One of the ways that electricity generators ensure safety is by making conservative decisions to resolve even the slightest safety concern. Rare and unexpected issues—even minor ones—in nuclear power reactors around the world can sometimes result in temporary outages to investigate and respond. When reactor operators choose to forego electricity production, which can represent tens or sometimes hundreds of millions of dollars, it shows how important safe operations are to them.
Operators sponsor or perform research to improve predictions of the behaviour of materials over decades so that there will be fewer unexpected issues that lead to down time. In this way, enhancing the reliability of power plants goes hand in hand with safety and cost-effectiveness.
One factor that can lead to cracks in metal components over long periods of time is called ‘creep,’ in which the shape of a metal deforms very slowly. The creep can affect the growth of cracks and eventually limit the useful life of high-temperature components, such as those near the core of an advanced gas-cooled reactor (AGR).
EDF Energy in the United Kingdom collaborates with Professor John Bouchard of the Open University to study the behaviours of materials relevant to AGRs, thereby generating knowledge for safety cases presented to the nuclear power regulator.
Stress data from the CNBC helped EDF Energy to explain observations of accelerated crack growth rates. Now, EDF Energy is continuing to use that data in further studies aimed at reducing unexpected plant down time, thereby enhancing the safety and cost-effectiveness of operations.
Prof. Bouchard’s team accessed the Canadian Neutron Beam Centre in 2012 to quantify how residual stress influences creep crack growth rates at high temperatures. The CNBC examined test specimens made by electron beam welding of a pipe made of Esshete 1250, a type of stainless steel, that had undergone prolonged exposure to high-temperature service conditions. Prof. Bouchard applied two complementary methods of determining a portion of the stresses, which agreed well with the neutron data and provided high confidence in the results. The full stress distribution along the direction of the crack, as obtained by the CNBC, was then used to show that electron beam welding introduces high stresses—which inevitably produce large effects on fatigue, fracture or creep crack growth behaviour.
EDF Energy then applied the stress measurements from the CNBC to calculate the degree to which stress contributes to the creep crack growth in the specimen. The findings demonstrated that the stress from such electron beam welds does indeed accelerate creep crack growth rates, helping EDF Energy to explain earlier observations of crack growth rates in replicas of pipe weldments. Now, EDF Energy is continuing to use the stress data in further studies that use computer modelling to simulate the effects of stress (along with other contributing factors) on crack growth—a step along the way to reducing unexpected down time and enhancing the safety and cost-effectiveness of its power plants.