Numerical model calculations
To evaluate the safe containment of highly radioactive waste in a deep geological repository over 1 million years, numerical model calculations are an important tool. If the essential processes are adequately represented, the behavior of a repository system can be simulated over long periods of time. Based on a systematic analysis of the repository system (long-term safety) and physics-based modeling, the safety-relevant processes and events are identified and transferred to a numerical model.
One focus of the BGR's work is on the numerical modeling of thermal, hydraulic and mechanical processes and their coupling mechanisms (computation methods) in barrier rocks. These are essentially triggered by the injection of heat-generating waste. An important aim of the calculations is to demonstrate the conservation of the containment properties of the rock, the so-called integrity, within the framework of safety analyses. The Federal Ministry for the Environment, Nature Conservation, Nuclear Safety and Consumer Protection (BMUV) has specified in the Ordinance on Safety Requirements for the Final Disposal of High-Level Radioactive Waste (Section 5 of the Final Disposal Safety Requirements Ordinance - EndlSiAnfV) what is to be examined with regard to the integrity of the containment providing rock zone. The following three criteria can be derived from this:
Dilatance criterion
According to this mechanically oriented criterion, it must be shown that the mechanical stresses in the rock do not lead to the formation or expansion of porosity, so-called dilatant deformation, as this could increase the permeability of the rock and thus affect the containment capacity.
Fluid pressure criterion
This criterion covers hydraulically induced dilatation or damage to the rock. It is considered to be met if the calculated hydraulic pressures in or at the edge of the barrier rock are not higher than the compressive stresses in the rock, as otherwise the rock could rupture.
Temperature criterion
According to this criterion, the barrier properties of the rock must not be significantly influenced by temperature. For example, mineral transformations that have an unfavorable influence on the retention capacity must be ruled out. Usually a limit temperature is set for the barrier rock, which must not be exceeded.
With the help of computational models and evaluations of these criteria, repository systems can be examined with regard to the potential impact on integrity. These criteria are applied by the BGR, for example, as part of extensive calculations to assess the integrity of the geological barrier for the Morsleben repository for radioactive waste (ERAM).
Assessment concepts and calculation methods for integrity have been developed and exemplarily applied in research projects for various host rock types and repository layouts (AnSichT, VSG, KOSINA, RESUS, PRESTIGE, CHRISTA II). These are usually based on “generic sites”, which represent a typical geological situation in Germany but do not have a direct reference to a specific site. Germany is currently in the process of selecting a site for a repository for high-level radioactive waste. The analysis of integrity will also be a component of the preliminary safety analyses in the site selection process.