Project Summary (the abstract of the RETROCK final report)

Geosphere retention of radionuclides can be an important component of the safety case for deep geological disposal of radioactive wastes. Methods to evaluate and model the migration of radionuclides in the geosphere have been developed intensively over many years. The RETROCK project, "Treatment of Geosphere Retention Phenomena in Safety Assessments", has examined how the retention and transport of radionuclides are dealt with in performance assessment (PA) models for deep geological repositories and how this could be further developed and improved. RETROCK is a part of Euratom's research and training programme and, as a Concerted Action, its work is based on existing background knowledge.

The environment considered in RETROCK is saturated, sparsely fractured rock in the so-called ‘far-field’ (between the repository and the biosphere), where the interconnected fracture network provides flowpaths for groundwater that could transport radionuclides. The results of the project are also applicable, with some constraints, to other geological environments. The phenomena in the focus of RETROCK are related to matrix diffusion, sorption and the distribution of groundwater flow. These, along with radioactive decay, provide the key terms to the transport equations of PA models. Some other processes are discussed but more cursorily: colloid-facilitated transport, the influence of microbes, gas-mediated transport, precipitation and dissolution, off-diagonal Onsager processes, effects originating in the near-field and the impacts of climate change.

The RETROCK participants acknowledge that the currently applied priorities to develop the treatment of the processes under discussion are well-reasoned – considering also that the project did not reveal new issues that could call for significant reprioritisation. It looks likely that further work on retention mechanisms and modelling will be at the level of gradual improvements within the existing main directions. The participants believe that sorption, matrix diffusion, the distribution of groundwater flow, and radionuclide decay continue to be the main processes that need to be dealt with in future PAs. A high level of consensus exists on geosphere retention and transport processes, their definition and their generic importance.

In recent years, performance assessments have been able to handle these processes adequately, if not in all cases explicitly, at particular stages of programmes. Basic data may be lacking or site-specific data may be difficult to obtain also for processes for which the level of understanding is good. In such cases, comprehensive utilisation of the available information to substantiate the choice of parameters for each set of PA calculations is essential. Supporting models are expected to handle processes with better coverage of phenomena and with more realism as more data will become available. They can be utilised in selecting PA parameter values so that the top-level PA models produce conservative results, which is a necessity in the demonstration of compliance.

On the whole, the assessments have been considered fit-for-purpose at the time they were produced and have not subsequently been significantly challenged. However, as a repository programme moves towards implementation there will be a greater need for completeness and adequacy of the models and data as well as for supporting arguments.