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

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 59Ni(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 59Ni(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 59Ni(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 59Ni(II); Cu has the potential for a much stronger effect. However, their individual effects at reducing the retardation of 59Ni(II) through sorption competition do not sum up linearly. In the various scenarios calculated, an upper limit for the reduction of the retardation of 59Ni(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 59Ni(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 59Ni in the vicinity of a high-level nuclear waste repository were assessed.