A Model of Coupled Heat and Moisture Transport in an Annular Clay Barrier

Rossi, Louis F.
Inyang, H. I.
Graham-Eagle, J.
Pennell, S.
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Department of Mathematical Sciences
The design of repository seals for deeply buried high-level radioactive wastes incorporates densely compacted clayey barriers around metallic waste canisters. In this paper, a mathematical model that is based on conservation of thermal energy and mass is developed to describe the locations of moisture and temperature fronts within a barrier, around a cylindrical waste canister of 1-meter radius, containing radionuclides with half-lives that range from 100 – 10,000 years. The solution developed is axisymmetric: the moisture fraction, w, and temperature T, vary only with time t, and radial distance r from the axis of the cylindrical waste canister. The model produces parabolic partial differential equations (PDEs). The spatial domain is discretized such that ordinary differential equations (ODEs) that result are solved. Computations using a uniform mesh of 0.1 meters and a cooling coefficient of 6.7 x 10 -2 with assumed but typical data on material properties, indicate that coupling of transport processes would be negligible in the case of radionuclides with long half-lives. Also, a quasi-steady vaporization front can form and propagate outward over the course of several decades after waste emplacement. The evolution of the front is somewhat insensitive to the half-life used and the permeability of the clayey barrier material.
radioactive waste barrier material , illitization , coupled transport , waste containment