A model for asymmetric ballooning and analyses of ballooning behaviour of single rods with probabilistic methods

Plastic deformation behaviour of zircaloy cladding is extensively examined during past and can be described best with a model for asymmetric deformation. Asymetric deformation behaviour of the cladding is explained by the fact, that slight displacement between the pellet and cladding will always exist and this will lead to the formation of azimuthal temperature differences. The ballooning process is strongly temperature dependent and as a result of the built up temperature differences, differing deformation behaviour along the circumference of the cladding results. Calculated ballooning of cladding is mainly influenced by its temperature, applied burst criterion and the parameters used in the deformation model. All of these influencing parameters possess uncertainties, leading ultimately to deviations and representation of the results by bands. In order to quantify these uncertainties and to estimate distribution functions of important parameters like temperature and deformation the Response Surface Method was applied. For a hot rod the calculated standard deviation of cladding temperature amounts to 50 K. From this high value the large influence of the external cooling conditions on the deformation and burst behaviour of cladding can be estimated. (orig.)