An experimental study of the bundle critical heat flux at near-critical pressures

The supercritical-cooled-reactors currently being developed adopt a once-through direct cycle as a heat removal system. In the once-through direct cycle, the Critical Heat Flux (CHF) must occur at some location during the reactor start-up and the associated temperature increase must be considered in the design. Because the CHF decreases fast near the critical pressure region, information on the CHF in this region is very useful for the protection of the nuclear fuel from melting. CHF experiments are carried out for the 5x5 bundle geometry at the Freon R-134a Thermal Hydraulic Experimental Loop developed at KAERI. The experiments were performed in the range of pressure of 3200-4030kPa (Critical pressure of R-134a=4059kPa), the mass flux of 150-1500kg/m2s, and the inlet subcooling 40-70kJ/kg. We observed that the CHF decreased very fast with the system pressure near the critical pressure region. At the supercritical pressures, the CHF phenomenon disappears; the coolant temperature is raised without a phase transition and the wall temperature of the heater rod is increased linearly with power. In the bundle experiments, there exists a threshold pressure that limits the CHF phenomenon. Over the threshold pressure, the behaviour of the wall temperature of the heater rod resembles the behaviour in the supercritical pressure condition. (author)