Leaching of Np and Tc From Doped Nuclear Waste Glasses in Clay Media: The Effects of Redox Conditions
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Leaching of Np and Tc From Doped Nuclear Waste Glasses in Clay Media: The Effects of Redox Conditions.
Véra Pirlet, Karel Lemmens, Pierre Van Iseghem, Waste and Disposal Department, SCK•CEN Mol, Belgium ABSTRACT 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 (FourgesCahaignes-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 FoCaclay, 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. INTRODUCTION Disposal of vitrified high level waste (HLW) forms in geologic formations is under consideration in many countries. In Belgium, Boom Clay is the candidate geologic formation and amongst the engineered near-field materials, FoCa-Clay is under consideration as a backfill material. Evaluation of the safety of the disposal of HLW requires the determination of the source term for the most critical radionuclides that will be released from the waste forms and the characterisation of all the mechanisms controlling their behaviour. Despite the many studies of radionuclide release from vitreous waste forms [1-6], the behaviour of the radionulides depends on a number of parameters that complicates interpretation of their behaviour in sub-surface environments. 237 Np and 99Tc are amongst the main critical radionuclides for vitrified HLW in Belgium. Np and Tc are redox sensitive elements. Redox conditions applied during disposal will strongly influence their behaviour and the chemical species finally found in the leachates and consequently, the solubility limits in the media. As determined by thermodynamic calculations, under oxidising conditions, Np(V) and Tc(VII)
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