Calculation of the effect of gaseous thermodiffusion and thermogravitation processes on the relative humidity surrounding a high level nuclear waste canister

Thermal diffusion causes a gradient in water vapor pressure in air–vapor mixtures in a temperature gradient. Estimates of the thermal diffusion constant for air–vapor mixtures permit the magnitude of the thermal diffusion effect to be estimated using conventional Onsager relations. When expected thermal gradients in an air gap adjacent to a high level waste container in a hypothetical geological repository with optimized geometries are considered, the thermal convection of the gas phase (thermogravitation) can result in a significant vertical separation of water vapor and air. For geometries including a vertical container surface and an air gap on the order of 2 cm wide and 600 cm tall, the relative humidity at the top of the convective system is up to 30% higher than when the gas mixture between a waste container and nearby backfill is assumed to be homogeneous. Such a difference may have important consequences, in particular, for the rate of corrosion of the waste container. However, high thermogravitational separation in the vicinity of the waste container is restricted to particular geometries, and waste repository can be designed to minimize the effect. Geometries that preserve an air gap of more than 5 cm or geometries prohibiting free convection are expected to eliminate thermogravitational separation of water vapor and air.