Assessment of ASYST and RELAP5 for predicting stability and limit cycles of a low-pressure natural circulation loop with flashing instability

Two thermal hydraulics system codes, ASYST VER3 and RELAP5 MOD3.3, are assessed for their prediction of stability and limit cycles of low-pressure natural circulation with potential flashing instability. The benchmark conditions are from a nearly five-meter-tall water loop whose experiments have generated a published dataset covering both stability and limit-cycle oscillations. These conditions are simulated by a model of the entire system, and the target asymptotic behaviors are approached through sufficiently long transients under prescribed operational settings. ASYST is found not conservative for stability prediction, in the sense that the unstable operational range is underpredicted with discrepancy mainly in the high-subcooling stability boundary. Qualitative flow changes across the island of instability are however simulated satisfactorily, and for conditions correctly predicted as unstable, reasonable prediction is achieved for the oscillation period, time-averaged flow rate, and peak flow rate. Further comparison against experimental void fraction confirms that ASYST simulates physical two-phase behaviors in the limit cycles. RELAP5 predicts a much more stable system whose island of instability is significantly smaller than that of reality. Its simulated flow oscillations also noticeably deviate from the typical patterns of flashing instability, resembling sinusoidal waves following periods different from the experimental measurement. Underprediction of flashing and overproduction of subcooled boiling are identified as potential major defects in RELAP5, which calls for future model calibration and further investigation. In general, the current assessment contributes to the awareness of potential uncertainties when adopting ASYST and RELAP5 for predicting flashing instability and its induced transients.