Probabilistic safety assessment model for near surface radioactive waste disposal facilities

A probabilistic safety assessment model has been developed for assessing the performance of near surface disposal facilities for low-level radioactive waste. Two modes of disposal such as single dump and multiple dump are considered in the model. The model is composed of four components: source term, repository failure, geosphere transport and radiological assessment. The source term contains low-level radioactive waste equivalent to 50 GWe.y energy production (1 GWe.y=3.2×1016 J of electrical energy production) either disposed at an instant or distributed over 50 years. The endpoints of assessment are expressed as radioactivity release rate, radionuclide concentration in ground water, radiation dose to a member of the critical group through drinking water pathway and total risk to critical group due to disposal practice. Sensitivity analysis is carried out to identify the critical parameters, which have maximum effect on the assessment endpoints. Uncertainty analysis, based on random selection of all main parameters, is also carried out to identify the effect of overall variation of parameters used in the model in relation to a reference level. The reference level delivers a maximum annual effective dose of 1.2×10−2 mSv to a member of the critical group after about 1.25×102 years of disposal mostly from 129I, the long-lived and less sorbing radionuclide. The corresponding risk to the public due to the disposal practice (9.0×10−7 y−1) is found to be lower than that due to natural background radiation. The most critical parameters as indicated by the sensitivity analysis are the distribution coefficient of radionuclides, seepage velocity in the unsaturated zone between the facility and the water table, dispersivity in ground water and thickness of the unsaturated zone. The uncertainty analysis shows that 129I is the critical radionuclide delivering maximum dose in most cases though it constitutes a low percentage in the low-level radioactive waste inventory. The annual effective doses derived from the uncertainty analysis fall in a log normal distribution with a geometric mean of 3.6×10−2±3.9 mSv and the most probable annual effective dose to a member of the critical group works out to be 2.8×10−4 mSv.