Western University engineers have taken a major step toward solving a long-standing challenge in nuclear power. A study, published in the high-impact journal Nature Communications, describes for the first time how hydrogen can combine with zirconium to form tiny, brittle structures that create intense stresses in CANDU reactor pressure tubes. These stresses may cause the surrounding metal to deform and even crack.
CANDU (Canada Deuterium Uranium) reactors are Canadian-designed, pressurized heavy-water nuclear reactors that use unenriched uranium as fuel and heavy water as a moderator and coolant. Inside the reactor, zirconium alloy pressure tubes (long, high-strength metal pipes) surround and hold fuel bundles and coolants for safety.
Over time, hydrogen can build up in the zirconium and combine with the metal to create a hydride (a chemical compound formed when hydrogen combines with another element). In this case, these tiny, brittle structures develop inside the zirconium and can create significant stresses in the pressure tubes.
The study, led by mechanical and materials engineering professor Hamid Abdolvand, offers a better understanding of how stresses develop and could help improve computer models used to predict fracturing in nuclear reactors. These findings may also be used to determine when reactor components are in need of replacement, helping improve their safety, reliability and lifespan.
“Nuclear engineers and scientists need to understand how these reactor components deform over time so they can accurately determine when they need to be replaced,” said Abdolvand. “This discovery proves why something happening at a scale invisible to the human eye matters to a reactor the size of a building.”
For the study, Abdolvand and his team used powerful X-rays at the European Synchrotron Radiation Facility in Grenoble, France to examine zirconium and hydride structures at a resolution of about 200 nanometres in three dimensions. The researchers found significant stresses inside the hydrides, exceeding one gigapascal.
“That is significant because these stresses can influence how cracks initiate and grow in the material. Having a three-dimensional picture of those stresses gives us a much better understanding of what is actually happening inside reactor components, such as fuel cladding or pressure tubes,” said Abdolvand.
A problem hiding in plain sight
The zirconium-alloy pressure tubes in CANDU reactors are only about four millimetres thick, yet they must withstand high pressures and temperatures while operating for decades. Hydrides can form repeatedly throughout a CANDU’s lifetime, so their effects can add up over time.
“If there is a small flaw in the zirconium-alloy component, the associated localized stresses can cause hydrides to form around it. As they form, they change the state of the material and potentially contribute to cracking,” said Abdolvand.
These localized stresses within the reactor may lead to delayed hydride cracking, a phenomenon that has been the subject of extensive research since a pressure tube failure at Pickering Nuclear Generating Station in the 1980s.
Seeing inside
The challenge with studying hydrides has always been seeing what is happening inside. Nuclear engineers have long had computer models that predict hydride stresses, but hydrides themselves are tiny – typically hundreds of nanometres to a few micrometres – making them extremely difficult to study experimentally, especially in 3D.
The experiment, led by Abdolvand, was conducted at the European Synchrotron Radiation Facility in France, which produces extremely bright and powerful X-rays that can probe materials at scales inaccessible to conventional imaging techniques. The Western team, which included Abdolvand and postdoctoral research associate Saiedeh Marashi, used a newly developed scanning 3D X-ray diffraction technique to map the grains and crystallographic structure within the zirconium alloy. The experiment generated about 10 terabytes of diffraction data and turning it into something meaningful required five years of analysis, computer modelling and collaboration.
Marashi, Masoud Taherijam and former PhD student Alireza Tondro, all contributed to the work, along with researchers at the European Synchrotron Radiation Facility and Lund University in Sweden.
Overall, the new technique gave the researchers something they had never had before – direct experimental evidence of the intense stresses inside hydrides their models had predicted.
Marking the importance of the discovery, the research team is making the work available to others. Along with the paper, the researchers plan to publicly release the raw diffraction data, cleaned and processed data, numerical results and source code.
“We are here to make a better place for the next generation,” said Abdolvand. “The next person who wants to look at this problem doesn’t have to start from zero. They can begin where our work stopped and take the science somewhere new.”
Darlington Nuclear Generating Station (Robert T. Bell)
From Western to the world
The five-year project also illustrates something Abdolvand considers just as important as the scientific finding: what happens to the students who do the work.
Marashi, a PhD student in Abdolvand’s Multiscale Deformation Lab when the project began, took on the extensive data analysis and numerical modelling. She ultimately led the project and wrote the paper. After working for more than a year as a postdoctoral research associate, she is now moving into the nuclear industry, joining CANDU Energy Inc., part of AtkinsRéalis.
And Tondro is now a scientist at Canadian Nuclear Laboratories, a world leader in developing nuclear technology for peaceful and innovative applications.
For Abdolvand, seeing students move into jobs where they use the skills developed through their research is not simply a measure of academic success or validation of the work. It’s a sign of real-world impact.
“Students spend years developing difficult, highly specialized skills. When they graduate and industry wants those skills, that’s real impact,” said Abdolvand.
That connection is especially important as Canada looks to extend the life of its existing CANDU fleet while developing new nuclear technologies, including small modular reactors for the Darlington New Nuclear Project.
Abdolvand has also seen knowledge move in the other direction – from industry into the university and back again.
“Researchers and engineers collaborate, students are trained on real problems, and the resulting knowledge can move into the next generation of reactor technology,” said Abdolvand.
Learn more about how Western is turning curiosity into solutions.
