X-Ray Computed Tomography helps ensure that fuels, materials, and components will perform safely, effectively, and efficiently under the extreme environment of a nuclear reactor
Reeghan Osmond is a fuel research scientist specialized in X-Ray Computed Tomography imaging at Canadian Nuclear Laboratories, pictured here holding a vial of surrogate TRISO fuel particles
Reeghan Osmond has an eye for the tiny.
As a child, she was drawn to the little details most would overlook: the scuttling legs of a ladybug, the single sparkle left behind by an art project, the maze-like circuits etched into a thumbnail-sized microchip. Nowadays, she’s studying a type of nuclear fuel that’s no bigger than a poppy seed, working as a fuel research scientist at Canadian Nuclear Laboratories (CNL).
They’re called Tri-structural Isotropic (TRISO) fuel particles — the nuclear fuel of choice for several companies aiming to build small modular reactors (SMRs) and microreactors here in Canada.
However, Canada has never used this type of fuel at the demonstration or commercial reactor scales before, especially because it’s designed for advanced reactors that haven’t been deployed here. Therefore, many questions remain around how exactly the country could use these tiny, 1-millimetre-diameter spheres to power our communities.
Osmond is part of a team of over 30 researchers at Canada’s national nuclear laboratory gathering the data needed to inform the country’s next steps regarding TRISO fuel and the role it could play to support net-zero targets by 2050. This research will support the federal nuclear regulator, the Canadian Nuclear Safety Commission (CNSC), in building a Canadian knowledge base on TRISO-based fuel — how to make it, study its properties, prove how it performs in reactors under normal and potential accident scenarios, and manage it at the end of the fuel cycle.
A significant part of that research involves closely examining the insides of these particles that are around the size of a grain of sand, without obliterating them in the process.
And this is where Osmond and her lab’s two technologists excel.
“Just like a medical doctor can use a CT scan to understand what’s going on inside their patient’s body, we can use the same technology to look inside tiny TRISO fuel particles, without cutting them open and damaging them in the process,” explains Osmond.
CNL advanced fuel researchers prepare to scan surrogate TRISO fuel particles using the XCT machine. Pictured are fuel research scientist Reeghan Osmond (left) and technologist Jason Budgell (right)
The trio specialize in a non-destructive imaging technique called X-Ray Computed Tomography (XCT), which uses x-rays to “slice” through a sample and generate thousands of cross-section images of it. Computer technology then sews those images back together to form a complete 3D scan of that sample.
In the case of their TRISO fuel research, the team analyzes these scans to identify and study details in them that indicate how well the advanced fuel was fabricated and how it could perform in a reactor environment.
The uniqueness of TRISO fuel particles
All nuclear fuel serves the same primary purpose inside a reactor: to produce heat that can then be converted into the energy we need for electricity or for heating.
Both CANDU® fuel pellets and TRISO fuel particles do this through a process called nuclear fission, where the neutrons in a reactor interact with the fuel, causing its uranium atoms to split apart and release energy in the form of heat and “fission products” — the radioactive leftovers from splitting these uranium atoms. “Even though they both work to produce heat, these two types of nuclear fuel are different in how exactly they’ve been designed to do that,” says Osmond.
To start, a CANDU® fuel pellet is solid uranium dioxide all the way through, while TRISO fuel particles only have a “kernel” uranium-based fuel at their very center.
And though nuclear fuel is designed to act as the first of many barriers that contain the fission products they generate, these two fuel types do so at a different capacity.
For CANDU® fuel, the pellets themselves are considered the first barrier, while the hollow, zirconium-alloy sheaths they’re stacked inside act as the second barrier. Comparatively, TRISO particles, themselves, contain both the first and second barrier that prevent fission products from escaping, with the uranium kernel as the first and the multiple coatings surrounding it as the second.
A colourized XCT scan of a 1-mm-diameter surrogate TRISO fuel particle
Directly surrounding the fuel kernel is the “buffer layer” made of porous pyrolytic carbon, which provides a tough but flexible radiation-resistant barrier. This layer accommodates the fuel as it swells and generates gases during the fission process.
Another denser layer of pyrolytic carbon surrounds that buffer layer, adding structural strength and preventing fission gases from moving outward.
Then, surrounding that is a layer of a ceramic compound frequently used in bullet-proof armour, called silicon carbide. This ceramic material is extremely strong, durable, and capable of resisting damage from radiation, corrosive conditions and high temperatures. It gives TRISO particles the structural strength to stay intact inside a reactor and serves as the main “plate of armour” that prevents the main fission products from escaping.
And finally, another layer of dense pyrolytic carbon. This layer not only protects the silicon carbide, but it also provides a surface that can bond to a graphitic matrix — the material used to hold and shape thousands of particles into more practical pellet-shaped “compacts” or a billiard ball-shaped “pebbles.”
Different types of XCT scans of 1-cm-diameter TRISO compacts that visualize the particles (left) and fuel kernels (right) contained within the graphitic matrix
Automating the process of analyzing XCT images
Though they provide extremely valuable insights, XCT scans of TRISO fuel — in either their particle, compact, or pebble form — have to be meticulously analyzed to identify the significant details that indicate how the fuel would perform.
“It’s a labour-intensive and, consequently, slow-going process”, explains Osmond.
To pull out all the measurements the team would typically be looking for in a TRISO compact — like the shape of the fuel kernels and the spacing between each particle embedded in the graphitic matrix, for example — it could take a trained professional anywhere upwards of three hours. And that doesn’t even include the time it takes to manually plot all the data points and compile everything into a neat and tidy report.
Because of this, Osmond has been exploring how to automate this process and make it faster.
Advanced fuel researchers Reeghan Osmond (left) and Jason Budgell (right) preparing an XCT scan
This summer, she successfully tested the Python code she developed that “looks at” an XCT image of a TRISO compact and automatically produces a report with key measurements needed to better understand the specific technique used to make the compact, the quality of that technique, and how that compact could perform in a reactor.
And, with the power of automation, it only took around 40 minutes to generate the report — a significant time reduction from the usual three hours. The early success represents a key step towards more streamlined XCT imaging in support of Canada’s TRISO fuel research and, more broadly, supports the larger international effort into efficient and strategic clean energy research during a time when climate change demands nothing less.
“This work that we’re doing — that the nuclear industry and research community are doing as a whole — has the potential to shape what the rest of our lives look like for many generations to come. There’s a lot riding on clean energy right now, and it feels good to be part of something that’s working to make the world a cleaner, safer, more sustainable place,” says Osmond.
TRISO fuel research at Canadian Nuclear Laboratories
CNL’s TRISO fuel research program has continued to grow for over four years now.
With support from Atomic Energy of Canada Limited’s (AECL) Federal Nuclear Science & Technology (FNST) Work Plan, CNL has rapidly expanded its experimental and modelling capabilities in TRISO fuel, building upon its eight decades worth of expertise in nuclear fuel, more broadly.
This effort of over 30 researchers at Canadian Nuclear Laboratories has resulted in new experimental setups and testing protocols, contributions that helped close knowledge gaps, and collaborations with Canadian universities and other international partners, explains Cathy Thiriet, CNL’s technical manager of advanced fuels and materials research.
“TRISO fuel is a foundational part of next-generation reactor designs. And its enhanced safety features make it especially ideal for small modular reactors and microreactor concepts,” says Thiriet, who has led CNL’s TRISO fuel research program since its inception. “I’m incredibly proud of our talented, enthusiastic team and the remarkable progress we’ve made over the past few years. It’s an honour to guide research that directly supports Canada’s top priorities.”
This research is funded by Atomic Energy of Canada Limited’s (AECL) Federal Nuclear Science & Technology (FNST) Work Plan, which connects federal organizations, departments, and agencies to the nuclear science expertise and facilities we have at Chalk River Laboratories.
Under the FNST Work Plan, researchers at Canadian Nuclear Laboratories (CNL) carry out projects to support the Canadian government’s core responsibilities and priorities across the areas of health, safety and security, energy, and the environment.
