The Nuclear Regulatory Commission and a scientists' group critical of safety at nuclear power plants agreed yesterday that existing formal fire safety regulations are inadequate at the nation's 65 nuclear power generating stations.

The Union of Concerned Scientists, a nonprofit group which says it has 43,000 supporters, asked the NRC to shut down the plants and half construction until the ruies are changed and implemented. The commission's staff said since a 1975 fire it has been requiring additional safety measures without waiting for new guidelines to be issued.

Robert Polland, a nuclear engineer who heads the Union of Concerned Scientists, said NRC tests at the Sandia Laboratories in Albuquerque, N.M., last July showed that the cables of reactor safety systems and the systems' backups burned freely under test conditions despite full compliance with existing safety guidelines.

He provided reporters copies of internal NRC documents and reports describing the test results, which he charged had not yet been made known to federal licensing boards studying pending construction or poerating permit applications. NRC staff members told a congressional hearing in october that all reports of difficulty are routinely transmitted to the boards.

The NRC's Vic Stello, director of the division of operating reactors, said, "There was absolutely no surprise in (the New Mexico) test." It showed the inadequacy of chemical flame retardants and a five-foot gap between cable sets, only confirming what the NRC had known since a March, 1975, cable fire at the nuclear platn in Brown's Ferry. Ala, he said.

"If you have an exposed fire under the cable, indeed it will burn through the cable," he said. Stello noted that the July test did not include fire barriers, smoke and heat detection systems sprinklers and other fire-halting devices proposed in new guidelines now under NRC considerations nd already being implemented nationwide.

"We began a program of backfitting all these plants in 1975," he said. "We haven't been waiting for these guidelines to come out."

Retrofitting can cost up to $25 million per plant, depending on how early in construction the task begins, Stello said. All power plants should be equipped within the coming year, he said. "We didn't have to twist arms. We hven't found the need to impose a new regulation for this purpose."

The dissident scientists' group said in its petition for NRC action tht other Sandia tests found electrical connectors (specialized plugs or joints) on backup safety systems to be inadequate to survive under conditions of a caustic chemical release, extreme heat and pressure that would occur in the events of an accident involving a breakdown in a nuclear reactor's cooling system.

The connector failure could result in radioactive leakage if thee were such an accident, the NRC documents said. "The Sandia results indicate that (the situation) is recurrent and may be very widespread," the scientists' petition said.

The NRC staff responded that the connectors would not be required to function as the scientists' group contended. "The Union of Concerned Scientists has miseonstrued the safety significance of the test results," the NRC statement said.

Pollard, who once said he left the Nuclear Regulatory Commission staff in 1976 because he "couldn't sleep for worrying" about safety conditions, rejected the NRC staff arguments.In the petition to shut down all nuclear plants, he argued that NRC rules require a three-level "defense-in-depth" against fire, in which all three levels must each be regarded as fully reliable: fire prevention, fire control and prevention of damage to safety functions.

Since standards for prevention and control were set when it was thought any fire could be kept from spreading to damage safety equipment, the Sandia tests now require "substantial additional protection" for safe reactor operation. Pollard said, and therefore the plants should be closed until it is provided.