Numerical Simulation of Fluid-thermal-force Coupling Characteristics of Helical-cruciform Fuel Assembly

As a new type of reactor fuel assembly, helical-cruciform fuel (HCF) assembly has some advantages. The cruciform section shortens heat conduction path and the twist structure strengthens inter-channel mixing, which can upgrade core power density with enough safety margin. The periodic self-supporting structure fixes the assembly instead of spacer grid to reduce flow resistance. However, because of the small contact area at self-supporting structure the stress concentration is prone in contact position, which has a negative effect on the integrity of cladding. To solve this problem, the thermal-fluid-force coupling characteristics of typical HCF assembly were studied in this research. The flow and heat transfer of coolant were simulated by ANSYS Fluent to obtain velocity, temperature and vapor fraction distributions at single-phase and two-phase flow. Steady-State Thermal and Static Structural module in ANSYS were used to analyze the thermal and mechanical behavior of HCF assembly, respectively, and the differences of stress and strain distributions between single-phase flow and two-phase flow were found. The thermal-fluid coupling was two-way, and the data were exchanged in System Coupling module. Because the deformation of HCF assembly was too small to change flow and heat transfer of coolant, the thermal-force coupling was one-way. The results show that the cross flow intensity of coolant near the cladding is greater than that in the center of channel, and the direction of vortices at the two places is opposite. The cross flow affects the coolant temperature and vapor fraction distribution and makes the vapor fraction fluctuate with a period of 45° along flow direction at two-phase flow. The temperature at outer surface of cladding and pellet center also fluctuates with a period of 90° along flow direction except for the entrance part at single-phase flow and two-phase flow. Because a small amount of bubbles at the cladding surface can enhance the heat transfer, the heat transfer coefficient at two-phase flow is twice as large as that at single-phase flow. The maximum Mises stress of 300 MPa is in the lobe region of HCF assembly with 0.1 plastic strain. The thermal stress of HCF assembly in the lobe region is mainly related to the contact constraint, but in the valley region it is mainly related to temperature gradient inside HCF. At the cladding of HCF assembly, the Mises stress in the valley region and the plastic strain in the lobe region at single-phase flow and two-phase flow are the same, but in the lobe region the Mises stress at two-phase flow is 30 MPa smaller than that at single-phase flow.