Scientists have long believed that solid plutonium dioxide (PuO2), a compound used in nuclear reactors and warheads, is the most stable form of plutonium. But in tomorrow's Science, a new report shows that water can oxidize PuO2 and make it more soluble, increasing the risk of leakage from storage facilities. The find may cause safety experts to rethink their strategies for containing plutonium waste.
Last year, astonished scientists discovered that trace amounts of plutonium had migrated more than a kilometer away from a cavern created by underground nuclear weapons testing in Nevada. Based on what they knew then about plutonium dioxide's solubility in water, the toxic element shouldn't have been able to travel more than 10 meters from the cavern, says chemist John Haschke, a consultant who used to work at the Los Alamos National Laboratory in New Mexico. Now, Haschke thinks he has found an explanation for plutonium's high mobility.
In 1995, Haschke and his collaborators exposed a sample of solid PuO2 to moist air. Ever so slowly the compound reacted with the vapor, stealing oxygen atoms from the water molecules. It took the team 4 years to collect a measurable amount of hydrogen gas that had been liberated from the water. Most disturbing, the oxygen-enriched compound (it has about 2.27 oxygen atoms for every plutonium atom) is far more soluble than pure plutonium dioxide. That probably allowed it to move rapidly through the aquifer near the Nevada site.
Other scientists are taken aback by the explanation. "Water is the oxidizing agent? This is tremendous, water is everywhere!" exclaims chemist Charles Madic of the Commissariat d'Energie Atomique in Saclay, France. He predicts the finding will give safety experts new headaches, as they may have to design more durable, watertight containers to store plutonium waste.