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[en] IVR-ERVC (In-Vessel Retention by External Reactor Vessel Cooling) is one of the severe accident management strategies, and removes the decay heat through the reactor vessel lower head by supplying cool-ing water to the lower head. Therefore, estimating HTC (Heat Transfer Coefficient) of the outer wall is im-portant to evaluate the ERVC heat removal capability. From previous researches, severe accident analysis code (MAAP and MELCOR) used Rohsenow correlation to calculate the HTC, and SBLB experiment was conducted to measure the HTC. However, the Rohsenow and SBLB experiment couldn’t consider effect of the reactor vessel lower head surface geometry (especially radius curvature) and material. These geometry and material of heater could affect the HTC. Accordingly, in this research, the HTC measurement experi-ment was conducted with SA508 heater which had 2.5 m radius curvature.This research had two main objects. First was experimentally producing the HTC database with SA508 heater which has 2.5 m radius curvature. Second was suggesting the HTC correlation by modifying the existing correlation. To simulate the flow boiling condition of ERVC, water loop was prepared, and the loop controlled working fluid (DI water) temperature and mass flux. The reactor vessel lower head outer wall was simulated by the SA508 plate heater. The experiments were conducted at four angular range to cover whole reactor vessel lower head (from bottom (0∘) to top (90∘)). The heater surface temperature was meas-ured by IR camera, and finally the HTCs were calculated. LLOCA was used as base accident, to determine working fluid inlet condition, mass flux and the heater heat flux. In this research, the HTCs were measured at 10-90∘ of the heater inclination angle, 300-500kg/m2s of the working fluid mass flux and 100-950kW/m2 of the heater surface heat flux. Modified Chen correlation was used to develop the HTC correlation
[en] Two-phase closed thermosyphon (TPCT) is vertically oriented wickless heat pipe that has working fluid in the interior. The TPCT transports a large amount of heat from evaporator to condenser by phase change of working fluid, and the working fluid passively returns to evaporator by gravity. Due to these advantages of the TPCT, the TPCT is considered as method of PRHR (Passive Residual Heat Removal) system in nuclear system. Parametric studies have done to investigate the heat transfer characteristics of the TPCT. Different working fluids such as water, ethanol, methanol and acetone were used at various filling ratios and at different operating temperatures to find maximum heat transport capabilities of TPCT. Effect of heat transfer rate, filling ratio and aspect ratio were investigated. Inclined angle effect was investigated at several filling ratios and working fluids. This study is interested in silicon oil effect on the TPCT. To carry out the experiment, experimental apparatus is designed and manufactured. In design process, the TPCT operation limit is considered This study is interested in silicon oil effect on the TPCT. Experiments were carried out at three oil weight percent with three input power. Effect of oil on the TPCT is evaluated by inner wall temperature distribution and thermal resistance. In this study, silicon oil effect on TPCT was investigated. The TPCT was operated with several oil weight percent and input power. From experiment, overall, the silicon oil reduced evaporator thermal performance, but enhanced condenser thermal performance. However, the TPCT total thermal performance was reduced by 100 c St silicon oil
[en] To evaluate heat transfer capability of the ERVC, estimating heat transfer coefficient (HTC) is important. In this study, the HTCs were experimentally measured, and large break loss of coolant accident (LLOCA) was used as basic accident. At the lower head outer wall, heat transfer phenomenon was downward facing flow boiling heat transfer. Because, natural circulation occurred. Hence, to simulate the flow boiling, water loop was designed. The reactor vessel lower head was simulated as 2-D slice main heater. To simulate the heat transfer characteristics of material and geometry, the main heater was made of SA508 consisting the reactor vessel, and its radius curvature was 2.5 m. The main heater outer surface (facing to air) temperature was measured by infrared (IR) camera, and the inner surface (facing to working fluid) temperature was calculated by solving conduction equation of main heater. The main heater heat flux was under CHF value of previous research. The results of 60 .deg. and 90 .deg. were used as representative angular location data. LLOCA was used as basic accident. Through this experiment, the HTC data was produced for SA508 heat transfer surface material and 2.5 m of radius curvature. The HTCs result shown different trend at each angular location. The HTCs commonly increased with heat flux increment, but the trends were different for angular location.
[en] The passive safety systems mean that the systems can operate without active component, and therefore uses natural force (gravity, density difference and etc.) as their driving force. The PAFS (Passive Auxiliary Feedwater System) is one of the passive safety systems and is operated by natural circulation of density difference. The PAFS aims to remove decay heat by cooling down the secondary side of steam generator. There are two trains of PAFS in a NPP, because of single failure criteria. The PAFS consists of PCHX (Passive Condensation Heat Exchanger) and PCCT (Passive Condensate Cooling Tank). Operation time of the PAFS is limited by capacity of the PCCT, because the PCCT is final heat sink of PAFS. The current PCCT is designed for 8 hours operation after a PAFS actuation. This paper designs the new PAFS which can operate infinitely and passively, and we call it the advanced PAFS in this paper for convenience. Dry cooling concept is considered for the advanced PAFS. The dry cooling uses the atmosphere as its heat sink which is the infinite heat sink. In addition, it can be operated by natural circulation. It means that the advanced PAFS can operate infinitely and passively. However, the air heat transfer coefficient is not compare with the water. Therefore design change of the PCHX is indispensable. This paper presents the conceptual design of the advanced PAFS and estimates its geometry and capacity. This paper studies concept of the advanced PAFS to overcome the current PAFS operation time limit. The dry cooling is considered for the advance PAFS. During first period of the reactor shutdown, the water removes the high decay heat. After all water is evaporated, the air removes the decay heat. The shift from the water heat sink and the air heat sink is automatic. The air heat transfer coefficient is smaller than the water heat transfer coefficient. Therefore, the PCHX design change is indispensable. The tube geometry and total number are changed to fulfill the thermal requirement. The advanced PAFS has the chimney to make proper natural air circulation. This study not only presents the concept design of the advanced PAFS, but also estimates its geometry and capacity. The amount of heat removal increases with chimney height and tube length. However, Total tube length also increases. Large chimney height and small tube length make minimum total tube length
[en] For sever accident mitigation, In-Vessel Retention through External Reactor Vessel Cooling (IVR-ERVC) strategy is used to remove the decay heat of the molten corium. The Critical Heat Flux (CHF) is one of the most important criteria in determining the success of the IVR-ERVC strategy. Amount of molten corium is depend on accident scenario, and hence maximum heat flux location is also changed by it. Therefore, to assess success of the IVR-ERVC, the location-specific CHF database is required. In this study, the CHF experiments were conducted to evaluate the effect of heater inclination angle and material. For the SUS304 heater, 30 °, 60 ° and 90 ° inclination angle experiments were conducted. For the SA508, 60 ° and 90 ° inclination angle experiments were conducted. Through the experiments, the lower heater inclination angle made the lower CHF value. The reason was the vapor escaping time was getting longer for the lower inclination angle. The CHF of SA508 heater was higher than one of SUS304 for all inclination angle, because the oxidized SA508 surface had better surface wettability and it enhanced the CHF.
[en] To prevent the fuel assembly melting, emergency core cooling systems are immediately initiated to flood the fuel assembly from the bottom. It is expected that the heated fuel temperature during the reflooding phase is higher than Leidenfrost temperature of water. When water rewets the fuel surface at the temperature higher than Leidenfrost temperature, quenching phenomenon occurs and a lot of droplets are generated near the quenching front. Dispersed droplet flow is formed above the quenching front. Direct contact heat transfer of droplets with the heated wall likely contributes to the cooling of the upper part of the fuel rods. However, the full understanding of complex fluid flow and heat transfer characteristics during droplet-wall collision is very difficult during LOCA. In this regard, this effect has been neglected in the previous studies, and thus prediction of fuel rod temperature has still a large uncertainty. A few years later, some research groups studied the effects of droplets on heat transfer and developed the prediction models for the fuel temperature. However, these model considered heat transfer mechanism without consideration of dynamics. Therefore, it is necessary to study the heat transfer mechanism considering the dynamic effects on single droplet-wall collision. We used a technique with the spatially and temporally synchronized high-speed camera and IR camera. The high-speed camera is used to measure droplet diameter, collision angle and velocity. IR camera is used to examine temperature changes on wall. Droplet-wall heat transfer phenomena are observed on the basis of wall temperature and collision angle. This study reviewed the previous our studies about the effects of wall temperature and collision angle on heat transfer characteristics of single droplet-wall collision. We compared the experimental data with the prediction by droplet-wall direct heat transfer correlation proposed by Bajorek and Young
[en] In-Vessel Retention through External Reactor Vessel Cooling (IVR-ERVC) is one of severe accident mitigation strategies. The ERVC’s main objective is keeping molten corium in a reactor vessel. The ERVC removes the decay heat by externally cooling the reactor vessel. Amount of molten corium is determined by the accident scenario. It means the maximum heat flux angular location is different for the scenario. Therefore, the location-specific CHF database is required to assess the IVR-ERVC success. Occurrence of the CHF is determined by liquid film dried out. The liquid film thickness is affected by the pressure and heater surface inclination angle. At high pressure, volume of generated vapor is small, and hence the film is thick. It means that the CHF is increased by the high pressure. On the other hand, at the low inclination angle, the film is thin, because vapor buoyancy force is toward the heater surface. It means the CHF is decreased by the low inclination angle.