New method for assessing ageing of reactor components

Extending the operation of reactors beyond their originally permitted operating lifetimes requires monitoring the condition of many of their critical components to ensure that damage from heat and radiation has not led, and will not lead, to unsafe cracking or embrittlement.

A specific kind of degradation, called spinodal decomposition, can occur in austenitic stainless steel, which is used for components such as the large 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.

Testing of a reactor's stainless steel components currently requires 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. Alternatively, 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.

Researchers have now developed a new, inexpensive, hands-off test that can produce similar information about the condition of these components, with far less time required during a shutdown.

The 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 nuclear power plants showed that the waves produced a distinctive double-peaked spectral signature when the material was degraded.

Cast austenitic stainless steels like those used in reactor components are what is known as duplex steels, actually a mixture of two different crystal structures in the same material by design. 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.

The tests conducted by the researchers used large lab-based lasers and optical systems, so the next step, which they are now working on, 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.

The team included researchers at Massachusetts Institute of Technology (MIT), Idaho National Laboratory, Manchester University and Imperial College London in the 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 (KAUST) 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.

The findings have been reported in a paper in the journal Acta Materiala.

"Every day that your nuclear plant goes down, for a typical gigawatt-scale reactor, you lose about USD2 million a day in lost electricity," said Saleem Al Dajani, who did his master's work at MIT on this project and is now a doctoral student at KAUST, "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."