Nuclear power – the materials challenge
The general debate around nuclear power may be ongoing, but the materials challenges involved have broader implications and UK engineers and scientists are on the case. Laurie Winkless looks at the key issues.
Britain is pushing ahead with a new generation of nuclear reactors. Although it’s uncertain when these plants will be built, one thing is clear – ensuring the safe operation of existing nuclear plants is paramount. Understanding materials performance is key to that and, thanks to the work of materials scientists, the current cohort of power plants will perform well beyond their design life. But the next generation of reactors is likely to operate at much higher temperatures than conventional systems (approaching 1,000°C rather than 300–500°C), in order to improve the plant’s efficiency and economy. This will pose a series of challenges for the materials used throughout the plant, that scientists and engineers across the UK have set firmly in their sights.
The materials used within a power plant are varied – from the uranium dioxide used in fuels to the stainless and ferritic steels used in pipework and pressure vessels. Depending on where they are used in a power plant, materials may have to cope with everything from elevated temperatures and high pressures to corrosive or radioactive environments. Historically, this has led plant and component designers to a series of alloys made with high-performance steels, nickel or zirconium.
Alloying the circuit
One of the reasons that nickel-based alloys have found widespread application in the industry is because, unlike most metals, their mechanical strength improves as temperatures increase. It’s all down to their microstructure – although their basic structure is similar to that of stainless steel, they have an additional strengthening phase that helps to maintain their structure, even at high temperatures. This also helps to offer better stress corrosion cracking performance, an important property when you consider that nickel alloys are used in the primary circuit of some light water reactors and within steam generators.
But as we see a move toward high-temperature next-generation nuclear plants, understanding the behaviour of these materials will become both more complex and more crucial. Lindsay Chapman, Senior Research Scientist at the National Physical Laboratory (NPL), argues that the scarcity of rare earth metals is causing alloy producers to look in detail at the role of the materials’ microstructure. Given the difficulty of characterising this at temperatures beyond 800°C, Chapman’s expertise in metrology saw her become a leading figure in Metrofission, an EU project that looked specifically at this problem. Working with NPL and other European National Measurement Institutes, the project developed a range of reference materials with similar properties to those used in the primary circuit. The thermal and mechanical properties of these materials were measured in both custom-built, high-temperature systems and standard commercial set-ups. On completion of the project, the Metrofission team had demonstrated good agreement between the systems, providing materials engineers with a high level of confidence in the materials properties, and in the systems used to characterise them.
A further challenge is that nickel alloys rarely exist in isolation in the primary circuit. Ben Britton, a Royal Academy of Engineering Research Fellow at Imperial College, says it is difficult to define the properties of a composite system, where alloys are bonded to steels. Understanding the influence of heat treatment and the evolution of residual stress in such materials is a question for modelling experts to answer, and one in which the aerospace industry has historically been a leading figure. Working with Rolls-Royce, Britton’s experimental work feeds into advanced materials models, with the aim of becoming less reliant on the expensive experiments typical of the nuclear industry. This approach has also been taken by the National Nuclear Laboratory (NNL), whose materials modelling stretches from production to end-of-life. NNL’s Chief Scientist, Andrew Sherry, says, ‘Materials modelling offers us the ability to simulate components from cradle to grave. We can now model everything from the fabrication of material welds, through to microstructural degradation and crack development.’ Sherry suggests that we may be moving towards a ‘virtual prototyping capability for materials’, offering further insight into their behaviour, without the cost of creating numerous test samples.
The materials metrology offered by the combination of modelling and material design has resulted in a new approach to manufacturing these critical alloys. According to Chapman, ‘In the “plumbing end” of nuclear plants, materials have definitely developed – we are optimising the composition, have a much better understanding of the causes and effects of impurities and can cast materials with increasingly long lifetimes.’ And the materials metrology offered by the combination of modelling and material design offers an economic benefit, too. A fuller understanding of the crystal structure achieved by heat treatments helps potential suppliers to optimise their production, first time.
Moving closer to the heart of a nuclear plant, there are materials considerations around storing the fuel. Zirconium alloys, made with small additions of tin, iron, nickel and chromium, are used as the cladding for the hundreds of cylindrical rods that contain the nuclear fuel (in the form of pellets of uranium oxide). Cladding material has two main roles to perform. Firstly, it must maintain excellent mechanical properties, in a harsh environment – in water at around 160 atmospheres and 300°C and, secondly, it must have a low thermal neutron absorption coefficient to allow the neutrons produced by the uranium to pass through and interact with other nuclei, sustaining the nuclear chain reaction. Zirconium has been used in the industry for decades, for its corrosion resistance, high hardness and ductility, and its transparency to neutrons.
Over time, the zirconium alloy oxidises with the water that surrounds the fuel rods, releasing hydrogen gas. This partly diffuses into the alloy and forms zirconium hydrides, but, according to Britton, as long as the temperature of the water is maintained, ‘hydrogen will live quite happily in a solution’. However, if the temperature inside the reactor decreases rapidly, the hydrogen can come out of solution, causing blistering and cracking that can result in failure of the cladding.
So, developing an in-depth understanding of the microstructure of the alloy and its oxide are of critical importance. According to Britton, ‘If we can understand the process of hydride formation better, it will give us an even higher level of confidence in the material, and may even allow us to run tubes that are either thinner or can be run for longer.’
Given that Generation IV reactors will run hotter than current plants, this consideration is more relevant than ever. ‘The nuclear industry is understandably conservative,’ says Chapman, ‘they need to fully understand the material performance in both normal
operation and accident conditions, and for a new alloy this can take up to 15 years.’ For Sherry, the UK is in a good position to meet the challenges. ‘Having operated the hottest reactors in the world for some time, we have lots of experience in understanding the performance of materials in high-temperature reactors,’ he says. The importance of materials in nuclear power has been further highlighted by the Nuclear Innovation Research Advisory Board (NIRAB). Chaired by Dame Sue Ion, NIRAB has made specific recommendations to government for R&D into materials for Generation IV reactors.
Whatever the final decision around the next generation of nuclear power plants, one thing is sure – work from today’s materials experts has been instrumental in ensuring the safety, and extending the lifetime, of those already in operation.But, as in all industries, progress will lead to new questions, and developing an in-depth understanding of how materials behave in all environments sits high on the research agenda. Identifying and reducing measurement uncertainties, as well as developing advanced modelling tools, will be key in the design of new materials. As well as answering the safety question, clever materials design will also have an impact on the economic argument around nuclear power, by reducing the cost of experiments while increasing the reliability of the alloys.