X-ray imaging detects pores as small as 80 nanometers in nuclear fuel cladding

Researchers at Switzerland’s Paul Scherrer Institute (PSI) used a new X-ray imaging technique to examine corrosion inside nuclear fuel rod cladding in 3D, detecting pores as small as 80 nanometers.

The study investigated whether the protective layers surrounding fuel rods remain effective after years of exposure to cooling water and radiation.

Working with samples from cladding tubes that had operated in a nuclear power station, the team used PSI’s Swiss Light Source to map internal structure. The tubes remained leak-tight after service, and the measurements showed that protective oxide layers continued to perform their function.

The findings help assess fuel rod safety and establish whether conventional laboratory corrosion tests accurately represent material exposed to reactor conditions. The supplied announcement did not specify when the experiments took place.

Tiny samples reduce radiation exposure

Fuel rod cladding must remain sealed throughout reactor operation, despite conditions that can cause corrosion. Researchers therefore need to understand how quickly corrosion layers form and how porous they become.

The cladding tubes typically contain more than 98 percent zirconium, with small amounts of other metals added to improve their properties. It encloses stacked uranium dioxide fuel pellets, forming the fuel rods used in nuclear reactors.

The study brought together three PSI research centers covering nuclear engineering, photon science, and neutron and muon sciences.

At the institute’s Hot Laboratory, staff used precision saws inside shielded hot cells to cut wafer-thin sections from spent fuel rod cladding. And then used a focused ion beam to shape the material into cylinders.

The samples came from the zirconium oxide layer formed on the cladding surface during operation.

“In the end, our tiny zirconium oxide cylinder had a diameter of just 20 micrometres – making it barely visible to the naked eye. A human hair is three to four times as thick,” said first author Okan Yetik.

Their size kept radiation emissions low enough for examination outside specially shielded hot cells.

X-ray scans reveal protective layers

The team examined the cylinders using ptychographic X-ray computed tomography, or PXCT, developed at PSI’s Swiss Light Source. An X-ray beam scans the sample point by point from different angles, generating high-resolution three-dimensional images of its internal structure.

“We were even able to identify pores as small as eighty nanometres,” said co-author Andreas Menzel.

The measurements also showed that the oxide layers retained their protective function after prolonged use.

“The measurements taken at the SLS showed that the protective oxide layers on the cladding tubes remain intact and continue to fulfill their purpose even after years of use,” said PSI researcher Liliana Duarte.

Laboratory tests reflect reactor conditions

Researchers compared the used material with non-irradiated cladding samples exposed to high pressure, high temperatures, and water inside an autoclave.

The density of the reactor-used samples was practically identical to that of the autoclaved material, supporting the value of standard corrosion tests.

This means researchers do not always need spent cladding for investigations. The findings can also help operators refine reactor water chemistry, and manufacturers tailor alloys and heat treatments.

The technique opens possibilities for similarly detailed examinations of components for small modular reactors and nuclear fusion reactors.

The researchers published their findings in the Advanced Engineering Materials journal.