Modelling of Rim-layer features in frames of START-3 code development

'Rim-effect' is a conventional term implying a number of microstructure and thermal physical phenomena, oriented from the periphery of a highly irradiated LWR fuel pellet, also called 'rim layer', 'rim', 'high burnup structure' (HBS) etc. One of the rim-effect consequences, important for fuel reliability, is evident intensification of fission gas release (FGR) and increase of rod internal pressure after certain threshold of fuel burnup. Results from post irradiation examinations (PIE) of WWER fuel demonstrate this tendency in a range of rod average burnup from 42-45 MWd/kgU to 60-65 Mwd/kgU. A recently developed dynamic model of FGR behavior based on analysis of a number of interrelated microstructure processes, differing by their rates and magnitudes and depending on external operational conditions, is briefly described in this paper. A verification of the model is performed using experimental data on matrix xenon EPMA and on fuel porosity. The verified model is integrated in the START-3 code and full-scale calculations of FGR are performed with the code. The developed model has demonstrated a good prediction for some important parameter of rim microstructure and for values of integral fission gas release in highly irradiated WWER fuel rods