Effect of Surface Characteristics of Zr-4 and Subcooling Degree of Coolant on Heat Transfer of Quenching Boiling

Abstract

During a postulated accident in water-cooled nuclear reactors, such as loss-of-coolant accident, the reactor core can be partially or completely uncovered from coolant and consequently becomes overheated. The cold liquid is reinjected into the nuclear reactor core to effectively cool the superheated fuel rods. In this process, the quenching boiling will be encountered in the fuel rod surface. The reflooding quenching boiling of nuclear fuel rod is an important accident mitigation measure to avoid the core melting after the loss-of-coolant accident of the core of pressurized water reactor. In this work, quenching experiments of Zr-4 alloy with different roughness were performed in subcooled distilled water pool at atmospheric pressure. The surface of Zr-4 was polished by abrasive paper with different mesh numbers. The effects of Zr-4 surface roughness and coolant subcooling degree on cooling rate and boiling heat transfer during the quenching boiling were studied. The visualization experiments of quenching boiling were carried out on different surface roughness test sections, and the internal temperatures of test sections were measured at the same time. The image processing technique was used to obtain the evolution of vapor film thickness during quenching boiling of fuel rod and the inverse heat conduction problem was employed to obtain the surface temperature and heat flux during the quenching. The experimental results show that the surface roughness has little effect on the film boiling heat transfer. However, the surface with higher roughness triggers the surface-liquid contact earlier and strengthens the quenching boiling. When the roughness is small, the effect of roughness on quenching boiling is weak. When the roughness is further reduced, the duration of quenching boiling increases due to the increase of the surface contact angle. In addition, with the increase of the liquid subcooling, the vapor thickness of film boiling decreases, the minimum film boiling temperature to maintain stable vapor film increases, and the quenching rate increases. Based on the experimental data, the relationship between the minimum film boiling temperature (Tmin) and the coolant subcooling degree (Tsub) is established with the average error of 2.4%. This study provides a theoretical basis for the boiling heat transfer mechanism during the reflooding process of nuclear reactor core at a loss-of-coolant accident.

Keywords

• quenching boiling
• roughness
• boiling heat transfer
• minimum film boiling temperature