The tunnel found Tuesday with a partially collapsed roof was unusual even for Hanford.
The nuclear reservation near Richland produced plutonium for the nation’s nuclear weapons program from World War II through the Cold War by irradiating uranium fuel and then using huge chemical processing plants to extract the plutonium.
PUREX was the last of five processing plants built. They were sometimes called canyons because they were long and narrow buildings with high ceilings.
Irradiated uranium was brought to the massive PUREX plant by rail, but the railroad tracks also had a second use.
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A 500-foot-long extension of the rail line was built to access a novel disposal system for large pieces of highly radioactive equipment.
Two tunnels were built to store the waste.
A remotely controlled electric engine pushed railroad cars loaded with highly contaminated items from the plant into the tunnels, according to a Hanford risk review prepared in 2015 by the Consortium for Risk Evaluation and Stakeholder Participation, or CRESP.
8 rail cars with radioactive waste
360 feet long
22 feet high
19 feet wide
The tunnel where the breach was discovered Tuesday is the oldest of the two.
It is 360 feet long and was built using primarily creosoted timbers arranged side by side. The Department of Energy said concrete also was used.
Then it was covered with about eight feet of soil. It’s 22 feet high and 19 feet wide, according to Heart of America Northwest, a Seattle-based Hanford watchdog group.
Between June 1960 and January 1965, eight rail cars loaded with radioactive waste were pushed into the tunnel.
A second, stronger waste tunnel was built in 1964 with internal steel I-beams attached to reinforced concrete arches. It has a steel liner.
The second tunnel is 1,700 feet long and holds 28 rail cars of equipment, although it was built for 40.
The last rail car apparently was pushed inside in the early 1990s.
Between 1995 and 1997, water-filled doors of both tunnels and the outer PUREX railroad tunnel door were sealed, according to the CRESP report. Water can serve as a shield against radiation.
Last year a new legal deadline was set, requiring DOE to decide what process it would use to assess the integrity of the tunnels by September of this year.
The cleanup of the tunnels is part of central Hanford work that is required to be finished by 2042.
Those deadlines were among Tri-Party Agreement deadlines for central Hanford revised in 2016, as work had fallen behind schedule there because efforts and budget were focused on cleaning up contamination nearer the Columbia River.
The PUREX plant was the workhorse of Hanford’s processing plants.
It went into operation in 1956 and separated plutonium from fuel rods from 1956 to 1972, and again from 1983 until 1988.
The plant processed more than 70,000 tons of uranium to yield about 75 percent of the plutonium produced at Hanford.
It is longer than three football fields and stands 64 feet above the ground and extends another 40 feet below ground. Concrete walls were built up to six feet thick to shield workers from radiation.
Plans call for eventually decontaminating and demolishing PUREX. The option of grouting the rail cars in place — encasing them in concrete — has been considered.
Removal of the cars would entail extreme worker safety hazards, DOE has said.