A real alternative is available to the idea of venting radioactive gases trapped inside the crippled Three Mile Island nuclear power plant, but the utility and nuclear regulators are trying to ignore it, Rep. Allen Ertel (D-Pa.) charged yesterday.

Many of Ertel's constituents around the Middletown, Pa., reactor are frightened at the prospect of releasing the gases into their air, no matter how small the amount of krypton 85 that is involved, Ertel said.

He asked the Nuclear Regulatory Commission to reconsider its earlier action in writing off an alternative to venting that his staff and one NRC commissioner, Victor Gilinsky, are convinced could handle the krypton: a process called selective absorption.

Metropolitan Edison Co., which owns Three Mile Island, is pressing the NRC for permission to vent the gases soon, to reduce radioactivity inside the reactor building enough to allow maintenance crews to enter. Vital instruments and ventilation equipment have been running in highly humid conditions without any maintenance since the accident occurred March 28, 1979, and Met Ed is worried that sudden failure could allow the reactor to begin reheating.

The NRC has been obtaining public comment on the idea of venting and expects to decide the issue next month. The NRC staff already has recommended venting, on grounds that the radiation released would be much less than other reactors release routinely and would pose no public health danger. The NRC staff also says no other option is available that could be in place in less than a year.

But Ertel said yesterday that selective absorption equipment could remove the radioactive krypton without venting and could be in place in six months.

"It's simple, it was developed for this purpose, there is a pilot plant in operation . . . and we can get rid of all the krypton or reduce it substantially before the gas is vented," Ertel told a news conference.

The pilot plant at Oak Ridge, Tenn., would have to be redesigned somewhat and increased 10 to 15 times in size, but the parts, personnel and technology are either available or could be made available quickly, Ertel said.

"If Met Ed had spent [the last 14 months] putting this in place instead of trying to convince people to vent, we would have had it ready and the gases out of the building by now," he said.

An off-the-cuff cost estimate by Department of Energy experts came to $1 million to $4 million, Ertel said. But he said he had not asked for detailed figures. "Met Ed has told us cost is not a factor and I accept them at their word," he said.

The selective absorption system involves passing the gases through a separator that removes 99.9 percent of the radioactive krypton 85 and then either returns the gas to the building or vents it. Returning the gas and recycling it to remove the last bit of krypton would take nine weeks, while a once-through process would take three weeks, Ertel said.

The Energy Department told the NRC that the process of designing, constructing and installing the system could take two or four years. "Those estimates were not as good as they should have been," said NRC Commissioner Gilinsky. He said he would seek to have the option reevaluated after DOE turns in an updated report on it that Ertel requested.