Effects of fuel rod uncertainty on the LBLOCA safety analysis with limiting fuel burnup change

Effects of fuel rod uncertainty to the PCT were assessed as a limiting fuel burnup for LBLOCA analysis changed from 0.5 MWd/kgU to 30 MWd/kgU. For this, total 36 uncertainty parameters that were related to the manufacturing, model and power were taken into account. Studies of sensitivity as well as combined uncertainty were carried out by utilizing FRAPCON-3.4/FRAPTRAN-1.4 fuel codes with a statistical treatment. Analysis results revealed that the important uncertainty parameters to the PCT were varied with changing the limiting fuel burnup. At the fuel burnup of 0.5 MWd/kgU, the parameters related to the gap width induced strong influence. But, at the fuel burnup of 30 MWd/kgU, burnup related parameters appeared as a dominant factor. As expected, fuel thermal conductivity resulted in a dominant impact regardless of fuel burnup. Results of combined uncertainty showed that with increasing the fuel burnup not only the PCT of base case but also the third highest PCT among 124 PCTs increased altogether, and the effects of combined uncertainty were strengthened. The importance of sampling probability in the statistical treatment was discussed also.