Leaching of Np and Tc from doped nuclear waste glasses in clay media: The effects of redox conditions

Boom Clay is the candidate geologic formation in Belgium for disposal of vitrified high level waste. In the waste glass, ²³⁷Np and ⁹⁹Tc are some of the principal radionuclides. The leaching behaviour of these nuclides has been studied in dissolution tests involving glass doped with ²³⁷Np and ⁹⁹Tc and was found to depend strongly on the redox conditions of the media contacting the glass. Static tests with the reference R7T7 glass and the reference PAMELA SM539 glass were performed in two clay media that may interact with the glass during the geological disposal. The first test medium consisted of FoCa-clay (Fourges-Cahaignes-clay), which is an oxidised clay. The second test medium was a mixture of backfill material, consisting mainly of FoCa-Clay, with metal corrosion products and Boom Clay. Lower Np and Tc concentrations were found in Boom Clay compared to FoCa-clay. In FoCa-clay, Np and Tc were predominately in their oxidised form. Although Tc is mostly present in the soluble pertechnetate form, the Np concentrations are lower than expected for the chemistry of Np(V) in the medium. The concentrations of the radionuclides are steady-state concentrations rather than thermodynamic concentrations. Different mechanisms can be invoked to explain these Np concentrations. In Boom Clay, the measured Np concentrations are close to the solubility limit whereas the Tc concentrations are slightly higher than those calculated in Boom Clay at thermodynamic equilibrium. The initial specific activity of the radionuclide in the glass was found to influence the soluble concentration. Np and Tc were not found to be retained in the glass reaction layer for both media.