Study on dissolution behavior of plutonium from Pu-doped glass

In order to study the migration behavior of plutonium from high-level vitrified waste in a deep repository condition, plutonium doped simulated high-level vitrified waste was prepared in a laboratory. Pu-doped glass was prepared with 1 wt.% of PuO2. The simulated high-level vitrified waste was crushed to less than 75 μm and put into platinum crucible (φ 17mm x 80mm) with PuO2 powder sieved to less than 75 μm. The crucible was set into an electric furnace and the temperature was raised up to 1300degC and kept for 2 hours. The melt was cooled down after annealing at 650degC for 2 hours. The specimen was cut and homogeneity of Pu in Pu-doped glass was observed by α-autoradiography. As a result of observation, plutonium was distributed homogeneously in the middle of the glass, but was distributed heterogeneously in the bottom of glass. And then the whole of the glass was crushed and melted again in the same way. Leaching experiments of Pu-doped glass and solubility experiments of Pu were carried out under aerobic conditions at room temperature. Leaching experiments of Pu-doped glass were carried out by using batch method with NaNO3 solution. The particle size of glass powder was under 75 μm and pH were adjusted by HNO3 and NaOH in the pH range 4-10. The experimental period was 151-340 days. Solubility experiments of Pu were also carried out from over saturation in 0.1 M NaNO3 solution and pH were adjusted by HNO3 and NaOH in the pH range 4-12. The experimental period was 42-233 days. These leachate were filtered through a 10,000 molecular weight cut off (MWCO) ultrafilter to separate solution and solid. These experiments were carried out in the teflon vessel. The Pu concentration in leachate was analyzed by α-spectrometry. The Pu concentration from the Pu-doped glass was around 10-6 mol/l in the pH range 4-6, and decreased to 10-9 mol/l with increasing pH. The leaching experimental results indicated that the aqueous behavior of Pu from Pu-doped glass was controlled by Pu(OH)4(am) solubility in the pH range of 6-10. On the other hand, solubilities of Pu from over saturation decreased from 10-5 to 10-11 mol/l with increasing pH. These results were similar to solubilities of PuO2(c). Based on these experiments, it is suggested that initial aqueous behavior of Pu from vitrified waste would be controlled by the solubility of pure solid phases of plutonium in the pH range of 6-10 under the above mentioned experimental conditions. Experimental results were compared with geochemical calculation results using PNC-TDB. Calculated results shows that the leaching and solubility experimental results can be interpreted by using thermodynamic data of Pu(OH)4(am) and PuO2(c), respectively. (author)