Analysis of nitrogen intrusion from ECCS accumulators in SBLOCA scenarios at LSTF test facility

Abstract

After the injection of borated water during accidental sequences, the intrusion of nitrogen gas from the accumulators once emptied can degrade heat transfer mechanisms and thus hinder core cooling. The non-condensable gas that enters the cold legs transfers and accumulates in the U-tubes of the steam generators and their outlet plenums. This can cause flow stagnation and even prevent reflux cooling. Furthermore, a temporary and fluctuating increase in primary pressure may occur, disabling the low-pressure injection system pumps to restore the inventory. In this scenario, it is essential to confirm the effectiveness of the accident management measures under the influence of non-condensable gas. To date, several experimental studies have been conducted within the framework of OECD/NEA programs at experimental facilities such as LSTF, PKL, or ATLAS (all related to PWR-type designs) where the issue of non-condensable gas intrusion has been studied, but always as a secondary objective of the analyzed sequences. In this work, three SBLOCA in the reactor pressure vessel of the LSTF facility are analyzed to determine the effect of nitrogen intrusion in the primary system. For calculations, the thermal-hydraulic code TRACE5 patch 5 is used. Results include the evolution of the main thermal-hydraulic parameters (pressures, mass flow rates and temperatures) and sensitivity cases for different scenarios of nitrogen intrusion. Based on the analysis of three experiments, it can be concluded that while nitrogen accumulation in the U-tubes is a common effect, the activation of safety systems such as LPIS depends on the location, size of the break and action management measures.