The United States is becoming increasingly sensitive to the security of nuclear facilities, adding guards and practicing defenses against mock terrorist attacks.

For example, at the Department of Energy's massive enrichment plant near Portsmouth, Ohio, where weapons-grade uranium is produced for the nation's nuclear arsenal, car searches are no longer limited to interiors. Under-the-hood inspections are now mandatory.

No longer are visitors allowed to walk through unclassified parts of the plant with one escort. Armed guards accompany the escort.

No longer are terrorist attacks against the plant brainstormed by security guards in classroom sessions. They are carried out against the guards in mock night attacks by members of the Army's Special Forces dressed in black and armed with laser weapons that shoot light beams to score "kills."

And the Department of Energy has its ultimate burglar-stopper -- a last barrier to anyone breaking into a container or vault with nuclear material. It is a thick, sticky, molasses-like material that oozes all over the lock mechanisms and halts any further tampering.

There was a time when authorities did not take seriously the prospect of terrorists stealing a nuclear weapon or enriched uranium or plutonium. But terrorism has become not only more widespread, but more sophisticated.

In recent years there have been some indications that nuclear targets are not off-limits to terrorists. The last few years have seen what Sen. Jeremiah Denton (R-Ala.), chairman of the Judiciary subcommittee on security and terrorism, calls the "warning signs that the nuclear aspect of terrorism is real and has to be faced."

French terrorists blew up parts of a French nuclear plant in 1979, causing $20 million in damage. Three years later, members of the same group, called the Pacifist and Ecologist Committee, fired five rockets at France's Superphoenix (Phenix in French) plutonium-producing fast breeder reactor.

When U.S. Army Brig. Gen. William Dozier was rescued in Italy from his Red Brigade kidnapers, he told Italian authorities his captors interrogated him at length about where U.S. nuclear weapons were located in Western Europe. Members of Germany's Red Army faction have been arrested carrying maps and drawings of U.S. nuclear weapons depots and the routes security patrols take outside and inside the depot gates.

Accidents may contribute to the problem. The United States goes to great lengths to prevent the loss of an atomic weapon, but it has lost a few, two of which are still missing.

A nuclear bomb was aboard a Navy A4 fighter-bomber that rolled off a carrier deck into the Pacific Ocean in 1965. Pilot, plane and weapon sank to the bottom 500 miles from the nearest land. They are still there.

A bomb was jettisoned over Wassaw Sound in Georgia in 1958 and sank to the bottom. Luckily, it was not armed with its nuclear components, but did have its high-explosive detonator.

Eight other bombs were lost at sea but recovered by salvage crews.

Nuclear terrorism has not been confined to other countries. There were 14 bombings of U.S. nuclear installations in the last 10 years, plus 300 threats. The bombings caused no serious damage or injury, and the threats so far have been cut short.

One of the most dramatic cases was in 1979, when an employe of a General Electric plant in Burlington, N.C., that fabricates uranium fuel rods for civilian nuclear power plants stole two steel drums containing 145 pounds of uranium in powder form, enriched, but not to full bomb grade. He demanded $200,000 from GE. If he did not get the money, he said, he would disperse the uranium over two unidentified cities in the United States. The FBI moved in on the case at once and caught the man before he could carry out his threats. He was sentenced to 15 years in prison.

"I don't want to be an alarmist, but someday somebody is going to try to steal something" more critical than powdered uranium, retired Air Force general William W. Hoover, now assistant secretary for defense programs at the Energy Department, said in an interview. "We must be ready for that threat."

In preparation, DOE has in the last five years quadrupled its budget to safeguard nuclear weapons and materials to $700 million a year and tightened security dramatically at the 12 DOE installations holding nuclear warheads or components.

These include the Pantex plant in Amarillo, Tex., where nuclear weapons are assembled; the Rocky Flats, Colo., plant where plutonium warheads are made and the Oak Ridge, Tenn., plant where uranium warheads are built; the Nevada Test Site, where atomic weapons are tested, and the Savannah River, S.C., and Hanford, Wash., plants where plutonium is made.

"Anybody who thinks terrorists aren't cunning or ruthless enough to pull off a nuclear attack has forgotten the Munich Olympics, the showdown at Entebbe and the shooting of the pope," Rep. Richard A. Gephardt (D-Mo.), chairman of the Democratic Caucus, told the Conference on International Terrorism sponsored last month by the Nuclear Control Institute. "And anybody who thinks an outlaw country won't help terrorists 'go nuclear' hasn't been to Tehran or Tripoli."

The guard force alone at those DOE installations has been increased by 500 men in the last 18 months. Stronger fences have been built around the plants and more electronic sensors have been added to detect intruders. A year ago, DOE established its Central Training Academy at Kirtland Air Force Base at Albuquerque, N.M., where 800 guards undergo Special Forces training every year.

DOE now recruits guards from the elite of the U.S. armed forces and requires that everyone in its nuclear guard force of 4,500 men run a mile in less than 8 1/2 minutes and 40 yards from the prone position in under eight seconds.

"I'm 53 years old and I went out and passed every test they're asked to pass," Hoover said. "I want my men to be in shape if they ever have to go up against a gang of lean and hungry terrorists."

When an atomic weapon travels by train in the United States, it moves in a gray metal car whose two-ton steel top is locked in place by massive bolts. If the weapon is transported by road, it travels in a truck whose wheels can be locked and whose armor-plated sides can resist any weapon short of bazooka antitank shells.

The NRC does not allow multiple shipments of even small quantities of weapons-grade uranium or plutonium to be on the road simultaneously anywhere in the country, if they add up to what the commission calls "trigger quantities" for either nuclear material. Shipments must reach their destination before new shipments can leave.

The department never transports a nuclear weapon in anything less than an armed caravan, whether it is moving by train or truck. The vehicle moving the weapon is tougher than any commercial armored car and has multiple electronic locks, including a fail-safe lock based on primitive technology..

Instead of relying on tumblers or tricky electronics, this lock employs an older principle of how things become impossible to open. When the lock is tampered with, it "releases a substance that's stickier than molasses into the locking mechanism," Hoover said. "We call it sticky foam, and we think it works."

The United States has managed to export its concern over nuclear terrorism to Europe and Japan, where safeguards against theft are not as strict as they are here. Take the case of a Japanese proposal in 1982 for shipping 557 pounds of plutonium of U.S. origin from France to Japan -- enough for 30 atomic weapons of the same force as the plutonium bomb dropped on Nagasaki in World War II.

Japan proposed that the French package the plutonium and ship it to Britain, where it would be repackaged and put aboard a British container ship. Suggested route of travel was through the Suez Canal, then on through the Straits of Malacca, where piracy is still practiced, then up through the China Sea to Japan. Security was to be provided on deck by two armed men. The Energy Department was appalled and succeeded in delaying the shipment long enough to obtain new security procedures.

When the plutonium left France last October aboard the Seishin Maru, it slipped out of Cherbourg at night with three British warships escorting it through the English Channel, after which it was picked up by a French task force that took it into the open sea.

Once at sea, the Japanese vessel linked up with two American satellites in geostationary orbit 22,300 miles above the Earth that served as radio relays between the ship and the nearest land. A U.S. spy satellite was assigned to follow the ship across the Atlantic Ocean. An armed guard of 40 extra men stood watch as the ship moved across the Atlantic and 10 U.S. warships kept watch along the way until the vessel reached the Panama Canal.

The U.S. Coast Guard took the Japanese ship through the canal, where three U.S. warships took up the task until she was halfway to Hawaii, where the Coast Guard took over again. Once more, satellites positioned over the Pacific and warships along the way watched the vessel's passage to the entrance to Tokyo Bay. There, Japanese patrol boats met the ship and escorted her to a berth in an obscure corner of the harbor a day earlier than called for by her manifest.

Once berthed, the cargo of six containers was unloaded at night under heavy police guard after cranes lifted the massive hatches covering each hold. Each container was put on a separate tractor-trailer with an armed guard in the cab, another in the rear and five police cars leading and trailing the convoy of six trucks to its destination 60 miles northwest of Tokyo.

Security against nuclear terrorism is being taken seriously as never before.