In the previous chapter we explored the methodology for determining the temperature field for a single fuel pin. Since a typical fast reactor core comprises several thousand fuel pins clustered in groups of several hundred pins per assembly, a complete thermal-hydraulic analysis requires knowledge of coolant distributions and pressure losses throughout the core. This chapter will address these determinations.We focus first on the coolant velocity and temperature distributions within an assembly. Then the discussion is broadened to include coolant velocity and pressure distributions throughout the reactor vessel. Having established these temperature and flow fields, the important design question of hot channel factor determination is then addressed.
Assembly Velocity and Temperature DistributionIn Chapter 9 we presented a simple relation for the average axial coolant temperature rise, based on an average bundle mass flow rate [Eq. (9.37)]. The problem is actually more complicated, for several reasons. First, the flow areas for the three types of channels in Fig. 9.13 are different, so the velocity and mass flow rates in each are different. Second, there is a radial power distribution, or power skew, across individual fuel assemblies. Third, there is crossflow, i.e., flow transverse to the axial direction, between assembly channels. In the first part of this section a simplified approximate velocity distribution will be derived that accounts for the variation in flow area between channels. In the second part the more complex set of equations accounting for crossflow and turbulent mixing between channels will be presented.
Approximate Velocity DistributionAn approximate velocity distribution, or flow split, between flow channels in Fig. 9.13 can be obtained from the physical fact that the axial pressure drop across each channel of a fuel assembly must be the same since the flow begins and ends in common inlet and outlet plenum regions. This technique was used by Novendstern [1] in his method for evaluating pressure drop in a fast reactor fuel assembly and A. Waltar (B) Pacific Northwest