MODELLING ~(226)RA, ~(222)RN, AND ~(210)PB MIGRATION IN A PROPOSED SURFACE REPOSITORY OF VERY LOW-LEVEL LONG-LIVED RADIOACTIVE WASTE

摘要

A generic concept for disposal of very low-level long-lived radioactive waste is currently being evaluated for radium bearing wastes that originated from a historical radium extraction plant at Olen, Belgium. A total volume of approximately 217 000 m~3 of waste with an average radium content of 7 Bq/g has to be disposed of. Upon request by the Belgian National Agency for Management of Radioactive Waste and Nuclear Fuels, NIRAS/ONDRAF, a generic disposal concept was evaluated for the purpose of identifying the minimum disposal concept which guarantees long term safety. Such an analysis would provide useful input to the final design of the disposal concept, as the contribution of the different engineered barriers to the overall safety will have been assessed. The analysis focussed on the migration of ~(226)Ra, ~(222)Rn, and ~(210)Pb to groundwater owing to infiltration of rainwater and the diffusive radon transport via the gas phase to the atmosphere. The generic design considered a waste dump with the contaminated material completely enclosed by a clay barrier. To protect the low-permeability clay from degradation by water and wind erosion, frost and desiccation, burrowing animals, and plant roots, among others, a multi-layer cap was designed to meet these requirements. In the performance assessment calculations, various cases were considered in which one component of the disposal concept was changed at the time. Cases considered included absence of clay layers, effects of hot spots, lower adsorption capacity of various materials, and the effect of separating the radium contaminated material from nearly uncontaminated material. Unsaturated flow calculations were done first to estimate the steady-state water content profile. Knowledge on the degree of water saturation is of paramount importance for radon transport through the gas phase. Based on the steady-state water content profile, advective dispersive transport calculations were done considering the decay chain reaction of ~(226)Ra. In addition to the radon gas flux to the atmosphere, fluxes of ~(226)Ra, ~(222)Rn, and ~(210)Pb to groundwater were also produced. Groundwater flow and transport calculations yielded radionuclide concentrations in a hypothetical well nearby the planned disposal site, whereas biosphere modelling provided the annual doses to the public considering the groundwater pathway and direct inhalation of radon in case of the atmospheric pathway. On the basis of the calculated radionuclide fluxes and doses the importance of the various model parameters and concept components will be evaluated and discussed.