Radium sorption on crystalline rock; spatial distribution and sorption modeling study

Spent nuclear fuel from the Finnish and Swedish nuclear power plants will be permanently disposed of in deep geological disposal facilities located on the coast of the Baltic Sea. As a part of the disposal's safety case, the migration properties of the safety -relevant spent nuclear fuel -borne radionuclides, such as the radionuclide progeny of the waste component 238U, 226Ra, need to be assessed in the lithosphere surround the facility. To this end, the sorption of Ra on disposal -relevant rock and groundwater types was studied in batch sorption experiments in a wide Ra and Ba concentration isotherm (2.6 x 10-9 M to 1 x 10-3 M). The experimental results of the batch sorption experiments were further interpreted using a PHREEQC geochemical model with ion exchange and surface complexation reactions considered. Additionally, the sorption of Ra was studied with thin section sorption studies where autoradiography analysis was combined with mineralogical data obtained in SEM/EDX to establish spatial distribution and mineral specific sorption of Ra on the studied rocks. In the study, it was observed that Ra sorbs well on all tested rock types and water conditions of low ionic competition, regardless of the rock type's projected sorption affinity based on its highly sorbing mica mineral content. In conditions of high ionic competition, Ra tends to stay in the sorption solution, facilitating migration in the disposal context. The obtained experimental and modeled Ra sorption data will be used in the safety case calculations and modeling of the Finnish and Swedish spent nuclear fuel disposal.