COCOSYS code validation against measured data during unintended single main safety valve opening at Ignalina NPP

Abstract The analysis of an unintended main safety valve opening at Ignalina NPP was performed with COCOSYS code in order to assess its capability in simulation of the transient processes that occur inside Accident Localisation System of Ignalina NPP. COCOSYS has several user-selected options, e.g. zone model (EQUIL._MOD, NONEQUILIB), water flow model (BAL_DRAIN, DRAIN_BOT), etc for nodalisation development. The influence of a zone model selection, a water overflow model selection and efficiency of heat exchanger in Condenser Tray Cooling System was investigated and presented in the paper. The performed analysis supported introduction of new water overflow model in COCOSYS code and showed that COCOSYS code can be applied for the analysis of Accident Localisation System of Ignalina NPP.