Government agencies have failed to develop adequate evacuation plans for population centers near the nation's 68 nuclear power plants even though 300,000 deaths could result from a single nuclear plant accident, according to a study released yesterday by a national environmental group.

The study by Peter G. Cleary, staff physicist for Chicago-based Citizens for a Better Environment, found that only four states - Connecticut, South Carolina, Washington and New Jersey - have any sort of emergency evacuation plan and alleges that those probably wouldn't be effective.

"No federal planning is being carried out by anyone, least of all the agency that should be doing it, namely the Nuclear Regulatory Commission," Cleary said at a press conference. "The NRC disclaims any responsibility for ensuring that the public is evacuated in the event of large radio-active releases that would result from an accident," he said.

Robert DeFayette, reactor safety engineer in the NRC's Office of Emergency Preparedness, acknowledged yesterday that nuclear plants are permitted to operate in states without evacuation plans.

"I don't know what to tell you," DeFayette said. "We don't have any legal clout over them." The NRC and environmentalists agree that the most likely type of accident is a reactor core meltdown, which would release 1,000 times as much radioactivity as released by the atomic bomb dropped on Hiroshima.

The NRC has estimated that the odds of such an accident are one in 17,000 reactor years, meaning that, with 250 nuclear plants expected to be in operation in the United States by the year 2000, one could occur every 68 years.

NRC has estimated that, in the event of a meltdown, only 3,300 persons would be killed immediately by radiation and 45,000 more would die of long-term illnesses like cancer.

But NRC acknowledges that estimate is based on the assumption that there is an adequate evacuation plan and that residents would be warned of the accident before any radioactivity was released.

The Union of Concerned Scientists, a group which includes several prestigious researchers, estimated in a recent study, that in the absence of an evacuation plan, a meltdown would cause 300,000 immediate and long-term deaths.

David Comey, executive director of the environmental organization, said, "An effective evacuation plan is a complete pipe dream."

Comey said that in two recent minor nuclear accidents, authorities were not notified until days after the emergency had ended.