Phenomenological Study of Fuel Relocation Behavior and Formation of Debris Bed in Metal-fueled Sodium-cooled Fast Reactor

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Phenomenological Study of Fuel Relocation Behavior and Formation of Debris Bed in Metal-fueled Sodium-cooled Fast Reactor
Heo, Hyo
Bang, In Cheol
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Graduate School of UNIST
Metallic fuel has been proposed as one of fuel types in sodium-cooled fast reactor (SFR). The metallic fuel has advantages in terms of safety issues. Especially, it has been known that severe accident for metal-fueled SFR might be terminated earlier due to its safety characteristics. Hypothetical core disruptive accidents (HCDA) are considered as the accident which causes fuel melting and cladding failure in SFR. When cladding bleach occurs in initiating phase of HCDA, molten fuel is relocated debris is formed in fuel assembly. Severe accident consequences are determined depending on these phenomena. Thus, it is necessary to study fuel behavior and solidification phenomena in severe accident. However, the studies related to the metallic fuel in severe accident condition were partly performed. There is still a lack of fundamental knowledge about the severe accident phenomena and related physics. Therefore, the present study focuses on fuel relocation behavior and debris formation phenomena, which is in specific accident scenario, with experimental approaches. The fuel relocation behavior has been not studied clearly, especially in initiating phase of HCDA. To understand the phenomena and provide physical insights, there were visual studies for various experimental conditions. The studies were performed as a parametric study so key parameters affecting the fuel relocation behavior were selected. The parameter discussed in the experiment are coolant boiling, channel condition, initial melt temperature, and initial melt ejection pressure etc. In the fuel relocation experiments, simulants were used instead of metallic fuel. Wood’s metal and gallium were used as the simulant for the metallic fuel in most of experiment. Since the fuel behaviors are driven by force balance, Froude and Weber numbers were compared to investigate similarity. High speed video camera was used to observe the fuel relocation behavior and visual analysis methods were applied on the parameter study. There were two steps to conduct the visualization experiments. Firstly, possible fuel relocation behaviors were observed in small-scaled experimental facility. The experimental facility is called UNIST molten core and coolant interaction experimental facility (UNICORN)-B (baby). From the experiments, it was found that sodium boiling could be powerful driving force for fuel dispersal regardless of channel condition. Then, there were additional visual studies using UNICORN-C (child) which was established considering actual scale of fuel assembly in metal-fueled SFR. As a result, it was clarified that debris bed formation was highly dependent on the fuel relocation behaviors. Although fuel is ejected into coolant channel, core could be cooled from hydrodynamic point of view. If high porous debris bed is formed, decay heat would be removed using natural circulation flow. UNICORN-A (adult) was established to simulate actual fuel ejection condition in HCDA. Simultaneous occurrence of unprotected transient over power (UTOP) and unprotected loss-of-flow (ULOF) event was selected as a target scenario. A severe accident code SAS4A was used to calculate initial experimental conditions. Radiographic images were obtained to analyze melt relocation behavior. The experimental results show that melt was not swept out from active core region. Mass fraction of frozen melt was investigated along axial distance. It showed melt was radially dispersed rather than the axial melt dispersal. In addition, most of melt was frozen near cladding failure point where the melt was directly ejected out. Debris bed seemed like agglomerated shape leading to local flow blockage. Since debris bed porosity affects to pressure drop and flow rate of fuel assembly, the porosity was evaluated after the experiment. The porosity was measured with two methods; classical method and post-processing method of radiograph image. The classical method was performed using volume fraction of test section before and after the fuel relocation experiment. Debris bed was measured 0.89 of its porosity with these methods. It could ensure that core has coolability by natural circulation. Thus, it was verified that there was a possibility of early termination of severe accident. The characteristics of debris bed are mainly determined from physical form of individual debris. Especially, debris bed porosity is affected by debris morphology. In severe accident condition in SFR, metallic fuel had ligament-like shape of debris. This morphology made characteristics of debris bed porosity. The metal fuel had relatively high debris bed porosity. In previous research group, quenching experiments were conducted using molten metal droplet. The debris morphology was analyzed quantitatively. It was insufficient to qualitatively investigate the debris morphology. The present study was performed with experimental works based on theoretical model regarding rapid solidification. It is suggested that debris morphology is attributed to freezing point and instantaneous contact interface temperature between melt and coolant. From rapid solidification experiments, it was shown that high porous debris bed would be formed above porosity of 0.83 based on the reactor accident conditions in ULOF.
Department of Nuclear Engineering
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