The U.S. Department of Energy in Richland declared an emergency at the Hanford nuclear site in Washington on Tuesday morning after a portion of a tunnel containing rail cars storing nuclear waste collapsed.

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

An emergency was declared at 8:26 a.m. after employees noticed the collapse in the 200 East Area during routine surveillance, said Hanford Emergency Center spokesman Destry Henderson.

About a 400-square-foot area of the ground sunk in near where two underground tunnels join, the Department of Energy said. The tunnels, one 360 feet long and the other 1,700 feet, originally were used in the 1950s to store contaminated equipment. The tunnels have 8 feet of soil covering them.

A 20-foot section of one of the tunnel's roofs caved in, Henderson said.

No workers were in the tunnel.

Six employees at the site of the tunnel collapse were evacuated, Henderson said. All employees in the Hanford site were told to take cover after inspectors found the collapse.

It's still unclear what caused the cave-in, Henderson said.

Officials were using a robot to get closer, up to a half-mile, to monitor "radiological and industrial hygiene," the Department of Energy said.

The Northwest News Network reported about 3,000 employees work in the 200 East Area.

"At the moment we're focusing on the safety of workers and making sure there's no release beyond immediate site," Washington Gov. Jay Inslee told reporters at the scene.

.@GovInslee on #Hanford: "At the moment we're focusing on the safety of workers and making sure there's no release beyond immediate site." pic.twitter.com/TrDo88ZT8L — Molly Solomon (@solomonout) May 9, 2017

Everyone is accounted for and there is no initial indication of worker exposure or airborne radiation, the Department of Energy said in a statement on Twitter.

Nearby roads weren't affected, officials said.

The collapse initially triggered an alert but was later upgraded to a "Site Area Emergency," which means the incident could affect people in the site but not beyond its boundary. Residents of nearby Benton and Franklin counties weren't required to take any action, the Department of Energy said.

The Oregon Department of Energy activated its emergency operations center in response to the accident as a precaution, the department said in a tweet. Nuclear experts at the Oregon department said the incident didn't affect Oregon.

"While there are special precautions being taken on-site at Hanford, Oregonians do not need to take any special precautions or protective actions," said department Director Michael Kaplan said in a statement. "We want people to be aware that we're closely monitoring the incident at Hanford and will continue to provide more information as soon as it becomes available."

The sprawling Hanford site is near Richland and is half the size of Rhode Island. It is about 35 miles north of Umatilla.

The tunnel collapsed near the Plutonium Uranium Extraction Plant, a facility built in the 1950s that is no longer used today, according to the Department of Energy. The plant processed plutonium from 1956 to 1972, and again from 1983 to 1988 -- more than 70,000 tons of uranium fuel rods, about 75 percent of the plutonium at Hanford.

Some scientists believe no other building on the planet processed more plutonium, according to the Department of Energy website. The building has been vacant for more than two decades but remains highly contaminated, according to the website.

Hanford is the largest depository of radioactive defense waste that must be cleaned.

U.S. Sen. Ron Wyden criticized the Department of Energy's past handling of the nuclear site in a statement Tuesday.

"Today's cave-in at the PUREX plant should remind everyone that the temporary solutions DOE has used for decades to contain radioactive waste at Hanford have limited lifespans," Wyden said. "The longer it takes to clean up Hanford, the higher the risk will be to workers, the public and the environment."

The site contains about 56 million gallons of radioactive waste, most of it in 177 underground tanks.

Several rail cars used to transport the fuel rods were buried inside the tunnel near the plant.

Employees north of the site's Wye Barricade and outside the 200 East Area were sent home from work at noon as a precaution. Later, non-essential employees in the 200 East Area were sent home, officials said on Twitter. Swing shifts north of the Wye Barricade were canceled.

Hanford employees should follow the site's Facebook page and Hanford's alert system for updates on the status of Wednesday shifts, Henderson said.

Officials continue to monitor the air and are discussing how to fix the collapsed tunnel roof.

— The Oregonian staff and wire reports