Xcel Energy said that extreme cold weather in late January left diesel generators inoperable at the 550 megawatt (MW) Unit 2 of Prairie Island nuclear power plant in Minnesota.

Cold temperatures exceeded the design limit of -30° F for the two diesel engines that provide backup power should the unit unexpectedly trip offline. The incident was reported in a filing with the Nuclear Regulatory Commission (NRC).

The condition was short-lived. After around 30 minutes, the Unit 2 diesel generators returned to an operable status as outside air temperature rose above the low-temperature design limit.

Two pressurized water reactors at the 1,100 MW Prairie Island facility generate about 20% of the electricity used by Xcel Energy’s customers in the Upper Midwest.

Also due to the cold, the Unit 2 reactor at the Salem nuclear station in New Jersey manually tripped offline from 100% power due to icing conditions that required removing four circulating water pumps from service. Salem Unit 1 shut down one circulating water pump and reduced power to 88% as frazil ice developed. Frazil ice forms in water that is too turbulent to freeze solid.

And Dresden Station heater boiler B tripped while placing a second heater boiler in service. With cold temperatures, the density of the supply air increased and contributed to a greater quantity of air entering the reactor building than what was previously supplied with a heating steam system in place.

Defense in Depth

Xcel Energy said that multiple layers of defense-in-depth measures were in place to ensure safety, the utility said. Prairie Island has five sources of offsite power. All of them remained available during the cold weather.

Diesel generators at the plant's Unit 1 reactor remained available and were capable of being cross-connected to Unit 2. That unit's diesel generators were supplied by a different manufacturer with different air temperature limits. Additional equipment capable of responding to beyond-design-basis events also was available on site, the utility reported.