High-level radioactive waste generated in the reprocessing of spent fuel is melted together with glass beads and disposed of as vitrified product. The molybdenum (Mo) in the waste has a low solubility in borosilicate melt, so separation of Mo phase and borosilicate melt occurs during the melting. This Mo phase is called yellow phase. Because the yellow phase incorporates radioactive elements and is water soluble, it can significantly reduce the chemical durability of waste form. Various glass compositions have been studied to increase chemical durability and MoO3 solubility in glass. In this study, we investigated effect of V2O5 on the MoO3 solubility and chemical durability of the glass in the system SiO2-B2O3-Al2O3-ZnO-CaO-Na2O-Li2O.
For this work, three glasses (Table 1) were synthesized by melt quenching method. To ensure phase separation, an excess amount of MoO3 (13 mol%) was added to all samples. The 19A2 is made so that the composition of silicate glass after phase separation is close to that currently assumed to be used in Japan. The 21E and 21E2 are the composition of which V2O5 and V2O5+B2O3 are added to the 19A2, respectively. The mixed reagent was placed in a platinum crucible, held at 1200℃ or 1000℃ for 24 hours and then quenched by water. The collected samples were observed and analyzed by using EPMA, ICP-AES and XRF.
In all samples, borosilicate glass and MoO3-rich white precipitate formed by phase separation were observed. Table 2 shows the analytical results of the borosilicate glasses. At 1200℃, MoO3 solubility of 19A2 was 4.3 mol%, while 21E2 (adding V2O5 and B2O3) was 8.6% and 21E (adding V2O5) glass was 5.6%. MoO3 solubility was also increased in the V2O5 containing glass at 1000℃.
Chemical durability of glasses with the same composition as Table 2 and simulated waste glass with simulated waste components (Bead glass) were examined by MCC3 method. Then, 2mol% of MoO3 were added in all glasses (Table 3). To the leachate, 1 L of ultrapure water was adjusted to pH 9 (at room temperature) using KOH, and 0.235 g of glass powder with a particle size of 40-75 μm was added. Surface area (SA) of the sample was determined using BET nitrogen adsorption. The SA/V ratio was 10 m-1. The experimental temperature was 90℃. Leachate was collected after 2, 4, 6, 8, 24, 30, and 48 hours. Leachate concentrations were measured by ICP-AES, and normalized leaching rates were determined.
Figure 1 shows results of normalized leaching rate of boron as a function of time. The leaching rate of V2O5 containing glasses (21E and 21E2) were higher than 19A2 and bead. The bead glass had the slowest leaching rate in the early stage, but there was a reversal after one day, the result after two days test showing the highest leaching rate.
In this study, we have shown that the V2O5 can effectively improve the MoO3 solubility, but decrease the chemical durability of borosilicate glass. The chemical durability is considered to have decreased due to the change in the ratio of network forming components. Leaching behavior of the bead glass is clearly different from other glasses, suggesting that the waste components have a significant impact on glass corrosion and formation of gel layer.