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The spacer grid models are important models for simulating the thermal-hydraulics phenomena during reflooding process. The spacer grid pressure drop model, wall heat transfer enhancement model, interfacial heat transfer enhancement model, droplet breakup model, spacer grid re-wet model and grid temperature model are integrated in COSINE multiphase sub-channel code. In this paper, the typical rod bundle heat transfer (RBHT) reflooding tests were selected to assess the spacer grid model in the code. The calculation results show that the spacer grid models can significantly enhance the code predicted heat transfer capacity, accelerate the quench rate and reduce the peak cladding temperature (PCT) during the reflooding process, and the calculated results are in good agreement with the experimental measurements. In addition, the deviation of the PCT predicted by the code is reduced from 10% to 7% due to the adoption of the spacer grid models. The spacer grid models have the ability to enhance the interfacial heat transfer between vapor and droplet, therefore the calculated results with the spacer grid models show a significantly faster decrease in vapor temperature than that without the spacer grid models. And the calculated vapor temperatures with spacer grid models are in good agreement with the experimental measurements. The trend of the code predicted spacer grid temperature agrees well with the experimental results, but the code predicted spacer grid temperature is slightly higher than the experimental measurements. Furthermore, the early wetting of the temperature probe located on the spacer grid in the experiment results in a rapid decrease in the spacer grid temperature, however，the code predicted spacer grid temperature does not show as sharp a decrease as the experimental results due to the interpolation method in the calculation of the spacer grid temperature. The spacer grid models adopted in the code can simulate the breakup of droplets across the spacer grid, the diameter ratio of the shattered droplets predicted by the code is in good agreement with the experimental measurements, and the calculation results of the Cheung model are more accurate than those of the Paik model in the test case 7151. In addition to this, the code can predict the trend of droplet velocity near the spacer grid during reflooding process, but the calculated droplet velocity decreases significantly faster than the experimental measurements, and it needs more work in future. In general, the calculated results with the spacer grid models are in good agreement with the experimental results, and the spacer grid models adopted in the code are reasonable and reliable.

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COSINE software package provides various functions in thermal-hydraulic analysis of pressurized water reactor (PWR) after years of development. In order to meet the engineering design and safety analysis requirements of the advanced passive reactor CAP1400, the multiphase field sub-channel code with higher calculation accuracy was developed based on the homogeneous sub-channel program (cosSUBC) in COSINE software package. The multiphase sub-channel code contained 11 basic equations including the mass conservation equations of the vapor, continuous liquid, and droplet, the axial momentum conservation equations of the gas, continuous liquid phase, droplet phase, the transverse momentum conservation equation of the gas, continuous liquid, and droplet, and the energy conservation equations of the gas and total liquid phases. The continuous liquid and the droplet were in energy equilibrium states in one cell was assumed, therefore the continuous liquid and droplet were merged into total liquid phases in energy conservation equation. The constitutive relation models between different phases were introduced in the code to realize the closure of the basic equations, the staggered discrete format in space and a semi-implicit time discrete format were adopted in multiphase field sub-channel code. The typical test cases and representative experiments were modeled and calculated in this paper to verify the calculation capability of the code. First, the test of droplet behavior was modeled to verify the correctness of droplet model adopted in the code. Then the turbulent mixing experiment was modeled to verify the correctness of turbulent mixing model used in the code. In the end, the wall heat transfer experiment and the reflooding process experiment were modeled to verify the simulation ability in the heat transfer regime especially in the post critical heat flux (post-CHF) condition. The results show that the droplet behaviors including the generation and the flow distribution of droplet phase are in good agreement with the reference code. And the code can well simulate the flow distribution phenomenon according the comparison between the calculation results and experimental data in flow distribution experiment and the turbulent mixing experiment. Besides, the code can well simulate the distribution of the wall temperature and the reflooding process in the post-CHF and heat transfer condition. Therefore, the calculation of the thermal-hydraulic phenomena in the core of reactor by the multiphase field sub-channel code is reasonable and right, and the code has the ability to calculate and simulate the thermal-hydraulic conditions in the core of PWR.