One of the benefits of working at Canada’s national nuclear laboratory is the opportunity to do first of its kind research. Take for example the 3D printing of uranium dioxide (UO2) parts by fuel researchers at Canadian Nuclear Laboratories (CNL). Most know 3D printing as a process of creating a three dimensional object layer-by-layer using plastic and a computer generated design. Well, CNL researchers are using the same process, but instead of plastic – their method uses nuclear fuel. And in uncovering a technique to print new geometries of this most commonly used nuclear fuel in reactors today, they’re helping to pave the way to improve the safety and performance of future nuclear fuel.
“We first began looking at 3D printing fuel in 2017 to produce new ceramic fuel designs to advance how fuel could more efficiently and safely perform in a reactor,” says James Crigger, an R&D Technical Officer in CNL’s Advanced Fuels & Reactor Physics Branch. “A conventional power reactor fuel is a pellet made by pressing UO2 powder into a cylindrical shape followed by sintering at high temperature. This process would be extremely difficult with intricate geometries (designs) given that it is already challenging to work with; ceramic is hard, brittle and melts at a very high temperature. Plus, 3D printed ceramic fuels had never been done.”
The team, led by former CNL researcher Andrew Bergeron, understood that these ceramic properties would be a challenge for 3D printing methods, which typically use heat to form model geometries out of filaments or powder. However, unlike plastic materials commonly used in 3D printing that melt around 200˚C, UO2 melts at 2,800˚C. Yet, even before testing UO2, the team had successfully 3D printed thorium dioxide (ThO2) parts – a relatively easier material to 3D print and is also used as a nuclear fuel and being considered for several novel reactor concepts. This research was supported under Atomic Energy of Canada Limited’s Federal Nuclear Science & Technology Work Plan and published in 2018.
How could a novel UO2 fuel design make a difference?
Inside a nuclear reactor, the current cylindrical design pellet reaches high temperatures of 1400 ˚C for CANDU® and 1600 ˚C for Light Water Reactors. One disadvantage of UO2 is that it is an extremely poor conductor of heat, hence there is huge temperature gradient across fuel pellets during operation. What does this mean exactly? For a pellet that is only 12 mm in diameter, the centre of the pellet becomes extremely hot (>1400˚C) while the outer edge of the pellet that comes into contact with the coolant is only “warm” (400˚C). The current reactors are designed to keep the fuel cool under normal operating conditions. However, there is an interest to increase the reactor coolant temperature and to have fuel that is even more tolerant to high temperature in the unlikely event of accident conditions.
“A higher coolant temperature is always desirable for any thermal power plant (coal, natural gas, diesel, nuclear, geothermal, etc.) since that means a higher efficiency, i.e., more electricity for same fuel expenditure,” says CNL’s Fuel Scientist, Anil Prasad, presently coordinating the 3D printing activity and trials. “Currently, the coolant temperatures of nuclear power plants are dictated by safety considerations.”
This is because if the temperature of the pellet/coolant interface becomes too hot, for example under extreme accident scenarios, the centre of the pellet could reach temperatures at which the pellet cannot retain its shape, and deform or worse, melt.
The 3D printing of new geometries of UO2
Nuclear fuel researchers are trying to improve the heat transfer between the UO2 fuel pellet and coolant by various approaches, including adding high thermal conductivity materials such as silicon carbide, diamond or refractory metals to the UO2. However, this results in less uranium per volume within the fuel pellet, which is not ideal. “Another approach to improve heat transfer is to modify the geometry of the fuel, such as creating an annular (ring-shaped) pellet, so that the coolant can also flow through the center of the pellet, increasing the contact area between the fuel and the coolant, and transferring more heat,” adds Prasad.
Although the team has yet to fabricate the ring-shaped pellet design, they were successful in printing many other shapes. Due to intellectual property considerations, the technique used to successfully 3D print the fuel remains a secret for now. So how is the research team able to deem the 3D printing successful? An important measurement of nuclear fuel is density. A CANDU reactor fuel pellet has a density that is 96% of the theoretical calculated maximum. The geometries of 3D printed UO2 fuel measured 94% – a very impressive result. The 3D printed parts were also imaged using an X-Ray Computed Tomography machine that lets you see inside objects without damaging them to verify the quality of the printing.
The capability to 3D print UO2 with representative density is a significant breakthrough that will lead to additional prototyping and fabrication of nuclear fuel. The team is certainly excited to expand on their success and to be contributing to new innovations in the nuclear and additive manufacturing industries.