Numerical Investigation of Effect of Ring-Type Spacer Grid on the Thermal Hydraulics Aspects of 5 X 5 Rod Bundles

The fuel assemblies of the Pressurized Water Reactors (PWR) are constituted of rod bundles set in a regular square configuration by spacer grids placed along its length. With the remarkable progress in the computer processing power, computational fluid dynamics (CFD) methodology can be useful for investigating the thermal–hydraulic characteristics phenomena in the nuclear fuel assembly. A methodology for the simulation and analysis of single-phase coolant flows around one or a row of spacers is presented. It is based on the multidimensional single-fluid mass, momentum and energy balance equations and use of proper turbulence models. This investigation presents results of flow simulations performed with the CD-Adapco code Star-CCM+ in a PWR 5 X 5 rod bundle part with a ring type spacer grid.A typical k-ɛ model has been used as a turbulence model and the symmetry condition is set as boundary conditions. It is confirmed that the turbulence in the sub-channel is drastically promoted by spacer and mixing devices. However, their effects quickly decayed to a fully developed level after passing about 10 times the hydraulic diameter downstream of the spacer. The static pressure of the fluid in the flow direction drops rapidly, then in a very short distance rises up, followed by a decrease near to the linear slope downstream. The simulations have also been compared with those performed by [1].