Radioactive fuel turns to goo during nuclear meltdown

Researchers have gotten the first atomic-level glimpse of what happens to radioactive fuel during a nuclear meltdown — inside the hot mess of uranium dioxide goo.  

In the heat of a doomed reactor, uranium dioxide’s oxygen atoms turn oozelike, and the compound’s uranium scaffolding folds and collapses into a reactive blob, researchers report in the Nov. 21 Science. Understanding the melting process of uranium dioxide, the most common nuclear fuel in use today, may help scientists predict and prevent subsequent chemical reactions during a nuclear disaster, the authors say.

In extreme cases, such as the Chernobyl nuclear power plant disaster of 1986, molten uranium dioxide fuel can react with concrete, steel and zirconium metal coatings in the reactor, says engineer Lawrie Skinner of Stony Brook University in New York, who led the study. “This is the first step to understanding that.”

In its solid form, uranium dioxide’s atoms form a tidy cubic structure — a uranium lattice filled in with oxygen atoms.  In that stacked arrangement, each uranium atom is surrounded by eight oxygen atoms, which are fixed in place. Chemists had come up with a variety of models to predict how that atomic layout would change in a reactor meltdown, but never carried out the dangerous experiments in labs.

After about a year of designing a special containment system, Skinner and his colleagues managed to get the fuel to melting temperatures, upwards of 2,397° Celsius —about half the temperature of the sun’s surface. In the custom cage, the researchers levitated a 3-millimeter pellet of depleted uranium dioxide using pressurized gas. (Keeping the melting pellet afloat prevents it from touching, and thus reacting with, anything else.) With a zap of a laser, the researchers heated the pellet and then measured its atomic structure using X-rays.

The heat turned the uranium dioxide into a molten, lavalike blob. Within it, oxygen atoms generally drew closer to the uranium pillars.  But overall the structure expanded, which is quite unusual, Skinner says.

The oxygen atoms also seemed to move around within the structure. “The oxygen behaves a little bit like a liquid in this solid lattice of the uranium,” Skinner says. Instead of eight oxygen atoms surrounding one uranium atom, as in the solid state, an average of about 6.7 oxygen atoms ringed each uranium in the liquefied fuel. The result is a wrinkled atomic structure, similar to unfolded tin foil.

“From a regulatory perspective, you never want to get the fuel melting,” says nuclear engineer Dale Klein of the University of Texas at Austin and former chairman of the U.S. Nuclear Regulatory Commission. “But if you do, you want to know the science behind it. These kinds of experiments help us understand nuclear reactors with the end goal to make them safer than they already are.”

Skinner and his colleagues next plan to melt uranium oxide alongside zirconium, a common coating in reactors, to see how the two react together.