Looking closer: Nuclear waste viewed in new light

Those stainless steel drums contain metal-clad spent uranium embedded in concrete, and they are highly radioactive. The only way to handle them safely is from behind 2-to-3-meter-thick concrete walls and leaded glass windows using automated equipment.

Yet a very small number of these drums have begun to bulge after many years in storage, raising questions about what is happening within. The only way to know for sure is to sneak a peek inside.

A team led by Tom Scott, a reader in nuclear materials at the University of Bristol, has found a way to do just that. Scott will present the team's findings in an invited talk at the AVS 61st International Symposium & Exhibition in Baltimore, Maryland on Tuesday, Nov. 11, 2014.

According to Scott, the most likely cause of the bulges is uranium corrosion. Corroded uranium has a much greater volume than uranium itself, so as it grows, it cracks the surrounding concrete and forces it apart. If the concrete contains enough uranium, the expansion could cause the steel drum to bulge or possibly even split.

"When uranium corrodes, it can form either uranium oxide or uranium hydride. Both have the same low density, but the hydride has the potential to cause fires, and nobody wants a radioactive fire," Scott said.

"Uranium oxide (UO2) is the normal product of uranium corrosion. It forms relatively slowly when uranium reacts with oxygen or water. But when water corrodes uranium, the reaction also releases hydrogen. If the concentration of hydrogen rises high enough, flammable uranium hydride (UH3) will start to form instead of the oxide."

To see if this was happening, Scott had to measure how fast these reactions were progressing and learn why some drums bulged while others did not.

That meant creating models of the 500-liter waste drums small enough to measure in a laboratory. So he embedded matchstick-sized bars of uranium in concrete and deliberately corroded them in wet, dry, and hydrogen-rich storage conditions.

He also needed a way to look inside. Conventional X-rays were too weak. Instead, Scott turned to synchrotron X-rays, powerful beams generated by particle accelerators.

He used a technique called X-ray diffraction to measure how the samples deflected the X-rays. Each material has its own deflection signature, and he used this to identify the different corrosion products inside the packages.

He also did tomography, X-raying the test packages while rotating them 360 degrees. Software then stitched the X-rays together to create 3-D models of the packages.

"We've now proven that it is possible to distinguish between uranium dioxide and uranium hydroxide, and to resolve the size and shape of the materials inside the package with phenomenal precision," Scott said.

Scott has been observing corrosion inside the test packages for about two years. He found that while the uranium oxide barrier reduces the amount of corrosion (and swelling) from water as it grows thicker, it does not prevent hydrogen from reaching the uranium.

While some scientists thought the uranium hydroxide might revert to less dangerous uranium oxide, Scott's experiments have shown that it persists in some instances.

Scott plans to continue the experiments for a total of 10 years to understand the evolution of uranium chemistry inside the waste drums. He also wants to scale up the system and use gamma rays, which are more powerful than synchrotron X-rays, to look inside actual waste drums.

"Once we have enough observations to predict how corrosion will progress, we can develop a response plan. We might need to cut some drums apart and reprocess them, but in other cases, the bulges may not be getting any worse. We want to find the safest solution, long before there is any chance of a problem," Scott said.