Analysis of operator actions during the accident caused by total loss of secondary heat sink

The accident with total loss of secondary heat sink was analyzed for the Krško nuclear power plant using the relap5;mod2 computer code and considering the emergency operating procedures foreseen by operator actions. Particular attention was given to the phenomenological description of thermal-hydraulic processes which may take place in the reactor coolant system during the accident. The investigation aimed to evaluate the minimal success criteria for various availabilities of plant safety systems and operator response times. The longest allowed operator response time for successful accident management was shown to be strongly dependent on the initial event. Among potential initial events, two secondary side originating events were demonstrated to be the boundary cases; the turbine isolation results in the longest allowed operator response time and the loss of main feedwater in the shortest allowed operator response time.