ROCKVILLE, Md. — The Nuclear Regulatory Commission on Thursday tightened radiation-exposure limits for nuclear plant workers and residents near atomic power facilities. The revision was the first in more than 30 years.

The agency said the new regulations, by reflecting recent scientific studies, will provide an extra margin of safety. Yet, the commission said, the current standards do not put workers at risk.

Anti-nuclear groups immediately criticized the new regulation, which does not take effect for two years. Critics argued that the tightened limits still put people at too high a risk of cancer and are based on outdated health information.

“The commission is straining its credibility by adopting an obsolete standard,” said the Nuclear Information and Resource Service, an anti-nuclear umbrella group. It said the new standards reflect recommendations made by leading scientists 13 years ago.

The NRC’s revised standards, approved unanimously by the commission, will replace radiation-exposure limits in effect since 1957 for atomic plant workers and people living near nuclear facilities.

The new rules require that, by January 1993, the nuclear industry assure that:

The maximum exposure to the general population from atomic facilities be no more than 100 millirems a year, a fifth of what is now allowed.

Atomic workers be exposed to no more than 5 rems of radiation a year. Now, some workers can be exposed to four times that amount under certain conditions, an NRC official said.

Plant operators be required to adopt policies keeping radiation doses “as low as reasonably achievable” instead of merely being urged to do so, as is now the law.

According to government studies, the average American can expect to be exposed to about 360 millirems of radiation from various sources a year. Radiation from an X-ray is about 6 to 7 millirems.

Atomic plant workers on average are exposed to an additional 400 millirems or so annually, although that amount can be considerably higher, NRC officials said.

The new rules also establish tighter exposure standards for pregnant workers at atomic facilities, limiting exposure to 500 millirems over the course of the pregnancy.

A rem, which is the standard measure of radiation, equals 1,000 millirems.

The new rules “reflect changes in the basic philosophy of radiation protection” and provide “a substantial increase in safety,” said Donald Cool, chief of the NRC’s radiation protection and health effects branch.

The requirements will affect commercial nuclear plants, various laboratories and facilities involved in uranium production.

Cool said most of the nuclear industry has procedures in place aimed at meeting the tighter standards, although some uranium-processing facilities may have to alter their operating procedures.

The cost of compliance for the nuclear industry was estimated at $170 million.

The new requirements are based on recommendations to limit radiation exposure made by the International Commission on Radiological Protection in 1977.