Exelon Nuclear has recently installed medium voltage adjustable speed drives (ASDs), or variable frequency drives (VFDs), in place of the original motor-generator (MG) sets for their boiling water reactors (BWRs) at Quad Cities. Exelon’s Dresden, Limerick, and Peach Bottom sites are also scheduled to replace the MG sets with ASDs. Other utilities have also installed these drives in the recirculation pump speed control application.

These ASDs have a design feature that allows a faulted power cell to be bypassed. The process requires that the output of the drive be turned off, that is, current goes to zero. During this downtime, the drive calculates new phase angles and voltage magnitudes required to provide a balanced output circuit. The maximum period for this calculation to occur is 250 msec. The drive is then restarted and catches the spinning load. The current design has the drive catch the spinning load and hold at or near the low-end speed of the transient. These ASDs have the capability to auto-recover from this transient and return speed to its original value, assuming no torque limits are encountered due to the quantity of cells bypassed. The drive will perform at 100% speed with two cells bypassed.

This paper evaluates the effects caused by various cell bypass speed profiles on a typical BWR-3 core (Dresden Unit 3) loaded with Westinghouse SVEA-96 Optima2 fuel. Cases with and without recovery to the initial pump speed at the onset of the cell bypass are considered. The BISON computer code, which is routinely used for BWR reload licensing transient analysis, is used to predict the system response. Average Power Range Monitor (APRM) and thermal power responses are the key parameters of interest. Sensitivities to recovery rate, time in fuel cycle life, and other operational constraints, such as single-loop versus dual-loop operation, are considered. The core physics (APRM/ void feedback caused by the resulting core flow changes) as well as safety and operational acceptability of the resultant reactivity events are explored with regard to this high availability medium voltage drive inherent design feature.

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