WASHINGTON (Reuters) - The nation's 103 nuclear power reactors are vulnerable to acts of terrorism and the government should immediately station soldiers and missiles around each plant for protection, two watchdog groups said on Tuesday.

Nuclear power plants are located in 31 states and provide about 20 percent of the nation's electricity supply.

The Washington-based Nuclear Control Institute and the Los Angeles-based Committee to Bridge the Gap urged the government to immediately station 30 to 40 National Guard troops around each nuclear plant to protect it from attacks.

The watchdog groups also said the government should be prepared to deploy anti-aircraft weapons to shoot down attack planes. Another needed measure is to carefully re-check the background of all nuclear plant employees and contractors to prevent internal sabotage.

U.S. soldiers would have about seven seconds to fire a missile and destroy a commercial airliner that is one mile from a reactor and traveling 500 miles per hour, the groups said.

The groups, which monitor the spread of nuclear weapons, said they prepared a detailed analysis of which U.S. nuclear plants were most vulnerable. However, that report will be given only to the Nuclear Regulatory Commission (NRC), they said.

NUKE PLANTS IN DANGER?

"It is prudent to assume, especially after the horrific, highly coordinated attacks of Sept. 11, that (Osama) bin Laden's soldiers have done their homework and are fully capable to attack nuclear plants for maximum effect," Paul Leventhal, president of the Nuclear Control Institute, told a news conference.

The groups underscored what they see as an immediate danger by noting that nearly half the U.S. nuclear plants in routine NRC-supervised tests failed to repel mock attacks.

"The new threat should now be evident to all, and the country can afford to wait no longer," said Daniel Hirsch, president of the Committee to Bridge the Gap. "The vulnerabilities at these plants can, and must be, closed now."

U.S. plants stepped up security after the Sept. 11 attacks, which left nearly 7,000 people dead or missing.

"We take the security threat very seriously," said NRC spokesman Victor Dricks. "In light of the terrorist attacks, it's only prudent that we look at our security regulations to make sure they're adequate to meet the challenge."

The NRC has acknowledged that it is unsure if U.S. nuclear power plants could withstand the crash of large, commercial airplanes, such as the kind that attacked the World Trade Center and Pentagon. The nuclear facilities, all of which are more than 30 years old, were designed to withstand tornadoes, hurricanes and earthquakes.

AIRPLANES NOW SEEN AS THREAT

A direct, high-speed hit by a large passenger jet "would in fact have a high likelihood of penetrating a containment building" that houses a nuclear power reactor, said Edwin Lyman, scientific director of the Nuclear Control Institute.

A plane's fuselage would likely crumble on impact but its engines are made of stronger steel and would probably break through a reactor's concrete shell, according to the groups.

In such an event, the release of radiation could result in widespread effects downwind of the plant. Many of the nation's nuclear plants are located near large cities, Lyman said.

Nuclear power reactors are enclosed in concrete walls of up to 4.5 feet (1.35 meters) thick. Concrete shielding the reactor domes is typically up to 3.5 feet (1.05 meters) thick.

U.S. power plants located along airline flight paths were built with the toughest concrete shells.

The industry group representing nuclear power plants, the Nuclear Energy Institute, defended security measures. In the few cases where plants were found vulnerable in NRC-supervised tests, each adopted stricter safeguards, the group said.

"These mock attacks are part of a robust regulatory program for security fulfilled by all nuclear power plants," said Joe Colvin, president of the industry group. He said the watchdog groups' claims were incorrect and alarmist.

Lynnette Hendricks, a NEI nuclear licensing expert, said utilities have begun rechecking all workers and contractors at plants.

However, NEI does not support the idea of placing soldiers at all plants because only the federal government has the intelligence information to determine if and when extra protection is needed, Hendricks said.

The nation's nuclear plants were already protected before Sept. 11, but if the government believes troops are needed as a visible deterrent, NEI would support such a move, she added.