Hanford's C Tank Farm was evacuated shortly after noon Tuesday as a precaution after a small plume of smoke or steam was spotted coming from a new ventilation system.
Workers were allowed back into the area, which has 16 underground tanks, about 90 minutes later. No worker was near the equipment when the plume was spotted, and no worker was exposed to radiation, Washington River Protection Solutions reported.
The Hanford Fire Department is investigating the source of the plume.
Hanford's tank farms hold 56 million gallons of radioactive and hazardous chemical waste in underground tanks arranged in groups called "farms."
A new ventilation system was being tested at the C Tank Farm to prepare for pumping radioactive waste out of Tank C-107 -- a single-shell tank -- and transferring it to a newer double-shell tank. The system had not been connected so there was no radioactive contamination.