UPDATE: (NBC News) - Some 200 workers at the Hanford Nuclear Site in Washington state were ordered to "take cover" Tuesday after a tunnel containing "contaminated materials" collapsed, the U.S. Department of Energy reported. 

The alert was declared at 8:26 a.m. local time after the cave-in covered "railroad tunnels near a former chemical processing plant," the feds said.

All workers at the plant, which sits along the Columbia River, were ordered inside and access to the affected area is now "restricted to protect employees." 

"There are no reports of injuries, no reports of radiological release," said Destry Henderson, deputy news manager for the Hanford Joint Information Center. "I would underscore this is confined to a small area of the Hanford site." 

"I can confirm we are investigating a small area of soil that had sunken," Henderson added. "This soil covers a tunnel used to access a former chemical processing facility." 

Citing a source, the local NBC affiliate reported road crews working nearby might have created enough vibration to cause the collapse. 

Hanford was dubbed the "Most Toxic Place in America" last year in an NBC News expose.

This used to be where plutonium was produced for America's nuclear arsenal. Now it's run by the Department of Energy and its contractor, Washington River Protection Solutions, and is in the midst of a massive $110 billion cleanup of 56 million gallons of chemical and nuclear waste that is stored in 177 underground tanks. 

The job is expected to take at least 50 years to complete.

PREVIOUS STORY: (KING-TV) - Hundreds of workers were in "take cover" position after a tunnel in a plutonium finishing plant collapsed at the Hanford Nuclear Reservation early Tuesday morning.

The tunnel was full of highly contaminated materials such as hot radioactive trains that transport fuel rods.

A source said that crews doing road work nearby may have created enough vibration to cause the collapse.

A manager sent a message to all personnel telling them to "secure ventilation in your building" and "refrain from eating or drinking."

A source also said that Vit Plant employees are in cover mode as well.

The AP reports "there are concerns about subsidence in the soil covering railroad tunnels."

Randy Bradbury, a spokesman for the Washington state Department of Ecology, told the AP there apparently has been no release of radiation and no workers were injured.