Steady Radionuclide Release Rates from a Bentonite-Protected Waste Canister

Abstract

A bentonite-based packing or engineered barrier is one ingredient of the multibarrier concept for underground storage of high-level nuclear waste. This paper investigates the possible effectiveness that such a smectiteladen barrier might have in controlling steady, solubility-limited release rates from a cylindrical waste canister.Darcy's law and the convective-diffusion equation in two dimensions are solved numerically with and without an annulus of packing material. Reduced fractional release rates quantifying the role of the packing are studied over wide ranges of ground water velocities, packing amounts, and packing permeability and effective diffusivity. Release rates are shown to be strongly dependent on the physical properties of the packing, particularly the effective diffusivity.Although the low permeability of a bentonite-rich packing negates convection next to the canister, calculated release rates of the packing-protected waste canister do not differ dramatically from the those for the unprotected canister, except at high ground water velocities. To afford more protection, consideration must be given to reducing the porosity of the packing material.