Parametric study with COMETHE

The objective of this parametric study is two-fold: (1) to analyse the basic behaviour of LWR rods during steady state operation and power ramping, by separating the effects of various parameters; (2) to generate basic functions to be used in a simpler calculation model to simulate the interaction between fuel and cladding at different power levels and to evaluate the margin to failure. In a first attempt this interaction was assumed to depend on the difference between the actual power density and an interaction threshold. It appeared quickly that it is not possible to analyse the mechanics of a fuel rod ignoring the thermal aspects. Fission gas release is a very sensitive mechanism which can increase rapidly. Its thermal effects are very important for thermal expansion and fuel gaseous swelling. It follows that there is no simple relation between fuel temperature and power density. The threshold power has therefore been replaced by the gap as a basic variable for the calculation of the pellet-cladding mechanical interaction. The present study shows an illustration of this new method applied to the analysis of different LWR fuels. (author)