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. The stochastic results indicate 90-percentiles generally at around two times the mean values, but up to 6 times for very skew distributions. It is also indicated that the statistical comparison with the dose distributions from background sources and of the lower detection levels of analytical results are other important criteria to consider. 1. Introduction In order to address uncertainties in assessment parameters, a stochastic safety assessment was performed for the post-closure conditions at the Rössing Uranium mine. The stochastic assessment was very similar to a deterministic assessment, which was first performed through a set of coupled spreadsheets, but instead of using fixed assessment parameters, parameter values were presented as so- called assumption distributions. The assessed doses were hence also provided as distributions (so- called forecast distributions), obtained through random (Monte Carlo) selections using the software package Crystal Ball, [1] from the assumption distributions. The forecast distributions were then evaluated statistically. Apart from presenting some measure on the uncertainties of the assessed doses, the stochastic assessment also allowed statistical significance tests on the dose contribution of mine- related sources to doses assessed for sources of natural background. Details on an extensive data acquisition programme for the assessment and on the safety aspects of the assessment results have been presented elsewhere [2, 3]. While this presentation again summarizes important aspects of both the parameters used and the broad assessment methodology, it focuses on the methods used and problems experienced to specify assumption distributions for the stochastic assessment and on the results of statistical significance tests. 2. Site Description The Rössing uranium mine is situated in the Namib desert of Namibia about 65 kilometres inland from the coastal town of Swakopmund. It performs open pit mining of low-grade ore (around 350 ppm uranium). Following various crushing and milling operations, extraction of uranium as ADU and calcination to U3O8 is performed at the mine. Apart from the open pit, the main impact features are various ore stockpiles and the tailings dam. The mine is situated next to the non-perennial Khan river and three dry gorges (Dome gorge, Panner gorge and Pinnacle gorge) drain from the mining area into the river. About 30 kilometres from the mine the Khan river flows into the Swakop river, which again 1 flows into the Atlantic ocean at Swakopmund. Arial pictures of the mine site and surrounding region are indicated in Figures 1 and 2 respectively. FIG.1. Arial picture of the Rössing uranium mine site FIG.1. Arial picture of the region surrounding the Rössing uranium mine 3. Source Terms Generally the major source is the tailings dam, which is a source of radon, dust and seepage water. While the complete dam is regarded as a single radon source, salt patches remaining after water evaporation from the water-collection ponds on the dam contain higher concentrations of some radionuclides and are considered as separate dust sources, which will be mitigated to various degrees by covers of waste rock. Seepage water is presently collected in seepage dams and recycled. After closure seepage water will continue to be recycled to evaporation areas during an active institutional control period. 2 3.1. Radon Sources The main radon sources are: — The open pit, including the waste rock dumps. — The crushing circuit, including the coarse and fine ore stockpiles. — The tailings dam. — Contaminated areas around the plants — Background locations including areas with enhanced levels. 3.2. Dust Sources The fugitive dust sources identified were the entire tailings dam surface, waste piles, the salt deposits on the tailings dam, and areas where fine material is deposited from seepage. 3.3. Aquatic Sources The primary sources were the tailings dam run-off and seepage. 4. Pathway Analysis The atmospheric and the aquatic pathways are the two major pathways considered. At the points of impact at the receptors, the contributions from the atmospheric and aquatic pathways provide source terms for the secondary pathways. It is at these points where the public can get exposed to radiation through various modes, notably ingestion, inhalation and external exposure. 4.1. Scenario Description The scenarios presented below represent both present as well as potential future scenarios with respect to human habitation in and around the mining grant. The eight scenarios are: 4.1.1. Scenario 1 Scenario 1 represents the present scenario of the township of Arandis whose residents live and work in the town. Arandis is located approximately 15 kilometres to the north-west of the mine site. The atmospheric pathway is of primary importance. 4.1.2. Scenario 2 Scenario 2 is similar to Scenario 1, except that the actual critical group is assumed to live and work in the immediate vicinity of Arandis airport. The atmospheric pathway is of primary importance. 4.1.3. Scenario 3 Scenario 3 considers a small farming community, living on the banks of the Khan River immediately downstream of the confluence to the Dome gorge catchment, receiving a potential dose through human inhalation, human ingestion, as well as external air and soil exposure. All the water is obtained from the Khan River as groundwater and the human ingestion pathway is extended to include the dose contribution from vegetable, fruit and animal consumption. Both the atmospheric and groundwater pathways are of primary importance. 4.1.4. Scenario 4 Scenario 4 is similar to Scenario 3, but the small farming community is located on the banks of the Khan River immediately downstream of the confluence of the Panner gorge. 3 4.1.5. Scenario 5 Under this scenario, it is assumed that a small community lives and works on the old Khan Mine site. It is assumed that all water at this site originates from the West Coast water supply scheme. As for scenarios 1 and 2, the atmospheric pathway is of primary importance. 4.1.6. Scenario 6 Scenario 6 is similar to Scenario 1, except that instead of working within the township of Arandis, it is assumed that some small industries make use of the existing office infrastructure present at Rössing after mine closure. This scenario therefore assumes a population living in the township of Arandis, but working (for an average of 2000 hours per annum) in an office block on the old Rössing mine site. The atmospheric pathway is of primary importance. 4.1.7. Scenario 7 This scenario involves residents of Swakopmund, 60 km from the mine who would be exposed through the atmospheric pathway only. Drinking water is pumped from the supply system of the aquifers of the Omaruru and Kuiseb rivers, which are not in any way linked to catchment area of the mine. Food is mainly brought in from outside the country and only very limited quantities of food are sourced from the market gardening done in the vicinity of the town. During wind events small quantities of radon and dust could potentially be dispersed from the mine to the town. 4.1.8. Scenario 8 A number of smallholdings are situated on the banks of the Swakop river, around 50 kilometres downstream of the mine. Farmers engage in market gardening and animal products which are mainly sold in Swakopmund. Drinking water for humans and animal comes from the aquifers of the Omaruru and Kuiseb rivers. Irrigation water is pumped from the Swakop river. It is estimated that 30 % of the fodder is produced at the farms. Exposure occurs through the atmospheric and the aquatic pathway. The aquatic pathway assumes that 50 % of the farmer’s diet would consist of farm products. Water quality is assumed to be worst-case, that is mixed, according to hydro-geologically modelled mixing proportions, of the Khan and Swakop river groundwater. 4.1.9. Scenarios 9 This scenario looks at the background dose near the mine. It is similar to the radon and aquatic pathway parts of scenario 3, but uses the identified radon background sources and analytical water data for the Khan river upstream of the mine.