Recycling process of defective aged uranium dioxide pellets

The work was devoted to study the recycling process of the unirradiated defective uranium dioxide pellets stored during 10 years. The effect of the temperature on the kinetics of the oxidation and reduction of dioxide uranium was investigated, respectively, in the range 250 °C–600°C and 300 °C–800 °C. For the oxidation process, the kinetics is low below 350 °C, and is fast at higher temperatures. The same phenomenon is observed for the reduction process, where the rate accelerates at above 500 °C and the reduction completed in shorter time. Using a laser particle size analyzer and a Brunauer, Emmett, Teller (BET) analyzer, it was determined that the oxidation at 400 °C gives a triuranium octoxide powder with an adequate particle size (33 μm) and a specific surface area of 0.9 m²/g in reasonable time (210 min). Reducing the triuranium octoxide to uranium dioxide powder at 600 °C in pure hydrogen was completely achieved after only 16 minutes, without affecting its characteristics. To ameliorate the specific surface area, several oxidation–reduction cycles were performed on the obtained uranium dioxide powder. It is found that after five cycles, the specific surface area of uranium dioxide was improved to more than 2.5 m²/g, minimum value required to the powder sinterability.