The Reagan administration yesterday unveiled its plan for revitalizing the nuclear power industry and proposed that utilities no longer be required to make changes to increase the safety of their atomic plants "if a plant is operating at an acceptable level of risk."

Under the new procedures, an Energy Department official said, "if a plant had a good record for 10 years," proposed safety changes might no longer be imposed on it.

Energy Department officials said their plan was designed to shorten the hearing and licensing process for nuclear power plants, and that it has already been "approved and accepted by the administration."

The administration said modifications now routinely ordered by the Nuclear Regulatory Commission to increase the level of safety at existing atomic power plants would no longer be required if the plants had been "deemed to be safe" under the old criteria.

A newly discovered "increase in risk that was significant," the administration said, would not require remedial action by utilities operating older atomic power plants as long as it did not create an "unacceptable level of risk."

This provision and other elements of the plan came under immediate attack from members of Congress and the NRC.

Rep. Edward J. Markey (D-Mass.), chairman of a House oversight subcommittee, said the administration's proposal regarding safety changes "would dramatically curtail the government's ability to require the nuclear industry to make their plants safer.

"Given the history of belatedly discovered problems and 27 currently acknowledged 'unresolved safety issues,' such a policy is clearly a step in the wrong direction," Markey said.

While NRC Chairman Nunzio J. Palladino, a Reagan appointee, declined comment on the administration's proposals yesterday, Commissioner Victor Gilinsky, who was reappointed by President Carter in 1979, faulted what he termed the administration's "cockeyed" premise that "the regulators are what is at fault, and what is needed is to stop the regulators."

"I think it's a terrible mistake to weaken the safety regulators," Gilinsky said. "If nuclear power depends on anything, it depends on the public having confidence that its safety is being taken care of. Nothing is going to be so detrimental to nuclear power as the public deciding they can't rely on their regulators. There are enough people who believe that already."

But Dr. Shelby Brewer, assistant secretary for nuclear energy, while conceding that "regulatory reform is not the only fix" for the problems that have beset the American nuclear industry, said he thought the licensing and regulatory reforms could lead to a "renaissance of orders" for new atomic power plants in the mid-1980s.

Brewer noted that where it was possible in the early 1960s to build and license an atomic power plant in five years, it now takes an average of 12 to 14 years. He said the administration's proposals were designed to reduce the process to seven years.

In addition to the two changes involving safety modifications and hearings, which he said could be implemented administratively by the NRC, Brewer said the administration would submit legislation -- probably in the next Congress -- to provide for:

* One-step licensing. Utilities now must first seek a construction permit, and then apply for an operating license after construction is completed. The administration would give utilities the option to obtain a construction and operating license in a single pre-construction hearing, and estimates this would cut 8.7 months from the process.

* Early site approval. Utilities now must obtain approval of a proposed site for a nuclear power plant. The administration would authorize NRC approval of sites as suitable for prospective nuclear plants "in advance of a utility's decision to apply for permission to construct," and estimates this would cut 5.6 months from the process.

* Pre-approval of designs. This would allow the NRC to approve, in advance, generic designs for entire atomic plants or major subsystems, so utilities could save an estimated 8.4 months by avoiding the need to seek NRC approval of a custom-designed plant.

Rep. Richard L. Ottinger (D-N.Y.), chairman of a House energy subcommittee that has been holding hearings on public participation in the nuclear licensing process, fired off an angry letter to Energy Secretary James B. Edwards complaining that his staff had been informed only Thursday that the administration's proposed changes would not be made public until after the first of the year.

Ottinger was particularly critical of the administration's proposal to substantially restrict the hearings held by the NRC and its licensing boards, and to greatly curtail the cross-examiniation of witnesses now permitted.

"If they are not going to address one of the most critical issues facing the nuclear power industry, which is the whole question of public confidence, they are going to meet an incredible amount of opposition to construction of nuclear power plants even if the economics should turn around," Ottinger said.