News in Science

First look at nuclear fuel in a meltdown

Nuclear Meltdown Scientists have managed to take their first close-up look at what happens to nuclear fuel when it becomes molten, as it would in a nuclear reactor meltdown.

In an innovative lab experiment, they discovered that uranium dioxide fuel behaves differently when molten than in its solid state.

The findings, reported in the journal Science, may help researchers improve safety at nuclear power plants, by better understanding uranium dioxide's behaviour under extreme temperatures.

"In extreme events like Fukushima and Chernobyl the uranium dioxide literally melts, and we wanted to study the material to really understand it," says the paper's lead author Dr Lawrie Skinner of Stony Brook University in New York.

"We can now pin down a little bit more accurately what the properties and temperature of the melt will be. Any sensible reactor design should take into account the real structure, physical properties, and behaviour of this melt."

Too hot to handle

Until now, the extreme heat and radiation has made it impossible for scientists to study uranium dioxide's characteristics and structure in a molten state.

Uranium dioxide melts at over 3000°C, far too hot for most furnace container materials which would melt and react with the test samples.

Skinner and colleagues got around the container problem by floating a tiny 3 millimetre bead of uranium dioxide in a gas stream and heating it with a laser.

They were able to study the relative positions of the atoms in both hot solid and molten uranium dioxide beads using high energy synchrotron X-ray diffraction.

"We didn't really know what to expect, it's not something we've measured before," says Skinner.

The authors found molten uranium dioxide showed distinct and unexpected differences in how the oxygen atoms were organized around the uranium atoms, compared to the solid state.

"We found that when the uranium dioxide melts, the average number of oxygens surrounding each uranium atom is reduced quite dramatically, affecting physical properties like viscosity and diffusion of the atoms at these extreme conditions," says Skinner.

"In uranium dioxide each uranium is surrounded by eight oxygen in little cubes, and many of these little cubes connect together to form the crystal structure. However in the molten state, there's a mixture of six and seven oxygen around each uranium atom."

Nuclear meltdown

The melting of the uranium dioxide fuel represent the first stage of any nuclear meltdown.

"We just wanted to understand the first step, the highest temperature part, when the uranium dioxide first melts," says Skinner.

In a real nuclear reactor core meltdown, such as occurred at both the Chernobyl and Fukushima Dai'ichi plants, the molten uranium dioxide melts and reacts with the zirconium metal cladding on the fuel rods, and with the surrounding steel and concrete structure, forming a lava which scientists have called corium.

These corium lavas are often referred to as Medusa because it would be fatal to be close enough to see them.

"Next we want to measure more complicated compounds like uranium zirconium oxide which forms when the uranium dioxide melts and reacts with the zirconium fuel rod cladding," says Skinner.