US Nuclear Power Plants Fast Facts

The North Anna, Virginia, #1 and #2 nuclear power generation stations operated by Dominion Virginia Power are seen in this aerial photo on March 24, 2011, at Lake Anna, Virginia. (PAUL J. RICHARDS/AFP/Getty Images)

(CNN)Here's a look at nuclear power plants in the United States.

The nuclear power reactor units provide the United States with about 20% of its electricity.
Other Facts:
The Nuclear Regulatory Commission or NRC maintains a threat assessment branch, which assesses security threats to nuclear facilities throughout the country.
    Sixty-five reactor units are pressurized water reactors and 34 are boiling water reactors. These two types of reactors the only reactors that are in commercial operation in the United States.
    There are 31 licensed and operating research reactors in the United States. The majority are located at universities.
    Since September 11, additional security measures have been put in place at nuclear facilities, including increased patrols, additional physical barriers and enhanced officer training.
    Timeline:
    May 26, 1958 - As a part of President Dwight D. Eisenhower's "Atoms for Peace" program, the first commercial nuclear power plant opens as the Shippingport Atomic Power Station.
    March 28, 1979 - The Three Mile Island nuclear plant near Middleton, Pennsylvania partially melts down due to a combination of factors including design flaws, equipment malfunction and human error. The accident remains the most serious nuclear power incident in the United States.
    January 1990 - Fuel removal from Three Mile Island is completed.
    August 23, 2011 - The North Anna Nuclear Power Station near Mineral, Virginia shuts down due to a 5.8 magnitude earthquake.
    October 22, 2015 - The Watts Bar Unit 2 nuclear power reactor, located in Tennessee, receives a full operating license from the NRC. Watts Bar is the first US reactor authorized to operate since 1996 and is also the first plant to comply with new orders from the NRC on Fukushima-related safety strategies.
    Region I - oversees 25 reactor units. (The Indian Point Energy Center in New York has two reactors in one facility that the Energy Department counts as two separate plants).
    Region II - oversees 33 reactor units.
      Region III - oversees 23 reactor units.
      Region IV - oversees 19 reactor units.