What we know about the Hanford nuclear emergency

— -- The Hanford Nuclear Reservation site in Washington state went into immediate lockdown mode Tuesday after an 8:26 a.m. alert revealed that a tunnel used to house contaminated radioactive materials was breached, according to U.S. Department of Energy officials.

Employees on the 500 square mile site were led to precautionary sheltering and later released to go home Tuesday after officials said there was no indication the contamination had spread. With zero employee injuries and reports of the site moving now from "emergency phase to recovery phase," here are some details about the emergency accident at the Hanford site, which was built as part of the Manhattan Project, for nuclear production.

Details of the breached tunnel

During a routine surveillance of the area on May 9, a 20-foot-wide hole in the roof of a tunnel was discovered near the Plutonium Uranium Extraction Plant, also known as PUREX, the Hanford Joint Information Center reported. According to Associated Press reports, the routine inspection occurred during a massive radioactive waste cleanup that has been underway since the 1980s, which costs more than $2 billion a year.

Immediately following the discovery, an emergency was declared, access to the 200 East area of the Hanford site where the incident took place was restricted, and at least 12 employees in the area were evacuated. Crew officials later ordered all employees to take precautionary shelter while the scene remained under investigation.

After officials confirmed that the contamination had not spread, sheltered employees and non-essential employees of the facility's 9,000 worker labor force were sent home, according to AP. An employee advisory is still in effect for workers today as crews prepare to fill the hole with new soil.

The Energy Department also confirmed that no action was required for the nearly 300,000 residents in the surrounding Benton and Franklin communities, according to AP.

Hanford tour

A public tour was in progress when the breach in the tunnel was discovered. According to Hanford officials, the tour was cut short for precautionary reasons and no one was reported injured.

Hanford tours, which started about two weeks ago, are open to the public and occur on most weekdays.

History of the tunnel and Hanford

The 60-year-old tunnel, constructed of wood and concrete, is next to the Plutonium Uranium Extraction Plant, according to the Hanford Joint Information Center.

PUREX is longer than three football fields, stands 64 feet above the ground, and extends another 40 feet below ground, according to details on Hanford's site. It was a nuclear chemical processing plant, as well as a site for radioactive waste disposal.

The tunnel was built to house rail cars that were filled with contaminated equipment and moved into the tunnels during the Cold War, the center said. It was sealed in the mid-1990s and is checked periodically, it added.

"That tunnel feeds into a longer tunnel that extends hundreds more feet and contains 28 rail cars loaded with contaminated equipment," the center said in a statement Tuesday night. "The hole opened up in the shorter tunnel near where it joins the longer tunnel."

For decades, Hanford made plutonium for nuclear weapons and is now the nation's largest depository of radioactive defense waste, with about 56 million gallons of waste, most of it in 177 underground tanks, according to AP.

Recovery phase

Hanford officials confirmed that preparation to stabilize and fill the hole started around 8 p.m. PST Tuesday, with personnel laying down a gravel road that leads to the collapsed section of the tunnel. Approximately 50 truckloads of soil are slated to help to repair the tunnel.

Access is still restricted to the immediate area where the accident took place and workers performing the recovery work are wearing protective suits and breathing masks.

The Department of Energy confirmed to ABC News that there is still no indication of the contamination having spread, despite the sinkhole itself being radioactive. Officials say it may take some time to determine what exactly happened. There is now more concern that some of the other 2000 feet of tunnels, which were built nearly 60 years ago, may also be vulnerable.

ABC News Karma Allen, Matt Gutman and Morgan Winsor contributed to this report.