Dec 19 2018
Traditionally, a method called “cook and look” was used to assess a material’s ability to withstand the high-radiation environment inside a nuclear reactor. Here, the material is exposed to high radiation and then taken away for physical examination. But that process is so slow it delays the progress of new materials for future reactors.
The ion beamline at Sandia National Labs where the new radiation damage measurement system has been installed and tested. The radiation damage process is observed in a target chamber located behind the black-box laser enclosure on the right of the image. (Image credit: Cody Dennett)
At present, scientists at MIT and Sandia National Laboratories have created, tested, and made available a new system that can observe radiation-induced changes uninterruptedly, providing more valuable data much faster than traditional techniques.
With a number of nuclear plants reaching the end of their operational lifetimes under present-day regulations, knowing the state of materials inside them can be important to assess if their operation can be safely extended and if so by how much.
The new laser-based system can be used to monitor changes to the physical properties of the materials, such as their thermal diffusivity and elasticity, without altering or destroying them, the scientists say. The findings are reported in the journal Nuclear Instruments and Methods in Physics Research Section B in a paper by MIT doctoral student Cody A. Dennett, professor of nuclear science and engineering Michael P. Short, and technologist Daniel L. Buller and scientist Khalid Hattar from Sandia.
The new system, based on a technology known as transient grating spectroscopy, uses laser beams to probe small variations at a material’s surface that can disclose details about alterations in the structure of the material’s interior. Two years ago, Dennett and Short adapted the method to observe radiation effects. Now, after broad testing, the system is all set for use by scientists examining the development of new materials for next-generation reactors, or those seeking to extend the lives of current reactors through a better insight of how materials degrade over time under the severe radiation environment inside reactor vessels.
The old method of testing materials for their reaction to radiation was to expose the material for a period of time, then take it out and “bash it to pieces to see what happened,” Dennett explains. Instead, “we wanted to see if you could detect what’s happening to the material during the process, and infer how the microstructure is changing.”
The transient grating spectroscopy technique had already been formulated by others, but it had not been employed to study the effects of radiation damage, such as alterations in the material’s ability to conduct heat and react to stresses without cracking. It took years of development to adapt the method to the unique and tough environments of radiation.
To mimic the effects of neutron bombardment—the type of radiation that causes most of the material degradation in a reactor setting—scientists usually use ion beams, which create a similar kind of damage but are much easier to manipulate and safer to work with. A six-megavolt ion accelerator facility at Sandia was used by the team as the basis for the new system. These types of facilities quicken testing because they can mimic years of operational neutron exposure within a few hours.
By applying the real-time monitoring ability of this system, Dennett says, it is possible to pinpoint the time when the physical variations to the material begin to accelerate, which tends to take place quite suddenly and progress quickly. By halting the experiment just at that point, it is then possible to study fully what occurs at this critical moment. “This allows us to target what the mechanistic reasons behind these structural changes are,” he says.
Short says the system could undertake comprehensive studies of the performance of a particular material in a matter of hours, while it might otherwise take months simply to complete the first iteration of finding the point when degradation begins. For a full characterization, conventional approaches “might be taking half a year, versus a day” using the new system, he says.
In their tests of the system, the team used two pure metals—tungsten and nickel—but the facility can be used to test all types of alloys as well as pure metals, and could also test numerous kinds of materials, the scientists say. “One of the reasons we’re so excited here,” Dennett says, is that when they have talked about this technique at scientific conferences, “everybody we’ve talked to says ‘can you try it on my material?’ Everybody has an idea of what will happen if they can test their own thing, and then they can move much faster in their research.”
The actual measurements made by the system, which triggers vibrations in the material using a laser beam and then uses a second laser to monitor those vibrations at the surface, directly probe the thermal properties and elastic stiffness of the material, Dennett explains. But that measurement can then be used to extrapolate other related features, including flaw and damage accumulation, he says. “It’s what they tell you about the underlying mechanisms” that is most important.
The unique facility, currently in operation at Sandia, is also the subject of current work by the team to further enhance its capabilities, Dennett says. “It’s very improvable,” he says, adding that they want to add more types of diagnostic tools to investigate more properties of materials during irradiation.
The study is “a clever engineering approach that will allow researchers to characterize the response of a variety of materials to irradiation damage,” says Laurence J. Jacobs, professor and associate dean for academic affairs at the Georgia Tech, who was not involved in the research.
An outstanding piece of research on a noncontact, nondestructive evaluation technique that enables the real-time, in situ monitoring of the mechanical properties of a material subjected to ion beam irradiation.
Laurence J. Jacobs, Professor and Associate Dean for Academic Affairs, Georgia Tech
The U.S. Department of Energy, the MIT-SUTD International Design Center, the U.S. Nuclear Regulatory Commission, and the Center for Integrated Nanotechnologies at Sandia National Laboratories supported the research.