| 摘要 |
PROBLEM TO BE SOLVED: To suppress discharge of FP gas by increasing the grain size of crystal only at the outer circumferential part of a pellet thereby retarding microcrystallization while simultaneously decreasing the porosity. SOLUTION: Thickness C at the outer circumferential part 1 surrounding the core part 2 of a columnar pellet is set about 20μm or above, preferably in the range of 20-2500μm. The core part 2 has ordinary mean grain size of 10μm or less and the outer circumferential part 1 has grain size two times as large as that at the core part 2. More specifically, mean grain size at the outer circumferential part 1 is set at 20μm or above, preferably in the range of 50-200μm. Consequently, microcrystallization can be retarded at the rim part at the time of high specific burnup and discharge of FP gas can be suppressed by decreasing the porosity at the time of producing a pellet. The pellet is produced by sintering a green pellet, prepared by adding U3 O8 to UO2 powder, in oxidizing atmosphere having partial pressure of oxygen of 20ppm or above for 2-6 hours at 1000-1300 deg.C and then sintering the green pellet in reducing atmosphere for 1 hour or longer at 1000 deg.C or above.
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