Nuclear power is responsible for more than one-fifth of the electricity used in the United States and is the second-largest source of energy generation in the country. The production process is relatively inexpensive and does not release the environmentally harmful emissions produced by other popular methods of energy production. But the process is not without risks; a radiation leak in Pennsylvania's Three Mile Island reactor in 1979 and the 1986 Chernobyl meltdown in the former Soviet Union demonstrated the dire consequences of a plant malfunction. After the 9/11 terrorist attacks, a range of observers expressed concern over the security of U.S. nuclear facilities. President Bush's new Advanced Energy Initiative, which calls for sizeable investment in the nation's nuclear power facilities, has again focused attention on the vulnerability of nuclear plants to terrorist attacks.

Most nuclear facilities are well fortified and difficult for terrorists to attack. But they remain attractive targets because of the potential to inflict devastating damage. An attack on a nuclear plant could release a high level of radiation that would gravely endanger public health. A 2004 study by the Union of Concerned Scientists says a successful attack on the Indian Point nuclear facility thirty-five miles north of Manhattan could cause as many as 44,000 near-term casualties, and 500,000 long-term deaths from cancer.

The 9/11 Commission Report noted that both Mohammed Atta, one of the 9/11 pilots, and Khalid Sheikh Mohammed, the mastermind of the attacks, "considered targeting a nuclear facility." In October 2001, U.S. officials shut down operations at the Three Mile Island plant in Pennsylvania for four hours and suspended flights at nearby airports, citing a "credible threat" of terrorism. The alert turned out to be a false alarm.

U.S. nuclear power facilities are privately owned, and each plant operator is required to provide security systems and personnel. Though they may receive peripheral assistance from the National Guard and Coast Guard, the private forces are responsible for plant security. Security measures must conform to the guidelines set by the Nuclear Regulatory Commission (NRC)—an independent federal commission charged with overseeing civilian use of nuclear materials. The NRC lists terrorist capabilities nuclear sites must be able to defend against, known in the industry as the design basis threat (DBT). The DBT includes scenarios for a ground attack, a waterborne assault, and an attack by an insider, but does not include an airborne attack. The DBT specifies details such as the number of attackers, their tactics, and the weapons and explosives at their disposal. To check compliance, the NRC periodically conducts "force-on-force" inspections in which NRC officials and security personnel use laser guns to simulate an attack on a nuclear plant.

Even prior to the 9/11 attacks, nuclear plants had extensive security measures in place. Each plant has a trained security force and a series of physical barriers. Security personnel undergo thorough background checks and submit to lengthy personal searches when entering and exiting the plant. The physical barriers consist of an "owner-controlled" buffer zone of land around the facility, a restricted-access "protected area," and a further restricted "vital area." Double fences, barbed wire, and surveillance systems are common. The containment vessels for nuclear reactors are among the world's sturdiest man-made structures. The vessel at the Indian Point plant, for instance, is made of three-and-a-half-foot thick concrete reinforced by three-inch thick steel bars.

After 9/11, the NRC began a top-to-bottom review of its security requirements, and in 2003, issued new orders to tighten security. Some $1.25 billion was spent on these measures, which included adding security barriers and detection equipment, creating more rigid access control, and increasing the number of security personnel by 60 percent. The NRC also revised the DBT to include what it claims is "the largest reasonable threat against which a regulated private guard force should be expected to defend." While details are classified, experts say this covers an assault by multiple armed attackers.

A 2002 report by the Aircraft Owners and Pilots Association suggests a nuclear reactor would remain intact if crashed into by an average private aircraft. Even a much larger commercial jet, such as a Boeing 757, would not cause critical damage to a reactor, the report concluded. NRC studies have reached similar conclusions. Peter D. Zimmerman, a professor at King's College, London who was among the authors of a National Academies report on spent fuel storage, says, "I think there are some reactors where it is possible to fly an airplane into the spent fuel pool and crack it open." He says, however, that the consequences of such an attack are unclear.

Nuclear facilities are not required to have plans in place to repel such an attack. The U.S. Transportation Security Administration, the agency charged with protecting the nation's transportation system, is responsible for preventing a 9/11-style airborne attack. Nuclear plant operators are in close contact with the national air defense command, and, in the event of such an attack, the two groups would cooperate to head off the assault or mitigate the impact.

Experts disagree. At an April 4 hearing of the House Committee on Government Reform, the Government Accountability Office (GAO) submitted a report saying recent revisions to the DBT appeared to be "based on what the industry considered reasonable and feasible to defend against rather than on what an assessment of the terrorist threat called for."

"One of the critiques is that the DBT does not encompass a 9/11-scale threat," says CFR Science and Technology Fellow Charles Ferguson. While the specifics of the DBT are classified, experts in the field say nuclear plants are only required to withstand an attack by a handful of well-armed terrorists, possibly working with one or two insiders. Critics say plants should be able to withstand an attack by at least nineteen terrorists, the number of men who carried out the 9/11 attacks. Another point of contention is that the DBT does not require security personnel to prepare for terrorists armed with rocket-propelled grenades (RPG) or .50 caliber sniper rifles with armor-piercing rounds. Such sniper rifles are legally sold in many states, and RPGs can easily be found on the black market.

In their defense, nuclear industry officials claim the cost of defending against a more robust DBT is too great. Testifying at the House hearing, Marvin Fertel, vice-president of the Nuclear Energy Institute, acknowledged that the current DBT does not represent the full spectrum of threats, but said, "Nuclear power plants are the most secure commercially owned facilities in the country." But not all plants have passed their inspections when audited under the current DBT standards. The authors of the GAO report observed one inspection in which a plant's security "was at best questionable in its ability to defend against the DBT."

Most experts suggest revising the DBT to more accurately reflect the terrorist threat nuclear plants face. Beyond that, there are a variety of suggestions to improve security. One of these is better management of the spent nuclear fuel. Used fuel rods are currently placed in containment pools, which experts say could be vulnerable to a terrorist attack. One way to reduce this risk is to move spent fuel into more secure storage containers. While this cannot be done immediately, Zimmerman says "it is wise to move the fuel to dry storage casks sooner rather than later."

Ferguson recommends addressing the nuclear industry's complaints about the high cost of extra security. "We need some hard numbers to determine how much additional money we are talking about," he says. If the price tag proves too high, he recommends an assessment of all the nation's nuclear plants to identify the most vulnerable sites, which would then receive additional resources.

Daniel Hirsh, president of the nuclear watchdog Committee to Bridge the Gap, suggests nuclear plants implement a two-person rule whereby "no single person can be left alone in a vital area." While this doesn't completely prevent the possibility of sabotage, it would greatly reduce the likelihood, he says. He also advocates the construction of beamhenge shields, large steel cages constructed around nuclear plants to offset the risk of attacks from airplanes.

Around each nuclear power site is a ten-mile emergency planning zone (EPZ). Within the EPZ, nuclear plant operators are required to maintain emergency sirens and conduct evacuation drills. Since 9/11, some critics have called for an expansion of the EPZ, especially for plants near large population centers.

One of the main effects of an accidental or terrorist release of radiation from a nuclear facility would be the dispersal of large quantities of radioactive iodine, which can concentrate in the human thyroid gland and cause serious health problems. Administering iodine orally prior to exposure can prevent the absorption of the radioactive iodine. In 2002, the NRC began distributing iodine pills to populations within several EPZs. Iodine pills do not protect against other forms of radiation, and this program is only available in states that have requested it.