The Nuclear Regulatory Commission has declared that irradiated animal carcasses and some laboratory equipment no longer must be classified as low-level nuclear waste.

The action affects about 15 percent of the country's low-level nuclear waste, which now may be buried at only three sites -- in Washington state, South Carolina and Nevada.

It is the first time that disposal controls on substances used in the laboratory have been removed.

The carcasses and so-called "scintillation vials," in which irradiated tissue samples are tested, now can be burned or buried in ordinary landfills, the NRC said.

This will save hospitals and research laboratories $13 million annually in transport, packaging and disposal costs while posing no danger to the public, the NRC said.

At the same time, the NRC sextupled the amount of radioactive tritium and carbon 14 that it will permit to go into public sewage systems, calling the increase from one to six curies "a negligible addition to the amount of radioactivity from these materials already present in the natural environment."

The Critical Mass Energy Project immediately criticized the action as "a step in the wrong direction." Experience has shown that low-level wastes can migrate through underground streams to contaminate nearby sites, director Richard Pollock said. "There need to be set standards and careful site selection for any radioactive waste," he said.

An estimated 80,000 cubic feet of burial space at existing low-level waste dumps is used every year for 55-gallon drums containing carcasses of rats, mice and other animals in which radioactive trace elements have been used to help trace effects of experimental drugs.

Another 400,000 cubic feet is taken up by drums containing vials in which the animals' blood or tissue is analyzed.

These two items comprise half of all radioactive medical waste. A large research hospital might produce 750 cubic feet per year of carcasses and vials, enough to fill 100 of the 55-gallon drums, according to Richard E. Cunningham, director of the NRC's fuel cycle and material safety division.

Cunningham said the rest is composed of X-ray film, wiping rags and other materials. All of it contains "very low levels" of radioactivity, mostly in the form of tritium or carbon 14, he said.

A person living about 125 feet downwind from an incinerator burning only carcasses and vials from a major hospital, he said, would receive one million of additional radiation per year. A coast-to-coast round trip by jetliner gives three millirem of additional radiation.

"Historically, it was cheap and simple to send everything to a waste burial ground," Cunningham said. About two years ago, disposal costs and transport charges began rising as burial space dwindled, and the three burial sites closed down temporarily in protest over lax enforcement of waste packaging rules.

"At that point, the labs recognized this as a problem, and we did, too," Cunningham said. Burial itself often posed difficulties because decomposition gases occasionally ruptured buried drums. The new policy will solve those problems by permitting conventional disposal methods to handle the wastes, Cunningham said.