Abstract
Quenching is critical for nuclear safety. After a loss of coolant accident, the emergency core cooling system in a nuclear power plant will drive water into the core to cool down the fuel rods at high temperature. The Fukusima accident renews the interest of studying quenching at high temperature. The objective of this study is to investigate the film boiling of two metallic spheres, stainless steel and zircaloy, in de-ionized water and sea water respectively. The diameters of the spheres are 17.5 mm. Each individual sphere is heated up to an initial temperature of ~ 1000 °C in a radiant furnace, and finally plunged into the quench pool by a connected pneumatic cylinder with pressurized air. The temperatures at four corners in the pool are measured with thermocouples. The forming of film boiling and its collapse is visualized and determined by a high-speed camera simultaneously with the temperature. In de-ionized water, with the occurrence of film boiling, the heat transfer measurement is poor as expected, and temperature corresponding to the minimum heat flux is so-called Leidenfrost temperature. The duration of film boiling has an average of 15.3 seconds for the stainless steel and 8.3 seconds for the zircaloy, with high repeatability and reproducibility. The differences of quenching period are mainly due to the diversity of heat capability and metallic density, and such differences also lead to a higher critical heat flux of stainless steel sphere. On the other hand, the average Leidenforst temperature of stainless steel ball is 598℃, and 678℃of zircaloy. The images from high speed camera clearly determinate the forming and the collapsing of the vapor film and the comparison are shown by the graphs. On the other hand, quenching in sea water shows a totally different result for both metallic shperes. Through the observation of the high speed camara, film boiling collapses within 0.1 second for both spheres and demonstrates a relatively high Leidenfrost temperature and critical heat flux. Because of the high concentration of salt in sea water, surface of the sphere react with the surrounding coolant vigorously, and thus alterd the surface condition into a rough and more hydrophilic state. The increase of the surface roughness and wettability enhances the heat transfer capability and results in a higher critical heat flux to shorten the cooling time. This could be a great benefit by using sea water as the coolant in nuclear power plant during a very serious situation, such as that occurred at Fukushima Daiichi nuclear power plant. The study also conveys the effect of subcooling of the coolant on the Leidenfrost temperature and the duration of film boiling. The increment of subcooling tends to shorten the duration and raise up the Leidenfrost temperature. Furthermore, non-condensable gas bubble were found in the quenching of zircaloy, as long as the surface temperature was high enough. It is very possible that such gas is hydrogen.