As part of the Swedish Radiation Safety Authority (SSM) initial phase review of the Swedish Nuclear Fuel and Waste Management Company (SKB) SR-Site safety assessment for a spent nuclear fuel repository in Sweden, this technical note documents Center for Nuclear Waste Regulatory Analyses (CNWRAÒ) review of potential effects on SKB dose calculations from radioactive elements associated with uranium over long time periods. Uranium is present in both nature and spent nuclear fuel. Radioactive decay creates elements with differing chemical and radiological properties. CNWRA evaluated whether the SKB approach to account for the process of uranium radioactive decay, in particular in groundwater transport and biosphere dose calculations, has omitted any aspects that could significantly affect dose results. CNWRA reviewed pertinent sections of the SR-Site safety assessment report (SKB, 2011), Biosphere Analyses for the Safety Assessment SR-Site Synthesis and Summary of Results (SKB, 2010a), Radionuclide Transport Report for the Safety Assessment SR-Site (SKB, 2010b), and other relevant documents.
CNWRA performed scoping calculations on the potential for disequilibrium of uranium decay products during geosphere transport. A common modelling assumption (by organizations working in radioactive waste management) for the movement of radionuclides in a natural system is that the products of radioactive decay exist in equilibrium with the parent material. Because different materials (or elements) can have different chemical properties and movement behaviour in the natural system (referred to as transport characteristics), it is possible for radioactive decay products to exist at different concentrations along the transport path compared to the parent material. This disequilibrium can enhance the concentration of specific elements at certain locations along the transport path and may result in higher estimated concentrations relative to a model based on assumed equilibrium between the radioactive parent and its decay products. By confirmatory calculations, CNWRA verified specific SKB calculations of outdoor and indoor doses from an assumed radon release and the SKB parameter that converts radon exposure to dose.