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.
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.
Text: Jan Berndorff
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.
Advanced Engineering Materials
Experimental study
Not applicable
Density Measurement of Irradiated and Non-Irradiated Zirconium Oxide by Ptychographic X-Ray Computed Tomography (PXCT)
11-Oct-2026