A robot dubbed the "crib crawler" has been sent into an underground trench at Hanford's Plutonium Finishing Plant where no person has been in more than 30 years.

It's taking pictures of one of the nuclear reservation's most unusual waste disposal trenches, where liquid contaminated with plutonium was dumped into the ground from 1955 to 62.

The trench once had so much plutonium built up at its bottom that Hanford workers feared an uncontrolled nuclear reaction.

Now the Department of Energy and Fluor Hanford are collecting information to develop plans to clean up the radioactively contaminated trench, called the Z-9 Crib.

"It's the only one like it," said Caroline Sutter, the project manager for Fluor Hanford.

Most waste disposal trenches at Hanford are built something like the leach field of a septic system with piping punctuated with holes and surrounded by gravel.

But for reasons lost to history, this trench, or crib as Hanford calls unlined soil disposal sites, was built by digging a 20-foot-deep hole with sloping sides that measures 30 feet by 90 feet at its bottom. Then a concrete slab supported by six columns was placed over the top.

A pipe near the underground trench's roof discharged about 1 million gallons of waste water from RECUPLEX, an early facility at the Plutonium Finishing Plant. To gather more plutonium for the nation's weapons program, it retrieved plutonium from scrap left over from processing plutonium.

In later years, liquid waste from retrieving plutonium was sent to Hanford's 177 underground tanks with other plutonium processing waste to await treatment for permanent disposal. But during the Z-9 Crib years, the waste was dumped into the trench to filter through the soil.

Much of the plutonium bound to the dirt in the top 12 inches of soil in the trench.

In fact, so much plutonium was captured there that as early as 1959 Hanford workers became concerned that it could cause a nuclear reaction, and samples of the soil were collected through a hole drilled through the concrete pad above the trench.

After tests were conducted using an infrared scanner in the '70s, a program was started to add a liquid solution of cadmium nitrate to the waste, according to research by Michele Gerber, a Fluor senior communications specialist. Cadmium nitrate absorbs or "poisons" neutrons.

There also were environmental concerns about the build-up of plutonium in the trench, said Matt McCormick, DOE assistant manager for central Hanford.

In the late '70s, contractor Atlantic Richland Hanford Co. spent several years digging up the top foot of dirt in the trench. A control room measuring about 8 feet by 8 feet was suspended from the roof of the crib to operate the shovel, called a clamshell for the way its two halves came together to scoop up the dirt.

The soil that was removed included 127 pounds of plutonium, Gerber reported.

However, the soil mining did not solve another problem at the trench. The liquid dumped in the trench included the carcinogenic solvent carbon tetrachloride, which contaminated ground water in central Hanford.

The initial step in cleaning up the Z-9 Crib is to remove its buildings, which include a glove box for the mined soil and the control room.

But engineers needed to know more about the condition of the 50-year-old slab over the trench before making cleanup plans, said Andrea Hopkins, project engineer for Fluor Hanford.

Rather than send workers into the trench to take a look, Fluor asked Pacific Northwest National Laboratory to develop a robot to do the job.

It came up with the crib crawler, a 50-pound robot equipped with halogen lights and a camera with a powerful zoom lens to creep around the bottom of the trench. It's used with a second, stationary camera in the trench to get a close up look at conditions.
The pictures show support columns with marks where liquid once stood more than a foot off the base of the trench. Stairs that were used for maintenance remain in place, along with the clamshell shovel and a conveyer used to lift plutonium-contaminated dirt to the glove box.

The ceiling of the trench has lost much of the acid-protective coating that covered terra cotta tiles and in places the tiles also are missing.

"Right now engineers are poring over the first set of pictures" taken in June, said Jake Tucker, senior engineer for the national lab.

DOE has a legal deadline to remove structures from the Z-9 Crib by Sept. 30, 2010. A decision about what to do with the underground portion of the crib will come later. – Tri-City Herald