UNIGRAF: Understanding and improving graphite for nuclear fission

Lead Research Organisation: University of Bristol
Department Name: Interface Analysis Centre

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.

Publications

10 25 50
 
Description 1. Assisted with the interpretation of service-acquired and test reactor-acquired X-ray diffraction data for AGR reactor core graphite. This is enabling both novel interpretation and understanding for evaluating long-term integrity. 2. Microstructure-based FE modeling has successfully predicted the role of both neutron damage and CO2 oxidation on the deformation and fracture of AGR reactor core graphite. These data have been made available to EDF energy as a basis for improved understanding and inputs into their safety cases.
First Year Of Impact 2016
Sector Energy
Impact Types Economic

 
Description Graphite X-ray diffraction data analysis , Phase 1
Amount £40,000 (GBP)
Funding ID 4840479529 
Organisation EDF Energy 
Sector Private
Country United Kingdom
Start 01/2017 
End 08/2017
 
Description Modelling at the test specimen length-scale deformation and fracture of unirradiated and irradiated Gilsocarbon graphite
Amount £30,000 (GBP)
Organisation EDF Energy 
Sector Private
Country United Kingdom
Start 02/2016 
End 10/2016
 
Description Modelling the deformation and fracture of irradiated Gilsocarbon graphite
Amount £27,000 (GBP)
Organisation EDF Energy 
Sector Private
Country United Kingdom
Start 09/2015 
End 01/2016
 
Title Resistivity measurement under load and temperature of graphite 
Description Measurement of resistivity of tensile test specimens over a temperature range developed in conjunction with Dr Bryan Roebuck at NPL 
Type Of Material Improvements to research infrastructure 
Year Produced 2017 
Provided To Others? No  
Impact Provides new insights on the fracture and deformation of reactor core type graphite 
 
Title Microstructural based FE model developed and applied in conjunction with Profressor E Schlangen Technical University of Delft 
Description Model has been applied to predict the deformation and fracture properties of un- and irradiated reactor core graphite and will be applied to the Sinosteelgraphite used in the current project [ 
Type Of Material Computer model/algorithm 
Year Produced 2016 
Provided To Others? Yes  
Impact Allows prediction of properties of reactor core graphite when subject to service environment....temperature , neutron dose and oxidizing environment 
 
Description Graphite X-Ray Diffraction Data Analysis 
Organisation EDF Energy
Department EDF Energy Nuclear Generation
Country United Kingdom 
Sector Private 
PI Contribution ABSTRACT Dr Mark Bradford (EDF Energy) provided examples of x-ray diffraction data acquired from specimens of Gilsocarbon graphite in unirradiated and irradiated conditions. These have been subject to different temperatures and neutron doses. A preliminary evaluation was undertaken of these data to establish if there was the potential to determine the macro-strain (stress), micro-strain (stress) and crystallite size. The analyses, including application of the procedure described by Williamson and Hall, indicated that it may be possible to evaluate these parameters from a detailed consideration of x-ray diffraction (XRD) peak position, peak shape and width. CONCLUSIONS A preliminary examination of the x-ray diffraction 2? traces has been undertaken to further address changes that arise in the Gilsocarbon graphite when subject to neutron irradiation. The approach has centred on application of the Williamson-Hall method to allow separation of coherence length (crystallite size) and micro-strain. Within the constraints when this method is adopted, the measured changes with increasing neutron irradiation include: • An expansion of the c lattice parameter. • A reduction in the a lattice parameter. • A reduction in average coherence length (crystallite size). • An increase in compressive micro-strain. However, to implement this analysis required assumptions relating to individual peak profiles and omission of the 101 diffraction peak. As a consequence, there is a need to provide a rationale for both of these assumptions, which can be linked to the complex atomic structure of the graphite. RECOMMENDATIONS The work described in this report meets the contract specification to provide a preliminary evaluation of specimens selected for the Blackstone programme. Both unirradiated and neutron-irradiated x-ray diffraction 2? traces have been further analysed. This demonstrates that it is possible to obtain a measure of coherence length (crystallite size) and micro-strain using the Williamson-Hall procedure. However, in order to implement the procedure assumptions have been required to achieve realistic measurements. The analyses have identified that there is more information embedded in these x-ray diffraction traces that can be further interrogated. Hence, as a minimum the following could be rigorously revisited: • A detailed analysis of specific peak profiles, such as the 002. • Overlap of the 100 and 101 diffraction peaks, their origin and the basis for exclusion of the 101 peak from the Williamson-Hall analyses. One potential interpretation of the overlap is that it is a result of complex interactions between hexagonal graphite and the turbostratic structure. • Further exploration of the correlation between the atomic-scale structure in both unirradiated and irradiated graphite, and the resultant x-ray diffraction traces. To achieve this requires a consideration of atomic-scale model predictions and the associated impact on the specific diffraction peak positions and profiles. Certainly, there is a need to deconvolute contributions from neutron dose, temperature and mass loss if appropriate data are available.
Collaborator Contribution Supplied data.
Impact Oral presentation at the 6th EDF Graphite Conference, 15-18 October 2018, Kendal, UK: Analyses of X-Ray Diffraction Data Obtained from Unirradiated and Irradiated Gilsocarbon Graphite James E Darnbrough1,†, Keith R Hallam1, Peter EJ Flewitt1, Mark R Bradford2 1University of Bristol, Interface Analysis Centre, School of Physics, Tyndall Avenue, Bristol, BS8 1TL 2EDF Energy, Barnett Way, Barnwood, Gloucester, GL4 3RS †Current address: University of Oxford, Department of Materials, Parks Road, Oxford, OX1 3PH Journal paper in preparation: The nano-structure of unirradiated and irradiated synthetic graphite James E Darnbrough1,†, Keith R Hallam1, Peter EJ Flewitt1*, Mark R Bradford2 1University of Bristol, Interface Analysis Centre, School of Physics, Tyndall Avenue, Bristol, BS8 1TL 2EDF Energy, Barnett Way, Barnwood, Gloucester, GL4 3RS †Current address: University of Oxford, Department of Materials, Parks Road, Oxford, OX1 3PH
Start Year 2016
 
Description Project progress meetings 
Organisation EDF Energy
Country United Kingdom 
Sector Private 
PI Contribution Technical advice
Collaborator Contribution Plant knowledge
Impact Ongoing
Start Year 2016
 
Description Graphite Core Committee (EDF Energy, UK) 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Industry/Business
Results and Impact Specialist expert group.
Year(s) Of Engagement Activity 2015,2016,2017
 
Description Vice-Chair (Nuclear), Energy of Materials Group, IoM3 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact --
Year(s) Of Engagement Activity 2015,2016,2017,2018