Radiation helps break down toxic waste.

The legacy of the Cold War lies buried underground. At sites across the United States, hundreds of concrete, steel-lined tanks hold toxic mixes of radioactive metals and nonradioactive compounds--organic and inorganic--left over from production of nuclear weapons.

As millions of gallons of hazardous chemicals stew, scientists know little of what is occurring in these cauldrons. A new finding may explain how the wastes decompose and how they produce dangerous gases.

High-energy gamma rays, produced copiously by the radioactive decay of waste elements such as cesium and strontium, can activate common minerals also found in the tanks. These activated particles help organic compounds break down and form gases faster than they otherwise would, according to the recent study.

George Adam Zacheis and Kimberly A. Gray of Northwestern University in Evanston, Ill., and Prashant V. Kamat of the University of Notre Dame (Ind.) report their findings in the April 8 Journal Of Physical Chemistry B.

Information from such studies will help resolve safety questions and "ultimately feed into ways to process the waste," says Donald M. Camaioni, a staff scientist at the Pacific Northwest National Laboratory in Richland, Wash. He works on tank-waste safety programs at the Hanford Nuclear Reservation near Richland. With 54 million gallons of waste in 177 containers, Hanford is the largest nuclear-waste storage site in the United States and the one with the most cleanup problems (SN: 12/20&27/97, p. 410).

The recently demonstrated breakdown of organic compounds makes the waste less hazardous, but the gases generated create a serious problem for underground tanks. So far, scientists haven't found ways to prevent the gas from being produced.

Hydrogen gas builds up in the tanks at Hanford, says Camaioni, but vents allow it to escape, relieving the pressure and the danger of the gas igniting.

Kamat, Gray, and Zacheis conducted their study on a simple system including aluminum oxide, or alumina, a major component of tank wastes. They coated nanometer-size particles of the mineral with the organic pollutant hexachlorobenzene and exposed the powder to gamma rays. They monitored the decomposition of hexachlorobenzene with infrared and ultraviolet spectroscopy and extracted the breakdown products for analysis.

When an alumina particle absorbs high-energy radiation, "it starts a whole series of events," says Kamat. The gamma rays excite electrons in the particle, causing them to migrate. This movement separates positive and negative charges. The charged alumina surface strips chlorine atoms off the hexa-chlorobenzene.

Normally, organic contaminants in very low concentrations do not decompose by themselves, says Kamat. They need a catalyst like alumina to help them along. "This is the first study to look in a more mechanistic way at how alumina promotes degradation," he says.

Researchers have largely overlooked reactions taking place on particle surfaces, Camaioni notes. "It has only recently been appreciated that you can excite particles [with radiation] to do chemistry on absorbents."

Whether this particular process occurs in underground storage tanks remains unknown, but Kamat says that based on the composition of the waste, "my personal opinion is that some of this is occurring."

Camaioni says that Hanford scientists have not done studies that would indicate whether the new findings are relevant to the stored nuclear wastes.