A systematic approach to assess the impact of processes on the safety functions in a repository system for high-level radioactive waste

In Germany, the Federal Company for Radioactive Waste Disposal (BGE) is responsible for implementing and performing the Site Selection Procedure, which is regulated by law, for a repository for high-level radioactive waste. The procedure is organized in three phases. At the end of Step 1 of Phase I, ninety sub-areas with favorable geological conditions for safe disposal for one million years were identified. These sub-areas cover approximately 54 % of Germany and include three different host rocks: claystone, rock salt (halite), and crystalline rock.

In Step 2 of Phase I, the ninety sub-areas are currently reduced to a limited number of smaller areas that are suitable for exploration, so-called siting regions. Within this step, representative preliminary safety analyses (rvSU) are applied. Essentially, these evaluate whether the safe containment of radioactive waste can be achieved. As stipulated in the regulatory framework, a maximum limit of a fraction of 10^-4 in total and a fraction of 10^-9 annually of both the mass and number of atoms over 1 million years is allowed to be released outside the main barrier of the repository system. Quantitative assessment is based on a vertical 1D finite-differences code for modeling the transport of radionuclides in the subsurface. This method, however, is currently suitable for claystones only. Rock salt is considered impermeable, and the models do not apply with the data at hand. For crystalline rocks, the available data in Step 2 of Phase I is not sufficient for reliable transport modeling as well. As an alternative approach, a qualitative evaluation method has been developed. The EVENT method (Evaluation of developments in the rvSU) evaluates whether geogenic processes have an influence on the safety functions of the geological barriers (host rock and overburden) within the assessment period of one million years. Safety functions are defined within the preliminary safety concept. They include the geometry – for example, thickness – or hydraulic properties of the barriers. Geogenic processes include processes such as glacial processes, erosion, or volcanism. FEP (features, events, processes) catalogues are used to structure the interactions and dependencies of processes and components (barriers). To account for climate evolution, the one-million-year assessment period is subdivided into four periods: the container cooling, the remainder of the current interglacial, the first glacial, and the rest of the assessment period. Continuation of the glacial cycles as in the Pleistocene is expected.

For each period, the impact of each process on the safety functions is evaluated. Not all processes will take place during all periods. Processes may have a positive or negative effect on the safety functions. Positive effects are documented but not considered further. Negative impacts are classified as “negligible,” “significant,” or “very significant,” and justified. A very significant negative impact or a considerable number of significant negative impacts indicate safe containment is not ensured for the area.

Assessments are carried out first for each host rock and are specified and adjusted for each area. All assessments are documented and stored in a sophisticated in-house database.