Westinghouse is currently developing the MEFISTO code with the main goal to achieve fast, robust, practical and reliable prediction of steady-state dryout Critical Power in Boiling Water Reactor (BWR) fuel bundle based on a mechanistic approach. To achieve this goal, the code resolves the multi-film mass balance equations at the sub-channel level within the annular flow region while relying on a simple two-field (liquid/steam) sub-channel solution to provide the cross-flow information. The MEFISTO code can hence provide detailed solution to the multi-film flow in BWR fuel bundle while enhancing flexibility and reducing the computer time by an order of magnitude as compared to standard three-field subchannel analysis. Models for the numerical computation of the sub-channel multi-film flowrate distributions, including the treatment of cross-flows, part-length rods and spacers grids are presented in this paper. The MEFISTO code is then applied to dryout prediction in BWR fuel bundle using VIPRE-W as a two-field sub-channel driver code (based on a fast and robust four-equation model). The dryout power is numerically predicted by iterating on the bundle power so that the minimum film flowrate in the bundle reaches the dryout criteria. Dryout predicted powers (including trends with flow, pressure, inlet subcooling and power distribution) and predicted dryout locations are compared to experimental results, using a large Westinghouse SVEA-96 dryout database, and are shown to yield excellent results.

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