Report calls for more R&D into material degradation

Degradation of structural materials must be further researched in new nuclear designs and potential mechanisms characterized and understood, according to a recent report.

A technician inspects the blades of a turbine during a planned maintenance intervention at the Bugey nuclear power plant (Source: Reuters/Benoit Tessier)

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Managing material degradation in nuclear power plants has always been a priority, though the report, “Degradation in structural materials for net zero: A landscaping exercise,” notes that millions of dollars can be saved if degradation is managed more closely.

The report was conducted by the Henry Royce Institute for Advanced Materials and Frazer-Nash Consultancy during spring this year and supported by the Institute of Corrosion.

Nuclear reactor life extension through material degradation management can have a direct economic impact within Britain of £1.5 billion ($2.1 billion) a year and generate supply chain income of £650 million a year, the report says citing the French utility EDF.

“In the nuclear sector, structural integrity and material degradation are key considerations early on in the design process, as they impact the safe operation and potential return from the facility,” says Tom Purnell, Business Manager for Advanced Nuclear and Government for Frazer-Nash Consultancy.

This is especially true for the next generation of advanced modular reactors (AMRs) which will operate at higher temperatures.

“Achieving this requires a detailed understanding of material degradation at these temperatures, and large research programs have both been run and are underway to obtain this understanding and the required confidence to operate at these temperatures,” Purnell says.

The study was conducted through engagement with stakeholders via a survey, with responses from 41 experts from across 37 organizations which were then clarified and discussed in detail in a technical review meeting held with respondents.

Cost of corrosion as a percentage of GDP, where bubble areas are scaled based on the cost as a proportion of world GDP

(Source: Compiled using data from the NACE Impact Report 2016 and the World Bank.)

Part of the plan

Nuclear has been identified as fundamental technology in the British government’s 10-point plan for reaching net zero by 2050, including large-scale light water reactors (LWRs), small modular reactors (SMRs) and AMRs and, with Britain’s fleet of advanced gas-cooled reactors (AGRs) due to come offline by the end of the decade, research and development for new nuclear is essential, the report says.

Common, cross-sector degradation mechanisms were identified as corrosion, fatigue and creep.

AGRs’ degradation mechanisms, due to the combination of irradiation and high temperatures, will result in stress corrosion cracking (SCC), fatigue, creep, and creep-fatigue of the metallic and graphite components. 

These mechanisms need to be better understood, the report warned.

“These mechanisms are not only significant in achieving net zero but are also responsible for significant disruption and expense to society,” the report said.

“The survey results demonstrated that concerns exist that the UK currently lacks sufficient practitioners with in-depth knowledge of the degradation mechanisms when irradiation and high temperatures combine,” it said.

Beyond higher temperatures, new nuclear will need to be more flexible as it works in conjunction with renewables, offering a baseload against the intermittence of solar and wind power, but this may add extra stress on the nuclear plant’s components, says Chief Executive of the Henry Royce Institute and Professor of Nuclear Engineering at Bristol University David Knowles.

“One of the biggest challenges with managing degradation in a large thermal plant is when you get a large number of transients – significant load changes with starting and stopping.  These will cause thermal expansion and contraction through thermal gradients which can generate significant cyclic stresses, which ultimately drive mechanisms such as fatigue; exacerbated in the presence of corrosive environments and high temperatures,” Knowles says.   

Digital technology and real time monitoring

Knowles is also the Principle Investigator for a new EPSRC Prosperity Partnership SINDRI (Synergistic utilisation of Informatics and Data centric Integrity engineering) which is developing cutting edge digital models of material behavior to assess the condition of components within a nuclear plant.

The five-year Prosperity Partnership aims to reduce the cost of nuclear power by applying new materials modelling frameworks and high-fidelity validation experiments to create a new approach to predicting material performance in a nuclear plant.

SINDRI is approaching this through digital material representations which will capture and improve understanding of materials at a meso scale to enable the prediction of components in service, though Knowles says this is still very much a work in progress.  

Ultimately these materials models will form a part of the large-scale digital representation of the nuclear plant – digital twins – whereby an identical digital model of an asset is created to monitor that asset’s internal functioning in real time and predict future performance.

Achieving this, says Purnell, will help track and predict degradation using statistical frameworks to quantify certainty in the output.  

However, digital twin implementation requires significant upfront investment to succeed including material test programs, model development and validation, he says.

“The ultimate digital twin runs alongside the actual plant. We’re not quite there yet, but it’s being developed as we go,” Knowles says.

We also need the development of the right sensors and monitoring equipment for the operating plant, which feed key information back into any digital twin being developed, he says.  

The vast amount of understanding of plant conditions, which can be gained from the right sensors in the right locations, is also being researched by another nascent area of growth in energy infrastructure monitoring: machine learning and artificial intelligence (ML/AI).

ML/AI help sift through the massive quantities of data to recognize patterns and filter out noise. This data can be linked to known defects or degradation processes.

“Often, for example, when we do an inspection with ultrasonic detection, we throw away a lot of the data just because we don’t have the capacity to process it all. Now, you have the potential to capture all the information when you put on an ultrasonic probe and with the right data processing tools, such as machine learning algorithms, you can start to use much more of that information,” says Knowles.  

Learning from experience

British experience of AGRs has led to an unparalleled understanding of high temperature degradation of steels and graphite degradation within the nuclear reactor, and this will be important as the country moves to the next generation of high temperature reactors, says Knowles.

“These are very complex degradation processes, but the learning and understanding which we have gained from them is going to be absolutely invaluable when we look at the next generation of reactors. The UK has got a huge amount of real plant operational experience on those degradation processes which most countries don’t have,” he says.  

Taking learnings from all over the energy and transport sector will be key in advancing the project, and this study is a step toward exactly that level of cooperation needed to properly address the issues faced in the push to net zero, the report notes.

“We identified six areas where strategic investment and coordinated efforts could bring cross-sector benefits,” Purnell says, including design and manufacture, modelling and simulation, maintenance and inspection, characterization and testing, knowledge and data management and leadership and policy.  

“The report identified that material degradation issues present significant but resolvable challenges to reducing our contributions to greenhouse gas production and global warming … this is an opportunity for collaboration prompted through the challenges we face to achieve net zero,” says Purnell.

By Paul Day