What was billed Thursday as a potentially serious problem in the nation's nuclear power plants "peaked and became a non-problem" yesterday as the Nuclear Regulatory Commission accepted industry assurances that no hazard exists.

The NRC staff had been worried about new research findings that the covering of nuclear fuel rods, or cladding, might swell under certain conditions to the point of blocking the flow of cooling water around the reactor core. Urgently summoned to Washington, representatives of the five companies that manufacture nuclear fuel produced computer analyses that showed, they said, that those dangerous conditions never occur in a real reactor.

"The problem completely went away," said Darrell G. Eisenhut, chief of the NRC's operating reactors division. "We want to have it confirmed, but based on all the information we have in hand, we don't have a problem.

NRC Chairman Joseph M. Hendrie said that since the situation "occurred, peaked and became a non-problem" in the space of about 24 hours, perhaps another look ought to be taken at the regulations under which it arose.

Eisenhut agreed after briefing the five commissioners that the false alarm might further damage the NRC's already tarnished credibility, but that under the circumstances he had not had any choice about bringing it up.

"If people come in and say we may have missed this (calculation) by about 1,000 degrees (Farenheit), I don't have any option," he said. "We would certainly be under greater criticism if we had sat on this and done nothing."

Two years of research for the NRC by the Oak Ridge National Laboratories in Tennessee concluded last Monday that a loss of coolant accident (LOCA) in a reactor could cause temperature and pressure conditions threatening the fuel cladding. The cladding could swell more than had been predicted previously and could block the flow of cooling water around the reactor core.

That, Eisenhut had said Thursday, could mean the nation's 70 operating nuclear plants might be forced to cut their power output to avoid any chance of overheating the reactor cores.

But industry representatives convinced the NRC staff in meetings Thursday and yesterday that other factors in the way fuel rods are constructed, plus factors that control temperature and pressure conditions after a LOCA, mean there is no problem.

"I am confident today that we do not have a safety problem based on what we have seen here," said Harold R. Denton, NRC's reactor regulation chief, at the commission briefing.

NRC staff members confirmed that there had been some mathematical errors in their preliminary analyses that tended to inflate the initial, alarming conclusions. However, Eisenhut said, the errors were minor, involving at most a change of 20 degrees Farenheit, and would not have affected his decision to launch an all-out investigation.

The probe was required by an NRC regulation called Appendix K. It requires nuclear plants to be able to maintain certain temperature and pressure conditions after a LOCA. The ranges of numbers in Appendix K were formed "by not-so-good data" and let to unrealistically low limits, according to Roger Mattson, director of the NRC's systems safety division. a

"This crisis atmosphere is not warranted," Mattson told the commission. "It's highly disruptive to an orderly regulatory process."

Research director Saul Levine went further. Reexamination of Appendix K was moving at a glacial pace before Three Mile Island . . . and now it's at zero, he said. The entire false alarm was "an artifice of Appendix K," he said. "It was a non-issue."