(CNN) -- Here's a look at what you need to know about nuclear power plants in the United States.
There are 100 commercial nuclear power reactor units licensed to operate in the U.S. These operate in 31 of the 48 contiguous states.
The nuclear power reactor units provide the U.S. with 20 percent of its electricity.
Other Facts: The Nuclear Regulatory Commission or NRC maintains a Threat Assessment Team, which assesses security threats to nuclear facilities throughout the country. It also conducts classified security training simulations on a regular basis which include military and surveillance techniques. This team is on call 24 hours a day.
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 35 are "boiling water reactors". The two types of reactors are very similar in design. These are the only reactors that are in commercial operation in the United States.
There are 30 licensed Non-Power Reactor sites in the U.S. The majority are located at universities and research institutes. There are none in Alaska or Hawaii.
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.
Security since September 11 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.
21 nuclear facilities are located within five miles of an airport.
There is no "no fly zone" over the plants.
Nuclear Regulatory Commission regions The country is broken into 4 regions:
Region 1 - oversees 26 reactor units in 8 states, New Hampshire, Massachusetts, New Jersey, Connecticut, Vermont, New York, Maryland and Pennsylvania.
Region 2 - oversees 32 reactor units in 9 states, Alabama, Georgia, Florida, Kentucky, North Carolina, South Carolina, Tennessee, Virginia, West Virginia and Mississippi. As well as Puerto Rico and Virgin Islands.
Region 3 - oversees 23 reactor units in 8 states, Minnesota, Wisconsin, Michigan, Iowa, Illinois, Missouri, Indiana and Ohio.
Region 4 - oversees 19 reactor units in 8 states Arizona, Arkansas, California, Kansas, Louisiana, Nebraska, Washington and Texas.