UNIGRAF: Understanding and Improving Graphite for Nuclear Fission

Lead Research Organisation: Loughborough University
Department Name: Materials

Abstract

Graphite has been an important material used in nuclear energy since the first reactor at Oak Ridge Laboratory (ORNL) in the USA where it was used as a moderator to slow down neutrons and control the fission process. Graphite is also used in the existing gas-cooled reactors (AGRs) in the UK and is an important material for the next generation of nuclear reactors. However commercially produced graphite produced on a large scale for nuclear applications is not the perfect layered structure that is described in text books but has a complex microstructure which depends on the production process. It is not yet known which production process gives the 'best' type of graphite for nuclear applications as radiation damage depends critically on the type of microstructure. To understand how the different forms of graphite respond to radiation damage, a joint experimental and modelling programme will be undertaken. This will involve international project partners. Different forms of graphite will be produced by a chinese company, Sinosteel which will be irradiated with a neutron source at ORNL and analysed experimentally there, to avoid the problems of shipment of hot material to the UK. Samples of the graphite, produced by Sinosteel will also be irradiated in the UK using ion beams as a surrogate for neutrons and also at GSI Darmstadt in Germany using swift heavy ions. Various forms of experimental analysis will be undertaken at Loughborough, Oxford and Bristol to examine the microstructure and to determine the its effect on physical properties and thus the type of graphite that has the best radiation resistant properties. A complementary computer simulation investigation will help with the understanding of the basic science behind the radiation damage produced by individual collision cascades but will also examine radiation dose effects which have not been the focus so far of computational investigation.
The research will be of benefit to the UK both in terms of its application to existing AGRs but will also keep the UK in the loop for new reactor designs which are currently being planned internationally, where graphite is an essential component.

Planned Impact

This research will also contribute to, and have impact on: the advancement of academic knowledge; the development of new graphite materials for Gen IV reactors; the understanding of the role of irradiated graphite in existing advanced gas-cooled reactors (AGR).
The primary industrial beneficiaries will be those companies involved in the commercial production of graphite. In our case Sinosteel are producing the graphite samples for our study but the results of our research will be open source and available to other companies involved in commercial graphite production.
Society as a whole will benefit since the knowledge gained will help improve the understanding of how nuclear graphite is affected by irradiation and hence make nuclear energy more safe which could potentially extend the life of existing reactors, thus helping with the UK's carbon emission targets.
Training three young researchers in this field will add to the nuclear expertise in the UK and, depending on energy policy, help provide a basis of knowledge for future new reactors that the UK may wish to design and build.
Impact will also occur through the collaboration with GSI who have a future application of graphite as a target wheel for the production of radioactive beams at the Facility for Antiproton and Ion Research (FAIR) to be built within the coming years.
Our co-I, James Marrow (Oxford) advises the UK ONR (Office of Nuclear Regulation) on graphite issues. The work will be of direct benefit to them.
The work will benefit the UK Advanced Gas Cooled reactors, aiding the interpretation by EDF Energy UK of the observed differences in the ageing behaviour of the different graphites within the fleet that were produced by different manufacturers over a period of time; improved understanding of the underlying mechanisms will allow better use to be made of the limited data obtained by core inspection.

Publications

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Smith R (2017) A ReaXFF carbon potential for radiation damage studies in Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms

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März B (2018) Mesoscopic structure features in synthetic graphite in Materials & Design

 
Description We have first investigated and clarified the structure of the boundaries interfacing the perfect graphite crystallites. These interfaces are the key structural footprints in nuclear graphite and can be quantified with HRTEM and Raman spectroscopy. Neutron and ion irradiation lead to increase of such boundaries and reduce the size of graphite crystallites with the increase of irradiation, as evidenced by HRTEM analysis and Raman spectroscopy measurements. The change of this key atomic structure feature has a strong link with the change of physical properties of nuclear graphite when neutron irradiation is applied, and correlations are expected to be established when this research finding is applied in the following up study.

We have modelled the diffusion of point defects after irradiation events in graphite using high temperature molecular dynamics and adaptive kinetic Monte Carlo. The results show that monovacancies can diffuse both within the graphite layers and also between layers to form stable divacancy and trivacancy structures. Interstitials can also combine, first forming interlayer strings which transform to ring structures. Separated ring structures can also combine to form mobile platelets which can be the seed for new layer formation. When a defective lattice contains a local mixture of vacancies and interstitials, both recombination and larger defect clusters can form. At high temperature the graphite layers bend which has the effect of enhancing defect motion and changing the relative stability of monovacancy structures. The insight of the kinetics of vacancy and interstitial carbon is essential in understanding the irradiation creep mechanism of graphite, which by now, has not been well established.

We have also investigated the residual stress in the near surface of as-finished graphite. This residual stress can skew the neutron irradiation qualification data when the irradiated samples are too small. The knowledge is expected to be accounted when neutron irradiation qualification is designed.



The lamina dimensions along the basal planes, are generally much larger in the coke filler than those in binding carbon.


Atomistic modelling shows that the prismatic boundaries are composed of 5, 6 and 7 member carbon rings.
Exploitation Route The knowledge generated from this project will be exploited and applied in our on-going research projects funded by industry. Most of the key findings have been published in well-respected journals and some papers are in gold open access, hence the new knowledge can be used by both researchers and engineers conveniently.
Sectors Energy

 
Description Our finding that surface damage by sample preparation can influence the accuracy of measurements has dawn EDF attention. New research project is set for validating study. We have proposed "crazy paving" structure in nuclear graphite as the key mesoscopic structure, which will help understand the irradiation damage by neutrons. We have found that machining of samples for MTR testing led to significant damage of graphite structure and large residual stress was detected at a depth around 200 microns underneath the as-machined surface. We also found such mechanical damage topped up the graphite structure damage by high energy ions. Such findings make people believe that mechanical damage in the near-surface could skew the measurements after neutron damage in a MTR, which can lead to wrong judgement on the capability of a graphite component, particularly for those that are in service in a reactor. Now, we are working with EDF Energy to review their history data acquired through accelerated irradiation in MTR.
First Year Of Impact 2019
Sector Energy
Impact Types Economic

 
Description Non-destructive Bayesian Learning of Material Density Function, with Applications to the Inversion of Scanning Electron Microscopy Image Data of Nuclear Graphite
Amount £80,000 (GBP)
Organisation Loughborough University 
Sector Academic/University
Country United Kingdom
Start 04/2018 
End 03/2021
 
Description Quantitative Characterisation of Structure in Nuclear Graphite
Amount £20,000 (GBP)
Funding ID 1965674 
Organisation EDF Energy 
Department EDF Energy Nuclear Generation
Sector Private
Country United Kingdom
Start 10/2017 
End 09/2021
 
Description Quantitative Characterisation of Structure in Nuclear Graphite
Amount £20,000 (GBP)
Organisation National Nuclear Laboratory 
Sector Public
Country United Kingdom
Start 10/2017 
End 09/2021
 
Description Quantitative Characterisation of Structure in Nuclear Graphite
Amount £110,000 (GBP)
Funding ID 1965674 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 10/2017 
End 09/2021
 
Description Structural characterisation of advanced carbon materials and nuclear graphite
Amount £100,000 (GBP)
Organisation Loughborough University 
Sector Academic/University
Country United Kingdom
Start 10/2017 
End 09/2020
 
Title Kenny-Jolley/Graphene 
Description Kenny-Jolley added polycrystalline graphite script. 
Type Of Material Computer model/algorithm 
Year Produced 2018 
Provided To Others? Yes  
Impact don't know yet. 
URL https://github.com/Kenny-Jolley/Graphene
 
Description ORNL 
Organisation Helmholtz Association of German Research Centres
Department GSI Helmholtz Centre for Heavy Ion Research
Country Germany 
Sector Public 
PI Contribution Our research outcomes are shared by these partners
Collaborator Contribution ORNL provides neutron irradiated samples, and cooperative study in structural characterisation of irradiated graphite. GSI provides heavy ion irradiation of graphite samples, and cooperative study in structural characterisation of irradiated graphite. Sinosteel Advanced Materials Ltd provide all graphite samples before and after neutron irradiation for all researchers in this project.
Impact Too early to report.
Start Year 2016
 
Description ORNL 
Organisation Oak Ridge National Laboratory
Country United States 
Sector Public 
PI Contribution Our research outcomes are shared by these partners
Collaborator Contribution ORNL provides neutron irradiated samples, and cooperative study in structural characterisation of irradiated graphite. GSI provides heavy ion irradiation of graphite samples, and cooperative study in structural characterisation of irradiated graphite. Sinosteel Advanced Materials Ltd provide all graphite samples before and after neutron irradiation for all researchers in this project.
Impact Too early to report.
Start Year 2016
 
Description ORNL 
Organisation Sinosteel Advanced Materials
Country China 
Sector Private 
PI Contribution Our research outcomes are shared by these partners
Collaborator Contribution ORNL provides neutron irradiated samples, and cooperative study in structural characterisation of irradiated graphite. GSI provides heavy ion irradiation of graphite samples, and cooperative study in structural characterisation of irradiated graphite. Sinosteel Advanced Materials Ltd provide all graphite samples before and after neutron irradiation for all researchers in this project.
Impact Too early to report.
Start Year 2016