Leach testing at 50 °C of α-doped SON68 glass alteration gels

Gels formed by altering α-doped (Np, Pu, Am) SON68 glass at 300 °C were leached during 1 year at 44 cm−1 and 50 °C under oxidizing conditions (Eh/NHE ≈ +150 mV) and under reducing conditions (Eh/NHE ≈ −250 mV). After 3 days of leaching the gel dissolution was highly incongruent. The gel dissolution rate calculated from the silicon concentrations was 4.4 × 10−5 g m−2 d−1, except for the Am-doped gel, for which the rate was two times higher. During leaching, Np is weakly retained in the gel (35% under oxidizing conditions and 50% under reducing conditions) whereas Pu and Am are strongly retained (over 90%). The three lanthanides La, Ce, and Nd exhibit exactly the same leaching behavior, but different from that of actinides. Speciation and complexation calculations for neodymium showed that its solubility could be controlled by Nd(OH)3 for periods beyond 3 months. Conversely, no simple chemical compound appears to control the solubility of the actinides.