KESS—a modular program system to simulate and analyze core melt accidents in light water reactors Original Research Article

The research and development work program performed at the Institute for Nuclear Technology and Energy Systems and related to nuclear reactor safety includes, amongst other things, the development of detailed, mechanistic models for the analysis of severe accidents in light water reactors (LWRs). It is intended to describe the most relevant physical and chemical processes likely to occur in the reactor core, during heatup and degradation, degraded core behavior and fission product release. The models have been developed mainly as independent modules and are implemented into the KESS code system by means of the RSYST informatic structure, RSYST provides module sequences, initial and boundary conditions for the simulation, handles data interfaces between modules and includes pre- and postprocessing tools. Works has been focused on competition of model development and partial verification for early-phase accident phenomena and selected late-phase processes, such as debris bed behavior. The code version kess-III/mod1.1 has been applied to analyze selected integral experiments and to simulate reactor core behavior for specific pressurised water reactor (PWR) accident sequences. Selected kess modules have also been implemented into the system code athlet-cd (GRS, Germany) and the ester code package (European development). kess will be further extended in national and European research programs, including e.g. improvement of existing early-phase models and new model development for late-phase phenomena