Thermohydrological Effects on a Nonreactive Aqueous Species in Unsaturated Fractured Geologic Materials

The chemical and mineralogical conditions of the near-field, i.e., that area in the vicinity of the waste materials, may be significantly altered from ambient conditions by thermohydrological processes resulting from the placement of heat-generating radioactive materials in a geologic repository. Models are developed linking the thermohydrological effects simulated with TOUGH2 to a nonreactive aqueous species (chloride). Perturbations in near-field chemistry from the ambient conditions may have potential impacts on engineered barrier system (EBS) performance, waste-form degradation processes, and radionuclide transport. The results of thermohydrological simulations with TOUGH2 utilizing various conceptual models for fracture representation are coupled to simple chemical models (density and osmotic effects are neglected) to demonstrate the complexity and potential magnitude of thermohydrochemical (T-H-C) processes. The concentration of chloride in solution returning to the EBS following dryout, in extreme cases, is predicted to exceed 100 000 mg/l. The dimensionality of the problem and the rate at which the tuffaceous rocks rewet significantly affect the magnitude of the thermohydrological impact on chloride redistribution. A process metric (initial rewetting rate and distribution) that is ignored when evaluating thermohydrological response is very important when a more complex coupling (T-H-C) is considered