A small quantity of radioactive gas leaked inside one of the buildings at San Onofre nuclear power plant north of San Diego, according to a spokesman for the Nuclear Regulatory Commission.

The spokesman said the radiation levels were “barely measurable,” but the plant was shut down as a precaution.

“At no point were the public or our workers in any danger,” Southern California Edison spokesman Gil Alexander told ABC News.

Officials say the radiation leak likely occurred in the steam generator tubes of San Onofre’s reactor #3. The steam system, which is supposed to be shielded from exposure to radiation, was replaced in December 2010. Alexander said plant officials will be conducting an investigation into why the new steam tubes leaked.

Gary Headrick is part of the environmental group San Clemente Green and lives just eight miles away from San Onofre.