New 3D imaging technique supports detailed analysis of cladding stability

A team of researchers at Switzerland's Paul Scherrer Institute has published a paper outlining a novel method of assessing the structural integrity of zirconium alloy fuel rod cladding.

In the paper, published in the journal Advanced Engineering Materials, the team describes using a process called ptychographic X-ray computed tomography (PXCT) that was developed at PSI's Swiss Light Source facility. The lens-less scanning technique creates high-resolution 3D images of complex materials and builds the image without destroying the sample being analyzed-an advantage it offers over electron microscopy.

For their research, the PSI team explored the use of PXCT to examine the density of zirconium oxide present on nuclear fuel cladding tubes. Zirconium alloys have been used for decades to make cladding-the protective tubes that encase fuel pellets in a nuclear fuel assembly-because of zirconium's ability to withstand corrosion and high temperatures and its low absorption of the neutrons produced in fission reactions, which allows those reactions to be sustained within a light water reactor.

Exposure to a reactor's coolant water causes zirconium alloys to oxidize; a zirconium oxide film is the product. As the PSI team notes in its paper, because of chemical processes that occur during reactor operation, zirconium oxide "develops into a complex structure characterized by diverse microstructures, oxide phases, and cracks" as well as pores that vary with the thickness of the oxide layer. The layer serves a protective function, operating as a semiconductor and reducing the amount of zirconium that can be oxidized later; if the layer degrades, the structural integrity of the fuel cladding can suffer.

The challenge that the PSI team set out to resolve was the difficulty of measuring the density of zirconium oxide layers. An oxide layer on a zirconium alloy cladding tube is typically only tens of micrometers thick. (For comparison, a strand of human hair usually measures 50 to 100 micrometers thick.) Techniques such as impedance spectroscopy, scanning electron microscopy, and replica transmission electron microscopy have been used in the past, but the PSI researchers sought a technique that would produce a truer assessment.

For their work, the team sliced layers of zirconium oxide from the surfaces of fuel cladding tubes that been used in a pressurized water reactor and sculpted the layers into three-dimensional, cylindrical samples approximately 20 micrometers thick with the aid of a focused ion beam. They then subjected the samples to PXCT scanning. Additionally, they analyzed similar samples from cladding tubes that were run in an autoclave, exposing them to high-temperature and high-pressure conditions similar to those in a nuclear reactor but without the radiation.

On examination of the samples from the used fuel rods, the team found that they could detect pores as small as 80 nanometers in the zirconium alloy layer. The precise measurements that they recorded and the analyses that they were able to make using their knowledge of the properties of zirconium oxide led them to conclude, in the words of team member Liliana Duarte, that "the protective oxide layers on the cladding tubes remain intact and continue to fulfil their purpose even after years of use."

Furthermore, the samples taken from the cladding tubes placed in an autoclave measured about as dense as those that had come from used fuel assemblies. The similarities of the results suggest that some aspects of later research into fuel cladding materials possibly could be performed without radioactive material from used fuel rods.