According to the US Energy Information Administration (EIA), the US nuclear fleet produced more electrical energy than ever before in 2018. Last year, it produced 807.1 terawatt-hours (TWh) of electricity, barely beating its 2010 peak of 807TWh. But the US nuclear industry has been in a well-documented decline. So what gives?

The EIA says the explanation comes from a combination of scheduling serendipity and what's called "uprating," where older nuclear plants are permitted to output more power. In a post this morning, the administration wrote that we shouldn't expect this much nuclear power output from the industry again—at least not in the near future.

Since the last peak in 2010, more than 5 gigawatts (GW) of nuclear capacity has been retired. Some of that was offset by a new reactor addition: another 1.2GW of capacity came online in 2016 at TVA's Watts-Barr nuclear plant when reactor 2 was completed.

Then, between 2010 and 2018, many nuclear plants completed uprates. Since many older plants were designed extremely conservatively many decades ago, modern modeling and equipment can be applied to some of these nuclear reactors to allow them to operate safely while delivering more power. The EIA estimates that 2GW of power were added to the US nuclear fleet between 2010 and 2018 through uprating alone.

Uprating is important because new reactors are not being built as quickly as they're being retired, and making better use of existing reactors is a way to prolong a solid source of low-carbon energy. A group at Oak Ridge National Lab has specifically been working on ways to improve capacity at existing nuclear reactors; you can read about their efforts here.

Still, taken together, the US nuclear fleet only added 3.2GW of capacity against more than 5GW of capacity that was retired from the US nuclear fleet between the 2018 peak and the previous peak in 2010.

At this point, it's important to remember that these capacity additions and subtractions are measured in watts, not watt-hours. The EIA measures total annual power output from the nuclear fleet in watt-hours, which offers another explanation as to how the US registered a peak output as the nuclear fleet is in decline.

A watt-hour measures power consumption of one watt over one hour, so if we consume 1GW for 24 hours a day, 365 days a year, we will have consumed 8,760GWh that year. Since nuclear plants require downtime for maintenance and refueling, the total watt-hours produced in a year is almost always less than the plant's maximum possible watt-hour output (that is, capacity × 24 × 365).

But in 2018, the US nuclear fleet had the highest output, or capacity factor, on record. So even if there was less total nuclear capacity on the grid in the US, those remaining reactors were working harder, longer hours. In total, the US nuclear fleet experienced a capacity factor of 92.6 percent, mostly by shortening the amount of time reactors needed to be offline for refueling.

The EIA notes that nuclear reactors were offline for about 25 days on average for refueling, but nuclear plants only refuel every 18 to 24 months, so occasionally years will have very few total refueling days, while other years have a lot of days where outages occur because of refueling. In 2018, it appears the nuclear industry got lucky.

In the future, the EIA expects only 2.2GW of new capacity to come online at the Vogtle nuclear power plant. That plant has been at the center of significant controversy after the reactor maker, Westinghouse, went bankrupt. Westinghouse's bankruptcy drew scrutiny to the billions of dollars of cost overruns that have plagued the reactor construction projects. Vogtle's sister project, at the Summer nuclear power plant, was shut down. Vogtle remains on a tenuous path to completion.

On Friday, Department of Energy Secretary Rick Perry is expected to visit Vogtle to extend $3.7 billion in loan guarantees to finish the plant, on top of $8.3 billion in loan guarantees that the owners of the plant have already received.