Development and application of knowledge-based source-term models for radionuclide mobilisation from contaminated concrete

Concrete materials in nuclear facilities may become activated or contaminated by various radionuclides through different mechanisms. Consequently, decommissioning and dismantling of these facilities produce considerable quantities of these materials (e.g. concrete structures, rubble), which are at least potentially contaminated with radionuclides and which must be managed safely and cost-effectively. In this paper, we present results from a research project that aims at the development of source-term models for the mobilization of radionuclides from contaminated concrete. The objective of this task was to clarify whether a more realistic source-term description could be beneficial for optimization of the management of decommissioning wastes by reducing the amount of material for disposal as radioactive waste as well as by saving natural resources due to the recycling of building materials. To identify important parameters and processes that affect the release rates of radionuclides, we evaluated the chemical behavior and the solid speciation of radionuclides in concrete materials and the influence of factors like concrete properties, source/pathway of contamination, and the scenario-specific chemical environment and hydraulic regime. Furthermore, concrete degradation processes and their influence on contaminant mobilization were addressed. On this basis, source-term models were developed to describe the radionuclide release by (i) the dissolution of radionuclide containing solid phases, (ii) the desorption of radionuclides from surfaces, and/or (iii) the leaching of radionuclides from a solid matrix without disrupting its structure. These source-term models were parameterized for probabilistic simulations of various release options, including the reuse of recycled building materials, the disposal of rubble in inert and municipal landfills as well as the on-site disposal of concrete materials (e.g. foundations remaining in the ground, in situ burial of rubble). For some scenarios and radionuclides, the calculated release rates were between one and two orders of magnitude lower than those used in former generic calculations. Based on the results of stochastic simulations, the consequences of the use of a more realistic source-term for dose assessments with respect to clearance/recycling of contaminated concrete will be illustrated and discussed. (author)