A tunnel full of radioactive waste has caved in at one of the most contaminated nuclear sites in the US

The Department of Energy oversees Hanford. It's a Cold War-era facility that led US production of plutonium for use in tens of thousands of nuclear weapons.

The site no longer produces plutonium. However, millions of gallons' worth of radioactive waste is still stored there, and workers are carrying out a lengthy process of decommissioning the nuclear reservation, which The Seattle Times dubbed "the nation's most contaminated nuclear site" in 2007.

The 586-square-mile site was put under a precautionary "take cover" order on Tuesday because a tunnel containing radioactive materials partially caved in, according to a Hanford emergency bulletin. Nonessential employees were later permitted to leave, and "the last of the employees in the vicinity of the tunnels were released from work" around 4:35 p.m. EDT.

Officials are trying to determine how to address the busted tunnel, according to the bulletin.

What is the emergency?

On Tuesday at 11:26 a.m. Eastern, Hanford posted an emergency bulletin that called on employees to evacuate and take shelter near "a former chemical processing facility" in its 200 East Area — specifically, at a 200-acre facility called the Plutonium Uranium Extraction Plant, or PUREX.

Two long rail yard tunnels feed into the plant. About half-dozen workers discovered the hole in earth covering one of the tunnels this morning, triggering the evacuation, Destry Henderson, a spokesman for the Hanford Emergency Center, said during a live press briefing on Hanford's Facebook page on Tuesday afternoon.

"All personnel in the immediate area have been accounted for, they are safe, and there's been no evidence for a radiological release," Henderson said. "Upon an additional investigation, crews noticed a portion of that tunnel had fallen — the roof had caved in — about a 20-foot section of that tunnel, which is more than 100 feet long."

The later discovery of the roof cave-in triggered a broader safety alert, since such an incident may have led to a release of radioactive materials.

"All of the employees on the Hanford Site have been told to take cover. What that means is shelter in place," Henderson said at the time. "This is purely precautionary, because again, no employees were hurt and there was no spread — no indication of a spread of radiological contamination."

Whats inside the PUREX tunnels?

Soon after the collapse, Susannah Frame, a reporter with KING 5 TV in Seattle, said on Twitter that the tunnel was "full of highly contaminated materials such as hot, radioactive trains that transported fuel rods."

Lynne T., a member of the joint information center with the Energy Department, who would not provide a last name, told Business Insider that "the tunnels do contain contaminated materials," though "there's no evidence of any injuries, and all employees have been accounted for. There's also no confirmation that any radioactive material has been released."

She said the Hanford Fire Department was "on the scene and investigating it, and everyone is being sheltered," adding that no one appeared to be hurt and that fewer than 10 people were inside the PUREX facility.

Frame has posted pictures of the site, including one of a TALON military-grade robot that's being used to determine whether there's a leak of radioactive materials.

Some reports suggest vibrations from nearby road work may have caused the tunnel roof collapse.

However, Henderson said it was "too early to know what caused the roof to cave in," and "we may not know that for some time."

As of Tuesday evening, Hanford's emergency bulletin said that officials "are working on how they will fix the hole in the tunnel roof."

It added: "They are looking at options that would provide a barrier between the contaminated equipment in the tunnel and the outside air that would not cause the hole in the tunnel's roof to widen."

What was the PUREX plant used for?

The PUREX facility was the "workhorse" of Cold War-era production of plutonium-239 (Pu-239), a fissile material that can be used in bombs to create nuclear explosions.

Rods of uranium fuel were irradiated in nuclear reactors at Hanford to form Pu-239, then trained into the PUREX facility for processing. The fuel rods were dissolved with acid to sort any plutonium from leftover uranium fuel and radioactive waste.

The plant "is longer than three football fields, stands 64 feet above the ground, and extends another 40 feet below ground," according to the Hanford website, and "concrete walls up to 6 feet thick were used in the plant to shield workers from the radiation of the building."

According to Hanford:

"Built in the early 1950s, the facility went into operation in 1956. From 1956 to 1972, and again from 1983 until 1988, PUREX processed about 75% of the plutonium produced at Hanford. Some scientists believe that more plutonium was processed at PUREX than any other building on the planet, as it processed more than 70,000 tons of uranium fuel rods during its operations.

"The building has been vacant for nearly 20 years, but it remains highly contaminated. Its walls are surrounded by razor wire and barbed wire fences. Several rail cars used to transport the irradiated fuel rods from the Hanford nuclear reactors to the processing canyons are temporarily buried inside a tunnel near PUREX as a result of becoming contaminated.

"As with the rest of the Hanford structures, PUREX is slated to be decontaminated, demolished, and some of its debris removed. The rail cars buried next to the facility will also be decontaminated, removed, and permanently buried. Although, the option of grouting the rail cars in-place within the tunnel is being evaluated, since removal of the cars would entail extreme worker safety hazards and would be more costly than grouting in-place."

Cleanup costs for the Hanford Site may exceed $113 billion, according to an estimate from 2014.