U.S. officials expressed confidence yesterday that evacuations could be managed successfully if a "worst-case" nuclear power-plant accident occurs in this country, although critics contended again that such plans are inadequate to deal with a serious accident.

U.S. commercial nuclear plant reactors are surrounded by thick containment shells that would prevent rapid and voluminous radiation release such as that occurring near Kiev in the Soviet Union, the officials said.

The disaster has inflamed debate raging in communities nationwide since a small amount of radiation was released in a 1979 accident at the Three Mile Island (TMI) plant near Harrisburg, Pa.

Such communities include those surrounding the nuclear plants closest to Washington, D.C.: Calvert Cliffs 1 and 2 on the Chesapeake Bay in Calvert County, Md.; Surry 1 and 2 in Gravel Neck, Va., and North Anna 1 and 2 in Mineral, Va.

Within 15 minutes of what officials term an "unusual event" at 11-year-old Calvert Cliffs, state emergency management officials and their counterparts in three counties would begin mobilizing resources to evacuate thousands from within 10 miles of the plant, according to plans mandated by the Nuclear Regulatory Commission (NRC).

Civil defense teams in parts of Calvert, St. Mary's and Dorchester counties would go door to door, directing residents to one of a dozen shelters.

Workers at Calvert Memorial Hospital in Prince Frederick would begin establishing checkpoints for detection of radiation contamination, and the Calvert County chapter of the American Red Cross would call in volunteers to handle feeding and sleeping arrangements.

Every two years, evacuation exercises are held in conjunction with state agriculture, health and public-safety officials and "graded" by the NRC and the Federal Emergency Management Agency (FEMA).

"It takes us two months to gear up for the full-scale exercise," said Edward Murray, director of the Maryland Emergency Management and Civil Defense Agency. "We've got more than 400 people involved . . . . "

In the worst-case scenario, he said, about 20,000 persons would have to be evacuated from within 10 miles. Most would leave using their own transportation, and others would be bused to shelters, he said.

With exceptions, Murray said, the last full-scale exercise, in October, went well.

"There was some confusion between [air] monitoring teams from the state health department and Baltimore Gas & Electric," the utility that owns Calvert Cliffs, Murray said. U.S. officials also said the Calvert County command center was too cramped, he added.

State and local officials are notified each time an "unusual event" occurs at the plant, which could be anything from a normal shutdown to limited operation. In the last year, five such events occurred there, a BG&E spokesman said.

"Thankfully, they have never progressed anywhere," said Tom Forgette, the company's supervisor of emergency planning. "And we've never had an alert at Calvert Cliffs."

An "alert" would put state and local officials on increased readiness, Murray said, warning county officials within 50 miles of the plant about the problem. As mandated by the NRC, state workers would test soil, crops, water and milk for radiation contamination within that area, he said.

At the next readiness level, he said, the Federal Aviation Administration and Coast Guard would be alerted to keep nearby air space and bay waters clear of traffic.

When the TMI accident occurred, U.S. utilities were not required to have emergency plans. Although Pennsylvania officials suggested only that a few thousand pregnant women and children should be evacuated, about 144,000 people fled.

Since then, elaborate emergency evacuation plans and drills have been required at all U.S. commercial plants, but critics have said the plans are based on unrealistic assumptions about human behavior.

"Is the United States prepared to evacuate New York City? Obviously, the answer is no," said Robert Alvarez, a nuclear expert at the Environmental Policy Center, a Washington lobby group.

Should a serious accident occur at the Indian Point nuclear station on the Hudson River 30 miles from Manhattan, and winds spread radiation south, as much as "90 percent of the radioactivity could be deposited on the city" in rain, as has happened in parts of the Ukraine, he said.

Edward L. Jordan, the NRC's director of emergency preparedness, said Soviet and U.S. reactors are "vastly different." An accident such as that near Kiev would be "highly unlikely . . . but not impossible," he said.

"We do not plan for the absolute worst case," Jordan said. "The absolute worst case is no containment, no water [surrounding nuclear fuel rods] and a meltdown dispersed as it may," as in the Soviet accident.

If such an accident occurred at Indian Point, "it would be extremely difficult" to evacuate New York City, he acknowledged.

Openings of two major nuclear plants are being delayed in Shoreham, Long Island, 50 miles from New York City, and in Seabrook, N.H., 30 miles from Boston, because opponents contend that surrounding areas cannot be safely evacuated.

Nora Bredes, head of a Shoreham opponents group, said the Chernobyl accident might persuade the NRC not to license the plant without a "realistic" plan. If an accident happened, she said, "panic" would occur.

However, FEMA's Samuel Speck noted that more than 1 million people were evacuated successfully ahead of East Coast hurricanes Gloria and Elena last year.