Five federally operated nuclear reactors in the United States lack "containment" facilities to prevent release of radioactive material in an accident. One is an Energy Department reactor in Hanford, Wash., that uses graphite technology similar to that believed used in the Soviet reactor near Chernobyl.

Scientists have speculated that the absence of a containment facility like the steel-and-concrete buildings that encase most U.S. and foreign nuclear plants may have been a key factor in the widespread contamination that followed the apparent meltdown of the Soviet reactor.

According to Energy Department and private nuclear experts, four other DOE reactors operate without containment facilities, all for weapons-production at the Savannah River plant in South Carolina.

In a hastily arranged briefing for a House Interior and Insular Affairs subcommittee yesterday, Energy Department officials urged members of Congress not to focus on containment facilities until more information is available on the cause of the Soviet accident.

"It is possible there could be a bearing [on U.S. facilities]," said James W. Vaughn, acting assistant secretary for nuclear energy. "It is premature to speculate on that."

But environmental groups said the accident's impact is clear enough to underscore the risks of the federal facilities, which are not licensed by the Nuclear Regulatory Commission because of their defense-related work.

"The nuclear weapons program is self-regulating," said Robert Alvarez, a nuclear specialist with the Environmental Policy Institute. "It makes its own decisions, and it is not accountable to the same pressures as the commercial industry. If the NRC were asked to license those reactors, they would have to close them down."

The Savannah River reactors, constructed in the early 1950s, produce weapons-grade plutonium for nuclear bombs. All four are so-called "heavy-water" reactors, cooled by a special water made with deuterium instead of hydrogen.

The Hanford "N" reactor, which began operation in 1963, is a dual-purpose reactor that makes plutonium and generates electricity. Like the Soviet reactor near Chernobyl, it uses graphite to "moderate" or absorb neutrons during the fission process.

"There are some similarities, but there are also some differences," said Vaughn, who called the Soviet reactor "uniquely a Russian design, built in Russia to Russian standards."

Other experts said the Hanford reactor was "very similar" to the Soviet design and posed similar risks. Sketchy reports from the Soviet Union suggest that a major graphite fire still rages at the Chernobyl reactor. A similar graphite-moderated reactor in Britain caught fire in 1957 in what is considered one of the worst nuclear accidents in history.

Vaughn said the British accident prompted technological improvements, and "the quality of graphite is such that it is much less susceptible to fire." He was unable to say whether the Soviet reactor, constructed in the late 1970s, also used improved graphite technology.

Asked if the government had considered closing the Hanford reactor, Vaughn replied, "We don't have any information to suggest that's necessary. It would be premature to take action when we don't know whether it's relevant or not."

DOE officials acknowledged that the Hanford facility had no containment building. Asked if any other reactors lacked containment facilities, Vaughn said, "I don't have that knowledge at my fingertips."

According to other DOE officials and environmental groups, however, none of the Savannah River reactors is so equipped. "They have filters and some metal housing, but no concrete bubble," Alvarez said. "They were built in the 1950s, and if they put a containment on now, they would basically have to build a new reactor."

The Savannah River complex has long been shrouded in secrecy because of its military activities, but documents obtained by Alvarez and West German physicist Bernd Franke and published last year in the scientific journal Ambio indicate that a large radioactive release occurred there in March 1955.

Plant operators attributed high radioactive readings in the area then to above-ground nuclear tests in Nevada, although the test plume did not pass over South Carolina.

Only one commercial power plant in the United States uses a graphite technology, and questions were raised yesterday about its containment facilities.

The reactor, a 7-year-old, 330-megawatt facility operated by Public Service of Colorado in Fort St. Vrain, about 35 miles north of Denver, differs from the Soviet reactor in that it is cooled by helium instead of water and is considered a "low-power density" reactor that uses relatively small amounts of nuclear fuel at a time.

Manager Gary Reeves said the Fort St. Vrain facility has a concrete, steel and graphite liner as much as 15 feet thick. "It is a prestressed vessel, and it encompasses the entire reactor," he said. "Everything is totally enclosed."