Linking Microstructure to Neutron Irradiation Defects in Advanced Manufacture of Steels

Lead Research Organisation: Imperial College London
Department Name: Materials


The UK plans to build a new fleet of nuclear power plants starting with two units at Hinkley Point in Somerset. The UK government has also recently announced in the autumn 2015 statement that £250M will be set aside for in innovative nuclear technologies. More specifically it has stated that the UK will invest in small modular reactor designs. The large reactors and many small modular reactor designs are based around a reactor type called a pressurised water reactor. These reactor designs have a steel reactor pressure vessel to enclose the nuclear fuel and act as a key barrier to the release of radiotoxic materials to the environment. The integrity of the vessels is paramount to the safety and continued operation of the reactor. Unfortunately, neutron irradiation from the nuclear fuel damages the steels over their 40-60 year design life. Understanding the role of neutron damage to these steels is therefore key to continued operation beyond the design life.

This programme of work will study commonly used reactor pressure vessel forging grade steels (A508 class 3), under neutron irradiation damage, at the OPAL test reactor, at Lucas Heights in Australia. The steels will be manufactured by processes not commonly used in nuclear reactors i.e. hot isostatic pressing (HIP) of powdered material and then welded using electron beams (EB). These new manufacturing processes could potentially be used to manufacture parts for the reactor pressure vessels of future small reactor designs. As yet there is no information on how changing the manufacturing routes from arc welding of forged material to EB welding of HIPed material will change the neutron irradiation response of the material. In this case the chemistry of the material remains unchanged so the key variable is the so-called "microstructure" of the material.

It is planned to irradiate samples, at the OPAL reactor, for up to 1 year, to achieve doses of neutron embrittlement equivalent to 40-60 years reactor operation. The irradiated material will then be mechanically tested, in hot cells, at the Australian Nuclear Science and Technology Organisation before material is shipped to the new Materials Research Faclility at UKAEA Culham site in the UK. Here, it will be prepared for state-of-the-art characterisation, by atom probe tomography on the new LEAP 5000 atom probe recently installed at Oxford University, Chemi-STEM transmission electron microscopy at Manchester University, together with atomic scale models developed at Imperial College London and Manchester University. The project will also have management and input from the National Nuclear Laboratory and Rolls-Royce and international links to the University of New South Wales, University of California Santa Barbara and Oak Ridge National Laboratory.

The overall output from this work will be much improved mechanistic understanding and models of how neutron irradiation effects steels manufactured by HIP and EB welding, lead to a new generation of engineers in the UK who can perform work on irradiated materials and help direct the use of such technologies for the building of future small reactor designs. It will also be a crucial driver in the effort to rebuild the physical and knowledge based infrastructure, for dealing with neutron irradiated steels, that has been missing for a generation in the UK.

Planned Impact

The UK has just invited the Chinese to invest £6Bn on nuclear new build with EDF investing the remaining £16Bn. The UK plan to build six new PWR units as a result. The electricity generated by a 1.6 GWe reactor is worth about £1-1.5M per day and the design life of new reactors is estimated to be at least 60 years, with potential to extend to 80, or even 100 years. 20 years of life extension even at a modest load factor, for a modern reactor, of 90%, is worth about £7-9 Bn per reactor. The science to underpin such life extensions will largely focus on the irradiation embrittlement behaviour of the reactor pressure vessel (RPV), the most safety critical part of the system. Once the RPV is deemed not safe the reactor is shutdown.

In tandem the UK government in the Autumn 2015 statement said it will invest up to £250M in innovative nuclear technologies with a strong focus on small modular reactor designs to make the UK a competitive world leader in the nuclear power sector. The work proposed here is targeted specifically at small reactors but using material that is common for existing large reactors and hence will have an impact on the future safety cases of both. The other great societal impact comes from the huge savings in CO2 emissions that a 20 year life extension of a low carbon (only 16 gCO2e kWh-1) nuclear power station can make. With both improved air quality and climate change high on the agenda of the UK government new nuclear build and life extension research will have a huge impact.

The work will also impact on UK infrastructure for handling and understanding nuclear materials as there has been a generation of researchers in the UK without this capability. Therefore, this programme will have impact in rebuilding the UKs capability, in this area, and train a new generation of academics, Post-doctoral researchers and PhD students. We will be a big user of the recent government capital investment under the National Nuclear Users Facility at Culham maximising the impact this facility and UK Government investment in it will have. The Materials Research Facility, for active materials, at Culham will receive the material and allow the investigators the ability to store it and further prepare samples for advanced analytical analysis at Oxford and Manchester Universities. Here we will again leverage to maximum effect the use of government investment in new instruments such as the new LEAP 5000 atom probe at Oxford and the Titan-chemi-STEM machines at Manchester to perform state-of-the art investigations into the materials irradiated at the atomic scale.

Impact for the nuclear industry is also expected through utiliisation of the results at National Nuclear Laboratory and Rolls-Royce for the qualification of the neutron dose-damage relationships used to predict RPV life. The proposal will provide research to underpin current models and provide new models for new product forms.

The final impacts will be on manufacturing of nuclear components, by methods such as hot isostatic pressing and electron beam welding. These materials have different microstructures to the forged and arc welded materials and may therefore have different response to neutron irradiation. The response is critical to licensing them for new reactors, such as small modular designs, and therefore this research will provide the first insights into their behaviours in a reactor environment.
Description We have discovered that a phase known as the G-phase, common to steels was likely formed by a transformation from a body centred cubic crystal phase commonly seen in steels and that this transformation is likely below a threshold level of iron in the body centred cubic phase. We also showed that a previous paper that describes the way the G-phase transforms is likely to be a misinterpretation of the data, which we showed was inconsistent with our own more accurate quantum mechanical models of the system.

King et al. 2020 "G-phase strengthened iron alloys by design" showed that by manipulation of the alloying additions to steels, especially the inclusion of Mn and Zr, that G-phase precipitation can be encouraged within a matter of 24 hours as opposed to over many hundreds of hours and this was corroborated through experimental alloys through a collaborator in Harbin Institute of Technology, China. Thuis could be used potentially in future to produce a new class of super high strength steels.

The work of Whiting has shown that inclusion of triplet solute-vacancy cluster information from density functional theory is essential in fitting empirical potentials or for kinetic Monte Carlo simulations as only using pairs does not give accurate descriptions of the binding energies and furthermore that these discrepancies can be as large as 0.3 eV.

Work by Carter et al. showed that the radiation hardening in hot isostatically pressed (HIP) steel was not too dissimilar to fully more normally manufactured bainitic steels. This is important if this manufacturing method is to be considred in future for small nuclear plants. However there were some differences in the way the radiation damage between pure ferrite and bainite phases.The ferrite microstructure showed a greater percentage of solute atoms available to form irradiation induced nano-sized clusters than bainite, but it also contained a lower cluster volume fraction and number density compared to the bainite.

Gasparrini et al. then looked at the mechanical properties of these two different phases in the HIP steel of bainite and ferrite and showed for the first time that the radiation hardening was higher for the soft ferrite phase than the already harder bainite phase. By making tiny (smaller than human hair) size specimens we were able to get mechanical properties. By avearging the results of several of theses tiny specimens we were able to get a very similar result to normal laboratory sized specimen test results. This is important for testing of irradiated materials and it means we could potentially extract properties of irradiated, and therefore radioactive materials, using tiny specimen that can be tested without any additional protective measures and in non-radioactive laboratory facilities.
Exploitation Route In Carter and Gasparrini et al. we show a new novel rapid way to test neutron irradiated materials at low cost. This is likely to be used by many follow up proposals in the UK and is already part of a large bid for future fusion/fission nuclear plant steels with UKAEA that is in review with BEIS at present.

These results are pertinent to all users involved with irradiated steels of many types and also to certain classes of duplex stainless steels under thermal aging conditions. The latest work on G-phase precipitation can be used to develop a potentially new class of hardenable hgih strength steels.

Other workers who want to test micromechanical properites of materials, whetehr irradiated or not, may want to use the method developed by Gasparrini et al.

Manufacturers of steel may well use the results in Carter et al. as evidence that HIP steels can perform well in nuclear reactor environments.
Sectors Aerospace, Defence and Marine,Chemicals,Construction,Creative Economy,Energy,Environment,Manufacturing, including Industrial Biotechology,Transport

Description The method we have used for irradiating small scale specimens in the OPAL research reactor and then sending them to the Materials Research Facility in Culham and then on to university labs for rapid analysis of irradiated materials has been picked up by the UKAEA for a follow on proposal to BEIS for the development of high temperature steels for fusion and Gen IV fission reactors in the UK. The proposal is currently undergoing review with BEIS for funding in 2022.
First Year Of Impact 2022
Sector Aerospace, Defence and Marine,Construction,Energy,Environment,Manufacturing, including Industrial Biotechology
Description ANSTO 
Organisation Australian Nuclear Science and Technology Organisation
Country Australia 
Sector Public 
PI Contribution We are collaborating with the Australian Nuclear Technology Organisation to irradiate steel samples provided by Rolls-Royce. The work package involves two PDRAs from Imperial working with ANSTO to design a route to irradiate and ship radioactive samples from ANSTO to the UK MRF facility. We are also working with ANSTO to develop micromechanical testing of samples and compare these to proton irradiated material.
Collaborator Contribution We are getting training in how to prepare a case to irradiate materials, how to ship them, handle them etc.
Impact multidiscipline: involving materials, reactor physics, health physics, reactor operation.
Start Year 2017
Description Harbin Institute of Technology 
Organisation Harbin Institute of Technology
Country China 
Sector Academic/University 
PI Contribution We provided DFT data that helped to design a new iron alloy for testing
Collaborator Contribution The collaborator made up and tested and characterised a new alloy that we suggested.
Impact Acta Materialia publication (2020), King et al. G-phase strengthened iron alloys by design.
Start Year 2019
Description Manchester 
Organisation Manchester University
Country United States 
Sector Academic/University 
PI Contribution 1 PDRA has been collaborating in micromechanical testing and donating her time.
Collaborator Contribution Training in making miniature tensile specimens and experience of working with irradiated materials. Expertise from a Prof with 40 years experience in irradiated steels, who has contributed to a written paper. Experience and training in TEM.
Impact 1 paper in review stage.
Start Year 2017
Description NNL 
Organisation National Nuclear Laboratory
Country United Kingdom 
Sector Public 
PI Contribution We provide the research link to research being conducted at Imperial, Manchester and Oxford through the organisation of regular meetings.
Collaborator Contribution Provision of head of reactor operations to attend meetings and provide advice on both the technical aspects of the research and on transportation of irradiated materials.
Impact None
Start Year 2017
Description UKAEA 
Organisation UK Atomic Energy Authority
Country United Kingdom 
Sector Public 
PI Contribution We have been liaising with the UKAEA at the new MRF facility to get some of the first ever hot cell (neutron irradiated) material into the facility through our collaboration with ANSTO and Rolls-Royce.
Collaborator Contribution Help with developing a safe transport route for material from ANSTO to Culham MRF. Training in the use of active FIB facility and handling of radioactive materials.
Impact None. Multidiscipline of Materials and Health Physics.
Start Year 2017
Description UNSW 
Organisation University of New South Wales
Country Australia 
Sector Academic/University 
PI Contribution We have sent 1 PDRA to work as a visitor at UNSW and 1 PhD student
Collaborator Contribution Research supervision in new techniques with respect to study defect behaviour in metals.
Impact Not multidisciplinary. We have written 1 paper, which is in the review stage.
Start Year 2016
Description oxford 
Organisation University of Oxford
Department Department of Materials
Country United Kingdom 
Sector Academic/University 
PI Contribution 1 PDRA has been training in atom probe. We have provided materials
Collaborator Contribution Training in FIB and atom probe analysis
Impact None so far
Start Year 2017
Description rolls 
Organisation Rolls Royce Group Plc
Department Rolls Royce Submarines
Country United Kingdom 
Sector Private 
PI Contribution We are arranging the irradiation of material provided by Rolls-Royce at ANSTO and coordinating meetings with research partners of oxford and Manchester.
Collaborator Contribution Hot Isostatically Pressed reactor pressure vessel steels (A508 class 3) Materials, forged materials and electron beam welded materials. They also provide expertise through a regular member of Rolls-Royce attending our 6 monthly research meetings. They also trying to provide access to materials irradiated in the US ATR programme worth in excess of £1M.
Impact Materials have been sent to ANSTO for irradiation.
Start Year 2017