An emergency was declared Tuesday at the Hanford Nuclear Reservation in southeastern Washington after a portion of a tunnel that contains rail cars full of nuclear waste collapsed.
Soil collapsed over a 20-foot square section of a tunnel that was storing contaminated material at Hanford -- a former nuclear weapons production site in the south-central part of the state.
The collapse of the tunnel, made of wood and concrete, was spotted by workers doing routine surveillance.
So far, radiation dangers and injuries appear to have been averted, but tests and monitoring are continuing, with emergency responders on the scene, according to US authorities.
"This hasn't happened before," US Department of Energy spokesman Mark Heeter told the CNN. "There are various projects in this site and occasionally there is spread of contamination."
But never, he said, has there been a tunnel collapse.
Hours after the incident took place, authorities determined there was no initial evidence that workers were exposed to radiation or that there has been an "airborne radiological release."
"All personnel are accounted for, there are no injuries," Hanford emergency centre spokesman Destry Henderson said. "There is no evidence of a radiological release."
The accident sparked an alert at 8:26 AM, which prompted federal officials to activate an emergency operations centre at the breached tunnel.