What's the Matter With Nuclear Materials
by David Pescovitz
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Brian Wirth (left) joined the faculty in 2002. (courtesy the researcher)
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Today, about 20 percent of the electricity consumed in the United States is generated by approximately 100 nuclear fission power plants. Nearly half of these commercial reactors first went online in the 1970s. After 30 years of extreme temperatures and constant radiation, how safe are they? That's one of the questions that Brian Wirth, a UC Berkeley professor of nuclear engineering, hopes to answer through materials science.
"Even though I'm in the nuclear engineering department, I really consider myself a materials scientist," Wirth says. "I want to understand the physical processes that are responsible for defect production and evolution in materials so we can predict when nuclear facilities are no longer safe to operate."
In the United States , commercial nuclear reactors have a licensed lifetime of 40years, yet, Wirth says, "most utilities would like to renew their licenses for another 20 years." The key though is determining whether the reactors are up to the task.
One threat is embrittlement of the reactor pressure vessel, the steel cylinder surrounding the nuclear fuel. The constant bombardment of neutrons causes the steel to weaken. A sudden change in temperature could crack the vessel. For example, in the event of an emergency, some reactors are designed to flood the core with cold water. This kind of thermal shock to alreadyembrittled steel is akin to filling a glass that's still hot from the dishwasher with ice water.
"Embrittlement could lead to catastrophic failure of the vessel and potentially a meltdown," Wirth says.
The irradiation hardening and embrittlement of steel, used in the construction of reactor pressure vessels like the one depicted in this computer model, threaten to limit the operating lifetime of nuclear power plants worldwide. (courtesy the researcher)
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Fortunately, it's hasn't happened yet. That's due in part to stricter rules established by the Nuclear Regulatory Commission, Wirth says. Policy is best though when it's based in science. Using various scientific methods--from positron annihilation spectroscopy, a technique for studying the tiniest defects in solids, to experiments where pieces of steel are irradiated and studied to molecular modeling on supercomputers--Wirth and his colleagues hope to uncover the cause-and-effect between neutron exposure and material failure.
"We're trying to link up models of all the levels, from the interaction of electrons in the crystalline structure up to hundreds, thousands, and billions of atoms at the micron scale," he says.
Wirth already has a good track record. He's continuing a longtime collaboration with G. Robert Odette, his former graduate adviser at the University of Santa Barbara , to study how the copper used to weld the pressure vessel together causes nanoscale defects in the steel.
"It turns out that welding with copper was a terrible idea," Wirth says. "They also used recycled scrap metal from cars that contained copper."
Along with the science of keeping reactors of the past in service, Wirth is also focused on the future of nuclear energy. Nuclear fusion--smashing two hydrogen isotopes together to release energy--promises an effectively limitless fuel supply with orders of magnitude less radioactive waste than fission reactors.
"Fusion is a potentially clean energy source, but right now the commercial reactors are just designs on paper," Wirth says. "In addition to the plasma physics challenges, the materials challenges are so tough that we can't make fusion work yet."
To that end, he's developing multiscale models of how materials change in fusion environments. The greater researchers' fundamental scientific knowledge, he says, the more success they'll likely have in creating new alloys capable of containing the kind of energy that powers the sun.
"Materials are the limiting technological factor in a wide variety of industries," Wirth says. "It's where safety, performance, and economics come together. The more we learn of the fundamental science, the more insight we'll have when we try to create new materials."
Brian D. Wirth's home page
US Nuclear Regulatory Commission Fact Sheet on Reactor Pressure Vessel Issues
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