Full-core uncertainty analysis of nuclear fuel behavior

Fuel performance analysis can be used to show compliance with regulatory criteria, such as criteria related to fuel melting, rod internal pressure and cladding stresses and strains. A nuclear reactor core contains typically tens of thousands fuel rods in hundreds of fuel assemblies, and each rod experiences different generated power and coolant conditions during its irradiation. Therefore fuel performance analysis must be performed on each rod separately, resulting in tens of thousands of simulation runs. Additionally, the manufacturing parameters of the fuel rods, fuel performance code model parameters and even the determined power are fundamentally uncertain: they are not single, exact values but rather their true nature can be approximated with statistical distributions. Therefore also the results are distributions of values and not exact. Parameters of these distributions must be compared to regulatory criteria, and as a conservative approach some limiting values can be established which can be used to show whether a regulatory criteria is exceeded or not. A method widely in use in the nuclear industry is based on order statistics, and can be used to show that the 0.95-content tolerance interval with 95 % probability does not exceed the examined criterion. In this work a method applicable forfull-core fuel performance analysis is demonstrated with a calculation of a PWR 17x17 assembly. Fuel performance parameters such as maximum temperature, pressure and cladding stress are investigated.