. We will respond, for example, as part of the reconnaissance in advance during the excavation of the underground facility, which will provide an even more precise characterisation of the area or with additional deep drilling planned on the periphery of the Cigéo implantation area.

What additions need to be made regarding meteorological hazards from climate change?

Given Cigéo’s operational lifespan of about a century, it is impossible to rule out future climate evolution at the Cigéo site. This climate change could cause changes in input data such as rainfall, winds, snow, etc.

A series of meteorological data has been considered in Cigéo’s overall design and has been retained for the request for authorisation to build based on the best knowledge and a prudent approach. However, in a logic of continuous improvement, we continue—and this is the normal life of a facility—to monitor potential climate changes and to take measures if significant developments are observed (for example, more extreme winds than those accounted for in the request for authorisation to build). Moreover, in the normal life of a basic nuclear facility, safety reviews every ten years will consider possible climatic evolutions, which could also lead to necessary measures being taken.

For example, we have conducted and continue to conduct studies on 100-year climate scenarios concerning certain surface flood risks. Cigéo’s surface installations will be located on the Barrois limestones, a type of epikarst formation [2]. The design of the surface buildings, especially the nuclear buildings, has prudently considered the hydrogeological characteristics of these Barrois limestones to date. Adaptations could still be considered if there were significant changes in the hydrogeology of this formation in the future.

What details are required concerning the methods of sealing waste packages?

Cigéo is designed to be sealed at the end of its operational phase. This involves, among other measures, the installation of seals in certain parts of the galleries, shafts, and descents. The characteristics chosen for sealing the galleries must fulfil, among other things, two main functions. The first is to limit water flow within the installation using bentonite, known for its exceptional qualities as a hydraulic plug. The second is to allow enough gas to pass through, particularly hydrogen produced from the anoxic corrosion of metals in the storage after the facility’s closure, to limit pressure.

A sealing concept, with justifying elements incorporating these functions, was therefore presented in the request for authorisation to build. The IRSN requested a detailed definition of this object. This is the process we are currently undertaking. We have general elements at the stage of a request for authorisation to build; now, we are moving towards a detailed definition, especially given the sealing demonstrators planned in Cigéo from the pilot industrial phase. The idea is not to restrict ourselves at this stage to an exact definition but to progress gradually.

What are your expectations for the second phase of the instruction?

The second phase of instruction concerns the safety evaluation during Cigéo’s operation. What’s important is that this second phase, which has already begun, continues in the same manner as the first phase. I wanted thus to underline the strong commitment of all the actors. Given the stakes, the ASN, the IRSN, and the standing groups mobilise and advance with extremely rigorous and high-level evaluations. Andra responds with the mobilisation of its teams of engineers and scientists, in all the concerned areas, around, of course, safety.

This second phase is already well underway. This is the case, for example, for the organisational and human factors, which, as for the request for authorisation to build, have been the subject of interviews by the IRSN of a diverse panel of people within Andra. It was both to understand and evaluate the past organisation that led to the request for authorisation to build the current one, in terms of project control of Cigéo, but also the anticipation of future construction and operation phases.

Interview by François Terminet (Sfen)

Image: Conceptual diagram of Cigéo, Source: Andra

[1] Group of experts from different backgrounds and on different topics, which may be both nuclear and non-nuclear.

[2] Karstic zone, having undergone dissolution, characterized by a network of fissures and cavities that collect and transport surface water.