Experimental Investigation of Coolability Degradation by Fission Gas Release into Flowing Sodium in a Fuel Pin Bundle

A series of experiments was performed to assess the thermal effect of a burst-type fission gas release from fuel pins. Simulated fission product gas was injected continuously and transiently from the central pin of a 37-pin bundle. The opposite pin surface impinged on by the released gas showed an extreme temperature rise under high coolant-flow conditions. Comparison of measured temperature change data with analytical results by a simple computer code revealed that the ratios of the heat transfer coefficient after gas injection to those of sodium single-phase flow were in the range of 0.05 to 0.15, irrespective of the magnitude of the gas plenum pressure and the nozzle diameter. The estimated pin-surface temperature increased by gas release in actual reactor operating conditions was less than the saturation temperature of sodium. The measured pressure pulse at the transient gas release was <0.2 times the initial gas plenum pressure.