Following the 2011 Fukushima nuclear accident in Japan, the U.S. Department of Energy (DOE) initiated industry-led research and development programs aimed at increasing the tolerance of the fuel rods used in today’s nuclear power reactors in the event of a similar beyond-design-basis severe accident.
In the initial phase of the effort, the DOE funded a broad range of projects at universities, national laboratories and companies that explored multiple concepts for producing fuel rods to better withstand the extremely high temperatures that a nuclear reactor core can reach during the unlikely event of a severe accident.
After reviewing the results from those initial projects, the DOE and its industry partners then selected only a few of the most promising concepts for accelerated development in the second phase of the program.
A University of Wisconsin-Madison research project led by Kumar Sridharan, a distinguished research professor of engineering physics and materials science and engineering, in collaboration with Westinghouse Electric Company, is among the projects selected to advance to the second phase.
“It’s very exciting to have our concept selected for this next phase amid heavy competition,” Sridharan says. “We were really fortunate to have been picked by Westinghouse as a collaborator for this high-profile project, which aims to increase the temperatures that nuclear fuel can withstand and, coincidently, help solve a pressing economic challenge for the nuclear industry.”
In their development, Sridharan and his research group have employed a process called powder cold spray deposition, which can be used to apply an oxidation-resistant metallic coating to zirconium alloy fuel rods.
Current industry standard fuel rods are made out of a zirconium alloy. In a reactor, these uranium fuel-filled rods are bundled together, forming the reactor core. While zirconium performs well in a nuclear reactor under normal and most accident conditions, the material can exacerbate beyond-design-basis emergencies like the one that first occurred at Fukushima. In that instance, an unprecedented 9.5 magnitude earthquake and 45-foot-high tsunami caused the reactor cooling system to fail and the core to climb to very high temperatures.
“When zirconium heats up, it oxidizes exothermically, and the reaction produces a lot of heat, causing the core temperature to rise quickly in severe accident conditions,” Sridharan says. “Zirconium oxidizes very rapidly at high temperatures, so it is not fully suited for a ‘beyond-design-basis’ situation like what happened at Fukushima.”
Oxidation, which occurs when a metal reacts with oxygen to form an oxide, can cause problems in a reactor in case of an accident, where fuel rods can reach high temperatures, because it reduces the thickness of the metal cladding of the fuel rods, decreasing their overall structural strength.. In addition, oxidation of zirconium produces hydrogen gas, which was one causal factor of the explosions that occurred in Fukushima.
The UW-Madison team’s protective metallic coating would slow oxidization and hydrogen generation—potentially preventing a nuclear emergency or significantly mitigating its consequences, by providing additional operating time at high temperature, as well as time for the reactor operators to react.
“Using our cold spray process, we’ve deposited this metallic material on the rods, and in our tests we can see that this coating achieves significant protection for the underlying zirconium from oxidation, and that’s the main goal,” says Sridharan. “In a nuclear accident, every minute of coping time you can buy matters a lot, and this technology could help buy additional time for responders to bring in remedial measures.”
His cold spray process entails using a spray gun to shoot powder particles of a material at supersonic velocities onto the surface of the fuel cladding tube. As the solid particles hit the surface of the tube, they deform and flatten out like a pancake, forming a coating.
Notably, this coating process is extremely fast and cost-effective while also occurring at room temperature and atmospheric pressure—making it well suited for scaling up for large-scale manufacturing.
Since this approach doesn’t require any design or material changes for current light water reactors, it’s a promising near-term solution for the nuclear industry, which has very long timelines for adopting new materials or designs. Sridharan and his team are continuing to work on optimizing their coating and the process.
Under the DOE program, Westinghouse is currently testing the UW-Madison zirconium alloy cladding sections at its facilities in a prototypical light water reactor environment, as well as in steam at temperatures exceeding 1,300 degrees Celsius, to represent the most severe accidents.
The team’s coated claddings also are being tested under neutron irradiation in the Halden test reactor in Norway and in the Massachusetts Institute of Technology nuclear reactor. Sridharan is preparing more samples for neutron irradiation testing in the Advanced Test Reactor at Idaho National Laboratory.
“Reactor testing is very expensive and takes a long time, so the fact that we’ve been selected for testing in three separate reactors indicates that the DOE and industry are viewing this project very seriously and putting a lot of faith in what we’re doing,” Sridharan says.
Author: Adam Malecek