Additional workers from a New Mexico nuclear waste depository will be tested for radioactive exposure, following a leak at the site earlier this month, the Energy Department said.
The 13 employees were working at the Waste Isolation Pilot Plant on Feb. 14, when the leak was detected, The Wall Street Journal reports.
The Energy Department does not yet know the cause of the leak, which is the first reported at the site. The site has been storing waste material coming from research and production of nuclear weapons underground since 1999.
Initial tests reveal the workers were exposed to americium, a nuclear-weapon production contaminant.
While the levels that escaped from the plants are below what require notifying the Environmental Protection Agency, the department is taking care to make sure the plant is brought back online safely.
"This is still serious for us," said Joe Franco, of the Energy Department's Carlsbad, N.M., field office. "We're going to hold our contractor accountable."