Sensitivity analysis of the dry density distribution of a bentonite buffer in a deep geological repository for spent nuclear fuel

Bentonite buffers consisting of compacted blocks and pellet fills with different installation densities are used as engineered barriers in the KBS-3 deep geological final disposal concept for spent nuclear fuel. In this work, a sensitivity analysis was performed to identify the most influential parameters for the density homogenization of the bentonite buffer. In total, 16 input parameters related to 1) the THM models developed for compacted blocks and pellet fills, 2) the buffer design and 3) the thermal and hydraulic boundary conditions were studied. The output variable was the heterogeneity degree that was defined to quantify the remaining density differences in the buffer. For the analysis, 34 numerical full-scale simulations were performed with varying input parameter value combinations prescribed by a definitive screening design (DSD), which allows for the statistically reliable estimation of the effects of the different parameters. The results indicate that three parameters are most relevant for the homogenization in the buffer: the elastic stiffness of the inter-pellet pores related to the triple porosity model developed for pellet fills, the width of the outer pellet-filled gap and the microstructural swelling capacity of the bentonite. On this basis, recommendations for control of the homogenization in the buffer are given.