Monday, Oct. 06, 2008

Planned fire tests cap over Hanford radioactive waste

By Annette Cary, Herald staff writer

Hanford workers have burned over a cap placed above a former disposal site for radioactive waste in the name of research.

How such barriers, or covers, behave after a fire is one of the biggest unanswered questions about their performance, said Kevin Leary, a Department of Energy environmental engineer.

The caps are among the tools DOE is developing to protect the Columbia River from Hanford contamination. In some cases rather than cleaning up waste, a soil cap may be built over a contaminated area to keep water from driving pollution in the soil deeper toward the ground water.

For instance, at U Plant DOE plans to use the lower level of the 800-foot-long plant in central Hanford as a waste receptacle for its contaminated equipment and cover it with a protective earthen cap that would form a huge berm standing 40 feet high along the length of the former processing canyon.

To test the concept, 14 years ago DOE built a cap over a waste site in central Hanford that once was used to dispose of radioactively contaminated condensate from one of the site's 177 underground tanks. The tanks hold waste from the past production of plutonium for the nation's nuclear weapons program.

The upper portion of the test cap includes 6.7 feet of fine silt loam soil that's intended to act like a sponge, unlike the sandy, cobble-filled soil that's more typical in central Hanford. The silty soil was trucked in from the McGee Ranch portion of the uncontaminated security zone around the production portion of the nuclear reservation.

The soil acts as what DOE calls an "evapotranspiration barrier." Precipitation and snowmelt should be soaked up in the soil. Some then would be lost to evaporation and some soaked up through the roots of native plants topping the barrier and then released into the air through their leaves.

"Part of the study is to look at how quickly vegetation would come back naturally," said Laura Buelow, a scientist for the Environmental Protection Agency, the regulator on the project.

Among concerns is how long it would take vegetation to return if no agency still is keeping a watch over the cap and reseeding it after fires that may occur in the future. However, given the many years a cap would be expected to last, the months without plants would be a relatively short time, Buelow said.

The cap is intended to work under extreme weather conditions, such as heavy and sudden snow melt that would occur seasonally when there would be minimal reduction of water through evaporation and uptake from plants.

Researchers also want to learn from the test burn what kind of plants might return naturally and how wind and water erosion might affect a burned area. There's a possibility that plant materials high in waxes and other substances can burn and leave a residue in the soil that repels water, an effect seen after some California fires.

To help protect against erosion, the test cap has a six-inch layer of pea gravel in its upper portion, Leary said.

Tests will begin to be conducted this month after the area was burned by the Hanford Fire Department a week ago. Temperatures were measured at 1,400 degrees and about half of the one-acre cap was burned.

The cap includes many instruments, including monitors that allow researchers at Pacific Northwest National Laboratory to see how moisture moves through the soil there, Leary said.