Disposal of hazardous waste like high-level radioactive waste (HLW) is made by confining it in canisters surrounded by dense, ductile “buffer” clay for placement in deposition holes at a few hundred meters depth in crystalline rock. The concept favored by authorities in Sweden, Finland and Canada implies that the holes are bored from blasted or bored tunnels to be tightly backfilled by stacking compacted blocks of clay in the center and filling the remaining space between the blocks and the rock with blown-in clay pellets. The problem with this is that water flowing in from the rock can cause piping and erosion of the pellet filling, which can turn it into mud and disturb the placement of canisters and buffer clay. The controlling parameter is the rate of inflow of water per inflow point, which is determined by the structure and hydraulic conductivity of the rock. The paper describes a simple model of the mechanisms in penetration of water into the pellet fill and provides a basis for estimating the required rate of backfilling for avoiding critical conditions. The study indicates that such conditions will be caused irrespective of the rate of water inflow per point if the backfilling rate is low.