10 July 2009

Work to start on K reactors burial ground at Hanford

Work should begin this month to dig up another Cold War burial ground at Hanford used to dispose of boron balls once employed to soak up radioactive neutrons.

The boron balls were part of a backup emergency system at Hanford reactors starting in the 1950s to slow down or stop nuclear reactions.

The burial ground, which holds assorted wastes from Hanford's K reactors, includes 16 unlined trenches and 11 silos. The silos contain the boron balls, radiation-contaminated reactor equipment and pieces, and ash from burning radiation-contaminated waste.

Washington Closure announced Thursday that it has awarded a $9 million subcontract to Dance Designs of Pocatello, Idaho, for the work. Watts Construction Inc. of Kennewick and Babcock Services Inc. of Richland are major subcontractors to Dance Designs, which also has offices in Richland.

During the Cold War, K East and K West were among nine reactors along the Columbia River at Hanford that produced plutonium for the nation's nuclear weapons program. The K Reactors operated from 1954-71 and waste from them was buried nearby in the 118-K-1 Burial Ground until 1973.

The debris was to remain buried permanently, but it's being dug up to protect ground water and the nearby Columbia River. Most of the debris and contaminated soil will be trucked to a modern, lined burial ground for low-level radioactive waste in central Hanford.

Work already has been done at similar burial grounds near the B and C reactors and F Reactor. But the silos at the K reactors are more numerous, larger and deeper, said Dale Obenauer, Washington Closure project manager for the job.

The K reactor silos are from four to 10 feet in diameter and up to 32 feet deep.

Six were used for highly radioactive reactor hardware that was dumped into them and left to radioactively cool, Obenauer said.

Then the material would be retrieved and buried in the trenches.

Workers are prepared to find radioactive hardware still inside the silos because some of the radioisotopes have long half-lives.

Excavators used to dig up the corrugated steel pipe silos will be equipped with radiation detectors mounted on their arms to alert workers to unusual radioactive materials.

If radiation levels prove too high, Washington Closure is looking at bringing in equipment that can be remotely operated.

Three silos were used for ash from contaminated material, such as tar paper and protective clothing, that was burned, Obenauer said. At first a steel box with holes was set atop the silos and material was burned inside as the ash dropped into the silo. But later it appears both the silo and box were packed with combustible material that was then burned, Obenauer said.

The remaining two silos were used for the marble-sized boron balls.

The reactors had horizontal control rods that were inserted or pulled out of a graphite block core to control reactivity. In addition, the reactors had vertical safety rods that could be rapidly lowered into the graphite block to quickly stop the chain reaction.

The boron balls also could be dropped down the tubes of the safety rods to poison the reaction. Some may have been dropped into the reactors' graphite core, including during maintenance, and are expected to be radioactive.

To make an educated guess about what's in the trenches and silos, Washington Closure has studied operating logs and production records.

In addition, some excavation was done starting in 2006. Workers had removed about 130,000 tons by 2008 when a decision was made to shift to burial grounds near the H Reactor, where the Department of Energy faced a legal deadline to complete work.

In the meantime, DOE hired North Wind Inc. of Idaho Falls to test a process to determine the contents of the K reactor silos, which revealed some radioactive hot spots.

"The additional data allow us to be better prepared to do the job safely," Obenauer said.

Although the eight-acre burial ground served two reactors, it's smaller than burial grounds at other sites.

Workers have found many whole and partial pieces of irradiated fuel at other burial grounds and could find more at the K reactors. Excavation waste will be sorted and screened for such unusual materials, which can't be sent to the low-level waste landfill.

Dance Designs is expected to complete excavation in the fall of 2010. Washington Closure expects all cleanup work, including backfilling and replanting, to be done by spring 2012.

1 comment:

  1. dicksaun@gmail.comJuly 10, 2009 at 10:06 PM

    Thanks for the fine details.

    ReplyDelete