Modelling and code development for improved description of fission product and aerosol release in the reactor core of a LWR during core heatup and core degradation phase. Final report

In the frame of the terminated CEC research activity the FIPREM code has been further developed and improved at the ''Forschungsinstitut fuer Kerntechnik und Energiewandlung (KE) e.V.'' FIPREM serves to analyze the physical and chemical phenomena associated with fission product release from PWR cores in severe accedent situations. To the extent possible the FIPREM models hae been realized on a modular basis. They are integrated into the KESS-III code system for the description of LWR in-vessel processes, such as fuel rod a structure heatup and degradation as well as core thermonydraulics. Work was focused not only on modelling aspects of fission product release and chemistry, but also on the realization of interface requirements between the different fission product models and their adequate link with core/bundle behaviour models. In addition, the general KESS informatic structure (RSYST library and tools) was completely adopted, thus allowing calculations with the coupled KESS-FIPREM code system as well as stand-alone tests of the respective fission product release modules, but with the advantage of a full provision of input data, boundary conditions and post processing tools. Scoping calculations of fission product release and chemistry have been performed for PWR reactor cores and bundle tests, namely Phebus FP, identifying sensitive parameters in the models. Up to now no fully conclusive picture exists with respect to the comparison of release models with different approaches (e.g. empirical versus diffusion models) or even between models with the same level of detail but different solution techniques (e.g. analytical versus numerical solution). Further investigations are required to compare the model equations and features, their relevant physico-chemcal properties and parameters, e.g. in the frame of European or international benchmark exercises. (orig.)