A new method could greatly reduce the time and expense required for some important safety checks in nuclear power reactors. This approach can save money and increase total power output in the short term, and it may increase the safe operating life of the plant in the long run.

Many analysts believe that one of the most effective ways to curb greenhouse gas emissions is to extend the life of existing nuclear power plants. However, extending these plants beyond their originally permitted operating life requires monitoring the condition of many of their critical components to ensure that heat and radiation are not causing damage and unsafe cracking or embrittlement.

Today, testing of the reactor's stainless steel components - which make up much of the plumbing system that prevents heat from building up, as well as many other components - requires the removal of test pieces of the same steel, called samples, which are placed next to the actual components so that they experience the same conditions. Or, it requires removing a small piece of the actual running part. Both were done during costly outages of the reactors, extending those scheduled outages and costing millions of dollars a day.

Now, researchers at MIT and elsewhere have come up with a new, inexpensive, hands-off test that can produce similar information about the condition of these reactor components in far less time during outages. Today, MIT nuclear science and engineering professors Michael Short, Saleem Al Dajani '19 SM '20(who did his master's work on the project at MIT), Now a doctoral student at King Abdullah University of Science and Technology in Saudi Arabia (KAUST)) and 13 others at MIT and other institutions report these findings in the journal Acta Materiala.

The test involves aiming a laser beam at a stainless steel material to generate surface sound waves (SAWs) on its surface. Another set of laser beams was then used to detect and measure the frequency of these SAWs. Tests on the same aged material as the plant showed that the waves produced a unique bimodal spectral signature as the material degraded.

Short and Al Dajani started the process in 2018, looking for a faster way to detect a specific degradation, called spinel decomposition, that can occur in austenitic stainless steel, which is used in components such as the 2 - to 3-foot-wide pipes that carry coolant water to the reactor core. This process can lead to embrittlement, cracking, and possible failure in an emergency.

While spinel decomposition is not the only type of degradation that can occur in reactor components, it is a major concern for the longevity and sustainability of nuclear reactors, Short said.

"We are looking for a signal that can link material embrittlement to properties that we can measure and that can be used to estimate the lifetime of structural materials," says Al Dajani.

They decided to try a technique that Short and his students and collaborators had extended, called transient grating spectroscopy, or TGS, on samples of known reactor material that had undergone optical rotation decomposition due to its reactor-like history of thermal aging. The method uses a laser beam to stimulate and then measures SAWs on the material. The idea is that decomposition should slow down the heat flow through the material, and this slowing can be detected using the TGS method.

However, it turns out there was no such slowdown. "We went in with assumptions about what we were going to see, and we were wrong," Short said.

Instead, what is shown in the data is that while a material typically produces a single frequency peak of SAWs of a material, in the degraded samples, there is a split into two peaks.

"It was a very clear pattern in the data that we just didn't expect, but it was there, calling out to us in the measurements," Short recalled.

Cast austenitic stainless steels like those used in reactor components are so-called biphase steels, which are actually a hybrid design of two different crystal structures in the same material. But while one of the two types is fairly immune to spinel decomposition, the other is fairly susceptible. This difference shows up in the material's different frequency responses as it begins to degrade, and that's what the team found in their data.

The discovery, though, came as a complete surprise. "Some of my current and former students don't believe it happened," Short said. We couldn't convince our own team, with the initial statistics we had, that this was happening." So they went back and conducted further tests, which continued to reinforce the importance of the results. They reached a point of 99.9 percent confidence that the optical rotation decomposition does coincide with the crest separation."

"Our discussions with those who opposed our original assumptions ultimately took our work to new heights," Al Dajani said.

The tests they've done have used large laboratory-based lasers and optical systems, so the next step the researchers are hard at work on is miniaturizing the entire system to make it an easily portable test tool for on-site inspection of reactor components, reducing downtime. "We are making great progress, but we still have some way to go," he said.

But when they achieve the next step, he says, it could make a big difference. "For a typical gigawatt reactor, for every day your plant is offline, you lose about $2 million a day in power, so shortening the outage is a big thing in the industry right now," Al Dajani said.

He added: "The team's goal is to find ways to enable existing plants to operate for longer: to keep them out of service for shorter periods of time and be as safe or safer than they are now - not by cutting corners, but by using smart science to get the same information with less effort. And that's what this new technology seems to offer."

Mr Short hopes that by conducting frequent, simple and inexpensive tests of key components, it will help to extend power plants' operating permits for decades without compromising safety. "Existing large power plants" each generate just under $1 billion a year in carbon-free electricity, and it can take more than a decade to bring a new plant online, he said. To bridge this gap, keeping our current nuclear power online is the single biggest thing we can do to combat climate change."

The team included researchers from the Massachusetts Institute of Technology, Idaho National Laboratory, the University of Manchester and Imperial College London in the United Kingdom, Oak Ridge National Laboratory, the Electric Power Research Institute, Northeastern University, the University of California at Berkeley and the University of Science and Technology. The work was supported by MIT's International Center for Design and the Singapore University of Technology and Design, the Nuclear Regulatory Commission, and the National Science Foundation.