New method for inspecting fuel rods

Researchers at the Paul Scherrer Institute PSI have been examining the cladding tubes of nuclear reactor fuel rods using a new 3D imaging technique. They were able to determine with extremely high precision how the quality of the cladding changes over time as a result of corrosion and radiation exposure. The oxide layer on the tubes proved to retain its density even after years in operation. The new method allows this to be checked in great detail.

• Researchers at PSI have been investigating how the zirconium-alloy cladding tubes of nuclear reactor fuel rods corrode over time.
• Using the PXCT 3D imaging technique developed at PSI, they examined the material at a nanometre resolution and found that it remains leak-tight even after years in operation.
• The new imaging technique improves our understanding of the material properties of cladding tubes, thereby contributing to the safe operation of nuclear reactors. In future, the method could also be used to analyse the components of new types of microreactors and nuclear fusion reactors.

The cladding tubes of the fuel rods used in nuclear reactors must remain leak-tight – despite the risk of corrosion from the surrounding cooling water and despite the radiation they are exposed to during operation. It is therefore important to know how quickly corrosion layers form and how porous they are. Researchers at the Paul Scherrer Institute PSI have now used a new technique to visualise these changes in the cladding material in three dimensions for the first time, achieving a resolution in the nanometre range. Another distinguishing feature of their experiment is that the samples they examined were taken from cladding tubes that had actually been in service in a nuclear power station. The tubes were able to withstand the stresses involved: they were still leak-tight. The researchers have now published the results of their study in the journal Advanced Engineering Materials.

Tiny samples produce manageable radiation levels

The new research was carried out in a collaboration between three PSI research centres: the Center for Nuclear Engineering and Sciences, the Center for Photon Science and the Center for Neutron and Muon Sciences. At PSI’s so-called Hot Laboratory – a unique facility for the analysis of highly radioactive materials – staff from the Center for Nuclear Engineering and Sciences used special precision saws in shielded hot cells to slice wafer-thin samples from the cladding tubes of spent fuel rods. They then shaped them into cylindrical samples using a focused ion beam.

The samples were taken from the surface of the cladding tubes – which consists of a layer of zirconium oxide, due to oxidation 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,” explains Okan Yetik, first author of the new study. The research was part of his PhD, now completed, at the PSI Center for Nuclear Engineering and Sciences and the Center for Neutron and Muon Sciences.

“At this minute scale, the sample emits only very small amounts of ionising radiation,” adds Liliana Duarte, a researcher at the PSI Center for Nuclear Engineering and Sciences and one of Yetik’s PhD supervisors. “This meant we remained below the threshold above which such tests have to be carried out in specially shielded hot cells.”

This made it possible to analyse the sample at PSI’s Swiss Light Source SLS, using the ptychographic X-ray computed tomography (PXCT) technique developed there. The sample is scanned point by point from various angles using an X-ray beam, producing particularly high-resolution three-dimensional images of the material’s internal structure. “We were even able to identify pores as small as eighty nanometres,” says Andreas Menzel from the PSI Center for Photon Science, another co-author of the study.

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

The cladding tubes in nuclear power stations are made of special metal alloys that are highly resistant to corrosion. They usually consist of more than 98 percent zirconium. Small amounts of other elements such as chromium, iron, tin, niobium or nickel are added, depending on the alloy and the type of reactor. These tubes enclose the nuclear fuel: fissile uranium, usually in the form of uranium dioxide (UO₂). The fuel is compressed into tablet-shaped pellets, which are stacked inside the cladding tubes. Cladding tubes are typically four metres long, with an external diameter of approximately one centimetre and a wall thickness of about 0.7 millimetres. Depending on the type of reactor, between 50 and 300 fuel rods are bundled together to form a fuel assembly. A reactor core consists of several tens of thousands of fuel rods in total.

Comparison with standard tests possible

For the sake of comparison, the researchers also looked at samples of non-irradiated cladding tubes, like those used to date for quality testing: in an autoclave – a vessel that works much like a pressure cooker – samples are deliberately exposed to high pressure, high temperature and water.

It emerged that the density of the cladding tubes that had been used in reactors was practically identical to that of the autoclaved cladding tube samples. This indicates that the standard material tests carried out using non-radioactive samples from an autoclave do indeed yield meaningful results.

Good news for safety

This is very good news for the safety of cladding tubes and, consequently, of nuclear power stations. And it is also good news for future research into cladding tubes: it means that it is not always necessary to examine actual spent cladding tubes.

PSI’s research can therefore help nuclear power station operators to further optimise the water chemistry and operational management of their reactors. Fuel assembly manufacturers can tailor alloys and heat treatments more precisely to the specific requirements. Not least, the new procedure opens up the possibility of analysing the components of newer types of reactors – so-called small modular reactors – as well as those of nuclear fusion reactors in similar detail in future, and optimising them for use.

Dr. Liliana Isabel da Silva Duarte
PSI Center for Nuclear Engineering and Sciences
Paul Scherrer Institute PSI

+41 56 310 21 16
[email protected]
[English, German]

Dr. Andreas Menzel
PSI Center for Photon Science
Paul Scherrer Institute PSI

+41 56 310 37 11
[email protected]
[German, English]

Density Measurement of Irradiated and Non-Irradiated Zirconium Oxide by Ptychographic X-Ray Computed Tomography (PXCT)
Okan Yetik, Liliana I. Duarte, M.G. Makowska, Roland Brütsch, Andrej Bullemer, Ana Diaz, Mirko Holler, Andreas Menzel, Johannes Bertsch, Pavel Trtik

Advanced Engineering Materials, 11.10.2026 (online)
DOI: 10.1002/adem.202503145

About PSI

The Paul Scherrer Institute PSI is the largest research institute for natural and engineering sciences in Switzerland. It is the central location of Switzerland’s large research facilities, available to both the scientific community and industry. Its research spans future technologies, energy and climate, health innovation and fundamentals of nature. The annual budget is CHF 450 million. PSI employs 2300 people; around a quarter of these are postdoctoral researchers, PhD students or apprentices. PSI is part of the ETH Domain, which also includes ETH Zurich and ETH Lausanne, as well as the other federal research institutes Empa, Eawag and WSL. (Current as of 06/2026)