Metallographic Examination of 3 × 3-2 Fuel Elements With Higher Burnup

The metallographic examination of 3 × 3-2 fuel elements with the maximal burnup of 30 917 MW· d/t is carried out. The results show that the external zirconia of the fuel cladding is extracted seriously, the maximal thickness of the internal zirconia reaches 19. 46 μm; the mean gap between the pellet and the cladding is 24. 93 μm; the maximal picking-up of hydrogen is 190 μg/g; the mean grain size of the pellet is about 15. 65 μm, without the columnar crystals; the percentage of the pore is about 5. 52%, which includes 29. 86% of the pore with dimension less than 5μm. It demonstrates that the fuel element is still safe under this level of burnup.