A new way to assess radiation damage in reactors

A new way to assess radiation damage in reactors

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By Rich Pell

Researchers at MIT and elsewhere say they have come up with a new, inexpensive, hands-off test that can produce similar information about the condition of reactor components in nuclear power reactors than a traditional hands-on approach, with far less time required during a shutdown. Currently, testing of a reactor’s stainless steel components — which make up much of the plumbing systems that prevent heat buildup, as well as many other parts — requires either removing test pieces, known as “coupons,” of the same kind of steel that are left adjacent to the actual components so they experience the same conditions or it requires the removal of a tiny piece of the actual operating component.

Both approaches are done during costly shutdowns of the reactor, prolonging these scheduled outages and costing millions of dollars per day. The new test approach 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 nuclear power plants showed that the waves produced a distinctive double-peaked spectral signature when the material was degraded.

This approach, say the researchers, could save money and increase total power output in the short run, and it might increase plants’ safe operating lifetimes in the long run. The method stems from the researchers’ search for a more rapid way to detect a specific kind of degradation – called spinodal decomposition – that can take place in austenitic stainless steel, which is used for components such as the two- to three-foot wide 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, say the researchers, it is a primary concern for the lifetime and sustainability of nuclear reactors.

“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,” says Saleem Al Dajani ’19 SM ’20, 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 (KAUST) in Saudi Arabia.

The researchers decided to try a technique called transient grating spectroscopy (TGS) on samples of reactor materials known to have experienced spinodal decomposition as a result of their reactor-like thermal aging history. The method uses laser beams to stimulate, and then measure, SAWs on a material. The idea was that the decomposition should slow down the rate of heat flow through the material, that slowdown would be detectable by the TGS method.

However, say the researchers, it turns out there was no such slowdown.

“We went in with a hypothesis about what we would see, and we were wrong,” says MIT professor of nuclear science and engineering Michael Short. “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, say the researchers, 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,” says Short. “We just didn’t expect it, but it was right there screaming at us in the measurements.”

Cast austenitic stainless steels like those used in reactor components are known as duplex steels – actually a mixture of two different crystal structures in the same material by design. But while one of the two types is quite impervious to spinodal decomposition, the other is quite vulnerable to it. When the material starts to degrade, the difference shows up in the different frequency responses of the material, which is what the researchers found in their data.

“Some of my current and former students didn’t believe it was happening,” says Short. “We were unable to convince our own team this was happening, with the initial statistics we had.”

Further tests continued to strengthen the significance of the results, reaching a point where the confidence level was 99.9 percent that spinodal decomposition was indeed coincident with the wave peak separation, say the researchers. The tests they did used large lab-based lasers and optical systems, so the next step, which the researchers say they are hard at work on, is miniaturizing 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.

When they achieve that next step, say the researchers, it could make a significant difference.

“Every day that your nuclear plant goes down, for a typical gigawatt-scale reactor, you lose about $2 million a day in lost electricity,” says Al Dajani, “so shortening outages is a huge thing in the industry right now. Let them be down for less time and be as safe or safer than they are right now — not cutting corners, but using smart science to get us the same information with far less effort.”

The researchers say they hope that this could help to enable the extension of power plant operating licenses for some additional decades without compromising safety, by enabling frequent, simple and inexpensive testing of the key components. For more, see “Detecting Thermally-Induced Spinodal Decomposition with Picosecond Ultrasonics in Cast Austenitic Stainless Steels.”

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