Coupling of Chemical Processes in the Near Field
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Coupling of Chemical Processes in the Near Field B. Grambow1, E. Giffaut2 SUBATECH, Nantes (France) 2 Andra, Chatenay Malabry (France) 1
ABSTRACT Coupled modelling has been performed using geochemical/transport codes and radiolysis models to describe the chemical evolution of the waste forms “high-level waste glass” and “spent nuclear fuel” together with its waste package and engineered barrier surroundings. Near field processes considered include container corrosion, hydrogen generation, mass transfer for radionuclides and other waste matrix components in corrosion products and buffer materials, geochemistry of near field solution chemistry, sorption of radionuclides on surface sites in the nano-sized pore space of near field materials and the radiolytic decomposition of pore water. The rate limiting steps in waste form dissolution and secondary phase formation mechanism and the associated radionuclide mobilisation chemistry (solubility, solid solution formation, speciation, redox stability) are strongly influenced by the near field constraints.
INTRODUCTION Repository construction and waste emplacement will disturb the geochemical and hydrological environment in the close vicinity of disposed nuclear waste. Also after repository closure, environmental conditions of the “near-field” will continue to evolve due to diverse geochemical and geophysical interactions, heat generation and radiation effects. This “nearfield” consists of the excavated area near or in contact with the waste package, including filling or sealing materials (i.e. the engineered barrier system), and those parts of the host medium whose characteristics have been or could be altered by the repository or its contents. In order to assess the safety function “containment and minimisation of release” of the near field of disposed radioactive waste, it is necessary to investigate the coupling of the dissolution of the waste form with the dominant near field processes for relevant water access scenarios. Even though a comprehensive review of the couplings of all potential near field processes is outside of the scope of the present communication, various geochemical interactions/couplings between the waste forms and various near field materials are described in the following in an exemplary manner, using a generic near field constellation as reference, composed of the waste form (R7T7 type nuclear waste glass or spent light water reactor UOX fuel), a cast iron container overpack, a bentonite backfill and the presence of cementitious materials.
COUPLING OF WASTE FORM DISSOLUTION AND CONTAINER CORROSION Glass: Batch system Glass corrosion is studied since more than 25 year. Important findings are: Initial dissolution rates in groundwater are fast for few days and surface area normalised rates slow down with time by 4 orders of magnitude due to saturation of dissolved silica and formation of
protective surface layers to achieve a low lresidual rate which may be constant (conservative assumption, backed up with some observations) or decrease with time b
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