The operator of the Three Mile Island nuclear plant was told yesterday it could release into the atmosphere the radioactive gas that has been held inside the plant for the last 15 months.

In a unanimous vote, the five commissioners of the Nuclear Regulatory Commission gave permission to the Metropolitan Edison Co. to vent to the air above the plant as much as 57,000 curies of radioactive krypton to allow technicians in protective suits to enter the plant and begin decontaminating the thousands of gallons of radioactive water inside the plant.

"I know there's been great concern and strain over this but we've concluded it poses no physical health hazards," NRC Chairman John Ahearne said yesterday at a crowded meeting at NRC headquarters. "Venting to the atmosphere is the best choice we have before us."

Ahearne said the NRC would not issue the order that allows Met Ed to begin venting the krypton gas to the air until Thursday. He also said that Met Ed must give the public at least 10 days notice before venting the gas, which means the earliest release date would be June 22.

In defending their action, the commissioners concluded that venting the krypton is the necessary first step toward cleaning up the contaminated water inside the plant. Nuclear engineers agree that the contaminated water poses a far more serious health hazard than the gas that sits inside the plant above the water.

"The potential health hazards are associated with the water and the badly damaged fuel in this plant," Ahearne said. "There will be some people who believe otherwise but these are the conclusions we've made."

The subject of bitterly contested hearings and legal delays, the purging of the radioactive krypton from the stricken Three Mile Island reactor building was bemoaned by at least three women who came down from Middletown, Pa. to the NRC meeting yesterday. One wept at the end of the meeting; the others aired their feelings for radio and television newscasters.

In granting permission to Met Ed to purge the plant, the NRC laid down stiff guidelines for the utility to follow. The NRC ordered that Met Ed not release the krypton gas in any way to expose people standing outside to more than allowable doses of radiation.

"We estimate that to the maximally exposed individual, the risk of skin cancer would be equivalent to spending 30 minutes in the sun," the NRC staff said in a report to the commission. "The average individual in the population would have an added risk of skin cancer equal to about a half-second exposure to the sun's rays."

The NRC estimated that the restrictions will extend the purging operation to as long as 60 days.

"Purging will be done on a real-time meteorological basis; everything will depend on the winds," said Bernard Snyder, director of the TMI Program Office for the NRC. "Once purging begins, the conditions will be updated every hour."

Though there is as much as 57,000 curies of radioactivity in the krypton gas inside the plant, more than 99 percent is beta radiation, which does not penetrate the skin. A tiny fraction is gamma radiation, which does penetrate the skin.

"Even that is what we call a soft gamma ray," the NRC's Frank Congel said. "If you breathe it in, you're exhaling it and getting rid of it 10 minutes later."

Congel emphasized that the krypton to be purged from Three Mile Island is a small fraction of the krypton still circulating in the atmosphere from nuclear weapons tests conducted in the last 10 years by France and China. He also said it is no more than the krypton vented routinely to the atmosphere by European nuclear reprocessing plants.