LOCA analysis of high temperature reactor cooled and moderated by supercritical light water

Large break LOCA of a high temperature thermal reactor (SCLWR-H) with descending flow water rods is analyzed. The criterion of the Ni-alloy cladding temperature is set to 1260degC. The safety system of the SCLWR-H is similar to that of BWR. The safety principles of the SCLWR-H are (1) feedwater from cold-leg is kept' and (2) 'coolant outlet is opened at hot-leg'. To prevent closure of the hot-leg, automatic depressurization valves (ADS) are actuated without delay by the same signal as that of main steam isolation valves (MSIV) such as 'Flow rate low level 3 or 'Core pressure low level 2 or 'Containment pressure high', while in ABWR the ADS is actuated by 'Water level 1 and 'Containment pressure high' with 30 s delay. The low pressure core injection system (LPCI) is actuated by the same signal as that of the ADS with 30 s delay due to the starting of the emergency diesel generators. In the case of 100% cold-leg break LOCA, the core flow rate decreases and then the cladding temperature increases up to about 720degC during blowdown phase. After the ADS are opened, the core flow rate is recovered and the cladding temperature decreases. The core reflooding begins 78 s after the break. The peak cladding temperature in the reflooding phase is about 790degC. In the case of 100% hot-leg break LOCA, the core coolability is much higher than that in cold-leg break. The peak cladding temperature does not exceed that of the normal operation. In the reflooding phase, since the cold-leg loops are isolated by check valves, only upward flow in the core is established. It is a kind of forced cooling by the LPCI. The reflooding phase is also less severe than that of the cold-leg break LOCA. Thus, reflooding phase of hot-leg break LOCA is not calculated in this study. In summary, large break LOCA of the SCLWR-H is mitigated by MSIV, ADS and LPCI. The flow system of the 'descending flow water rods' makes the water inventory large and contributes to the high coolability. This is an advantage. But it should be noticed that the core coolability is small before the ADS are opened at cold-leg break LOCA. (author)