The startup transient was simulated in the Purdue University Multidimensional Integral Test Assembly (PUMA) facility for natural circulation boiling water reactors (BWRs) with different values of core inlet flow loss coefficient. The simulation was performed by means of pressure and power scaling, considering the core inlet subcooling and the structure-stored energy differences between the model (PUMA) and the prototype (Simplified BWR). A novel core inlet plate was designed and installed so that the loss coefficient could be easily changed without disassembling the reactor pressure vessel. The simulation experiments demonstrated the pattern of the geysering oscillations during the single-to-two-phase natural circulation transition phase. The core inlet flow loss coefficient was not observed to have a significant effect on the geysering oscillations.

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