High power natural circulation SBLOCA transient in LSTF (ROSA V): Simulation with TRACE5.0

Failure of scram in Small Break Loss-of-Coolant accident (SBLOCA) scenarios produced in pressurized water reactors (PWR) has been proved to cause high core power for a long time and thermal-hydraulic response that hinders core cooling, gradually decreasing the primary coolant mass inventory. In such scenarios, there is a chance of natural circulation to occur, meeting different conditions as high core power with supercritical flow in the hot legs and counter-current flow limiting (CCFL) at the inlet of the steam generator (SG) U-tubes that holds a large amount of coolant in the tubes. In such context, detailed study of the transition between subcritical and supercritical flows is crucial for reactor safety analysis, being the Froude number (a dimensionless value that describes different flow regimes of open channel flow) an essential parameter to consider. In this study, developed within the OECD/NEA ROSA Project Test 3-1 (SB-CL-38 in JAEA), a cold leg Small Break Loss-Of-Coolant Accident (SBLOCA) under the assumption of total failure of high pressure injection system has been studied in the LSTF (Large Scale Test Facility) and simulated via the thermal-hydraulic code TRACE5. The test reproduces PWR high-power natural circulation due to failure of scram during cold leg SBLOCA with a break size equivalent to 1% of the cold leg, under assumption of total failure of high injection system. TRACE5 model of the plant has been applied, and the chronology of events set to imitate test conditions. Different thermal-hydraulic models have been tested in order to fully comprehend the high-power natural circulation phenomenology, such as the reflood, the CCFL or the critical flux model. Special attention is devoted to Froude number estimation in hot legs due to the prominence of the transition between subcritical and supercritical flows in high power natural circulation phenomenology. Assessment of TRACE5 leads to the conclusion that the code can successfully reproduce complicated conditions of High Power Natural Circulation, during a cold leg SBLOCA. Deviations from the experimental results are seen during the accumulator activation. Despite this fact, TRACE5 offers a good predictability in this type of accident scenarios. (authors)