Most of the 675,000 cells are located in and around the fuel racks where the highest gradients and most significant phenomena occur. Cell height at the top of the racks is reduced to 1 cm through the use of grid adaption. This fine resolution is needed to resolve the large thermal and velocity gradients predicted in this region. Transitions from the very fine mesh to regions with larger mesh are constructed of tetrahedral elements with a controlled growth rate. This technique eliminates the skew in the hexagonal element regions and provides a smooth transition between cells of different sizes.
All external walls use the no-slip condition for the momentum equations.
No surface roughness is applied. All external walls are adiabatic except as noted in the sensitivity studies. The surface facing the reader in Figure 4 is a symmetry plane. Internal vertical walls are positioned within the fuel and rack region to separate zones of different fuel ages and to align the flow vertically. These internal walls use a no-shear boundary condition since all flow resistance within the racks and fuel is accounted for by the porous model.