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Coupled Model for Heat and Water Transport in a High Level Waste Repository in Salt

This report summarizes efforts to simulate coupled thermal-hydrological-chemical (THC) processes occurring within a generic hypothetical high-level waste (HLW) repository in bedded salt; chemical processes of the system allow precipitation and dissolution of salt with elevated temperatures that drive water and water vapor flow around hot waste packages. Characterizing salt backfill processes is an important objective of the exercise.  An evidence-based algorithm for mineral dehydration is also applied in the modeling.  The Finite Element Heat and Mass transfer code (FEHM) is used to simulate coupled thermal, hydrological, and chemical processes; new capabilities were added to the FEHM code.  Benchmarking exercises are presented that compare simulation results to recent experiments on the thermal behavior of salt and water release from clay dehydration.  Examination of sensitive properties, uncertainties, and coupled effects in excavated area and backfill of a generic system are presented.  A model for the pore-scale simulation of brine transport within aggregates of salt crystals  is presented, and multiphase reactive transport phenomena (phase transition between liquid-vapor phases) and dissolution/precipitation of the salt in the brine inclusion are simulated; effects of initial inclusion size and temperature gradient on the brine behavior are investigated. In addressing early phase post-emplacement in a generic repository, for the purpose of the current modeling, justification is provided to treat the EDZ as a single porosity medium with effective damaged porosity and permeability values, but the situation may differ for late post closure time.