Help with “screamingly radioactive” storage tanks

ALBUQUERQUE, N.M. — Synthetic goods are generally modeled on scarce but desirable materials — diamonds, fine wools, even fruit juices.

Jim Krumhansl’s offering to the world is a bit different. Krumhansl has created synthetic sludge.

Unappetizing, perhaps? You thought there was enough of the real thing? But the unusual product, which harmlessly mimics the deadly sludge found in underground nuclear waste storage tanks, could save taxpayers hundreds of millions of dollars in cleanup costs around the United States.

It will allow researchers to safely and cheaply determine which radioactive wastes will remain embedded in, and which will migrate from, the sludge found in waste storage tanks.

This should permit easy and much less costly decommissioning of some tanks at a cost of $10 million each, rather than “worst case” disposal, with maximum safeguards for every tank, of $65 million each. There are approximately 180 such tanks on Hanford Reservation in Washington State alone. A far fewer number are at the Savannah River site in South Carolina.

The work, to be reported at the American Chemical Society meeting on Aug. 23 in New Orleans, is an outgrowth of the February 1995 Galvin Report’s suggestion that national laboratories find new science to cut down on the very large costs of projected environmental clean-up bills.

The artificial sludges consist of “nonradioactive representations of screamingly radioactive materials” that precipitate as a matter of course during storage in giant underground tanks that may contain up to a million gallons of nuclear waste, says Krumhansl, a Sandia researcher.

The sludges created by Krumhansl’s research group are part of a joint project among the Department of Energy’s (DOE) Sandia National Laboratories, Pacific Northwest National Laboratory (PNNL) and the University of Colorado.

Sludge that won’t budge

Naturally occurring sludge sticks to the walls of tanks and could serve as a long-term source of radioactive contamination in the environment.

“When the tanks are emptied, some of the sludge won’t budge,” says Krumhansl. “The tanks will be sluiced, sloshed and squirted, but people won’t be sent inside to clean them up.”

While purists might prefer that scientists experiment on real sludge, workers feel differently. Many of these materials are so radioactive that working with them is both costly and requires taking extreme measures to protect the health of researchers performing the work.

Synthetic sludges, while chemically similar to radioactive ones, are not radioactive and can be handled without danger by lab workers attempting to quantify how much radioactive material sludges will store or release, and for how long.

Costs of decommissioning tanks, treating every material as a worst case in its potential to escape into the environment, could be hundreds of millions of dollars. “The decision how to treat these tanks ultimately depends on how much hazard there is from their residual radioactivity being able to move about. If virtually none of it goes any place, then you’re a lot freer to do simple decommissioning techniques,” Krumhansl says. “The question is what fraction of radionuclides in the sludge will stay there indefinitely and what fraction could become mobile and enter the groundwater.”

Some will be tied up in solid materials that constitute the sludge, says Krumhansl. Some should be stable indefinitely in an underground environment, even if left near the surface. The worse-case alternative is removing, to a new location, potentially corroded radioactive tanks that are 70 feet across, 55 feet high, with tops seven feet below the ground. The simpler alternative is to leave the tanks in place and seal off them off, says Krumhansl.

Wanted: the sludgehammer

“Imagine being the one to dig up seven feet of radioactive earth just to reach the top of the tank,” says Krumhansl, describing the worst-case ordeal. “Where do you put the dirt? How do you cut the tank into small enough pieces to move? Working in a lead-lined [hazardous materials] suit is like working in a sauna. It’s awkward, uncomfortable, you need a respirator, you need a cutting torch, you have to make sure the pieces you’re cutting don’t fall down and hit you. The job may take robots, because the radiation levels are so high. It’s a difficult and expensive task to do.”

To judge whether the synthetic sludges are realistic representations of their more lethal counterparts, group member Jun Liu of PNNL has done transmission electron microscope (TEM) work comparing the atomic structure of a small number of actual radioactive sludges with the group’s synthetic version. (The TEM uses such a small sample that the radioactivity constitutes no danger.)

In some cases the synthetic sludges contain nonradioactive elements from the periodic table that “weren’t exact matches for radioisotopes but provide useful insights into the way these things work, ” says Krumhansl. Examples of this include substituting rhenium for technetium and neodymium for americium. Other elements, like strontium, cesium, and selenium have nonradioactive isotopes that behave identically to the radioactive isotopes in the actual waste. Finally, the artificial sludges also contain nonradioactive components such as lead and cadmium present in the actual wastes that are of concern because of their heavy metal toxicity.

Recipes for harmless sludges

Step one of the process was to construct recipes for the different kinds of sludges that are likely to have resulted from the various chemical processes used to refine irradiated fuels over the past four decades. Then, says Krumhansl, the next step was to make the sludges and see what sticks to each mix. The third step, to be completed this coming year, is to find in what ways these radionuclide surrogates are released.

“We are just completing the second year of a three-year grant from DOE’s Environmental Management Science Program, and it has become pretty clear that not all sludges would be expected to pick up the same radionuclides,” says Krumhansl. “For example, aluminum-rich sludges exhibit an affinity for components in the waste that travel as negatively charged ions in solution such as selenium and technetium. Iron-rich sludges fail to scavenge these elements. On the other hand, all the sludges scavenged cadmium, lead, barium, strontium, neodymium, and cobalt, and none absorbed a significant amount of cesium.”

Current research efforts are focused on which of the chemical compounds in the sludges are responsible for retaining various radionuclides, and on assessing whether any of the radionuclide surrogates associated with the solid sludge would be released at a later date. Other members of the research project team are, from Sandia, Patrick Brady, Pengchu Zhang, Buddy Anderson, Sara Arthur, and Sheila Hutcherson; and Kathy Nagy, University of Colorado.