Investigators for the Nuclear Regulatory Commission found a second pocket of tiny radioactive particles yesterday about 300 feet from a nuclear research reactor in Gaithersburg.

The discovery of cobalt-60 particles near a drainage pipe outside the fenced-in reactor buildings at the National Bureau of Standards prompted bureau officials to renew assurances that none of the small bits of radioactive metal found since Tuesday pose any danger to the public or to breau employes.

But the source of the contamination has not been determined.

The search for the origin of the particles, which were found on two sides of the building and on its roof, is now focusing on a discarded heat exchanger which used to be attached to the reactor to help cool it. The exchanger has been lying outside the building for the last five years, a bureau spokesman said.

The highest levels of radiation discovered outside the building so far were located under the exchanger, which is about 15 feet long and 2 to 3 feet wide, according to Victor Stello, chief of investigations for the NRC.

"I'm not prepared to tell you that the [heat exchanger] is the source of the radiation," Stello cautioned yesterday. But he added, "It is clearly an area to look at very hard."

The heat exchanger rests behind the building, inside a perimeter fence marking a restricted area.

Cobalt-60, the radioactive metal involved, is not used in or produced by the bureau's small reactor, which is used for a variety of research projects. The metal, however, often is found as a trace element in steel, although it would not normally be radioactive.