An investigation of the neptunium complexes formed upon interaction of high level waste glass and Boom Clay media

Among the actinides which can be released from vitrified HLW, Neptunium-237 is considered as the most critical one in the Belgian performance assessment studies. Therefore, leaching experiments with Np-doped glass in humic acid containing Boom Clay water at different conditions were carried out. For the speciation of Np in the leachate, LPAS (Laser Photoacoustic spectroscopy) and UV-VIS absorption spectroscopy were used. The Np concentration in the leachates, after a slightly decrease, seems to approach a steady-state value at about 5 x 10-7 M after 6 months of glass corrosion. The highest Np concentrations were observed for the highest S/V value and for the glass doped with the highest Np concentration, the combination of the two separate parameters leading to even higher Np concentrations in solution. Np is mainly bound to colloids. The first spectra of the leachates obtained by LPAS confirm the potentiality of this technique to characterize the Np species at low Np concentration and show the presence of Np(IV) complexes which have still to be identified. The authors, can, therefore, anticipate that the Np steady-state concentration may be controlled by a Np(V) reduction/precipitation process