Finite Element Modelling Used to Clarify Separate Effects

1D and 1.5D fuel behaviour codes include either mechanistic or empirical models for the physical phenomena occurring in the fuel rods. Since it is in the nature of integral tests and of real-life situations (e.g. licensing) not to provide detailed, local pre-characterisation of the rods, some sort of statistical averaging is inherently included in the modelling of the processes. Often applied averaging leads to assumptions of e.g. • uniformity, • homogeneity, • axisymmetry. Finite element codes make it possible to account for 3D and local phenomena, e.g. • PCMI including the effect of cracks in the pellets, • PCI with a mixture of boded cladding areas. Moreover, the basic assumption on the homogeneity and uniformity of the cladding can be lifted and the effect of inhomogeneities and slight variation in thickness can be studied in e.g. LOCA conditions. MTA EK has started research to study such local phenomena in order to better understand and reproduce experimental data. The results are promising: the stress distribution in a cladding with bonded and unbonded areas differs significantly from the azimuthally symmetric case. The shape of the ballooned area of a fuel rod subjected to LOCA can only be reproduced if the above averaging assumptions are lifted, which leads to a new, second order approach. (author)