Understanding the mechanisms controlling low potential stress corrosion cracking in nuclear reactors

Lead Research Organisation: University of Oxford
Department Name: Materials


The UK currently operates a civil nuclear fleet consisting primarily of advanced gas reactors (AGRs), a pressurized water reactor (PWR) and a nuclear-energy powered submarine fleet of PWRs. Any new nuclear build in the foreseeable future will likely be a new PWR European Pressurized Reactor (EPR) by EDF/Areva and/or an ABWR by Hitachi. The nuclear industry has one of the highest commitments to safety in terms of investment in R&D and both the Universities of Oxford (UoO) and Manchester (UoM) have played a vital role in the past decades by collaborating with almost every nuclear player in the world. In particular, we have dedicated a great effort to understand and predict stress corrosion cracking (SCC), which is one of the most serious concerns for the industry and will be the main focus of this project.

SCC is a progressive failure mode which requires a specific environment (cooling water), stress (applied or residual) and a susceptible material (stainless steels or nickel base alloys). Several mechanisms have been proposed to explain its occurrence in nuclear reactors but, unfortunately, none has been capable of explaining or predicting it fully for the materials of interest. Some of the most accepted mechanisms involve preferential intergranular oxidation, local deformation around the crack tip or hydrogen embrittlement [1].

Over 50 years ago, Henri Coriou [2] identified a "safer" range of compositions with Ni wt% between 20 and 60 where alloys would be less susceptible to SCC. Initially, this study did not receive much attention, but it was later known as the "Coriou effect" and most recently it has been the subject of a comprehensive review [3]. The validity of this effect has been extensively investigated by Arioka (INSS), who autoclave-tested a series of samples with varying Ni, Fe and Cr levels at different temperatures. His work led to the conclusion that crack growth rates (CGRs) are indeed strongly affected by these parameters (chemical composition and/or temperature) [4] and thus validated the existence of the "Coriou effect".

A mechanistic explanation for this observed behaviour has yet to be realised, however we believe we are now in a position to formulate it. If we are successful, we believe we can unveal most opearting SCC mechanisms and their interplay. Our approach involves isolating the effects of single variables in SCC crack initiation or propagation, which has been instrumental in revealing the effect of cold-work, water temperature, alloy composition and stress level on SCC and the controlling mechanisms under low-potential conditions (PWR and ABWR) [5]. We plan to use a multi-technique characterization approach, involving state-of-the-art equipment and the combined expertise from the universities of Oxford and Manchester, to better understand the "Coriou effect" and whether H plays an important role or not. The proposed project will make use of one of the most ambitious and comprehensive set of samples ever tested, provided in kind by INSS.

1. Lozano-Perez, S., Dohr, J., Meisnar, M. & Kruska, K. SCC in PWRs: Learning from a Bottom-Up Approach. Metallurgical and Materials Transactions E 1, 194-210 (2014)
2. Coriou, H., Grall, L., Mahieu, C. & Pelas, M. Sensitivity to Stress Corrosion and Intergranular Attack of High-Nickel Austenitic Alloys. Corrosion 22, 280-290 (1966).
3. Feron, D. & Staehle, R. W. Stress Corrosion Cracking of Nickel Based Alloys in Water-Cooled Nuclear Reactors. , 384 (2016).
4. Arioka, K., Yamada, T., Miyamoto, T. & Aoki, M. Intergranular stress corrosion cracking growth behavior of Ni-Cr-Fe alloys in pressurized water reactor primary water. Corrosion 70, 695-707 (2014).
5. Meisnar, M., Vilalta-Clemente, A., Moody, M., Arioka, K. & Lozano-Perez, S. A mechanistic study of the temperature dependence of the stress corrosion crack growth rate in SUS316 stainless steels exposed to PWR primary water. Acta Materialia 114, 15-24 (2016).

Planned Impact

We will ensure that relevant groups and the nuclear industry benefit from this research by continuing and expanding our current collaborations, attending international conferences, holding targeted workshops and publishing in leading international journals. More importantly, this project will also further develop our expertise and international leadership, train of the next generation of expert scientist/technicians in a field critical to the energy security of the UK and provide the nuclear industry with a powerful research methodology.
Our research on SCC of austenitic alloy will have a direct impact on the safe operation and reliability of the new fleet of nuclear reactors in the UK. As an example, our ongoing relationship with EDF, collaborator in this project and responsible for the EPR reactor(s) at Hinkley Point C, will ensure that an efficient knowledge transfer and a two-way communication are in place.
The principal pathways to impact of this project will emerge through interactions with the project partners: the MAI/EDF (France), INSS (Japan) and other collaborating companies (already involved in collaborative projects) such as Rolls Royce (UK) and AMEC Foster Wheeler (UK). In addition, Oxford and Manchester have already strong links with the nuclear industry, which will directly benefit from any novel data or results through our frequent regular meetings, e.g. with NNL (UK), Areva (France), SENPEC (China) or EPRI (USA). In addition, we have setup a collaboration with Oxford Instruments that will give us access to state-of-the-art detectors and data analysis software which we will use in this project. These partners have expressed their commitment to this project through total in-kind contributions totaling £1.2M as outlined in the statements of support attached to this proposal.
Both the School of Materials at Manchester and the Department of Materials at Oxford University have large and successful school outreach programs. Through these programs, we will interact and engage with school children through either visiting schools or through University open days. This is a core component in the delivery of pathways to impact as it is vital that we excite the next generation of nuclear scientists at an early age.
Interactions with universities undergraduate students will also be undertaken. In particular, undergraduates will be encouraged to participate in Masters projects (at no cost to the project) in the field of environmental degradation in materials. There are already several projects proposed for this and next academic year. This is a significant pathway to impact aimed at generating more nuclear materials scientists and engineers and counteracting the general lack of specialists the UK is currently facing.
A project website will be set up, linked to the existing nuclear related projects based at Oxford University e.g. the Materials for Fusion and Fission Power website (http://mffp.materials.ox.ac.uk/), and the Bristol-Oxford Nuclear Research Centre. It will be expected that the public areas of these websites will be cross-linked highlighting the important role materials science has in the design and engineering of new nuclear builds.
The dissemination of results obtained as a direct product of this research to academia is a key factor in the success of the project. The area of this proposal - environmental degradation of nuclear materials - is an area in which the publication of results in international journals is highly desirable. Open access publication will be ensured as it is part of the university policy. This is vital in furthering public interest in work of this type as members of the general public will have access to cutting edge research that has previously been inaccessible. In addition, our research will be presented at relevant seminars, workshops and conferences.


10 25 50

Description This award would have finished this year but it has been extended until May 2022 due to the impact of the Covid-19 pandemic. Our findings are cumulative in nature, benefiting from parallel sub-tasks that have been providing key data to the project. We have already established how some previously unreported mechanisms control stress corrosion cracking in light water reactors. In particular, we have identified and quantified several precursors which have an effect on crack initiation and growth, which weight depends on alloy composition and operating temperature. These include intergranular oxidation, localized deformation around the crack tip, intensity of grain boundary migration amongst others. In addition, we have studied the role of Hydrogen, which affects local motion of dislocations (and, therefore, local deformation behaviour) and has a different effect on different alloys. Once we had developed a robust methodology for the high-resolution characterisation of environmental degradation and given the project had enough manpower and bandwidth, we decided to extend our characterization and analysis approach to other materials systems, used in Gen IV reactors, with very satisfactory results. These resulted in a series of well-received additional international publications and presentations.
Exploitation Route They are already being taken into consideration by some of our industrial partners. EDF (Energy sector) is using our findings to refine and improve their crack initiation and growth models which are essential to demonstrate plant operation safety and will sponsor further research in Oxford. INSS (Energy sector) has proposed a new explanation to the high-diffusion observed at PWR temperatures based on vacancy diffusion/availability based on our findings and have extended our joint collaborations. Framatome (Energy sector) has decided to use our approach to better understand the effect of industrial surface finish on materials degradation in nuclear reactors, sponsoring one of our new DPhil projects. They are also a regular user of our consultancy and characterization services.
Sectors Energy,Environment

URL https://nanoanalysis.web.ox.ac.uk/understanding-the-mechanisms-controlling-low-potential-stress-corrosion-cracking-in-nuclear-reactors#/
Description The nuclear industry has strong links with Oxford, dating back over 40 years. Our joint projects include reactor pressure vessel embrittlement under irradiation, Zirconium fuel cladding environmental degradation and, the topic of this particular project: stress corrosion cracking in the primary cooling circuit. The outcomes of this project have not only been publicised through international conferences and peer-reviewed publications but also been discussed in numerous meetings with our industrial collaborators. In particular: - EDF: Has used our findings in SCC mechanisms to validate, refine and improve their SCC initiation predictive model. A similar approach is currently being discussed to better understand IASCC of former baffle bolts and SCC of CW stainless steels. - Framatome: Has incorporated the characterization approach we developed in this project to their alloy testing and validation programme. They now collaborate in a newly created DPhil project and have started using our consultancy services for related quick turn around characterization. - INSS: Has used our findings in Alloy 690 long-term degradation (via intergranular cavitation) to propose a mechanistic model of degradation to the Japanese regulator which should better reflect microstructural and mechanical properties changes in this alloy during service Regarding cultural and societal uses of our findings, I would like to think that my outreach activities, public lectures and media interviews (E.g. after receiving the "UK Framatome prize in 2019", have resulted in a better understanding of the challenges and benefits of nuclear energy for the society as a whole. Besides, this project has produced a very educational narrative by showing how a systematic and careful materials characterization approach, often pushing the limits of what was achievable, has been able to generate very intuitive and graphical results that easily show how the mechanical and chemical changes in a very reduced region (of just nanometres!) around crack tips can explain macroscopic behaviour and, ultimately, failure. It has been a very rewarding experience to watch the audiences react to our results and see how they can follow our advances and understand our findings.
First Year Of Impact 2021
Sector Education,Energy,Environment
Impact Types Cultural,Societal,Economic

Description Collaboration with Shanghai Jiao Tong University 
Organisation Shanghai Jiao Tong University
Country China 
Sector Academic/University 
PI Contribution This collaboration provides Shanghai Jiao Tong University with access to our high-resolution characterization facilities in Oxford, annual visits by Prof Lozano-Perez to Shanghai to organize workshops and lecture series and visits by Shanghai Jiao Tong researchers to Oxford.
Collaborator Contribution Shanghai Jiao Tong University contributes to this partnership by providing free-of-charge access to their autoclave testing facilities. Our research relies on access to samples tested under nuclear-reactor simulated conditions and we don't have any means of testing samples in Oxford. Shanghai Jiao Tong has one of the most extensive testing facilities in China and we have established a very fluent communication and collaboration strategy that allows us to request specific samples/ testing conditions as part of our collaboration. This would cost hundreds of thousands of pounds if requested commercially.
Impact Mostly joint peer reviewed papers (listed separately in their section), conference contributions and joint proposals currently in preparation
Start Year 2018
Description Collaboration with Shanghai University 
Organisation Shanghai University
Country China 
Sector Academic/University 
PI Contribution We have hosted visitors from Shanghai University during 2019 where they have familiarized with our unique high-resolution characterization methodology. They have characterized samples from our collaborative projects and learn how to analyse the data. This will be the basis for future joint publications and joint projects.
Collaborator Contribution As an International Expert Group Member of Shanghai University, I visit China twice per year. During my visits we discuss current and future collaborations and review joint publications. At present, three manuscript have been prepared and submitted to international journals. The topics covered were already presented at international conferences (e.g. Int Symp on Environmental Degradation of Materials in Nuclear Power Systems)
Impact Joint contributions to the 19th Int Symp on Environmental Degradation of Materials in Nuclear Power Systems: -Coupling effect of charged-hydrogen and cold work on oxidation behavior of 316L stainless steel in deaerated high temperature water -Characteristics of oxide films formed on 309L and 308L stainless steels in simulated PWR primary water -Diffusing hydrogen effect on the oxide film on 316L SS in simulated PWR secondary side water -Stress corrosion cracking of stainless steel cladding layers in simulated PWR primary water -Effect of post weld heat treatment on microstructure and PWSCC of Alloy 52M weld metal in dissimilar metal weld joint -Effect of weld dilution and dendrite orientation on PWSCC behavior of Alloy 52M weld metal -Microstructural Evolution of 52M weld metal near the fusion boundary and oxide films formed in simualted PWR primary water
Start Year 2019
Description Interview for French national news 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Industry/Business
Results and Impact In Dec 2019 I was awarded the first Excellence in Nuclear Reactor Science in the UK Award by Framatome. The ceremony took place in the French Embassy in London and I was interviewed by the French TV, by Framatome for their podcast and by the written press. Extracts from these interviews can be found in YouTube, Facebook, LinkedIn, etc. I talked about our advances in understanding materials properties and problems by looking at atoms and how important the contribution from academia was for such important industrial challenges.
Year(s) Of Engagement Activity 2019
URL https://www.youtube.com/watch?v=sY739SUBgY4
Description School Visit (Botley School, Oxford) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact Y3 students were introduced to Microscopy and Exploring the Nanoworld as part of the Science Week (STEM). Around 60 students plus teachers were present. I used a presentation with videos and pictures from my research (nuclear energy, catalyst nanoparticles and sample preparation for electron microscopy). Importance of understanding materials by looking at atoms was explained. A 30 min discussion with Q&A followed. The students engaged well and have been asking questions ever since. A 2nd visit this year has already been planned.
Year(s) Of Engagement Activity 2019
Description Talk to university admin staff 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Industry/Business
Results and Impact The University of Oxford organizes staff team days where they become familiar with aspects of the university that they are not usually exposed to, for example research. This year, I was invited to talk about my group research and I chose the topic of "Understanding degradation of materials in nuclear reactors". I expanted the topic so that the audience could also appreciate how our characterization techniques allowed the understanding of "big" problems by looking at "small" volumes (where atoms are observed directly). I believe the audience enjoyed it very much, since there were many questions afterwards and, as a result, many members of the admin team now recognize me and my research and told me they feel more engaged and "valued".
Year(s) Of Engagement Activity 2019
Description Undergraduate demonstration of TEM capabilities 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Undergraduate students
Results and Impact Every year I organize demonstrations in our research labs to familiarized groups of interested undergraduate students with our rearch projects. I use the characterization of cracks in nuclear reactor materials as they are perfect to illustrate how the failure of a several tonnes component can be understood by looking at the changes of a few atoms around a crack tip. All students are enrolled in our Materials Science undergraduate degree and the main purpose of this activity is to establish tangible links between the topics they study in their lectures and lab practicals and the "real" research that goes on in the department (most of the time, unnoticed by them). The outcome is always very rewarding, since I schedule the demo for 1.5h per group, but the number of questions and requests aftwerwards easily take the session beyond the 2h duration. Many of the undergraduate students will hopefully be interested enough in my research area to apply for a DPhil project in my group.
Year(s) Of Engagement Activity 2018,2019