Two-phase pressure drop reduction BWR assembly design

摘要

In a boiling water reactor having discrete bundles of fuel rods (60, 65) confined within channel enclosed fuel assemblies, an improved fuel design of bundles of fuel rods interior of the channels is disclosed. Specifically, partial length rods (60) are utilized which extend from the bottom of the channel only part way to the top of the channel. These partial length rods (60) are shortened with respect to the remaining rods and are symmetrically distributed throughout the fuel bundle with the preferred disposition being in the second row of the bundle of fuel rods form the channel wall. The symmetrical distribution of the partial length rods is at spaced apart locations one from another. The partial length rods extend from the bottom of the fuel bundle and terminate within the boiling region. The total pressure drop is reduced, permitting more fuel rods in the design or a larger fuel rod diameter. The ratio of two-phase pressure drop to single phase pressure drop also is reduced, tending to increase the threshold for thermal hydraulic instability or coupled nuclear-thermal-hydraulic instability. Rod spacers (S2) and bundle tie-plates (40, 42) are provided with larger apertures overlying the partial length rods (60) for further channeling of the steam and reduction of two-phase pressure drop. Most importantly and during full reactor power output, the pressure drop in the two-phase region of the bundle is reduced without substantial corresponding degradation of the fuel assemblies thermal limits.