Development of phenomena identification ranking table for LONF-ATWS sequences in a Westinghouse PWR

Abstract

Anticipated Transient Without Scram (ATWS) sequences belong to the beyond design basis events. Within ATWS sequences, the most limiting for PWRs is the Loss Of Normal Feedwater (LONF). This sequence produces a pressure peak in the reactor cooling system that can compromise its integrity.To cope with this sequence, systems like the AMSAC were installed in PWR Westinghouse reactors, helping to manage the transient. However, the consequences of this sequence are still strongly dependent on several variables such as the systems availability, reactor coolant system characteristics and the human actions performance.In order to study this sequence, a sensitivity analysis focused on the most influential parameters is performed. The parameter selection for these sensitivities is based on the literature found, and the analysis is made using a 3-Loop PWR-W TRACE model.Moreover, a proposal of a Phenomena Identification and Ranking Table (PIRT) for the ATWS-LONF sequence is developed. The sequence is analyzed for two different time phases (before and after the pressure peak), focusing on 6 different systems and assessing 20 phenomena. In order to rank the importance of the different phenomena, a review of the experiments, proceduralized human actions, PSA and DSA is performed, with the additional information obtained in the sensitivity analysis. The results show that the important phenomena are those related to reactivity feedback, the energy stored in the core, the AMSAC actuation, and the mass and energy release through pressurizer safety and relief valves.