The Nuclear Regulatory Commission has put national security at risk by allowing newly hired personnel to start work without security clearances and by failing to reinvestigate many veteran workers, a government auditing agency said Wednesday.

The General Accounting Office said it found that since 1983 about 10 percent of NRC employees who began working before their security investigations were completed later left the agency because the checks disclosed drug-related or other personal problems that made them unacceptable security risks. It said nearly all workers are granted this security waiver.The report was the subject of a public hearing Wednesday before a House Government Operations subcommittee, which requested the GAO investigation in 1987.

At NRC headquarters in Rockville, Md., spokesman Robert Newlin said, "We are planning improvements in our programs that are responsive to the GAO recommendations." He said details of these changes would be revealed to the House panel.

The GAO report criticized the NRC for failing to reinvestigate nearly half of the approximately 10,600 people who hold security clearances. Only those holding the most sensitive jobs are subjected to reinvestigation, but the GAO said results of those probes indicate a need to extend the practice to all workers.

"Reinvestigations revealed cleared persons with histories of child molestation, drug abuse and criminal activities," the report said.

"Periodic reinvestigations can highlight these problems and provide NRC information needed to determine whether an employee could be a possible candidate for activities, such as espionage, that would endanger national security."

The GAO said it reviewed a sample of NRC's personnel security files for clearances since 1983 and found several cases that underscored the need for stronger security rules.

It cited an example of an NRC branch chief who was granted the highest level of security clearance in 1979 and then suspended in 1983 after the employee was indicted on 52 counts of interstate transportation in aid of racketeering. The worker was found innocent, but the NRC later fired him after learning that the worker had a business interest in an outcall massage and escort service.

The NRC said it does not reinvestigate workers with lower-level clearances because of the added cost and because those employees have only limited access to classified information.

The report was critical of the NRC's practice of allowing new hires to start before completion of background investigations by the Office of Personnel Management or the FBI. It said the security clearance process takes so long that the requirement is waived for about 99 percent of new employees, including those with access to nuclear weapons design data and other highly sensitive information.

"Waivers have become the rule rather than the exception," the report said.

The GAO investigators said a review of NRC files since 1983 showed that in about 10 percent of cases in which security clearance requirements were waived the employees later were forced out.