A group of scientists critical of nuclear power said yesterday that 16 nuclear power plants should be shut down and the rest re-examined in light of the government's recent repudiation of a major nuclear safety study.

"We are in a race against time," said Robert Pollard, a safety engineer for the Union of Concerned Scientists (UCS). The Washington-based group said defects in electrical systems make the 16 plants subject to fires that could knock out safety mechanisms and lead to a catastrophic release of radiation.

Continued operation of the power plants had been justified by a 1975 study called WASH-1400, or the Rasmussen report, which evaluated the mathematical possibilities of accidents, the UCS said.

However, the Nuclear Regulatory Commission (NRC) last week withdrew its endorsement of the Rasmussen report, saying it was unreliable and misleading. The action caused consternation in nuclear power circles, which had grown accustomed to citing the report as an assurance that nuclear power was safe.

"With the Rasmussen report estimates now invalidated, continued operation of these plants... is unjustified," Pollard told a news conference.

NRC Chairman Joseph M. Hendrie, responding to the charge, said the commission had asked for a staff evalulation of the degree to which the Rasmussen report had been used in licensing and other regulatory actions.

"Based on what they've looked at so far, the preliminary conclusion is that there has been no significant reliance on the [Rasmussen] safety study," Hendrie said. An NRC spokesman said the specific issues raised by the UCS had been reviewed "to some extent" and would be checked further.

Hendrie said the commission would take "whatever action is necessary" at the end of the staff review in the next few weeks.

Rep. Morris K. Udall (D-Ariz.), chairman of the House energy and environment subcommittee with jurisdiction over the NRC, said he would hold hearings beginning Feb. 26 on the commission's hadling of the Rasmussen report. "I'm concerned that we understand how this study was used and that corrective measures be taken if necessary," he said.

The scientists' group divided the 16 affected plants into two groups. Twelve, it said, "are now operating where a fire could destroy all the cables controlling the multiple systems intended to prevent a reactor meltdown."

Those are Brunswick I and II in Southport, N.C.; Haddam Neck, Conn.; Oconee I, II and III in Seneca, S.C.; Pilgrim I in Plymouth, Mass.; Rancho Seco at Clay Station, Calif.; H. B. Robinson II in Hartsvile, S.C.; There Mile Island I and II in Middletown, Pa., and Trojan in Prescott, Ore.

Pilgrim I and Haddam Heck and four other plants have safety equipment that was either not tested or faultily tested for ability to perate after an accident, the UCS said. The four other plants are D.C. Cook I in Bridgman, Mich.; Browns Ferry III in Decatur, Ala.; Yankee Rowe in Rowe, Mass., and Ginna in Ontario, N.Y.