Oxidized plutonium reaches a higher state

Expose a silvery piece of plutonium to air and it tarnishes, developing a patina that looks yellow or green, depending on the sample. For many years, scientists thought that this unpredictable film was plutonium dioxide, considered the most stable oxide of this radioactive element.

Now, a team of researchers has taken the luster out of this description. They’ve found that plutonium dioxide reacts very slowly with water and oxygen to form higher oxide phases. In air, these phases—intensely green in color—are actually more stable than plutonium dioxide, which is yellow. The additional compounds “were there all along, but no one realized it,” says John M. Haschke, a chemist and consultant from Waco, Texas.

The oxidation reaction produces hydrogen gas and increases the ease with which plutonium dissolves in water. The findings, appearing in the Jan. 14 Science, could have implications for the long-term storage of nuclear waste and plutonium dioxide’s use as a fuel for nuclear-power reactors.

Haschke collaborated with Thomas H. Allen and Luis A. Morales of Los Alamos (N.M.) National Laboratory. “When we were looking at corrosion of plutonium metal several years ago, we had indications that something unusual was happening,” he says.

They had seen signs that other oxides form along with the dioxide. To learn more, the researchers allowed pure plutonium dioxide samples to react in a sealed chamber with water vapor and oxygen. They monitored the production of hydrogen gas and used X-ray diffraction to deduce the crystal structure of the compounds. While plutonium dioxide has two oxygen atoms for each plutonium, crystals of the higher oxides have an oxygen-plutonium ratio of more than two but less than three, they conclude.

The idea that plutonium dioxide is the only outcome of plutonium oxidation was a “sacred cow,” says Haschke. Though other researchers had seen hints of the higher oxides, they couldn’t reconcile their data with the established belief.

Slow reaction times make the newly discovered oxides easy to miss. The Los Alamos group’s room-temperature test lasted 4 years. Such reaction times become relevant if buried nuclear waste is to remain stable until the next millennium. If water is present, hydrogen gas could build up in sealed containers.

Moreover, these oxides dissolve easily in water, explaining why plutonium migrates through the ground more quickly than had been expected. “This is very important input for people who model these migration processes,” says Louis J. Terminello of Lawrence Livermore (Calif.) National Laboratory.

“Plutonium dioxide has gotten into the environment over the years,” says Haschke, but “it was assumed that plutonium dioxide remains localized.” That assumption and others now must be reconsidered.