Behavior of UO2 in the RBMK-1000 spent fuel under oxidizing conditions

The oxidation of UO2-type spent nuclear fuel (SNF) in gaseous media was studied at different temperatures and oxygen contents using gravimetric and powder X-ray diffraction (XRD) techniques. The aim of the study was to determine the mechanism(s) of thermal-oxidation alteration of SNF during long-term dry storage. The samples used in the experiments were chips of RBMK-1000 fuel rods. Oxidation of UO2 with a mean bum-up of 10.7 and 19.73 MW d/kg in humid air was observed at a temperature as low as 150degreesC. At 200degreesC nearly all of the UO2 was transformed into U3O8 between 3500-4000 hours. In a humid nitrogen environment containing of 0.65-1.3 vol. % oxygen at 300degreesC, the UO2 completely transformed to U3O8 between 2500-3000 hours. Oxidation of UO2 in samples with small amounts of jacket damage (e.g., <0.04 mm(2)) progresses more slowly and after similar to3000 hours the oxygen-to-uranium ratio was 2.56. Stabilization of the oxidation process was not observed in the fuel samples up to an O/U ratio of 2.4, which may be attributed to the small bum-up of the fuel under investigation.