Multidimensional Water Flow in a Low‐Level Waste Isolation Barrier

Abstract

AbstractAn analysis was conducted of multidimensional subsurface moisture flow in a hypothetical low‐level radioactive waste disposal facility. The key design feature examined in the analysis was a sloping sand/gravel capillary barrier designed to route natural infiltration around a concrete vault. Three barrier slopes (1:25, 1:10, and 1:5) and three sand/gravel property combinations were considered. The slopes and material property combinations were selected to represent a range in barrier effectiveness. The porous media flow code used for this analysis was VAM3DCG, a three‐dimensional, finite‐element code which was able to use nonorthogonal grid discretizations and employed robust, efficient numerical techniques. Three‐dimensional modeling demonstrated that flow in the hypothetical design exhibited cross‐slope flow because of the pressure gradient produced in the third, cross‐slope dimension. Barrier effectiveness was shown to be highly sensitive to the sand/ gravel material properties. Barrier slope was less important, especially for the effective material combinations. The presence of three‐dimensional flow could be important in a performance assessment if the quantity of water predicted to breach the capillary barrier by a two‐dimensional model were different from that predicted by a three‐dimensional model. Comparative modeling demonstrated that a two‐dimensional analysis resulted in underestimation of barrier effectiveness. For the considered design, two‐dimensional modeling is a conservative, yet reasonable, approach in a performance assessment application.