Background For the disposal of high-level radioactive waste (HLW) in a deep geological formation such as the Boom Clay, safety assessment studies have shown that the long-lived 79Se (T½ presently reassessed to ~ 295 ka) is one of the more critical fission products. Therefore, the understanding of its migration properties (diffusion, retention, sorption, solubility) through the geological barrier is of prime importance. The migration behaviour of selenium strongly depends on its chemical speciation. Under the reducing conditions prevailing in Boom Clay, selenide, [Se(-II)], is the thermodynamically stable species, and HSe– is expected to be the dominant selenium species in solution. The selenium migration should mainly be controlled by the low solubility of iron selenide such as FeSe or FeSe2, or solid solutions with seleniferous pyrite. However Se species are often found in redox disequilibrium and more soluble higher oxidation state [selenite: Se(+IV), and selenate: Se(+VI)] might also coexist if their reduction is kinetically hindered. Due to the unknown oxidation state of selenium in the waste form and the uncertainties related to the redox disequilibrium it is important to study the behaviour of selenium in all its oxidation states.
For the disposal of high-level radioactive waste (HLW) in a deep geological formation such as the Boom
Clay, safety assessment studies have shown that the long-lived 79Se (T½ presently reassessed to ~ 295 ka)
is one of the more critical fission products. Therefore, the understanding of its migration properties (diffusion,
retention, sorption, solubility) through the geological barrier is of prime importance. The migration behaviour
of selenium strongly depends on its chemical speciation. Under the reducing conditions prevailing in Boom
Clay, selenide, [Se(-II)], is the thermodynamically stable species, and HSe– is expected to be the dominant
selenium species in solution. The selenium migration should mainly be controlled by the low solubility of iron
selenide such as FeSe or FeSe2, or solid solutions with seleniferous pyrite. However Se species are often
found in redox disequilibrium and more soluble higher oxidation state [selenite: Se(+IV), and selenate:
Se(+VI)] might also coexist if their reduction is kinetically hindered. Due to the unknown oxidation state of
selenium in the waste form and the uncertainties related to the redox disequilibrium it is important to study
the behaviour of selenium in all its oxidation states.