The objective of the efforts at GRS is the development of a polythermal thermodynamic model for the description of the activities of dissolved selenium species under reduced conditions. Within this frame GRS investigates the speciation and activity of selenide in NaCl and MgCl2 dominated solutions by spectrophotometrical titrations under inert gas atmosphere. This will be done in a wide pH-range as a function of concentration. Aim of the measurements are the determination of the protolysis coefficients of H2Se and H2Sen (n= 1….4) for later derivation of the protolysis constant at ionic strength “zero” as well as interaction coefficients of involved selenide ions.
The thermodynamic activity of selenium species (selenite and selenate), iodide and caesium by temperatures higher than 25°C will be described by developing a combined experimental predictive method. Hereby, calorimetric titration of dilution enthalpies and mixture enthalpies will be used to expand knowledge and models found in the previous project VESPA. Ternary parameters will also be identified by isopiestic measurements.
Furthermore, GRS will experimentally study, whether oxidized selenium species (selenite, selenate) are reduced und immobilized when these are moving by diffusive transport through corrosive layers of a containment. For this, batch experiments under inert gas atmosphere are planned with corroded iron chips and iron corrosion products (e.g. chukanovite Fe2(OH)2CO3 or Hibbingite (Fe,Mg)(OH)3Cl) which are representative for the geochemical conditions developing in a repository in claystone or rock salt.
Another topic to investigate by GRS is the development of geochemical reference scenarios for potential radioactive waste repositories in German claystone and rock salt formations. Initial geochemical conditions at the beginning of a radionuclide mobilization as well as further chemical processing of the near field will be described by these geochemical reference scenarios. The geochemical conditions are important for the assessment of speciation formation of radionuclides in the near field and influence their migration behavior significantly. They are characterized by chemical parameters like pH, redox potential, kind and quantity of sorbing secondary phases, main solution components or complexing minor components. The aim of this exercise is to describe some geochemical overall developments which should be representative and reasonable for the near field of a radioactive waste repository in case of a solution intrusion in clay stone and rock salt, respectively.
