In the boiling water reactor (BWR), closure of a turbine stop valve (TSV) on the main steam piping system, hereafter called main steam (MS) line, will reduce the steam flow velocity to zero in about 100 msec (0.1 Sec) and create a pressure disturbance that propagates backward through the MS line toward the reactor pressure vessel (RPV). When the compression wave reaches the RPV, it expands into a region at lower pressure, transmitting a compression wave at acoustic speed from the pipe geometry into a complex geometric region at reactor pressure, bounded by the steam dryer and RPV inner wall surfaces. The arriving compression wave expands on the dryer surface, creating a pressure force, while simultaneously steam in the steam line, compressed from the valve closure to a pressure higher than that in the reactor, flows backwards into the RPV, creating a jet impingement, or "backflow" force on the dryer. Simplified, conservative modeling is applied in this study to obtain reasonable bounding loads which allow for the dryer curvature and other complexities of the dryer-vessel geometry.
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