The influence of stable element inventory on the migration of radionuclides in the vicinity of a high level nuclear waste repository exemplified for ~(59)Ni

摘要

A generic modelling approach has been used to estimate the influence of the stable inventory, i.e. stable isotopes in a radioactive waste repository on the migration of radionuclides from waste canisters into the surrounding bentonite or Opalinus Clay. The model radionuclide chosen was bivalent ~(59)Ni(II); the stable isotopes Ni(II), Fe(II), Mn(II), Zn(II) and Cu(II) are considered to be competitive for the same sorption sites on bentonite or Opalinus Clay. A simplified one-dimensional modelling approach in space was used for reactive transport calculations using the MCOTAC code incorporating the 2SPNE SC/CE sorption model. Calculated ~(59)Ni(II) breakthrough curves in bentonite and Opalinus Clay are compared to estimates of the influences of the individual competing metals present in the porewaters. Generally, faster migration, i.e. a reduced sorption, for ~(59)Ni(II) was calculated – up to two orders of magnitude in arrival time at specified locations in the bentonite or Opalinus Clay. This influence is a maximum for highest specified stable isotope concentrations. Fe, Zn and Mn have approximately the same effect on the migration of ~(59)Ni(II); Cu has the potential for a much stronger effect. However, their individual effects at reducing the retardation of ~(59)Ni(II) through sorption competition do not sum up linearly. In the various scenarios calculated, an upper limit for the reduction of the retardation of ~(59)Ni(II) has been assessed for the combined sorption competition influence of all the stable isotopes. Although the calculated scenarios include several simplifications, they cover a wide range of combinations of sorption competition effects of stable isotopes present in a high-level waste repository on the migration of radioactive ~(59)Ni(II). More detailed scenario calculations would be possible if a more detailed ‘‘geochemical inventory’’ of radionuclides and stable isotopes were to become available. Nevertheless, upper limits for the effects of sorption competition of bivalent stable isotopes on the migration of ~(59)Ni in the vicinity of a high-level nuclear waste repository were assessed.