Actinide metals like uranium, and especially plutonium, pose both an environmental threat and a security risk¿if stolen, they could be used to build nuclear weapons. Researchers have known for a while that one way of safely disposing of surplus plutonium is to combine it with boron, turning it into the very stable and insoluble compound, plutonium boride. Combining the two elements, however, has proved difficult. Traditionally the process has involved heating the two materials to more than 3000 degrees centigrade, cooling them off and grinding them to a powder, which is then mixed and heated again. In some cases this process had to be repeated several times.
Anthony Lupinetti and his colleagues at LANL thus decided to look for other, less energy-intensive ways to combine actinides and boron. "We're using reactive compounds to overcome the problems of working these very complex reactions," Lupinetti notes. Instead of using elementary uranium and boron, they tried the compounds uranium tetra-chloride and magnesium-diboride.
What resulted were the desired uranium boride and the by-product magnesium chloride, which washed out easily. "By combining actinide metal halides, like uranium tetra- and tri-chlorides with molecular boron precursors like magnesium diboride or calcium hexaboride, we've been able to do reactions at much lower temperatures, in the 500 to 800 degrees centigrade range," Lupinetti explains.
Although their initial experiment was conducted on a very small scale, using only about 100 milligrams of material in a small quartz tube, the researchers are optimistic. "It's a very young field. We're still discovering what the rules are in combining these things¿using the entire periodic chart and wide variations of temperature with unusual materials like high-temperature solvents," Lupinetti observes. "There are so many variables, we're all really learning this together, so it's very exciting science."