Just four Sieverts per hour is a fatal dose – worst contamination found just a few feet from groundwater

Hanford officials have settled on a plan to clean up what may be the most highly radioactive spill at the nuclear reservation.

It depends on calling back into service the 47-year-old, oversized hot cell where the spill occurred to protect workers from the radioactive cesium and strontium that leaked through the hot cell to the soil below.

Radioactivity in the contaminated soil, which is about 1,000 feet from the Columbia River, has been measured at 8,900 rad per hour. Direct exposure for a few minutes would be fatal, according to Washington Closure.

Washington Closure Hanford has issued a notice telling companies that it plans to request bids in April for a major project that will call for an intensive design effort. While many of Washington Closure's bid awards go to small companies, this bid request will have no restrictions as the Hanford contractor looks for a company with the experience to handle a complex assignment.

The winning subcontractor will be required to design remotely operated equipment to be installed inside the hot cell. Using the equipment, it must take out the hot cell's floor, dig up the contaminated soil beneath it and transfer the contaminated soil to nearby hot cells to be grouted in place.

"DOE and their contractors developed this plan using both out-of-the-box creative thinking as well as the methodical, object evaluation of the alternatives," said Larry Gadbois, a scientist for the Environmental Protection Agency, a regulator on the project. "It provides a safe process for workers at the site and safe handling of the packaged waste."

The spill is beneath the 324 Building, which already was a complicated and hazardous demolition project because of its large hot cells contaminated from research with high levels of radioactive material. The building, used from about 1965 to 1996 and measuring 102,000 square feet, is just north of Richland.

Work began in the 1990s to remove as much equipment as possible from the building, and Washington Closure has been working to stabilize and prepare five highly contaminated hot cells for removal since 2005.

That included using remotely operated equipment to chisel out grouted waste material that fell to the bottom of one of the cells, Radiochemical Engineering Complex B-Cell, a room-sized, 30-foot-tall hot cell with 5-foot thick walls.

B-Cell was used for projects that included testing methods for grouting radioactive materials and for projects related to vitrifying, or glassifying, waste for disposal. Workers stood outside the cell and used controls to operate manipulators inside the cells that handled the radioactive material as they watched through leaded-glass windows.

In the 1980s, cesium and strontium spilled inside the hot cell, according to a 1993 report that referenced the spill. Germany needed a heat source to use for tests of a repository for radioactive waste, which emits heat, and the cesium and strontium were being fabricated into the sources.

"This was concentrated material," said Mark French, the Department of Energy's project director for Hanford cleanup along the Columbia River.

The grouted radioactive waste that fell to the bottom of the cell had built up in a trench and drainless sump -- a lowered area to collect liquid -- at the bottom of the cell, making the sump level with the rest of the floor and mixing with the spilled cesium and strontium.

As the grout was chiseled out and vacuumed up, Washington Closure got its first look at the bottom of the sump in 2009, revealing that the stainless steel liner on the floor of the cell was damaged, raising the likelihood of a leak into the soil beneath the hot cell.

Since then, Washington Closure has demolished a maintenance annex attached to the 324 Building to provide closer access to the spill site.

It used a series of 10 probes pushed almost horizontally beneath the building to collect radiation readings in the soil beneath the cell. It also used a drill rig to insert vertical probes beneath the cell to determine the depth of contamination. In June, after extensive mockups and practice with long-handled tools and containment sleeving, two soil samples were collected.

Little or no contamination was found beneath the center of the cell or beyond the cell footprint, said Don McBride, Washington Closure technical services manager for decommissioning and demolition. Instead, the cesium and strontium moved to the edges of the hot cell and down into the soil at the cell's expansion joints.

It migrated down in a open square shape, with the worst contamination down to five or six feet deep, McBride said. There is not evidence that it has reached the ground water which is about 54 feet below the ground there and about 42 feet below the bottom of the hot cell, which is partially below ground.

Because the contamination is underneath the building, precipitation hasn't reached it to carry it deeper in the soil.

The most contaminated of the soil will be dug up, working from inside B-Cell, and placed in neighboring hot cells. There it will be incorporated into grout that will be used to fill the hot cells.

The grout-filled cells then will be removed as monoliths and hauled by large transporters to the Environmental Restoration Disposal Facility in central Hanford. The thick walls of the cells will provide radiation-shielding as the waste is driven to the landfill.

After all the hot cells are removed and the building demolished, more of the contaminated soil will be dug up.

The project has many challenges, French said.

It's one of the three highest risk projects at Hanford along the Columbia River -- the other two are the 618-10 and 618-11 research waste burial grounds -- but Washington Closure has come up with innovative solutions throughout its contract, he said.

DOE's goal is to have the work completed by 2015, when it plans to have most environmental cleanup along the Columbia River at Hanford finished.