Analyses are reported for an Anticipated Transient Without Scram (ATWS) event in a Boiling Water Reactor (BWR). The plant response is investigated for a loss-of-feedwater transient without either hydraulic or mechanical control rod insertion. Reactor control is achieved by the injection of soluble boron. Calculations are carried out with the POLCA-T computer code. The computational tool and methodology for carrying out the analyses were established at Westinghouse Electric Sweden. The simulation details all critical phases of the event leading up to stable shutdown conditions. The analysis shows the possibility for reactivity excursions due to sudden dilution of boron in the core, although the safety challenges were found to be limited for the case considered. The reactivity excursions only occur for a short time and are due to the inherently complex events involved. The paper demonstrates the utility of the POLCA-T code to analyze this type of ATWS sequences.

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