Advanced subchannel analysis for BWR fuel bundle

A reload bundle with a large critical power margin introduces flexibility into reload fuel management. An important design objective is therefore optimization of the bundle and spacer geometry in order to maximize the bundle critical power. With a view to reducing the extent of the complex full-scale tests usually carried out to determine the thermal hydraulic characteristics of various bundle geometries, the subchannel analysis method was further developed with the KWU computer code RINGS. The code, qualified with test data from various 9x9 bundles, accounts for subchannel flow using an annular flow model with a liquid film on the heated rods and determines dryout by calculating the film thickness. (orig.)