U.S. Nuclear Power Plants

The North Anna, Virgina, #1 and #2 nuclear power generation stations operated by Dominion Virginia Power are seen March 24, 2011, at Lake Anna, Virginia, in this aerial photo. The Lake Anna Reactor is ranked 7th most at-risk for earthquake damage. According to the Nuclear Regulatory Commission, North Anna #1 and # 2 face an annual 1 in 22,727 chance of the core being damaged by an earthquake and exposing the public to radiation. The national average for US nuclear plants is a 1 in 74,000 chance. The top five most at-risk plants are all on the east coast: Indian Point, north of New York City; the Pilgrim Plant south of Boston, Limerick outside of Philadelphia, the Sequoyah plants near Chattanooga Tennessee and Beaver Valley near Pittsburgh. These five plants are at a higher statistical risk than those along fault lines in California, for example, because they were not designed for and built in presumed strong quake danger areas. Since they were constructed the US federal government has revised upwards the quake risks where they are. According to Jim Norvelle with Dominion Power, North Anna was designed to withstand a magnitude 5.9  6.1 earthquake.AFP Photo/Paul J. Richards (Photo credit should read PAUL J. RICHARDS/AFP/Getty Images)

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

There are 99 commercial nuclear power reactor units licensed to operate 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.

Reactors are massive structures, typically constructed with two to five feet of steel-reinforced concrete. The containments have an interior steel lining, and redundant safety equipment to add further protection.

65 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 that includes increased patrols, augmented security forces and weapons, additional security posts, heightened coordination with law enforcement and military authorities and additional limitations on access to the site of personnel in vehicles.

    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 built near Middleton, Pennsylvania goes into meltdown due to a faulty pressure valve in the Unit 2 reactor, design-related problems, and human error. The Three Mile Island accident remains the most serious nuclear power incident in the United States.

    1989 - The Shippingport Atomic Power Station becomes the first successfully decommissioned nuclear power plant in the country.

    January 1990 - After five years, 99% of the damaged fuel from Unit 2 reactor at Three Mile Island has been removed. The damage was so severe that the unit remains unusable and in storage.

    August, 29, 2011 - North Anna Power Station near Louisa, Virginia is damaged during the 5.8 earthquake that hits northeastern U.S. The epicenter of quake is less than 20 miles from power station. No radiation leaks are detected.

    February 9, 2012 - The Nuclear Regulatory Commission approves plans to build two new nuclear reactors near Augusta, Georgia. It is the first approval of new reactors since 1978.

    Nuclear Regulatory Commission regions
    The country is broken into 4 regions:

    Region 1 - oversees 25 reactor units in 7 states, Connecticut, Maryland, Massachusetts, New Hampshire, New Jersey, New York, Pennsylvania and Vermont.

    Region 2 - oversees 32 reactor units in 7 states, Alabama, Florida, Georgia, North Carolina, South Carolina, Tennessee and Virginia.

    Region 3 - oversees 23 reactor units in 6 states, Illinois, Iowa, Michigan, Minnesota, Ohio and Wisconsin.

    Region 4 - oversees 19 reactor units in 10 states Arizona, Arkansas, California, Kansas, Louisiana, Mississippi, Missouri, Nebraska, Texas and Washington.