Why the grid relies on nuclear reactors in the winter

This scheduled regularity may seem mundane, but it’s great that operational reactors are as reliable and predictable as they are. It leaves some big shoes to fill with next-gen technology to hopefully join the fleet in the next few years.

In general, nuclear reactors operate at constant levels, as close to full capacity as possible. In 2024, for commercial reactors around the world, the average capacity factor — the ratio of actual power production to the theoretical maximum — was 83%. North America rang an average of about 90%.

(I’ll point out here that it’s not always fair to just look at this number to compare different types of power plants — natural gas plants can have lower capacity factors, but that’s mostly because they’re more likely to be intentionally turned on and off to help meet uneven demand.)

These high capacity factors also reduce the true reliability of the fleet, as a lot of downtime is scheduled. Reactors need to be refueled every 18 to 24 months, and operators tend to schedule these outages in the spring and fall, when electricity demand isn’t as high as when we’re all running our air conditioners or heaters at full blast.

Take a look at this chart of nuclear power outages from the U.S. Energy Information Administration. There are some days, especially in the height of summer, when power outage rates are low, and almost all commercial reactors in the United States operate at nearly full capacity. As of July 28 of this year, the fleet was 99.6% operational. Compare that to 77.6% of capacity on October 18, when the reactors were shut down for refueling and maintenance. We are now heading into another busy season, as reactors come back online and shutdowns reach another low point.

This does not mean that all outages are planned. At the Sequoyah Nuclear Power Plant in Tennessee, a generator failure in July 2024 caused one of two reactors to stop operating, an outage that lasted nearly a year. (The utilities also did some maintenance work during that period to extend the life of the plant.) Then, a few days after the reactor started up again, the entire plant had to shut down due to low water levels.

And who can forget the incident earlier this year when jellyfish wreaked havoc at two nuclear power plants in France? In the second case, the squishy creatures entered the filters of equipment that sucked water from the English Channel for cooling at the Balwell nuclear station. They forced the factory to cut production by almost half, although it was restored within days.