Brine migration in salt in a thermal gradient

Abstract:

Salt deposits have long been considered viable repositories for long-term storage of high-level nuclear waste. However, brine trapped in salt tends to migrate up thermal gradients, such as can develop around radioactive waste storage containers, potentially promoting corrosion of containment structures. Brine inclusions move up the temperature gradient through the three main steps: 1) the dissolution of salt at the hot side of the inclusion caused by increased salt solubility, 2) ordinary and thermal diffusion of dissolved salt ions within the inclusion, and 3) precipitation of salt at the cold side of the inclusion due to local supersaturation. This process of brine transport through salt under a thermal gradient is generally referred to as thermal migration. Here we investigated thermal migration of brine inclusion in salts for a wide range of mean temperatures (~ 50 °C to ~200 °C) and temperature gradients (~ 10 °C/cm to ~57 °C/cm). With time brine inclusions moving towards the heat source become elongated parallel to the thermal gradient. We quantified the rate of brine migration as a function of mean temperature and thermal gradient using time-lapse optical microscope. X -ray and neutron tomography were used to visualize and quantify 3D spatial distribution of brine inclusion in a salt crystal at different stages of thermal migration. Migration velocities are shown to increase with temperature, temperature gradient and size of inclusion. We find an abrupt increase in migration velocity at certain time steps of thermal migration. Migration velocities of brine inclusions ranged from 0.1 m/year to 30.7 m/year. Empirical equations at different velocity regions for brine inclusions were obtained by fitting exponential equations to the experimental data with high coefficient of determination values (R²> 0.94).The experimental results are in good agreement with the theoretical migration rates obtained using a previous analytical model.