Experimental study of spacer effect on film flow in fuel bundles

Boiling water reactor (BWR) cores are designed to prevent a boiling transition. For this purpose, large-scale experiments have been conducted to obtain fuel bundle critical power (CP). However, in order to survey a wide variety of rod bundle designs, an analytical CP prediction model is necessary. One such model, a new subchannel dryout prediction model, is being developed. In this model, mass and energy distributions in a fuel bundle are calculated using subchannel analysis based on a slip model, and film flow rate along each fuel rod is calculated with an annular flow model that is similar to the Harwell annular flow model. Critical power is defined as the bundle power when on of the film flows along the fuel rods disappears. This new prediction model directly simulates dryout phenomena, and has the potential to be extended to cover a wide range of geometries by establishing a relationship between film flow and geometries. In the present study, the spacer effect on film flow along fuel rods is examined experimentally to produce a space model that could be incorporated into a CP prediction model.