MIDDLETOWN, Pa. (AP) _ Four workers at Three Mile Island breathed in a small quantity of radioactive particles inside the nuclear plant's Unit 1 reactor last month, the plant operator said Tuesday.

The technicians, who were not wearing respirators because the area was determined to be safe without them, spent periods varying from two to 27 minutes in the head of one of the reactor's steam generators, said Gordon Tomb, a spokesman for GPU Nuclear Corp.

He said each worker breathed in no more than 10 millirems of radiation.

A millirem is a measurement of radiation. The federal government says exposure to up to 5,000 millirems per year is considered safe.

Tomb said the four workers were also each exposed to between 50 and 260 millirems of external radiation, but that exposure had been expected.

The reactor has been idle since March 1979 when the adjacent Unit 2 was involved in the worst accident in U.S. commercial nuclear history. GPU has said it will be ready to restart Unit 1 by the spring.

Until the company finishes its investigation of the incident, all workers will be required to wear respirators inside the plant, Tomb said.

''We like to eliminate the use of respirators as much as possible'' to allow technicians to work faster, exposing them to less external radiation, he said.