Part 1 of 2 Parts
A new method has been discovered that could significantly reduce the time and expense of important safety checks in nuclear power reactors. This new approach could save money and increase total power output in the short run. In the long run, it might increase plants’ safe operating lifetimes.
Many analysts suggest that one of the most effective ways to control greenhouse gas emissions is to prolong the licenses lifetimes of existing nuclear power plants. However, extending these plants beyond their originally permitted operating lifetimes requires monitoring the condition of many of their critical components in order to ensure that damage from heat and radiation has not led or will not lead to unsafe cracking and embrittlement.
Stainless steel components of nuclear power reactors’ make up much of the plumbing systems that prevent heat buildup and many other parts. In order to test them, test pieces called coupons must be removed. These coupons are left adjacent to actual components containing the same kind of steel. In some cases, a tiny piece of the actual operating component must be removed. To apply either approach, the reactor must undergo an expensive shutdown. Testing prolongs scheduled outages and cost millions of dollars per day.
Researchers at MIT and elsewhere have come up with a new, cheap, hands-off test that can produce similar information about the condition of these reactor components. Far less time is required during a reactor shutdown. This research was reported today in the journal Acta Materiala in a paper by a MIT professor of nuclear science and engineering named Michael Short. Saleem Al Dajani who did his master degree work at MIT on this project and thirteen other researchers at MIT and other institutions also contributed.
The new testing technique involves aiming laser beams at the stainless steel material. This generates surface acoustic waves (SAWs) on the surface of the component. Another set of laser beams is then used to detect and measure the frequencies of these SAWs. Tests on materials aged identically to nuclear power plant components indicated that the waves produced a distinctive double-peaked spectral signature when the material was degraded.
Short and Dajani started on the new process in 2018. They were looking for a more rapid way to detect a specific king of degradation called spinodal decomposition. This degradation can occur in austenitic stainless steel (ASS). (ASS is one of the five crystalline forms of stainless steel.) ASS is used for components such as two-to-three-foot wide pipes that carry coolant water to and from the reactor core. This process can cause embrittlement, cracking, and potential failure in the event of an emergency. Spinodal decomposition is not the only type of degradation that can occur in reactor components. However, it is a primary concern for the lifetime and sustainability of nuclear reactors. Al Dajani 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.”
Short and Dajani decided to work on a technique that Short and his team had been developing called transient grating spectroscopy (TGS). They applied the technique on samples of reactor materials known to have experienced spinodal decomposition as a result of their reactor-like thermal aging history. The technique uses lasers beams to stimulate then measure SAWs on a material. The concept was that the decomposition should slow down the rate of heat flow through the material. The slowdown should be detectable by the TGS method.
Please read Part 2 next.