June 1, 2010

Variable-Frequency Drives Upgrade Reactor Circulating Pumps

Mark W. Harshman

A recent trend in nuclear power plant upgrades has been the replacement of the motor-generator (MG) sets that drive the reactor circulating pumps with variable-frequency drives (VFD). Siemens’ first application of VFDs in this industry began in 2000 with an installation of six VFDs at the Browns Ferry Nuclear Plant. The use of the VFD continues to expand, and upgrades were recently completed at several U.S. plants, including the Hatch Nuclear Plant in Georgia.

MG sets were originally used to drive the reactor circulating pumps (Figure 1). Their purpose was to control reactor reactivity by changing pump speed and therefore the rate of fluid flow through a boiling water reactor. Today, pumps retrofitted with VFDs permit much more precise speed control and have the potential for achieving speeds (and fluid flow) above the speed set by the line frequency of 60 Hz. An added benefit is the combination of an AC induction motor powered by a VFD, which results in a much more efficient combination than an MG set. The VFD can provide very slow acceleration rates when low-power operation is required. Because raising pump speeds raises reactor power, plants are very cautious about speed control.

Replacement of the MG sets addresses the obsolescence and maintenance issues associated with this older technology but also yields several advantages once VFDs are installed and operating.