Despite power plant closures, U.S. nuclear generating plants produced more electricity in 2018 than in any year before, according to preliminary annual data from the U.S. Energy Information Administration.

The federal agency attributed the uptick in nuclear generation to a combination of uprates, shorter outages and balance-of-plant thermal efficiency improvements but cautioned that the future does not offer as much room for improvement.

In a new report, "Annual Energy Outlook 2019," the EIA said U.S. nuclear generation surpassed its previous peak of 807.0 million MWh in 2010 with a slightly higher 807.1 million MWh for 2018.

"Although several nuclear power plants have closed since 2010, a combination of added capacity through uprates and shorter refueling and maintenance cycles allowed the remaining nuclear power plants to produce more electricity," the EIA said. "In the near future, however, EIA expects that U.S. nuclear power output will decline."

The EIA noted that the only new reactor to come online in the U.S. since 1996 was the Tennessee Valley Authority's 1,270-MW Unit 2 at the Watts Bar nuclear plant in Tennessee, which started operating in fall 2016. In contrast, seven reactors with a combined capacity of 5.3 GW have retired since 2013.

One reason the U.S. nuclear power fleet has been able to maintain generation levels of near 800 million MWh for over a decade despite the net loss of power plants is that several generators modified their facilities to increase capacity, the EIA said. "2.0 GW of thermal power uprates [occurred] between 2010 and 2018, nearly the equivalent of adding two new reactors similar to Watts Bar Unit 2."

"Nuclear power plants have also shortened the time they are out of operation for refueling or maintenance," the federal agency said. "Nearly all of the recent reduction in outage duration is attributed to shorter outages for refueling operations."

The EIA noted that the average reactor outage in 2018 lasted about 25 days. But as nuclear power plants typically refuel every 18 to 24 months, the EIA cautioned that some of the annual fluctuations in nuclear output are attributable to how maintenance cycles align across the fleet.

"The 2018 peak level of U.S. nuclear generation is not likely to be surpassed in the coming decades," the EIA said.

Only two new nuclear reactors are scheduled to come online in the near future: Units 3 and 4 of Southern Co.'s majority-owned Alvin W. Vogtle Nuclear Plant in Georgia in 2021 and 2022, respectively, with a combined capacity of 2.2 GW. That is not enough to offset the capacity expected to be lost in the next seven years, according to the agency. Twelve reactors have announced their plans to retire in that time frame, and U.S. nuclear capacity is expected to fall 10.5 GW by 2025 as a result, the EIA said.

At the beginning of 2019, the U.S. commercial nuclear fleet consisted of 98 reactors at 60 nuclear power plants, but Entergy Corp.'s approximately 683-MW Pilgrim nuclear plant in Massachusetts is slated to retire in June, and Exelon Corp.'s 829-MW Three Mile Island nuclear plant in Pennsylvania will follow suit in September.

Of those 98 reactors, 89 have applied for and received 20-year license extensions beyond their initial 40-year terms from the U.S. Nuclear Regulatory Commission. The agency also is expected to approve an operating license renewal for NextEra Energy Inc.'s majority-owned, 1,249-MW Seabrook nuclear plant in New Hampshire.

The EIA said opportunities for additional uprates of nuclear reactors are shrinking. Citing the NRC, the EIA said only 60 MW of thermal uprate applications are anticipated through 2020. The agency said unfavorable market conditions of relatively low wholesale electricity prices and flat energy demand growth are not providing the financial incentive nuclear plant owners need to invest in improvements that would increase the output from the existing fleet of aging nuclear reactors

The EIA projected in its report's reference case that as more nuclear plants shut down, net electricity generation from U.S. reactors will fall 17% by 2025. That loss is expected to be largely offset by power produced by new natural gas, solar and wind facilities, the EIA concluded.