EMIL Project - New GRS project looks into and refines tools for carrying out a long-term safety case for a repository
Storage in deep geological formations is discussed as one option of the disposal of radioactive waste. In this connection, special significance is attached to the so-called long-term safety case. In its course, it is investigated how a repository will evolve in the future, i.e. is over several hundred thousands of years from now onwards. In Germany, investigations into the long-term safety will amongst others be necessary once a selection process will deal with finding concrete sites for the disposal of radioactive waste.
In the EMIL (Entwicklung von Methoden und Instrumenten für den Langzeitsicherheitsnachweis – Development of methods and tools for the long-term safety case) project, GRS investigates and develops tools for the performance and evaluation of a long-term safety case. These tools can be adapted to the specific framework conditions of a concrete repository site. The Federal Environment Ministry finances this project, which comprises three key areas of work.
Geochemistry and transport processes
If materials, like for example radionuclides, are released in a repository, they can be transported as a result of several processes in the underground or they may be retained because they interact with the surrounding rocks and materials as well as with the materials contained in the groundwater. One effect of this interaction is e.g. precipitation. In this process, radionuclides are extracted or released from the water or rock through chemical reaction and precipitate in the form of flakes or crystals. In the work package “Geochemistry and Transport Processes “, the tools for analysing this interaction are to be refined. For that purpose, simulation codes are used that can model complex processes underground. Amongst others, GRS already disposes of the MARNIE, PhreeqC and ChemApp codes for such simulations. In the EMIL project, the codes are to be linked with each other for combined modelling. That way, the interaction of radionuclides during transport in geological formations is to be represented even better in a long-term safety case.
Density stratification and transport
Transport is also the subject of the second EMIL key activity. However, transport processes of groundwater are the central concern here. In the geological underground, there exists a so-called density stratification because usually the salinity increases with growing depth and as a result, the density of the groundwaters do so, too. This density stratification influences the velocity of groundwaters and also of the radionuclide transport underground: the speed of material transport decreases with increasing density. To put it simply: the deeper the underground, the higher the density of the groundwater, the slower the flow of groundwater and with it the mass transport. In the EMIL context, GRS now explores in depth the behaviour and the interactions of radionuclide transport in fresh- and salt water systems, taking the density stratification into account. In the course of this project, new models for processes are developed, existing ones are examined, and recommendations for their further development are given.
Development of scenarios
One element of a long-term safety case - and hence an essential basis for the assessment of the safety of repositories - is the determination of scenarios that may possibly lead to a radionuclide release. A scenario describes the possible evolution of a repository system and its safety functions. In the development of scenarios, mainly two approaches are pursued. In the “top-down" approach, scenarios are defined, based on which possible processes in the repository are analysed. In the “bottom-up“ approach it is the other way round: based on processes, possible scenarios are developed. Both approaches have already been applied by GRS in the predecessor projects (VSG, VerSi and ISIBEL) in the derivation and consideration of scenarios.
Different methodical approaches for scenario developments are pursued internationally. The German repository programme is aimed at the development of a commonly supported expert basis. The latter is discussed and further developed in the working committee on scenario development. This committee is composed of representatives from different institutions in this specialist field. Within the framework of the EMIL project, the working committee is to discuss and examine in particular methodical problems. The EMIL project will run until September 2016. Results will be published in a report.
Background: What is the subject of a long-term safety case?
Within the framework of a long-term safety case, the safety strategy is explained that is applied to a concrete repository site. This includes i.a. strategies of site examination, repository planning, design and optimisation, the system of safety functions and safety management as well as the actual verification. A central element of the actual verification is the site-specific analysis of the function of the repository system in regard to the radiologic consequences. With this analysis and adequate tools, the safety case seeks to demonstrate that the radiological protection goals can be kept. Among the protection goals are first and foremost the protection of man and the environment against the harmful effects of radioactive radiation.