Testing Safety and Performance Indicators in an Assessment of the Long-Term Performance of the Geological Disposal of Spent Fuel in Boom Clay

The classical numerical output, or indicator, from assessments of the long-term safety of geological disposal systems for high-level radioactive waste is the individual effective dose rate. This indicator is an estimate of the possible individual health detriment and it is commonly compared to regulatory limits for assessing the safety of other nuclear activities as well, such as medical and industrial activities. As a safety indicator, the individual dose rate provides an estimate of the overall safety of the disposal system. However, because of the time frames involved in safety assessments of geological disposal systems, the need arises of complementary safety indicators that could be less affected by uncertainties like those associated with future human behaviour or the effects of climate change on the biosphere and the aquifers. Such alternative safety indicators can be, for example, radionuclide concentrations in the groundwater or fluxes to the biosphere due to a repository. Safety indicators only tell how globally safe a disposal system is. For confidence building, performance indicators can be used in addition to tell how the system works. In particular, performance indicators such as fluxes, activities or activity concentrations of selected radionuclides can show how the different components of the system fulfil their safety functions and contribute to the overall safety. The SPIN project of the European Commission assessed the usefulness of seven safety indicators and fourteen performance indicators by testing them in four actual assessments of disposal systems in granite formations. In this paper, indicators calculated from an assessment of the disposal of spent fuel in the poorly indurated Boom Clay formation are presented. Conclusions from the SPIN project that hold for repositories in clays are highlighted, as well as results that illustrate differences between the granite and clay disposal options. Finally, various performance and safety indicators are combined into a logical sequence to comprehensively present, and explain, the results of a safety assessment.