A group critical of nuclear power plants said today that while it foresees no direct health effects from a government proposal to release radioactive krypton gas from the damaged reactor at Three Mile Island, it remains opposed to the plan.

The Union of Concerned Scientists said it believes that venting the gas into the atmosphere "may seriously exacerbate" psychological problems suffered by local residents since the accident at the plant last year.

The venting plan, recommended by Metropolitan Edison Co. and the Nuclear Regulatory Commission, has engendered angry opposition from people living near the plant, which is 10 miles south of here.

Recent studies have shown that between 10 and 20 percent of the 200,000 people living in the vicinity of the plant have suffered some type of physical or behavioral problems because of the accident.

The NRC staff and Met Ed have repeatedly said that the amount of krypton to be released into the air is so small as to pose no health hazard. The UCS, in a 63-page report released here, agreed with the assessment.

But because of the fears of residents, it recommended two alternative plans to release the gas high above the idle plant, greatly reducing the exposure to local residents.

Gov. Richard L. Thornburgh, who asked the UCS to study the venting issue, said he would announce his position on Friday. However, at a news conference, the governor strongly hinted that he is prepared to endorse the Met Ed-NRC plan.

"The mere fact that the Union of Concerned Scientists has found no direct adverse biological health affects will go a long way toward reducing the psychological stress in the area," the governor said.

"This report may well amount to an emancipation from fear for the people of this area."

About 57,000 curies of krypton-85, a product of nuclear fission, have been lodged in the containment building of the TMI plant since the accident on March 28 of last year. A curie is a measure of radiation.

Met Ed and the NRC staff have warned that the longer the gas remains in the plant, the greater the chance of unplanned leaks. Gas has periodically leaked from the plant since the accident. Also, they have said the gas must be removed before employes can enter the containment building to continue repairs on the badly damaged plant. The first human entry into building since the accident is planned for next Tuesday.

One UCS alternative would involve heating the krypton in a small incenerator and releasing it through a 250-foot stack. The release point would be about 1,000 feet above the ground.

A second proposal would channel the gas through a nylon-coated tube held 2,000 feet aloft by a tethered balloon. The report said the Air Force has found this idea feasible.