Equivalent Flow Rates from Canister Interior into the Geosphere in a KBS-3H Type Repository

Equivalent flow rates from the interior of a damaged canister in a KBS-3H type horizontal deposition drift into the geosphere are calculated as a function of the transmissivity of a rock fracture intersecting the deposition drift and its distance from the defect in the canister. Equivalent flow rate (litres/yr) gives the steady state release rate of a stable species into the geosphere in a case where a constant concentration of one unit per litre is assumed to prevail in the canister.

The results show that in a fully saturated state, where a swollen and homogenous buffer surrounds the canister and no significant erosion of the bentonite takes place, the boundary layer resistance between the stagnant porewater in the buffer and the groundwater flowing in fractures intersecting the deposition drift is a dominant transport resistance in the near-field. The diffusion resistance in the buffer is significant for anions which have low diffusivity in the buffer thanks to anionic exclusion. In similar flow and fracturing conditions, long-term release rates from a KBS-3H deposition drift are lower than those from a KBS-3V deposition hole, where the upper part of the deposition hole backfilled with the tunnel backfill as well as the tunnel may become important release routes for long-lived radionuclides, if significant flow of groundwater takes place through these parts of the near-field. This concerns especially cations having a high mobility in the buffer and backfill.