Repository research: How the construction and operation of a repository are related to long-term safety

© Bundesgesellschaft für Endlagerung mbH
It is unquestionable that a repository for high-level radioactive waste must be safe. This applies both to the phase during which the repository is being constructed and operated and to the phase afterwards, when the waste is enclosed and all drifts and shafts are sealed. In a joint research project, scientists from GRS and BGE Technology GmbH have investigated how these two phases are interrelated and influence each other.

What requirements must a safe repository fulfil?

Fig. 1: Different phases of a repository in the safety caseFor the design, construction, operation and sealing of a safe deep geological repository for high-level radioactive waste, a multitude of complex and interdependent aspects have to be taken into account. But what basic requirements must a repository fulfil in order to be considered safe? To answer this question, it helps to take a closer look at the phases a repository goes through. In research, a distinction is generally made between the pre-operational, operational and post-closure phase (see Figure 1). In the operational phase, the protection of the operating personnel, the general population and the environment plays a key role. In the post-closure phase, long-term safety is crucial. For this purpose, the waste is to be safely isolated from the biosphere for a million years. In order to ensure both operational safety and long-term safety, a number of safety cases, i.e. safety demonstrations, must be submitted for a repository before a licence is granted. 

These safety cases are tailored to the respective phases, whereby different safety aspects play a role or are weighted differently. For example, special operational features are irrelevant in the post-closure phase since occupational accidents can no longer occur in the drifts and shafts of the repository that are then sealed.

Recent focus of safety research increasingly on construction and operation

In the past, the focus in the development of strategies to demonstrate the safety of a repository was very much on long-term safety, i.e. on the post-closure phase. Accordingly, there are already very advanced safety concepts for the post-closure phase (for example, the concept described in the preliminary safety analysis for the Gorleben site), but no comprehensive concepts for the operational phase. This focus on long-term safety is quite obvious for a repository that is supposed to ensure safety for a million years. However, little attention is paid to the fact that a repository logically cannot be realised without safe construction and operation. Moreover, construction and operation have an influence on long-term safety that should not be underestimated. On the other hand, many requirements for demonstrating long-term safety in turn have an impact on the technical repository concept and on safety during construction and operation of a repository. The operational phase and the post-closure phase thus influence each other – especially when it comes to safety.

In recent years, repository safety research has therefore increasingly focused on the construction and operation phase – both in international research projects and here in Germany. For such scientific questions, the respective legal framework is also of interest. In Germany, very different legal frameworks are relevant, the interaction of which alone represents a major challenge. For example, in addition to the Atomic Energy Act, the Radiation Protection Act and the Radiation Protection Ordinance, mining law also defines fundamental requirements for the planning, construction and operation of a repository including practical requirements such as sufficient ventilation, i.e. technical measures to supply mines with fresh air, or at least two separate access points to the mine, all of which must be taken into account.

Safety can be impaired by internal and external hazards

The question of how the safe construction and operation of a repository and long-term safety depend on each other and how these interdependencies can ultimately be assessed was recently addressed by a team of researchers from GRS together with scientists from BGE Technology GmbH. For the BASEL research project, they developed a method with which these dependencies can be represented and analysed in the obligatory safety cases. The starting point for their analyses were questions such as the following:
  • What requirements result from having to ensure long-term safety for the evolution of the repository system during the construction and operation of a repository?
  • What are the potential hazards for repository operation and what basic measures for avoiding/controlling these hazards can be taken to reduce hazards to operational safety and incidents during operation (e.g. container crash or fire)?
  • Which aspects of operational safety and measures to ensure it have an impact on the long-term behaviour of the repository system?

To answer these questions, the researchers identified so-called “hazards”, i.e. impacts that can damage the facility and that can occur regardless of its operating state. Such hazards are not a new invention but are known from the nuclear rules and regulations. They are subdivided into external hazards and internal events.

Classic external hazards are, on the one hand, natural events such as earthquakes or external flooding and, on the other hand, man-made hazards such as blast waves or aircraft crashes that originate outside the site of the facility. The causes of internal hazards occur at the site inside or outside buildings and include, for example, fires, explosions or the drop of heavy loads.

External hazards, internal hazards and FEPs

In Germany, external hazards are defined in the nuclear regulations so that the external hazards relevant for repositories could therefore be adopted for the research project. This was more difficult for internal hazards. To identify internal hazards for a repository, the scientists used a tool that has already proven useful in long-term repository safety research: With the compilation of FEPs, complex contents can be documented and possible event sequences described.

The acronym FEPs stands for features, events and processes. Related catalogues are based on the internationally established FEP database of the Nuclear Energy Association and are already available for some disposal concepts discussed in Germany. Examples of such FEPs include metal corrosion, heat flow, radioactive decay, stress changes, microbial processes in the mine workings, solution inflow into the mine workings or sorption.

The researchers in the BASEL research project adopted this FEP concept for the description of possible event sequences during the construction and operation of a repository. For this purpose, they systematically analysed all components of a repository mine and examined possible processes on these components to determine whether they represent potential internal hazards.

Making hazards controllable through measures

Fig. 2: In underground structures, walls and ceilings are reinforced with anchors and metal meshes (Photo: Bundesgesellschaft für Endlagerung mbH)   Finally, they developed organisational and technical measures to prevent potential internal hazards in a repository mine or to make their consequences controllable or mitigate them in order to ensure operational safety. On the one hand, the scientists drew on their own expert knowledge and, on the other hand, on the experience gathered over the years from the Asse, Morsleben and Konrad repositories and the construction of the former Gorleben exploration mine in Germany.

In Germany, measures intended to ensure the safe operation of the repository must not impair the long-term safety of the repository system significantly or more than unavoidably. For example, the materials required to implement preventive measures (contour consolidation, anchor reinforcement, etc., see Figure 2 ) must not lead to gas formation in an unacceptable manner (e.g. through corrosion of metals). If the gas pressure builds up too quickly, it could, for example, negatively affect the rock in which the radioactive waste is emplaced.

In the research project, based on the systematic compilation of internal hazards and the derivation of associated measures, it was possible to carry out a comprehensive analysis of the effects of internal hazards on long-term safety, which has not been carried out in Germany in this completeness before.

For further work, a more concrete concept should be defined

The observations and results developed from them were limited by the fact that quite general disposal concepts have been considered in BASEL. The advantage of this is that the results can be applied as a starting point for discussion for a variety of possible disposal concepts and can therefore also be used in the next steps of the site selection procedure. Based on this, however, a “customised” safety concept must then be tailored to a specific site.

At this stage and with current knowledge, the research team could not go beyond a certain level of detail. It therefore recommends defining a concrete concept in advance to further work. This would allow the FEPs and, derived from them, the internal hazards to be elaborated in more detail and the impacts to be considered more thoroughly. It is also suggested that the FEP database, as already developed for the long-term safety case, should be adapted to cover the operational phase as well.

The project was sponsored by the Federal Ministry for Economic Affairs and Energy (BMWi); the synthesis report is available in the GRS publication database.

Project information

Title: "Bewertung der Abhängigkeiten zwischen dem sicheren Bau und Betrieb eines Endlagers für hochradioaktive Abfälle und der Langzeitsicherheit" (Assessment of the interdependencies between the safe construction and operation of a repository for high-level radioactive waste and long-term safety)

Term: 04/2016–09/2020

Duration: 54 months
Customer: Federal Ministry for Economic Affairs and Energy (BMWi)