The Nuclear Regulatory Commission has tentatively decided that the Three Mile Island nuclear power plant accident was not an extraordinary event.

If the finding stands, it will make it much harder for people who claim millions of dollars damages from the March 28 incident to press their cases in court.

The NCR has asked for 30 days of public comment on a staff-level decision that the incident does not meet the formal criteria for being labeled an "extraordinary nuclear occurrence."

The label was set up in 1968 as part of the Price-Anderson Act, which regulates nuclear industry insurance, as a way of expediting court action on claim from some big disaster.

If an event is declared an extraordinary occurrence, those claiming damages from it do not have to prove negligency on the part of the industry. The power plant or company involved cannot defend itself by saying that the person filing suit assumed some risk -- for example, by agreeing to live next door to the plant ot by refusing to leave. And the statute of Limitations on lawsuits is extended.

"the only way that victims or potential victims will be able to reasonably recovery damages is by having the NCR declare Three Mile Island an extraordinary nuclear occurrence," said Richard Pollock, head Mass, which has petitioned to intervene in the decision." "Otherwise Metropolitan Edison will be protected in such a way that the victims will be tied up in court for years."

Metropolitan Edison Co., which owns Three Mile Island, is a defendant in a consolidated class action suit pending in U.S. District Court in Harrisburg, Pa., along with its parent company, two other subsidiaries, the power plant builder (Babcock & Wilcox Co.) and its parent company, and a marntainance firm, Catalytic Inc.

There were originally 28 suits, but all were joined into one for trival purposes. Those pressing claims divided themselves into three groups for the suit: individuals, businesses and professional groups living within 25 miles of the plant who suffered economic damage; property owners in that area who evacuated or suffered economic damage, and, the most controversial grouping, individuals who suffered personal injury, medical expenses or "emotional distress," or who are "threatened with medical expenses" now and for "a reasonable number of years in the future."

These categories are subject to judicial review, which is the next step in the procedure. But if they stand, all they would have to do to collect after an "extraordinary nuclear occurrence" is proved that damage was suffered and that it was caused by Three Mile Island.

The tentative denial of that label by the NCR "is what we would expect," said John Harkins, a Philadelphia attorney defending the companies in the case. "there is no possibility that the criteria [for applying the label] would be satisfied . . unless there is new information that nobody has at the moment."

To reverse the preliminary decision would be catastrophic for the industry, according to a spokesman for the American Nuclear Energy Council, the industry's lobbying arm.

You'd be saying a guy coming down with cancer 20 to 30 years from now could claim it had to do with Three Mile Island. It would open up every kind of suit . . . It would just be catastrophic," the spokesman said.

Criteria for the label in NRC regulations are Very specific. The NRC would have to find that offsite radiation could have given a person at least 20 rems of exposure, or that offsite property received a complex high dosage of contamination, and in addition that at least $1 million in total damages was done or that five or more people were killed or hospitalized.

"Based on the information available to the NRC staff at this time, it appears that neither part . . . is statisfied" by the events of Three Mile Island, the staff said in a July 20 memo.

"Both personal exposures and property contjmination are presently considered to be far below the levels specified.

That, responded Pollock, is because "the criteria are hopelessly out of date. You have look at a whole new body of research on the biological effects of radiation."

He said Critical Mass would formally petition the NRC to update its criteria for an "extraordinary nuclear occurrence" finding if the tentative ruling should stand. A decision is expected sometime in September.