X-rays look at nuclear fuel cladding with new detail

Micro-beam measurements at the Swiss Light Source SLS have enabled insights into the crystal structure of hydrides that promote cracks in nuclear fuel cladding. This fundamental knowledge of the material properties of cladding will help assess safety during storage.

For over seventy years, zirconium alloys have been used as cladding for nuclear fuel rods. This cladding provides a structural support for the nuclear fuel pellets and an initial barrier to stop fission products escaping into the reactor water during operation. During its long history, which includes extensive research and development advances, reactor type zirconium alloys have proved themselves as an extremely successful material for this application.

Yet they have a well-known nemesis: hydrogen. When submerged in water during operation in a reactor, at the hot surface of the fuel rod water molecules split into hydrogen and oxygen. Some of this hydrogen then diffuses into the cladding. It makes its way through the cladding until – when the concentration and conditions are right – it precipitates to form chemical compounds known as zirconium-hydrides. These hydrides make the material brittle and prone to cracking. Now, using the Swiss Light Source SLS, researchers were able to shed new light on the interplay between cracking and hydride formation.

Using a technique called synchrotron micro-beam X-ray diffraction, the researchers could study the structure of hydrides during the growth of cracks in fuel cladding at a new level of detail. “Through thermomechanical tests, we could control extremely slow crack propagations. Discovering at such high spatial resolution which hydride formations actually occurred made all the challenges of the material preparation worthwhile,” says study first author, Aaron Colldeweih who designed the thermomechanical testing procedure as part of his PhD project at PSI.

One of the things they discovered was that an unexpected type of hydride was present at the crack tip. This type of hydride, known as gamma-hydride has a slightly different crystal structure and stoichiometry to the type more commonly present, known as delta-hydride, “There has been a lot of discussion about gamma-hydrides: whether they are stable and whether they exist at all. Here we could show that with certain applied stresses you create gamma-hydrides that are stable,” says Johannes Bertsch, who leads the Nuclear Fuels Group in the Laboratory of Nuclear Materials at PSI.

Read more on the PSI website

Image: Malgorzata Makowska, scientist at the MicroXAS beamline of the SLS, carefully positions a standard material for setup calibration on the sample manipulator in front of the X-ray beam.

Credit: Paul Scherrer Institute / Mahir Dzambegovic

Innovative fuels for Small Modular Reactors

If Canada is to meet its target of net-zero emissions by 2050, our country must transition to a diverse, innovative range of alternative sources of energy.

Mouna Saoudi, a materials scientist at Canadian Nuclear Laboratories (CNL), is using the Canadian Light Source at the University of Saskatchewan to explore how advanced nuclear fuels for small modular reactors (SMRs) could be used to help fill the gap between fossil fuels and renewables.

“SMRs would be an efficient way to reach net zero by 2050, which is an ambitious but hopefully achievable goal,” says Saoudi.

SMRs can power electrical grids, provide process heat, and offer energy solutions for various industries — such as remote mining operations.

Saoudi is currently investigating how types of advanced nuclear fuels behave under different reactor conditions.

“My main focus is characterization of advanced nuclear fuels for potential use in small modular reactors,” Saoudi says.

The advanced fuels combine uranium oxide — the main element used in nuclear fuel for decades —with the naturally occurring and abundant element thorium in oxide form. Saoudi says that there are many advantages to mixing the two elements, including increased efficiency and better in-reactor performance.

Using the HXMA beamline, Saoudi was able to confirm the similar distribution of the two elements, uranium and thorium, in the mixed fuel oxides. Saoudi believes this was the first time the CLS has been used for this type of study.

Saoudi has been working with USask researcher Andrew Grosvenor from the Department of Chemistry. Their findings were recently published in the Journal of Nuclear Materials.

The CLS allowed Saoudi and her collaborators to investigate the electronic and local structure of the fuel — crucial information needed to identify the optimum fuel composition that would have better in-reactor performance than that of uranium oxide.

Read more on the CLS website

Image: (Left to right) Dr. Than Do, Dr. Mouna Saoudi, and Dr. Julien Lang, R&D scientists at Canadian Nuclear Laboratories (CNL).

I am doing science that is more important than my sleep!

NSLS-II #LightSourceSelfie

Dan Olds is an associate physicist at Brookhaven National Laboratory where he works as a beamline scientist at NSLS-II. Dan’s research involves combining artificial intelligence and machine learning to perform real-time analysis on streaming data while beamline experiments are being performed. Often these new AI driven methods are critical to success during in situ studies of materials. These include next generational battery components, accident safe nuclear fuels, catalytic materials and other emerging technologies that will help us develop clean energy solutions to fight climate change.

Dan’s #LightSourceSelfie delves into what attracted him to this area of research, the inspiration he gets from helping users on the beamline and the addictive excitement that comes from doing science at 3am.

Using uranium to create order from disorder

The first demonstration of reversible symmetry lowering phase transformation with heating.

ANSTO’s unique landmark infrastructure has been used to study uranium, the keystone to the nuclear fuel cycle. The advanced instruments at the Australian Synchrotron and the Australian Centre for Neutron Scattering  have not only provided high resolution and precision, but also allowed in situ experiments to be carried out under extreme sample environments such as high temperature, high pressure and controlled gas atmosphere.

As part of his joint PhD studies at the University of Sydney and ANSTO, Gabriel Murphy has been investigating the condensed matter chemistry of a crystalline material, oxygen-deficient strontium uranium oxide, SrUO4-x, which exhibits the unusual property of having ordered defects at high temperatures.

“Strontium uranium oxide is potentially relevant to spent nuclear fuel partitioning and reprocessing,” said Dr Zhaoming Zhang, Gabriel’s ANSTO supervisor and a co-author on the paper with Prof Brendan Kennedy of the University of Sydney that was published recently in Inorganic Chemistry.
Uranium oxides can access several valence states, from tetravalent— encountered commonly in UO2 nuclear fuels, to pentavalent and hexavalent—encountered in both fuel precursor preparation and fuel reprocessing conditions.
Pertinent to the latter scenario, the common fission daughter Sr-90 may react with oxidised uranium to form ternary phases such as SrUO4.

>Read more on the Australian Synchrotron website

Image: Dr Zhaoming Zhang and Gabriel Murphy.