Inspectors for the Nuclear Regulatory Commission have found traces of radioactive material at three locations near the test nuclear reactor at the National Bureau of Standards complex in Gaithersburg, officials of both agencies reported yesterday.

The traces of the radioactive metal cobalt-60, which were discovered during a routine annual inspection of the grounds, present no danger to the public or to the bureau's 2,500 employes, according to Robert Carter, chief of the Bureau of Standards Reactor Radiation Division.

However, neither Carter nor Nuclear Regulatory inspectors have determined how the tiny pieces of cobalt-60, each no larger than a grain of sand, came to be clustered behind the reactor building, on its roof, and at a spot in front of the building.

"It's a very serious problem to us. The material that we have found does not represent a health or safety problem to the public or our workers, but the fact that it was there and we don't as yet know how it got there is very important to us," Carter said.

The bureau test reactor, which has been used for research projects almost continuously since it began operating 10 years ago, is about a hundredth the size of a power-generating reactor like the one the Baltimore Gas & Electric Co. operates at Calvert Cliffs, Carter said.

Cobalt-60 is not produced either as a main product or a byproduct of bureau experiments, Carter said, although he added that "it's possible some materials around the reactor may have trace elements of cobalt in them" and that this colbalt was made radioactive during the reactor's operation.

Small amounts of cobalt-60, most commonly known for its use in radiation therapy for cancer patients, are used at the bureau to test such calibration equipment as geiger counters, the same was a tuning fork is used to test the pitch of a musical instrument.

Carter and regulatory commission official Victor Stello, who directs the commission's office of inspection and enforcement, both stressed that inspections to determine the source of the cobalt-60 including examinations of lawn equipment and employes' -- shoes are still under way.

All the radioactive material discovered outside the fence surrounding the reactor builidng has been removed, bureau spokesman Matt Heyman said yesterday. The "final clean-up" of radioactive material inside the fence probably will be finished by today, he said.

Stello, the chief commission inspector, said that this was the first instance in his memory of radioactivity being discovered outside the perimeter of a local reactor. The maximum radiation emitted by the particles found on the grounds were 2 millirems per hour, and none of the radiation would be measureable outside the bureau compound, he added.