Counterpart test for LSTF SBLOCA at the RPV upper head using ATLAS test facility

This study presents a comprehensive analysis of the OECD-ATLAS2 B5.1 test, the counterpart for the LSTF SB-PV-07 test. This test simulates a 1% small-break loss-of-coolant accident (SBLOCA) with the break at the reactor pressure vessel (RPV) upper head. The transient scenario considered a total failure of the high-pressure injection (HPI) system. But it incorporated the accident management (AM) actions to mitigate the accident and considered the inflow of non-condensable nitrogen gas from the accumulators following coolant depletion of accumulator (ACC). This test scenario is distinguished by its complex coolant distribution and depressurization characteristics compared to typical cold-leg break accident. This test was designed to investigate the thermal–hydraulic response of the ATLAS (Advanced Thermal-hydraulic Test Loop for Accident Simulation) facility compared to the LSTF test result. In addition to that, the effectiveness of the accident management actions considered in this test scenario was another objective of this study. From the test results, the ATLAS facility successfully reproduced the essential thermal–hydraulic phenomena that can be expected in the SBLOCA conditions including loop seal clearing (LSC) and core heat-up even though several differences were found with the LSTF test result. Critically, the study demonstrated that manual accident management actions − comprising manual HPI actuation and rapid secondary-side depressurization − effectively mitigated the accident progression and cooled down the system successfully even under the influence of non-condensable gases.