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MIT Better World

By Mark Wolverton

Michael Short ’05, SM ’10, PhD ’10 found one such seed in the form of a neglected memo from more than 70 years ago that led him to the scientific question that now drives most of his work.

Short, the Norman C. Rasmussen Career Development Professor in the Department of Nuclear Science and Engineering whose lab is part of the MIT International Design Center, is fascinated by the fundamental definition of material damage at the atomic level. “We don’t have a way to measure radiation damage right now,” he says. “That makes it awfully hard to quantify.” Put a piece of metal into a nuclear reactor, he says, and despite any existing tests you might run on the material afterward, “you can’t tell me how much damage is left behind.”

Such damage—invisible, but with major implications for nuclear reactor technologies and a host of other applications— comes from high-energy particles like neutrons or ions knocking atoms out of place from the ordered atomic lattice of a material, a phenomenon that scientists generally describe using the term DPA (displacements per atom). But DPA doesn’t give the complete picture, as Short explains: “It’s a measure of how many times each atom gets knocked around by ionizing radiation, but it’s not a measure of damage, because most of the atoms pop back into place like nothing ever happened. Very few of them remain as defects.” Another problem is that DPA calculations are approximations rather than precise measurements. “I always ask experts in the field why we use the DPA, and their candid answer is, well, it’s not that good, but it’s the best we’ve got.”

Short decided there had to be a better way. Unexpectedly, he found the inspiration for one in a World War II–era memo. In it, Manhattan Project physicists Eugene Wigner and Leo Szilard were discussing a phenomenon that came to be known as Wigner energy or the Wigner effect, in which certain materials exposed to ionizing radiation might actually store energy in some way.

Short first learned of the memo from his nuclear science and engineering colleague, assistant professor Scott Kemp. Short also spoke with Ronaldo Szilard, a distant relative of Leo Szilard, who is a nuclear engineer at the Idaho National Laboratory. Using stored energy as a way to quantify damage had occurred to Short and his collaborators, but until hearing about Wigner and Szilard’s speculations, he’d found little encouragement to support such an idea. “We thought we were crazy until we realized this Nobel Prize–winning guy [Wigner] and this other couch-surfing theoretical physicist [Szilard] thought about it earlier.” Kemp showed Short a book noting the document’s location deep in the research library of the DuPont Corporation, where Short’s uncle, Cyril Milunsky, happened to work. “I was like, hey Uncle Cyril, go find this memo!”

Short realized that the stored energy concept could be expanded to encompass not only radiation damage in metals, but any sort of damage in materials. “Damage is defects, and it takes energy to make those defects,” he says. “Usually when you want to get rid of the defects in a material, you anneal it—you heat it up to a high temperature for a long time. If those defects go away, they should release the energy that it took to make them. That’s the crux of it, really.”

Short’s insight is that the energy is released as heat in a particular pattern—what he calls a “stored energy fingerprint.” Measured with exquisite precision with a nanocalorimeter, this energy fingerprint can provide a sharp picture of the defects in the material and the specific events that created them. This ability, applied to radiation damage and nuclear technology, has startling implications both for civilian and military uses.

“Scott Kemp has called this ‘radiation forensics,’ using radiation in different ways to reconstruct the historical usage of things,” says Short. “He and I are working together on using stored energy to reconstruct historical uranium enrichment. Scott and I think we can take, say, the centrifuges in Iran, and measure the stored energy in the walls of the devices and figure out how many bombs they’ve made.” That would provide a means of technical verification for international inspectors assessing nuclear deal compliance. For nuclear reactors used to generate energy, Short envisions a “cheek swab test” to check the health of steel reactor vessels, correlating the stored energy fingerprint to embrittlement and other important material parameters, and thus providing the technical reassurance required for extending the life of existing nuclear plants.

After a year of intense simulations, the project is now moving ahead to the dedicated experimental phase, using MIT’s research reactor and other facilities. For example, with funding from the MIT International Science and Technology Initiatives, Short is working with the research group of Oleg Maksimkin at the Institute for Nuclear Physics in Almaty, Kazakhstan, a collaboration he describes as indispensable, the 14-hour flights between continents notwithstanding. Maksimkin’s group has provided some of the theoretical explanations required to interpret Short’s calorimetric findings, confirming the measurements by their own magnetic techniques.

Recent data from Short’s group shows the energy released or absorbed once irradiated stainless steel is heated. Features in heat flow for the irradiated steel (red), compared to an unirradiated piece (blue), support Short’s premise that radiation creates a “stored energy fingerprint” in materials. Two unexpected energy absorption spikes above 400 degrees Celsius helped Short and his collaborators in Kazakhstan identify another, previously undocumented effect that could assist in identifying damage: some materials become magnetic when irradiated. Graph: Courtesy of the researchers

As a nuclear scientist, Short’s main focus at present is defining a standard unit for radiation damage, but he’s actively exploring other possibilities for the stored energy idea. “Let’s say you have a piece of steel that looks fine, but isn’t on the inside, and you can’t just cut it open and look at it because that would destroy it. Can you scoop out a microgram-sized sample of material and make a stored energy measurement to figure out what’s going on?” If Short’s work is successful, the possible applications range far beyond nuclear science to practically all areas of engineering.

For Short, the project has proven not only that good ideas are planted in unexpected ways and places, but also the value of persistence. “I’ve spent pretty much my whole life until now”— including 17 consecutive years at MIT, as a student and then a researcher—“thinking of things and finding out that somebody else has already thought them through,” he says. “Finally, after four years on the MIT faculty, I have an idea that no one else had already!”