Numerical Analysis Of Thermal-hydraulic Characteristics Of Spent Fuel Rod Assembly

Prerequisite for the rapid development of nuclear power is to ensure the safety andeconomic advantages. The range of reactor designs involved is wide, including reactorphysics, reactor thermal hydraulics, reactor control, reactor structures and materials.Thermal-hydraulic design objective is to ensure that the reactor core in the variety ofoperating conditions can be sufficient cooling, where the thermal-hydraulic numericalanalysis of fuel assemblies is one of the main methods. Spacer grid is an important structuralcomponent of the fuel assembly and directly affects the thermal-hydraulic characteristic. Thispaper studied the heat transfer characteristics of the fuel assembly with spacer grids by usingCFD method. The works were about the complicated structural of the model geometry andmesh generation, the numerical simulation with the core conditions and spent fuel poolconditions of the flow field and temperature field analysis.The model that has been analyzed is a5×5periodic fuel assembly. Three-dimensionalmodeling software UG was applied for building geometry model. ICEM CFD meshingsoftware was used for hybrid grid mesh generation. The only rod assembly part was dividedwith hexahedral, and the section of complex geometry part with spacer grid was generated byautomatic tetrahedral mesh method. Eventually the two parts were merged. The boundarylayer meshes were generated by Tgrid software. The polyhedron and hexahedral mesh wereused for the numerical simulation finally. The polyhedral mesh generation conversion wasdone in the FLUENT software.The results show the distribution of lateral flow and temperature distribution,cross-section lateral flow and temperature contours, loss coefficient and surface temperaturedistribution of rod assembly. Analyze the three models by comparing the calculated results,including only rod model, mixing vanes model, integrity model. The results show that in thecondition of the reactor core the fuel assembly with mixing vanes could significantly enhancethe cross flow and reduce the maximum surface temperature of the rod assembly; the springsand dimples of the integrity model could influence the flow field intensely. For the influenceof contact between the grid and rod, the distribution of surface temperature near that positionis complicated. The area of contact should be reduced during the spacer grid designing to prevent the temperature distribution from being worse. In the condition of spent fuel pool, theinfluence of spacer grids has became weakly. The springs and dimples of the integrity modelcould influence the flow field and temperature field hardly. Through comparing the majorvelocity and lateral velocity of the integrity model in two computational conditions, theinfluence of spacer grid has disappeared at the position of50mm from out of the spacer gridin the spent pool condition, but in the reactor core condition the lateral flow don’t disappear inthe out cross-section of the fuel assembly.