The facile, rapid, and complete reductive dissolution of CeO2 and ceria-based oxides, known to be highly refractive materials towards dissolution, is reported for the first time in very mild conditions.

PuO2 is considered an important material for current and future nuclear fuel; however it is a very refractive compound towards dissolution. Among other techniques, its reprocessing can be performed via complexing dissolution in concentrated and boiling nitric acid containing hydrofluoric acid, or via oxidant dissolution in the presence of reagents with redox couples having high potentials such as Ce(iv)/Ce(iii), or Ag(ii)/Ag(i). Reductive dissolution can be performed under softer conditions and is considered an alternative to these methods which may suffer from several drawbacks (corrosion, effluent management, compatibility with nuclear waste disposal, etc.). In this study, a sonochemical and reductive approach is investigated for PuO2 dissolution under relatively mild conditions. At the first stage, the experiments are performed with CeO2 as an inactive surrogate for PuO2. The quantitative dissolution of both oxides can be achieved under ultrasound (20 kHz, 0.35-0.70 W mL(-1)) in 0.5 M HNO3/0.1 M [N2H5NO3]/2 M HCOOH sparged with Ar at 33-35 °C in the presence of Ti particles as a generating source of reductive species. Ultrasound enables the depassivation of the Ti surface (usually strongly passivated in nitric solutions) through acoustic cavitation which then allows further generation of the intermediate Ti(iii) reductive species. Dissolution rates and yields can be further increased with the injection of dilute fluoride aliquots (NH4F or HF) in the sonicated solution to favor Ti chemical depassivation. The rapid and complete dissolution of PuO2 under selected conditions is accompanied by Pu(iii) accumulation in solution.

The reductive dissolution of cerium–lanthanide mixed oxides was investigated under mild conditions in weakly acid media. The dissolution is catalyzed by platinum nanoparticles (Pt NPs) deposited onto oxide surfaces. The deposition of Pt NPs was performed by sonochemical reduction of Pt(IV). The dissolution efficiency was studied as a function of different parameters such as stirring, sonication at 20kHz, dissolution media and oxide composition. The quantitative dissolution of these oxides can be performed in 0.25M HNO3–1M HCOOH–0.2M [N2H5][NO3] and 0.125M H2SO4–1M HCOOH under stirring at 40°C in the presence of argon, or in 0.125M H2SO4 under sonication in the presence of Ar–CO gas mixture at 40°C. In nitric acid solutions mechanical stirring is more efficient than sonication due to the secondary denitration triggered by ultrasound. The incorporation of trivalent (Gd, Nd) or tetravalent (Tb) lanthanides into the ceria matrix resulted in faster dissolution. After dissolution, lanthanides and platinum can be separated by simple filtration. The catalytic dissolution of ceria-based mixed oxides offers a simple way of lanthanide and noble metal partitioning which could appear of interest for catalyst recycling.

Ten borosilicate glass compositions consisting of a ternary sodium borosilicate containing increasing numbers of some of the key elements (Al, Ca, Zr, Ce) present in nuclear glasses were leached in pure water at 90°C and monitored for up to 14years. They were then characterized to establish correlations between the glass composition and the short- and long-term alteration rates. We first qualitatively explain the variations of the initial dissolution rate by structural considerations. Then we evidence a qualitative inverse correlation between the initial and residual rates. This counterintuitive result is in fact related to the effect of gel reorganization on the diffusive properties of the passivating layer. Since no equilibrium can be reached between glass and solution, these long-term experiments help in understanding how glasses behave once the solution is saturated with respect to the main glass formers. Very efficient synergy between Ca and hardener elements (Al or Zr) leads to the lowest residual rates, compared with glasses having only one of the two categories of elements. We also confirm the detrimental effect of precipitation of silicate minerals on the residual rate.