ABSTRACT
Nuclear fuel rods are smooth enough that the effects of surface roughness can be neglected when calculating the turbulent friction factor f. Rod spacers or grid spacers are used to hold reactor fuel assemblies together, and they keep the fuel rods inside of the assemblies from vibrating or bowing as the coolant flows by. In pressurized water reactors (PWRs), a small fraction of the coolant can flow between adjacent coolant channels in the radial or transverse direction, and it can also flow between adjacent fuel assemblies under certain conditions. In PWRs and boiling water reactors (BWRs), the upper and lower fittings which hold the fuel assemblies together can also function as orifice plates. In BWRs, the core-wide pressure drop is a larger percentage of the average operating pressure than it is in a PWR, but this is due to the fact that the coolant boils as it flows upward through the core.
