A STOCHASTIC APPROACH TOWARDS A POST CLOSURE SAFETY ASSESSMENT A Ramlakan1, GP de Beer1, A Abrahams2, R Schneeweiss2 1NECSA, PO Box 582, Pretoria 0001, South Africa, E-mail:
[email protected] 2Rössing Uranium Ltd, Private Bag 5005, Swakopmund, Namibia. Abstract. Following an extensive data acquisition programme to identify and characterize sources on the mine as well as some enhanced background sources around the mine site, a post-closure safety assessment has been performed for the Rössing uranium mine in Namibia. The transport of radioactivity from these sources to exposed critical groups was studied through atmospheric and simple aquatic pathway models together with analyzed data and estimated parameters. Using dose models the doses to pre-identified critical groups were assessed for the various pathways in a deterministic safety assessment. The effect of some dust mitigation options was also assessed, while the effect of post-closure institutional control of water seepage was considered in the aquatic model. A stochastic assessment was next attempted to provide information on the possible range and distribution of the assessed doses in terms of uncertainties in the experimental data and parameters used. While insufficient data caused some difficulties, various approximate distributions have been obtained for several of the parameters used. Techniques range from the utilization of the means and standard deviations of analytical data, published parameter ranges and other approximate methods to estimate parameter distributions. Using random selections from these distributions, the dose distributions for various exposure pathways and scenarios have been assessed. The deterministic doses were used to distinguish those critical groups where doses would likely be above the dose constraint from those where doses would likely be below the dose constraint or trivial.