The leak was discovered in an aboveground waste transfer line at the tank farm in the 200 West area of the south-central Washington site, where contractor CH2M Hill Hanford Group Inc. has been unearthing long-buried containers of waste.
The leak measured about 15 to 18 feet in diameter.
Environmental monitoring picked up the traces of radioactivity Friday morning, and at about 11 a.m. workers were ordered to remain in place as a precaution. By 3:30 p.m. they were being moved out of the area, said department spokeswoman Kim Ballinger.
Surveys of the area confirmed no hazard to employees, Ballinger said.
Authorities said they believed the leak started and stopped sometime Thursday night, and that no employees were in the area at the time. The contaminated ground was contained to keep the leak from spreading and officials were determining how best to clean the area, Ballinger said.
Typically, many more than 50 workers would have been on hand, but many had Friday off work, she said.