The United States Nuclear Regulatory Commission, or NRC, lists all active nuclear power reactor units in the country on its website, nrc.gov. As of 2015, the NRC has record of 61 operating nuclear power plants containing 99 reactors throughout 30 of the 50 states.
With three separate nuclear power plants, Illinois is the highest-ranking state in both net summer capacity, measured in megawatts, and net generation, measured in thousand megawatt hours. Pennsylvania and South Carolina rank second and third, respectively, in both categories.
The two oldest nuclear power reactors in the country, Oyster Creek and Nine Mile Point Unit 1, began operation in 1969, while the newest reactor, Watts Bar Unit 1, opened in 1996.