Loss of coolant accident simulation with RELAP5/SCDAPSIM/MOD4 for the Canadian supercritical water-cooled reactor

RELAP5/SCDAPSIM/MOD4 has been improved to accommodate supercritical water properties and transcritical fluid transitions and is used for simulation of the Canadian Super Critical Water Reactor (SCWR). The system response to several postulated Loss of Coolant Accident (LOCA) transients are studied. For a 100% (single-ended) cold-leg break located in between the feedwater pump and the inlet plenum, the system pressure immediately drops followed closely by a flow reversal with a rapid break discharge. A brief power pulse (178%FP) is observed under this non-equilibrium depressurization scenario. The transient simulations show the potential for two sheath temperature maxima, one early in the transient as a result of the power pulse and the subsequent flow-power mismatch, and the other later peak resulting from the fuel heat-up under near stagnant channel flow conditions as the heat transfer regime changes to radiation dominated. The Automatic Depressurization System (ADS) mitigates the later fuel heat-up by introducing forward channel flows. This effect is enhanced by additional coolant supplied from Low Pressure Coolant Injection (part of the Emergency Core Cooling system). Under the 100% break LOCA with coincidental failure of the Emergency Core Cooling System (ECC) transient, the core inventory is depleted even faster after the break. The highest maximum cladding surface temperature of 1331 K is achieved approximately at 141s and meets the safety criterion (1533 K). Beyond this time the sheath temperatures gradually decrease by heat transport to the Passive Moderator Cooling System (PMCS). LOCAs initiated by a range of 5% to 100% cold-leg break sizes are simulated with coincidental loss of ECCS. In this specific design, break sizes less than 15% are defined as SBLOCAs and show an early pressurization up to the Safety Relief Valve (SRV) setpoint, while breaks larger than this threshold do not undergo a pressurization phase. During SBLOCAs, the first MCST peak is more limiting than the LBLOCA cases because of insufficient fuel cooling caused by lower reverse flows. However, these lower discharge flows prolong the period of blowdown cooling and hence mitigate the secondary MCST peak. The worst LOCA occurs in the 15% break case with a highest MCST of 1450 K. The results showed the most sensitive parameters are delays associated with SDS action, emissivity and ADS actuation parameters.