The structure of fuel element codes

Abstract

Various models appropriate to the varying degrees of theoretical sophistication have been developed in fuel element structural analysis. Establishing these code systems required a tremendous amount of work relating to the mathematical/mechanical description to be done, and it seems that the standard of analytical modelling enables us to understand most of the intrinsic physical fuel rod phenomena. Nevertheless, comparisons of theory and experiment or comparisons between several codes often show rather poor results. Moreover the interaction of complex material behavior and complex structural analysis makes it very difficult to identify the real sources of discrepancies. In most cases a few experiments with very specific objectives were used to calibrate the codes against experiments. But in general a limited data base can be understood quite satisfactorily even by contradictory models and, consequently, wide scattering results if models are applied to new situations. In the present paper the basic structure of fuel element codes is discussed. The concept common to all fuel element structural analyses led to a general program structure which is presented in detail. The mechanical concepts of integral fuel element codes are reviewed, with special emphasis put on the modeling of pellet/cladding interaction and axial coupling. An attempt is made to classify material data and models, and the state of the art of accident analysis by means of fuel element codes is outlined. The objective of this contribution is a completion of code ranking already carried out in the past. In contrary to code ranking where inevitably subjective criteria have to be used besides objective criteria, a classification of fuel element problems enables the community of fuel element structural analysts to take advantage of a much larger data base and to harmonize physical models or, perhaps, fuel element modeling in general.