As of 2016, the United States had 99 operating nuclear reactors at 61 plants across the country, with a capacity-weighted average age of 37 years. The oldest operating nuclear reactor in the United States was built in 1969. Watts Bar 2, which entered commercial service in 2016, was the first new reactor added since 1996. An additional four reactors are currently under construction. Operation of nuclear plants at high capacity factors enabled them to contribute nearly 20% of total U.S. electricity generation in 2016 while only making up 9% of U.S. generation capacity.
Of the 99 gigawatts (GW) of total operating nuclear capacity in the country, 95 GW came online between 1970 and 1990. However, planned nuclear capacity additions began to slow as early as the late 1970s because of a number of factors, including slowing electric demand growth, high capital and construction costs, and public opposition. Costs, schedules, and public acceptance were all influenced by the accident at the Three Mile Island plant in 1979. From 1979 through 1988, 67 planned builds were canceled. However, because of the long times required for permitting and building new nuclear reactors, many plants that had begun the process in the 1970s continued to come online through the early 1990s.
U.S. nuclear plants are licensed for an initial operating life of 40 years by the Nuclear Regulatory Commission (NRC). Owners of nuclear power plants can apply for a license renewal, extending license expiration by 20 years. The decision to apply for a renewal is based on the economics of the capital investments required to extend the operating lifetime and estimated future revenues. As of 2016, the NRC had granted license renewals to 84 of the 99 operating reactors in the United States.
Nuclear capacity has decreased in the United States in recent years as plants have retired. The retirement of the Fort Calhoun Nuclear Generating Station in October 2016 marked the fifth nuclear retirement since 2013. Several other plants have announced plans to retire in the near future (including Oyster Creek, Pilgrim, Palisades Unit 1, and Indian Point Units 2 and 3) totaling more than 4 GW of capacity. In addition, Pacific Gas and Electric announced that it will not seek license extensions for its Diablo Canyon nuclear power plant, meaning Diablo Canyon Units 1 and 2, with a combined capacity of more than 2 GW, will be retired by the time their current licenses expire in 2024 and 2025, respectively.
Nuclear capacity can increase either by building new reactors or by instituting changes that allow existing plants to increase their generating capacity, known as uprates. Four new reactors are under construction and are expected to bring more than 4 GW of capacity online. Uprates of existing plants require NRC approval, and almost all U.S. reactors have applied and received at least one uprate. Through 2016, these uprates have contributed more than 7 GW to total U.S. nuclear capacity.
Nuclear plants have higher capacity factors than any other electricity generating technology, averaging 90% over the past five years. Because nuclear plants run near full capacity for much of the time they are operating, they serve as baseload generation. Refueling and maintenance outages at nuclear plants are typically scheduled during the spring and fall periods of lower electricity demand. Nuclear plants typically refuel every 18 to 24 months, and over the past few years these outages have typically lasted about six weeks.