Experimental investigation on transient boiling heat transfer during quenching of fuel cladding surfaces

Abstract

In case of postulated accidents in a nuclear reactor, the fuel rods uncovered from coolant can be overheated. Immediate quenching of overheated fuel rods is desired. Quenching experiment is performed at atmospheric pressure with rodlets of the FeCrAl alloy, one of the accident tolerant fuel cladding materials, and the Zircaloy-4, the conventional cladding material. In the experiment 600 °C rodlet specimens are plunged into the subcooled water bath. A high-speed camera is employed to record quenching phenomenon. With the embedded thermocouples the rodlet temperature is measured at the frequency of 50 Hz. One-dimensional inverse heat conduction problem is solved to obtain the surface temperature and heat flux. Influence of surface condition, solid thermal properties and liquid subcooling on transient boiling heat transfer is investigated. In the film boiling regime heat transfer is mainly affected by solid thermal properties and liquid subcooling. Small solid (kρCp)w and large liquid subcooling results in relatively thin vapor film and more efficient film boiling heat transfer. Roughness on the polished level shows no appreciable influence on the minimum film boiling temperature, TMFB, which increases with decreasing contact angle. Surface oxidation is slow on the FeCrAl surface and hence, does not significantly affect the boiling heat transfer. However, on the Zircaloy-4 surface the oxide layer with small (ρkCp)w increases TMFB and decreases the critical heat flux. Large (ρCp)w results in slow quench front propagation, long quench duration and weak subcooling effect. It is found that on each surface the vapor film collapses at the critical thickness which is irrelevant to liquid subcooling. Based on the critical thickness, a new approach for prediction of TMFB is proposed.