The Nuclear Regulatory Commission has discovered "a potential major problem" that could require drastic long-term power reductions at every nuclear power plant in the country.

The problem involves possible swelling under high temperatures of the cladding, or covering, around nuclear plant fuel rods. A two-year NRC research program concluded this week that the swelling could be much greater than had been thought, enough under some conditions to block the flow of cooling water around the reactor core.

The NRC summoned fuel manufacturers to an urgent meeting yesterday to discuss the situation through the weekend. A decision on the gravity of the problem and on any necessary action is expected early next week.

Darrell G. Eisenhut, director of the NRC's division of operating reactors, said in an interview that the research program involved only the behavior of the cladding metal, a zirconium alloy called ZircaLloy. He said it is possible that the five manufacturers have built their fuel rods so that other factors cancel out the potential problem of swelling. That is what the meeting is trying to determine, through analysis of computer models and equipment specifications.

"I would describe it as a potential major problem that may or may not result in a need for the plants to be derated," or reduced, in their power output, Eisenhut said.

Derating ensures that temperatures in the reactor core would not reach dangerous levels should there be a loss of coolant accident (LOCA).

It also means that the reactor could not produce electricity at full output; the amount of the reduction would depend on the characteristics of each plant. Nuclear plants account for 13 percent of America's generated power.

Ralph Nader and his Critical Mass organization immediately called for a congressional investigation to determine whether all operating reactors should be shut down. They planned to petition the NRC today for immediate shutdown while the situation is being evaluated.

The behavior of reactors under LOCAs is the subject of most reactor safety study. Since the March 28 accident at Three Mile Island, which involved a small LOCA, research has shown that any size LOCA generally uncovers the core for at least a few seconds.

"The swelling of the cladding occurs in those few seconds," Eisenhut said. Before this week, it had been assumed that the cladding on the fuel rods burst relatively quickly, thus releasing pressure and stopping the swelling before channels could be blocked around the rods.

The NRC research, which was conducted at Oak Ridge National Labratories in Tennessee, found that the cladding "will swell more than was previously thought before it bursts," Eisenhut said. Depending on the reactor structure and the time the core is uncovered, the swelling might or might not block the flow of cooling water down through the fuel assembly.

Any water blockage would cause internal temperatures to rise, and "under some conditions in some plants, the temperature could go above 2,200" degrees Fahrenheit, Eisenhut continued, possibly "significantly higher. . . on the order of hundreds of degrees more."

The cladding begins to crumble at 2,200 degrees, further blocking the cooling water. But the radioactive fuel itself is in no danger of melting until temperatures reach 5,000 degrees, Eisehut added.

The Three Mile Island accident invovled disintegration of the fuel cladding, but Eisenhut said he did not think a swelling problem was involved there. "It's certainly something we're looking at," he said.

Einsenhut said he received notification of the research findings on Monday and immediately called a meeting of the five firms that manufacture fuel rods: Combustion Engineering of Windsor, Conn.; General Electric Co. of San Jose, Calif.; Westinghouse Electric Corp. of Pittsburgh; Babcock & Wilcox Co. of Lynchburg, Va., and Exxon Corp.'s fuel manufacturing division in Richland, Wash. Nuclear plant-owning utilities were also notified of the meeting and about 15 of them are represented at the meeting in the NRC's Bethesda office, Eisenhut said.

He described the mood of the meeting as "interested, concerned. We're catching them a little by surprise."

The vendors were given "a very strong request" that they attend the meeting since the NRC has no official authority to compel them to do anything, Eisenhut said. Should the companies's computer analyses reveal potentially dangerous conditions at any particular plant, the NRC could order derating of those plants.