Experimental Investigation On Boiling Heat Transfer On High-temperature Accident Tolerant Fuel Cladding Surface

In the case of large break loss of coolant accident in pressurized water reactors,the reactor core can be uncovered,and the emergency coolant is injected from the bottom into the core in order to prevent overheating of the fuel rods.During the core reflooding stage,a stable vapor layer is formed due to the high temperature of cladding surface,and film boiling occurs.Generally,when the cladding surface temperature is decreased below minimum film boiling temperature,the coolant come into contact with the heated surface.The heat transfer regime is transformed from film boiling to transition boiling,which significantly enhances the boiling heat transfer.The quench rate of cladding surface is dependant on the rewetting of high temperature surface.Downstream of the quench front,the impact of entrained droplets on heated surface helps heat removal from the bare part and therefore reduce the cladding peak temperature.Accident tolerant fuel(ATF)cladding exhibits excellent high-temperature oxidation resistance,which can reduce the hydrogen generation and energy release during the reaction between cladding and cooling water under high-temperature conditions,and enhance the accident tolerance of reactor.However,research on the effect of ATF cladding on heat transfer performance under accident conditions is still in the initial stage.The experimental investigation on the heat transfer on ATF cladding surface,including SiC and FeCrAl,is conducted in present study,focused on the quenching and droplet impact cooling phenomena during core reflooding stage in case of LOCA.An experimental apparatus is developed to study the quench behavior of FeCrAl and Zr-4 in present study,and the effects of liquid subcooling,surface oxidation,surface roughness and solid thermal property on boiling heat transfer are analyzed in the quenching experiments.The quenching behavior of the test sample is visualized by a high-speed camera,and the quenching curve of FeCrAl and Zr-4 is measured by the embedded thermocouples.The surface temperature and heat flux are calculated based on the inverse heat conduction code,and the corresponding boiling curve is determined.The minimum film boiling temperature is determined at the point of minimum heat flux in film boiling.The increase of liquid subcooling results in the decrease of vapor layer thickness,and hence the film boiling heat coefficient increases.The thin vapor layer makes the vapor layer susceptible to collapse,and the minimum film boiling temperature is increased.Due to excellent oxidation-resistance of FeCrAl,an oxide layer is not observed on the surface during the sequential quenching experiments.Thus,the FeCrAl exhibits better reproducibility than Zr-4 in the sequential quenching experiments.Larger surface roughness leads to intensive heat transfer at the quench front,and massive vapor is supplyed into the vapor layer which make the vapor layer more stable.Therefore,the quenching time is increased,and the minimum film boiling temperature is decreased.Compared with Zr-4,FeCrAl exhibits larger thermal property.Thus,the quenching time is significantly increased,and the minimum film boiling temperature is decreased.Taking into account of above factors,coorelations of film boiling heat transfer and minimum film boiling temperature are proposed.As for the droplet impact cooling phenomenon,an experimental apparatus is designed to investigate the effect of Weber number,surface roughness,solid thermal property and oxidation on the impact behavior of droplet on SiC,FeCrAl and Zr-4 surface.The droplet impact behavior on test samples is recorded by a high-speed camera.According to the visualization results,the droplet impact behavior on heated surface is categorized into five regimes: i.e.deposition,rebound with secondary atomization,breakup with secondary atomization,rebound and breakup.Based on the impact behavior,the droplet impact regime maps of the test samples are determined.It is found that the droplet maximum spreading diameter and Leidenfrost temperature increase with Weber number.Based on the analysis of the impact regime map of CVD SiC with different surface roughness,the critical Weber number for droplet breakup and Leidenfrost temperature decreases with surface roughness.An oxide layer is formed on the FeCrAl surface after corrosion,and the Leidenfrost temperature is siginificantly increased for oxidized FeCrAl.Compared with Zr-4,FeCrAl and sintered SiC exhibits larger thermal property,and the Leidenfrost temperature is decreased.Based on the energy balance during the impact of droplet on heated surface,a correlation for droplet maximum spreading diameter is proposed.Based on the homogeneous nucleation theory,a correlation for droplet Leidenfrost temperature is developed considering the effect of droplet impact cooling.In summary,experimental investigation on the boiling heat transfer on ATF cladding surface during quenching and droplet impact cooling is conducted in present study,which makes up for the deficiency of research on the thermal hydraulic performance of ATF cladding under accident condition.The proposed minimum film boiling temperature correlation and Leidenfrost temperature model is helpful to understand the mechanism of the film boiing and improve the accuracy of safety analysis of system codes.