Keeping Reactor Electric Systems in Phase

Jake Zimmerman
Electrical Engineering Branch Chief
Office of Nuclear Reactor Regulation

Nuclear power plants generate electricity, but their systems need power from the grid to operate. That electricity is delivered through three lines, or “phases.” If one phase is lost, or “open,” that can challenge a plant’s ability to safely shut down. The NRC and U.S. nuclear power plants are working to ensure “open phase conditions” are resolved safely.

CalvertCliffs_DSC2302Losing one or two phases coming from the grid to a transformer can unbalance the voltage on the side of the transformer connected to plant safety equipment. Voltage problems can automatically trigger the plant to disconnect from the affected power line, but sometimes they don’t. In that case, the plant’s safety systems might not get switched to backup power sources and may lack enough power to function properly. An “open phase” can happen at all but one U.S. commercial reactor (Seabrook).

The issue got our attention in 2012, when the Byron plant in Illinois shut down safely after an “open phase” event. The plant didn’t automatically switch from the affected power line, but control room operators diagnosed the problem in eight minutes and manually swapped power sources.

The NRC examined the issue and determined a handful of plants had dealt with open phase situations that never challenged plant safety. The agency’s response included sending all U.S. nuclear power plants a Bulletin to confirm they could meet electric power system requirements.

Every affected U.S. nuclear power plant has temporary measures in place to reduce the risk associated with an open phase condition. These measures include ensuring control room operators are aware of the issue and are trained to respond, and modifying power source switching procedures to ensure plants have emergency power if needed. The NRC staff reviewed and agreed with the temporary measures, and regional inspectors plan to inspect the measures.

The nuclear industry’s proposed permanent fix would provide alarms to alert operators to the issue and start an automatic swap of power sources. The NRC staff has laid out four goals that plants should meet when they install the proposed fix. We’ve also developed objective guidelines for our reviewers to consider when examining the plants’ implementation, which the NRC expects will be done by January 2019.

The NRC recently received an enforcement petition on the open phase condition. The petitioners asked the NRC to require either immediate corrective actions or shutdown of operating nuclear power plants. While the agency concluded the plants’ temporary measures eliminate the need for additional immediate action, the petition was accepted for further review.