This code can simulate the phenomena of metal fuel SFR initiating phase event which is the migration of molten fuel in a simple way. The amount of melted fuel going to the upper and lower chambers is taken into account.
Research background and motivation
In the case of metallic fuel, it is proven that the accident is stopped early because of the inherent safety of the fuel itself in the event of an accident. However, in the case of FCI modeling, the behavior of the fuel can be calculated independently of the flare conditions. Calculations were performed at the top of the fuel, and the lower part from the cutoff point was not performed.
These vertical drop experiments simulate the impact of molten corium in the core during a severe accident on the lower part of the reaction. With the spreading of the fuel, modeling of the surface of the fuel is required. Here CIA1 is given as input, and the speed means the radius of the fuel particles.
A'fu is the heat transfer area of the molten fuel film in a generalized unit smear volume. In cases 1 and 2, molten fuel can be seen venting at the top of the fuel plug. Split the mesh in the axial/radial direction, and this calculates the mesh inside the cladding and the tube (hex wall) temperature in the energy equation.
These input conditions take advantage of the results previously performed in the other stages of detail. If the temperature of the fuel is calculated below the melting point, recalculate the mesh. In general, the temperature of the molten corium can be almost the same, but the difference in the discharge pressure remains significant.
Safety issue of SFR with metal fuel/oxide fuel
SFR severe accident code: SAS4A and SIMMER
Unlike these general accident analysis codes, severe accident analysis codes simulate CDA, further calculation of occurrences and movements of radioactive materials. Studies have been conducted focusing on the severe accident of the SFR, which uses metallic fuel.
Model description of SAS4A and scope of simulation
The Fuel Freezing model used in LEVITATE allows the structural formation of partial fuels. If the channel fuel temperature falls below the freezing temperature, the thrust temperature is reached.
Research objective and its scope
In the experiment, the Weber number did not have a large effect, and the higher the melt temperature, the smaller the dispersed form. In PLUTO2, heat transfer and kinetic exchange are calculated between the channel components.
ANALYSIS OF SODIUM-COOLED FRAST REACTOR SEVERE ACCIDENNT
SAS4A LEVITATE module analysis
The heat transfer coefficient between the above fuel and the liquid sodium is referenced in the Cho-Wright model. In this flow mode, the heat transfer area is much smaller than in the particulate flow mode between the fuel and sodium.
SAS4A single/multi-assembly calculation
As shown in Table 2.3, in cases 1 and 2, the top of the melt was ejected from the inside of the pin, but the resulting reduction in reactivity was small, resulting in ejection to the outside. This can be seen due to the density changes caused by the internal blowout of the pin. The results of these first and second tests resulted in the release of fuel from the inside of the fuel pin into the fishbowl due to the continued increase in power output.
Conversely, in the remaining cases, sufficient negative reactivity was obtained by the release of the fuel to the outside of the fuel, resulting in termination of the accident. Each graph shows the total reactivity, the programmed reactivity to give transients, the Doppler reactivity, the negative reactivity of the control rod and the reactivity of the displaced fuel. The horizontal axis of the graph and the vertical axis are different, so be careful.
One of the biggest differences between the two simulations is the contribution to the crash of negative stick reactivity. Comparisons between Cases 1 and 6 indicated that the accident could have ended outside the core of the activity area when fuel was ejected from Case 6 into the upper space.
MESFRAC (Metal Fueled Sodium-cooled Fast Reactor Accident analysis Code)
Fuel-coolant interaction mechanism
Zhi-gang Zhang et al observed the phenomenon in detail using a smaller amount of copper melt than in the aforementioned experiment. In this experiment, the contact temperature was lower than the melting point of copper, so the copper was very dispersed even though it solidified quickly. At contact temperatures above 1050 degrees Celsius, the melting point of copper, it was observed that the dispersion of a droplet of the same mass as another mass was little different, and the diameter was smaller as the superheat increased.
As a result of the experiment, the mass median diameter of the solidified particles was measured to be 0.3 mm. However, in these hydrophobic models, the larger the number of Webers, the smaller the particle size, which was not observed in the experiment. Furthermore, when measuring the dispersion distance of alumina, it was shown that the current typical correlation was between 60% and 70% lower.
These results suggest that this is due to the dominant thermal dissipation occurring before the hydrophobic model is sufficiently developed. The team developed a statistical correlation that can predict the length of decay of the Jet.
Limit of SAS4A and importance of FCI modelling
Preliminary development of MESFRAC; Simple_MESFRAC
Since the solidification of the molten corium is important, the heat transfer between the cladding and the molten corium was first modeled using a one-dimensional heat transfer equation in the radial direction. A node is determined in the diameter and axis directions, and the analysis begins by calculating the new fuel temperature with respect to the time interval dt. After calculating the new fuel temperature and fuel quantity, the fraction of voids per cell is calculated for each cell based on the fuel height in the innermost cell of each axial cell.
These void environments are used to calculate conduction or convection heat transfer in an energy equation. The initial conditions used the initial conditions of the experiment for the main fuel burst and are shown in Table 3.1. To determine whether such modeling is suitable for the heat transfer calculation, a comparison with the modeling result values was performed using the commercial numerical analysis code.
In case of phase change, the calculation of variation in the properties of the material and the latent heat at the melting or boiling point must be done sensitively. Since the conditions of the experiment were a drop in a simple liquid melt, we simply considered the velocity of the exhaust to pressure and the exhaust velocity to the drop height difference.
List of subroutines in MESFRAC
Fuel ejection from the pins in MESFRAC
In MESFRAC, the mass/motor/energy conservation equation is not calculated from all chain meshes, but is calculated for the specified domain. The variables that define these domains are HYDTOP and HYDBOT, which are responsible for the top/bottom of the calculated domain. In order to prevent the actual molten corium from spreading faster than the actual molten corium speed due to the relatively large mesh size, the molten corium movement distance according to the calculation speed within each bumper and the mesh size compared to the calculated travel distance determines the expansion or contraction of the calculated domain.
The split area is calculated by multiplying the area by the period, and the cycle length is calculated by dividing the area by the meter. The modified heat transfer coefficient is calculated using the surface and periodic coefficient calculated in the previous step, and the heat transfer in each component is calculated. Then, the corresponding area ratio inside the channel is calculated and the pressure value is calculated.
Simple_MESFRAC's calculation module is called for the 2-dimensional temperature calculation of the cladding and pipe to perform the calculation. 3.14, the calculation is performed by considering the material in the upper half of the boundary and at the lower half of the boundary.
Main conservation equations in MESFRAC
In this MESFRAC only molten fuel is considered, the heat transfer coefficient in the last line is very simplified. If enthalpy change is calculated, this enthalpy is used to calculate the temperature of the uranium. In this MESFRAC, the momentum sink term will consider wall friction and resistance between gas and molten fuel.
Timestep consideration of MESFRAC
Ex-pin molten metal fuel relocation and bond sodium effect
However, since this calculation requires us to see a difference in the behavior of the molten fuel due to a difference in pressure, the calculation was performed using a pressure that did not account for the effect of the sodium bond in the calculation. In each accident, the fuel temperature is not significantly different, so the behavior under the same BOEC or EOEC conditions is not significantly different. Related studies include a reassessment of the manufacturing sector of the largest fuel consumption emissions by FAIDUS in Japan's JSFR.
3.223.7 shows a contrast plot of the fuel disposal response to the invalid FCI response in the event of a severe accident in the US. The maximum reactivity of the 1M core void was set to $6 and the response was compared using SAS4A. Consequently, the high joint pressure of the EOEC from the fission gases, together with the rapid discharge of molten corium, resulted in a much greater upward drift than the BOEC, and the density within the core was reduced due to the mass of the fuel emitted outside the nucleus. , resulting in a safer accident outcome.
This MESFRAC can calculate the behavior of molten fuel in the coolant channel and outside the active core. Thermal interaction between the jet of molten metal and the sodium pool: effect of the main factors determining the fragmentation of the jet”, Nuclear technology249, 2005, pp. Grolmes, “A review of fragmentation models relating to the disintegration of molten UO2 when quenched in sodium coolant,” in: OECD Nuclear Energy Agency 1976.
Thermal interaction between molten metal beam and sodium pool: effect of main factors controlling fragmentation of the beam”, Nuclear Technology pp.
