Researchers at Massachusetts Institute of Technology (MIT) and other institutions have developed a remote test that can produce key information about the condition of stainless-steel reactor components, minimising outage time. The findings are reported in the journal Acta Materiala in a paper by MIT professor of nuclear science and engineering Michael Short; Saleem Al Dajani (who did his master’s work at MIT on this project and is now a doctoral student at the King Abdullah University of Science and Technology in Saudi Arabia); and 13 others.
The team included researchers at MIT, Idaho National Laboratory, Manchester University and Imperial College London (UK), Oak Ridge National Laboratory, the Electric Power Research Institute, Northeastern University, the University of California at Berkeley, and King Abdullah University of Science and Technology in Saudi Arabia. The work was supported by the International Design Center at MIT and the Singapore University of Technology and Design, the US Nuclear Regulatory Commission, and the US National Science Foundation.
Currently such testing requires removing test pieces of the same kind of steel that are left adjacent to the actual components to experience the same conditions, or removal of a tiny piece of the operating component. Both are done during shutdowns of the reactor, prolonging these scheduled outages and costing millions of dollars a day.
The new test involves aiming laser beams at the stainless-steel material, which generates surface acoustic waves (SAWs) on the surface. Another set of laser beams is then used to detect and measure the frequencies of these SAWs. Tests on material aged identically to NPPs showed that the waves produced a distinctive double-peaked spectral signature when the material was degraded.
Short and Al Dajani began their research in 2018, looking for a more rapid way to detect a specific kind of degradation, called spinodal decomposition. This can take place in austenitic stainless steel, which is used for components such as the pipes that carry coolant water to and from the reactor core. This process can lead to embrittlement, cracking, and potential failure in the event of an emergency.
While spinodal decomposition is not the only type of degradation that can occur in reactor components, it is a primary concern for the lifetime and sustainability of nuclear reactors, Short said.
“We were looking for a signal that can link material embrittlement with properties we can measure, that can be used to estimate lifetimes of structural materials,” Al Dajani noted.
They tried a technique Short and his students and collaborators had expanded upon, called transient grating spectroscopy (TGS) using samples of reactor materials known to have experienced spinodal decomposition as a result of their reactor-like thermal ageing history. Laser beams were used to stimulate and measure SAWs on a material. It was assumed that decomposition should slow the rate of heat flow through the material and the slowdown would be detectable by the TGS method. However, no such slowdown was detected. Short said: “You go in guns blazing, looking for a certain thing, for a great reason, and you turn out to be wrong. But if you look carefully, you find other patterns in the data that reveal what nature actually has to say.”
Instead, what showed up in the data was that, while a material would usually produce a single frequency peak for the material’s SAWs, in the degraded samples there was a splitting into two peaks. “It was a very clear pattern in the data,” Short recalls. “We just didn’t expect it, but it was right there screaming at us in the measurements.”
Cast austenitic stainless steels used in reactor components are duplex steels – a mixture of two different crystal structures in the same material by design. While one is quite impervious to spinodal decomposition, the other is quite vulnerable t. When the material starts to degrade, the difference shows up in the different frequency responses of the material. The team carried out further tests, which continued to strengthen the significance of the results.
“Our discussions with those who opposed our initial hypotheses ended up taking our work to the next level,” Al Dajani says. The tests they did used large lab-based lasers and optical systems. The next step is miniaturising the whole system into something that can be an easily portable test kit to use to check reactor components on-site, reducing the length of shutdowns. “We’re making great strides, but we still have some way to go,” he said. “Every day that your nuclear plant goes down, for a typical gigawatt-scale reactor, you lose about $2m a day in lost electricity,” he noted, “so shortening outages is a huge thing in the industry right now.”
Short hopes the new technique could help to enable the extension of NPP operating licences for some additional decades without compromising safety, through the frequent, simple and inexpensive testing of the key components.
The team included researchers at MIT, Idaho National Laboratory, Manchester University and Imperial College London (UK), Oak Ridge National Laboratory, the Electric Power Research Institute, Northeastern University, the University of California at Berkeley, and King Abdullah University of Science and Technology in Saudi Arabia. The work was supported by the International Design Center at MIT and the Singapore University of Technology and Design, the US Nuclear Regulatory Commission, and the US National Science Foundation.
Image: Massachusetts Institute of Technology (courtesy of MIT)
