Reassessment of the Importance of the Passive Equipment Availability in the NPP Accident Progression

Abstract

The paper presents the further results of the study for reassessment of the importance of the safety systems availability in the accident progression for Nuclear Power Plant (NPP), WWER-1000/V320 design. An Initiating Event (IE) for a Large Break Loss of Coolant Accident (LB LOCA) – 300mm equivalent diameter as an example is selected. Based on the Level 1 Probabilistic Safety Assessment (PSA) results, the criteria for successful recovery of this IE is actuation of 2 of 4 Hydro-accumulators (HAs - Passive Emergency Core Cooling System YT) without operator action. Five calculations are performed with the assumption of actuation of 2 of 4 HAs investigating all possible variations of injection into the reactor pressure vessel (RPV-YC). An additional calculation with actuation of all four HAs is run for comparison. All cases are performed with the assumption of loss of all Alternating Currency and Direct Currency (AC and DC) power simultaneously with the IE of LB LOCA to investigate the core degradation. The purpose of the calculations is towards two directions. One is for the verification of the success criterion (actuation of 2 of 4 HAs) to accomplish the safety function ensuring the core cooling (F3-2) for very early accident phase. The other purpose is an investigation the importance of the place of injection and the number of actuated HAs onto the delay of the RPV failure. The results could be used in several directions: in PSA, in the analytical validation of Symptom-Based Emergency Operating Procedures (SB EOPs) and Severe Accident Management Guidelines (SAMGs) for WWER-1000/V320 reactors, in the operator training for successful management of accident progressions.