The Nuclear Regulatory Commission was told yesterday that it will never know how much radiation escaped from the plant at Three Mile Island because the levels exceeded the capacities of the plant's instruments to measure them.

"All the radiation monitors in the vent stack, where as much as 80 percent of the radiation escaped, went off scale the morning of the accident," the NRC's Albert Gibson told the five NRC commissioners yesterday.

"The trouble with these monitors is they were never contemplated for use in monitoring accidents like Three Mile Island," he said.

"So we don't really know what went up there?" NRC Commissioner Victor Gilinsky asked. "Up through the vent stack?"

"That's correct," Gibson replied.

There are three radiation monitors in the vent stack, Gibson said, and another five monitors in the pathways leading to the stack. All eight instruments showed the highest radiation levels they could read during the accident, which released airborne radiation for three straight days starting March 28.

Gibson said an average of 30 measurements were made on Three Mile Island and within a three-mile radius of the island each day radiation was released, often when the wind was shifting or when escaping radiation was not at its peak.

On the day of the accident, 365 millirems of beta and gamma rays per hour were measured at ground level 1,000 feet from the vent stack. Directly over the vent stack, a helicopter measured levels three times that.

Both those measurements triggered emergency announcements, leading Pennsylvania Gov. Richard L. Thornburgh to advise pregnant women and pre-school children in a five-mile radius of the plant to stay indoors.

"Those measurements were very inconlusive," Gibson told the NRC. "All they will show will be does rates at the moments the measurements were made, and without knowing the precise weather patterns we don't know if they were made at the appropriate locations."

Gibson said most of the radiation that escaped through the vent stack came through open relief valves and leaking pipes and valves. Until yesterday, the NRC had thought most of the escaped radiation was gas that had bubbled out of contaminated water on the floor of the auxiliary building alongside the reactor.

Gibson told the five commissioners that radiation readings inside the auxiliary building were extremely high at all three levels inside the building, forcing an evacuation of the building the morning of the accident.

"The radiation monitors were all off scale," Gibson said. "One set of readings were greater than 1,000 rems per hour. We don't know how much greater, because the licensee didn't have instruments that measured more than 1,000."

The maximum exposure the NRC allows nuclear plant workers is five rems per year, less than 1 percent of the hourly radiation concenntrations being found inside the auxiliary building the morning of the accident.