Making Fake Sludge

With all the real waste lying about, it might seem strange for chemists to create artificial waste. But that's exactly what's being done as part of the effort to ameliorate the seemingly intractable problem of radioactive contamination at Hanford Nuclear Site in southeastern Washington State. Cleaning up 55 million gallons of radioactive waste stored in tanks is a key task at the Hanford site, which the site's owner, the Department of Energy (DOE), calls "the world's largest environmental cleanup project." The Hanford site houses aging reactors and other leftovers of a 40-year project to make plutonium for nudear bombs, induding tanks full of sludge, mixtures that formed from dissolved solid radioactive wastes, such as spent fuel rods, and the solutions used to process the wastes. The 177 tanks at Hanford contain different types of sludges resembling wet plaster, concrete, or peanut butter, says Jim Krumhansl, a geochemist at Sandia National Laboratories in Albuquerque, New Mexico. The huge tanks were not designed as permanent storage. The DOE says at least one million gallons of liquid from the tanks has already reached groundwater and may eventually threaten local groundwaters bisecting the Hanford site. In October 1999, the DOE's Office of River Protection reported finding high levels of radioactive technetium-99 in groundwater near one group of tanks. The tanks present a dilemma. Doing nothing perpetuates the threat to groundwater. But mixing or moving the sludge carries all the risks ofworking with large quantities of highly radioactive material (including spills and exposures to humans and the environment). And laboratory research is hobbled by expensive radiation-protection precautions. The cleanup costs at Hanford-for tanks and other projects-totaled $1.6 billion in FY 1999 alone. In order to research the chemistry needed to decommission the tanks, Krumhansl and colleagues have started brewing artificial sludge. The work is an outgrowth of the DOE's Environmental Management Science Program, started in 1996 to provide a basic research perspective on nuclear waste cleanup problems that might ultimately cut the cost of remediation. In addition to cutting costs, the research aims to answer two critical problems: what will happen to radioactive isotopes remaining after cleaning and how can their migration at a later date be minimized? Coinvestigator Kathryn Nagy, a geochemist at the University of Colorado at Boulder, is making simple sludge-chiefly iron and aluminum-in a basic, high-nitrate system. To represent technetium-99, an isotope found in tanks and groundwater at Hanford, Nagy is using the safer, nonradioactive rhenium, which behaves like technetium in w chemical reactions. One goal of her work iS to learn whether the radioisotopes are more concentrated in the liquid or solid portion of the sludge. She says this information will be useful in determining whether chemical or physical treatments are needed to maximize radioisotope removal from the tanks. The investigators are also trying to learn what will happen to radioisotopes in the sludge that remains after the tanks have been emptied. "The tanks will be sluiced, sloshed, and squirted, but people won't be sent inside to clean them up," says Krumhansl. "There will be some percentage that sticks to the bottom and sides." Krumhansl has made artificial sludge in which nonradioactive isotopes of cesium and strontium substitute for radioisotopes of those elements. "I try to come up with what it is about sludge that holds onto radioisotopes, and how much will leach out [from residual sludge]," he says. "This is a tool to figure out what is going to be left, and how much of that radiation will leave to get into the groundwater." Krumhansl says that doing this research on nonradioactive artificial sludges is much less expensive than working with the real thing, as well as immeasurably safer. "Once this information is available," he says, "we can assess just how clean we need to get the tanks, and not spend any more than necessary on the project." -David J. Tenenbaum