Performance Assessment of a High-Level-Waste Repository in Clay

Abstract

ABSTRACTPerformance assessment studies have been undertaken on the geological disposal of high-level waste in a clay layer in the framework of the CEC project PAGIS. The methodology applied consists of two consecutive steps : a scenario and a consequence analysis. The scenario analysis has indicated that scenarios of normal evolution, of human intrusion, of climatic change, of secondary glaciation effects and of faulting should be evaluated. For the consequence analysis as well deterministic “best estimate” as stochastic calculations, including uncertainty, risk and sensitivity analyses, have been elaborated.The calculations performed show that most radionuclides decay to negligible levels within the first fewjneters of the clay barrier. Just a few radionuclides, 99Tc, 135Cs and 237Np with its daughter nuclides 233U and 229Th can eventually reach the biosphere. The maximum dose rates arising from the geological disposal of HLW, as evaluated by the “best-estimate” approach are about 10−11 Sv/y for river pathways. If the sinking of a water well into the 150 m deep aquifer layer in the vicinity of the repository is considered together with a climatic change, the maximum calculated dose rate rises to a value of 3×10−7 Sv/y. The maximum dose rates evaluated by stochastic calculations are about one order of magnitude higher due to the considerable uncertainties in the model parameters. In the case of the Boom clay the estimated consequences of a fault scenario are of the same order of magnitude as the results obtained for the normal evolution scenario. The maximum risk is estimated from the results obtained through stochastic calculations to be about 5×10−8 per year. The sensitivity analysis has shown that the effective thickness of the clay layer, the retention factors of Tc, Cs and Np, and the Darcy velocity in the aquifer are parameters which strongly influence the calculated dose rates.