Research background and motivation
General literature review
The effect of the initial temperature of the coolant sodium on the fragmentation of the simulants was found to be negligible. Most studies conducted with metallic simulants in sodium concluded that the molten metals were heavily fragmented after liquid contact and that the dominant factors determining fragmentation were superheating and latent heat of fusion of the simulants38-44. It was found that the debris size was well distributed, but the average debris size is about 2-3 mm.
There is still a knowledge gap regarding the initial phase behavior of metallic fuel transfer within core structures in the case of HCDA.
Objectives and scope
In the steady state, the metal-driven SFR axially has an asymmetric temperature profile due to the sodium coolant, while there was a symmetric temperature profile in the oxide-driven LFR. When an unprotected loss-of-flow (ULOF) event is taken into account, the fuel dispersion is at most the density difference between the fuel and the coolant. The metal fuel is quietly heavier than the sodium, which would lead to upward and downward dispersion.
The movement of fuel is more affected by the drag force than the driving force, so the fuel would rise and travel with the natural circulation of the LBE, it indicates that the accident mitigation strategy should be prepared to prevent the serious accident.
OBSERVATION OF EX-PIN PHENOMENA
Experimental setup and procedures
- Experimental facility
- Test procedure and experimental methods
The distance between the upper end of the coolant channel and the melt inlet is 260 mm. The melt injection system was designed to determine the pressure of the melt with regulating noble gas. To calculate the Weber number, it is necessary to evaluate the interfacial velocity between the melt and the coolant.
Thus, the interfacial velocity is assumed to be the initial relative velocity between the melt jet and the coolant.
Experimental results
- Physical insights in ex-pin phenomena
Throughout the experiment, the possibility of melting fluctuation was investigated, even in the absence of the coolant flow. This indicates that the melt fragmentation caused by reaction of the melt with the coolant did not occur. The empty coolant area formed around the melt injection point, and the coolant was filled at the bottom of the coolant channel.
As the vapor pressure accumulated in the coolant channel, the melt at the bottom of the coolant channel was discharged upward.
FUEL RELOCATION EXPERIMENTS
Experimental setup and procedures
- Experimental facility
- Test procedure and experimental methods
To investigate the melt behavior and observe its distribution and fragmentation, most experiments were performed without rod bundle structure. The melt ejection system is designed to control the temperature of the melt and the melt ejection pressure to achieve the experimental conditions. Here the ejection pressure of the melt is controlled by pressing or pressing noble gas.
The initial temperature and pressure were measured with thermocouples or pressure gauges before the injection of the melt. The dispersion behavior of the molten wood metal in the coolant channel was observed using a high-speed video camera (1000 fps). The initial melt temperature and pressure are set in the melt injection system using an induction heater and noble gas.
During the process, the melt displacement behavior and fragmentation phenomenon are obtained using a high-speed camera. The experiments were conducted to observe the basic fragmentation behavior of the melt in the coolant channel. The initial relative velocity is determined by the velocity difference between the melt and the coolant, which means the vector difference between the two velocities.
The initial relative velocity between two fluids is directly related to the momenta of the materials at the surface, which determine the fragmentation behavior of the melt. Since the temperature of the melt and the coolant changes with the interaction between the melt and the coolant, the experimental conditions were chosen based on the initial conditions.
Experimental results
- Fuel relocation behavior
- Fuel levitation
The melt was continuously injected into the coolant channel and collided with the channel wall. The inertial force of the melt activated the fragmentation of the melt and dispersed the melt waste. Before the melt injection into the channel, the gas mushroom was formed in the channel.
As in previous cases, the gas was injected into the coolant channel in the early phase of the melt injection. Moreover, the inertial force caused the melt jet to collide with the channel wall. This agglomeration was formed by quenching the center of the melt jet at the channel wall.
Once the center of the melt was quenched, the melt agglomeration was expanded by continuous melt injection. This amount of melt injection means that the melt is injected into the coolant channel during a single pin failure condition. After air injection, the melt was injected into the coolant channel and collided with the channel wall.
However, the center of the melt jet was not quenched and collided with the coolant channel wall as a liquid phase. As a case of single pin failure, the air was injected into the coolant channel in the early phase of the melt injection. This means that the physical fragmentation due to the inertial force of the melt is promoted during melt ejection.
This could be seen by accelerating the peak discharge behavior of the melt in the later stage of melt ejection.
DEBRIS BED POROSITY EXPERIMENTS
Experimental setup and procedures
- Ex-pin experiment
- Quenching experiment
In the event of a severe PGSFR accident, melt ejection conditions can also be derived from the result of the SAS4A code. The experimental result of fuel transfer obtained from the melt ejection conditions could be used to validate the related modeling of the phenomenon. The test matrix was chosen to reflect the melt ejection conditions obtained from the SAS4A analysis.
Therefore, the melt extraction pressure obtained from the SAS4A analysis and the melt extraction pressure in the TREAT-M DB series experiment were selected as the upper and lower melt extraction pressure ranges, respectively. In addition, the effect of structure temperature on the melting behavior and porosity of the waste bed was examined. This is to analyze the melt discharge behavior and the axial distribution of the top discharge.
A total of 19 pins were placed in the test section of the hexagonal tube, and the center pin was selected as the broken pin to simulate the melt ejection. The copper coil for the induction heating can float the melt drop in the coil. With oscillating electromagnetic fields in the coil, there was a force to make the melt drop float.
Based on experimental results of fuel displacement experiment in Chapter 3, it was found that the melting temperature gave significant effect on the formation of the waste morphology. The melt formed nominal spheres of 10 mm diameter in the induction coil before quenching.
Experimental results
- Debris bed porosity
- Characteristic length of debris
The graph shows that the metal-type dirt increases the pressure drop compared to the bare condition, even when the porosity is high. The porosity of the debris bed is an important variable in determining the magnitude of the accident, as the pressure drop depends on the porosity of the debris bed in the fuel assembly. Because the porosity of the debris bed varies depending on the accident conditions, experiments on the porosity of the debris bed were conducted and the porosity range of Wood's metal debris bed was measured.
From previous fuel displacement experiments, it was determined that the debris bed was formed in the subassembly. In the effluent flow experiment with metallic fuel, a waste layer with a high porosity of 0.8 was formed. In order to investigate debris size and morphology in relation to debris porosity, debris characteristics must be quantified.
Thus, the characteristic diameter of the waste was determined by the volume of the waste on its surface. When the shape of the rubble is assumed to be uniform as a spherical shape with a characteristic length, the porosity of the rubble layer per unit volume would increase as the diameter increases. It was found that the waste morphology became relatively complex as the temperature of the instant contact interface increased.
It was clearly confirmed that the ligament remnant has a similar shape with an increase in the current contact interfacial temperature from the radiographic images. Therefore, it is expected that the metal fuel residue layer will have a highly porous residue layer, even if the shape of the residue is simple or the melt is not fragmented.
CONCLUSIONS AND RECOMMENDATIONS
Fuel relocation behavior
Characteristic length of debris
Recommendations
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