Plutonium solubility and self-irradiation effects in borosilicate glass

摘要

This article presents the limitations for the immobilization of plutonium in borosilicate glasses. A first one is related to the solubility of this element in glass. The effects of the temperature and redox conditions during glass processing were studied. Glass specimens containing plutonium and plutonium surrogates are fabricated. The results show that trivalent elements (La, Gd, Nd, etc.) exhibit greater solubility than tetravalent elements (Pu, Th, Hf). Fabricating the plutonium-doped glass samples under reductive conditions reduced the Pu to trivalent oxidation state and increased its solubility to 4 wt% PuO_2 without reaching the solubility limit. A structural approach based on the results of EXAFS and NMR spectroscopy suggests that the structural role of the trivalent and tetravalent elements corresponds to that of intermediate network modifiers and intermediate network formers, respectively. The second factor is the effect of actinide decay on the long-term behavior of the glass. Borosilicate glass samples were doped with different curium contents (0.05, 0.5, 1.5 and 4.1 wt% of CmO_2). The macroscopic properties (density, microhardness and initial dissolution rate) of the glasses were characterized up to 4 × 10~(18) αg~(-1). No significant effect on the initial alteration rate was detected. The glass swelled slightly, saturating at about 0.5% after receiving a dose of about 2 × 10~(18) αg~(-1). Further studies are ongoing to confirm the satisfactory long-term behavior of the borosilicate glass matrix at higher doses, and to determine the solubility limit of plutonium in reducing conditions.