Behaviour of actinides (Th, U, Np and Pu) and rare earths (La, Ce and Nd) during aqueous leaching of a nuclear glass under geological disposal conditions

Leaching experiments in aqueous media were carried out on non-radioactive and radioactive nuclear waste glasses to investigate the release behaviour of 4 actinides (Th, U, Np and Pu) and 3 lanthanides (La, Ce and Nd) from the solid into solution. The experiments were conducted under flowing conditions using synthetic aqueous solutions. The lanthanide and actinide concentrations were measured in the leachates before and after filtration to 0.45 μm and 1.8 nm. Over 98.5% of the lanthanides and Th released from the glass were retained in the alteration products on the glass surface, probably coprecipitated with a siliceous gel. The retention was enhanced by the presence in the gel of 20 wt% P oxides from the initial 5 × 10−3 M PO4 solution. In the leachates, more than 90% of the lanthanides and Th were associated with colloidal particles. Uranium and Np retention varied from 40% in a 5 × 10−3 M CO3 medium to 95% in a 5 × 10−3 M PO4 medium. In the CO3 medium, U and Np formed stable CO3 complexes that diminished the formation of precipitates; in the PO4 medium, precipitation of PO4 or more complex phases may have occurred. Pu exhibited atypical behaviour to the extent that it was strongly bonded to colloidal particles in solution, concentrated in the fraction exceeding 0.45 μm, and modifyed the calculated Pu leach rates. These particles may be the result of limited disaggregation of the alteration film. As most of the Pu was found at valence IV, it may be strongly retained in the alteration layer by precipitation or coprecipitation. Under oxidizing conditions, Np and U are found at high valences as Np(V) and U(VI), yielding charged forms in solution (NpO2+ and UO2+). The lanthanides, Th and Pu were present at lower valences as Ln(III), Th(IV) and Pu(IV), yielding Ln3+, Th4+ and Pu4+ in oxidizing media. The different behaviour of U and Np compared with the lanthanides, Th or Pu, observed during these experiments shows that the valence is the determining factor, and must be taken into account in assessing the evolution of radionuclides during the alteration of nuclear waste glass.