Next-generation claddings and perhaps coatings may head off future nuclear disasters, but only if the United States is willing to return the research to the front burner, says a scientist who is developing more protective cladding for nuclear applications.
Smoke rolls from Fukushima-Daiichi’s Reactor #4
after an explosion there March 15. The reactor had
been shut down since November for routine inspection,
but damage to the cooling system caused the pond
temperature to rise suddenly on March 14.
“There are materials that can withstand a few thousand degrees Celsius, but they have traditionally not been used for nuclear fuel cladding,” Dr. Hongbing Lu, of the University of Texas at Dallas, said in a recent interview.
Lu, a nanomaterials expert with the Erik Jonsson School of Engineering and Computer Science, is studying how new nano-based cladding might withstand future cracks in stressed fuel rods and other areas of nuclear reactors.
He has been working since last July on the issue, supported by an $875,000 grant from the Department of Energy.
And while the project began as an effort to improve fuel-rod efficiency, its safety implications have taken center stage since a series of disastrous explosions rocked multiple reactors earlier this month at Japan’s Fukushima-Daiichi nuclear power plant.
US Nuclear Regulatory Commission
Inspectors examine nuclear fuel
assemblies from US-operated reactors.
The plant was slammed by a 250-mile-long wall of water following a 9.0-magnitude earthquake that actually moved Japan nearly eight feet closer to the United States, according to researchers at Yale University.
‘Nobody Paid Serious Attention’
Lu says the plant actually survived the initial hit from the earthquake. But the facility’s designers never counted on a tsunami.
“Nobody paid serious attention,” he said. “They do factor in earthquakes in designing reactors. But when a tsunami comes, that’s something nobody predicted.”
Ironically, the hundreds of tons of water that dangerously exposed the fuel rods in the first place was insufficient to keep them cooled. That requires a continuous flow of water—a cooling system—which was unavailable since the tsunami knocked out power to the facility. Partial cooling has been restored, but engineers have far to go.
The U.S. Environmental Protection Agency has an extensive network of radiation monitors around the country and says it has detected no radiation levels of concern in the U.S. thus far. See www.epa.gov/radiation for more information.
More durable cladding materials could “definitively” make a difference in preventing or minimizing cracks in fuel rods in future disasters, says Lu. “Scientists might be able to develop fuel cladding to resist these temperatures.”
Ceramics, for example, can survive extreme heat, but they are currently far too brittle to withstand cracking.
Current fuel rod cladding is made of different alloys, with high concentrations of chromium to ward off corrosion in a highly corrosive environment, Lu explains.
During a reaction, he said, neutrons bombard the alloy and create “a lot of defects. Those defects are very small initially. But they will grow, and they will eventually form voids, like Swiss cheese. And those voids will join.”
‘Cladding Does Fail’
If rods can be cladded with a material that resists cracking, they may be able to withstand the ultimate disaster of a meltdown, in which the cladding becomes molten and cannot carry any load, Lu said. Current fuel cladding has a melt point of about 1,700 degrees Celsius, and nuclear reaction takes place at a much higher temperature, Lu said.
“The critical thing is to contain the temperature,” he said. “If you don’t cool down, that’s a problem.”
He added: “Nuclear fuel cladding does fail. If it’s just a small number that fail, it can be controlled. But if it’s a massive failure, it’s a disaster.”
Lu decided to pursue cladding in his research because, he said, few other researchers are.
“There’s no work to deal with the fracture of fuel cladding,” he said. “It’s not a well-known area, but a very critical one.”
Developing new materials presents enormous challenges, however. First, fuel rods in operation are not inspected or accessible, so Lu’s team has had to develop a code that simulates crack propagation under a number of situations in fuel cladding.
The larger problem, however, Lu contends, is the general lack of support for nuclear research in the United States. His grant is the first, and largest, in many years to fund such an effort.
“In the U.S., there has been no major nuclear engineering support in decades,” says Lu. “Recent energy discussions have re-started” some research, but the “US needs to educate a generation of scientists.”
“Nuclear energy is certainly the way to go,” Lu said. New reactors are faster and more efficient, and the spent fuel does not require 1,000 years of storage, he said.
“Renewable energy is not able to provide the amount of energy that is needed to replace fossil energy,” he said.
“Nuclear energy is not something that can be avoided. Nuclear reactions are happening underneath the Earth all the time. It’s not something that we can avoid. To avoid study will simply miss opportunities.”