PRObabilistic Safety Assessment, (PROSA). Uncertainty/sensitivity analysis of the advective transport of radionuclides in two generic models

Abstract

In the PROSA (PRObabilistic Safety Assessment) project, a study is made of radioactive waste disposal in rock-salt formations using probabilistic techniques. The purpose of PROSA is to determine the health risks of humans and the safety relevant characteristics of a waste disposal concept. The two generic local hydrogeological models considered in this project are a shallow salt dome with an overburden of 230 m and a deep salt dome with an overburden of 800 m. A prescribed hydraulic head distribution at the top of the models results in two infiltration zones at the edges and a seepage zone in the middle of the models, forcing the released radionuclides from the top of the salt dome through the overburden into the biosphere. Two different scenario's have been considered, a water intrusion and a diapirism/subrosion scenario. The sensitivity and uncertainty analysis of the groundwater compartment, performed with the UNCSAM package, is presented in this report. The velocity values are derived from a large number of groundwater velocity fields determined with the METROPOL-1 code. The considered input parameters are the thickness, porosity and permeability of the geohydrological units, the ratio of permeability between a fault present in the middle of the models and the adjacent layer, and the absolute value of the hydraulic gradient at the earth surface. Initial calculations with the UNCSAM package, including all the input parameters, resulted in unreliable values of the chosen sensitivity and uncertainty measures. To improve the analysis, a total of four modifications were implemented in the regression model including the use of the natural logarithm of the average vertical particle velocity value as the dependent variable and the elimination of the fault factor as a stochastic input parameter. The fault factor is very dominant and since it is also correlated with the permeability of the adjacent layers, it completely obscured the effects of all other parameters in the UNCSAM analyses. Recognizing the fault factor as the most influential parameter, it was left out in the UNCSAM analyses discussed in this report. Of all other considered parameters, the permeability of the clay layer (layer 1) is the most influential input parameter. When the clay layer is removed from the models as a result of diapirism/subrosion, the permeability of the layer with the cretaceous deposits and the permeability of the Brussel's Sand/Ieper Clay layer (layer 1''') become the most influential parameters.