The thermal hydraulics of SBLOCAs relative to pressurized thermal shock

The US Nuclear Regulatory Commission (NRC) Pressurized Thermal Shock (PTS) study had previously identified Small Break Loss of Coolant Accidents (SBLOCAs) as a risk dominate accident scenario due to (numerically calculated) primary loop flow stagnation at high pressure. The objectives of the present effort were two-fold. First to develop a physically-based understanding of controlling thermal-hydraulic phenomena producing such PTS SBLOCA stagnation scenarios. Secondly, to use these insights in developing a simple (computationally efficient) ''mapping'' tool to quantify the occurrence and thermal behavior of such high pressure flow stagnation regimes. Review of the previous (TRAC) calculations revealed that inaccurate modeling of vapor condensation erroneoudly produced flow stagnation and hence overly conservative rapid vessel cooldown rates. Using a corrected version of this code, our new calculations exhibit loop flow. Parametric analysis of less likely (more equipment failure-PORVs/HPI pumps) scenarios revealed that flow stagnation was indeed possible but could only occur at lower pressures. This sample mapping procedure has been favorably benchmarked against the (TRAC) system calculations. This tool is therefore useful for screening possible risk dominant SBLOCA scenarios in various PWR design. 9 refs., 32 figs., 3 tabs