Development of the methodology of the safety analysis performed by the coupled Kiko-3D/Athlet code system in VVER-440 type NPP

In the deterministic safety analysis, codes are required in order to provide evaluations of potential nuclear power plant accidents. In the fields of the core transient behaviour, the computer codes have achieved a high degree of realistic modelling. Nevertheless, some further tools for the investigations of the wide range of physical phenomena in the whole plant transient, such as modeling the ex-core detector signals and the malfunctioning of the emergency control system are unavoidable, too. The main objective of this work is to show the status of the code package based on the best estimate coupled code system Athlet/Kiko-3D and to present a safety analysis. The programs and methods used in KFKI-AEKI for safety analysis of VVER-440 NPP are presented. The accident analysis methodology for a boron dilution scenario, in which an inactive coolant loop is started, is shown. The cooling and the strong dilution increase the reactivity resulting in increasing power level especially in the affected sector. Due to the use of time dependent signals of the ex-core detectors the SCRAM (emergency shutdown) is delayed. Investigating the DNBR (burnout ratio) value by the TRABCO code, no dangerous hot spot was found