Fundamentals of current and future uses of nuclear graphite

Lead Research Organisation: University of Salford
Department Name: Sch of Computing, Science & Engineering

Abstract

Graphite is a key component of most UK operational reactors and for the most exciting designs of new high temperatures reactors that should one day produce the clean fuel, hydrogen. Graphite acts as a moderator to slow neutrons down and make them more effective for nuclear fission. It is also a structural component, so the otherwise slippery and weak single crystal graphite is not used but rather the components are polycrystalline (in the same way that a rock comprises many different interlocking mineral crystallites). In the course of its neutron moderation it becomes damaged, more porous and the individual crystallites change their shape. These changes are carefully monitored but we need to be able to predict the changes so that we can better gauge the life expectancy of our reactors. It will be an important step towards meeting the UK's commitments to carbon emission reduction to 2020 and beyond. In the longer term, High Temperature gas-cooled Reactors (HTRs) are internationally seen as an important source of power, in particular for hydrogen production, so we need similarly to show that future international HTRs could be capable of operating for 60-100 years. Materials Test reactor data for nuclear graphite are incomplete due to the early termination of irradiation experiments aimed at giving lifetime data for UK AGRs.When the original theories of graphite were formulated in the 60's and 70's, less was known about the hexagonal carbon nets that are the layers of graphite. We now know these nets can be isolated and studied on their own (the discovery of graphene in 2004 by Andre Geim and co-workers at Manchester), they can be rolled into tubes (discovery of nanotubes by Iijima in 1991) and they can form into balls (discovery of fullerenes by Kroto and coworkers in 1985). Thus, existing theories did not think to account for buckling or folding of the graphite layers, which we have shown to be important in radiation damage.In addition, electron microscopes were not as powerful then as now: we can get pictures of the layers of graphite in atomic detail. We can detect spectroscopic signatures of different structures from Raman and electron spectroscopy and even perform holography of the polycrystalline graphite with nanometre precision. Finally, the progress in computer software and hardware means that we can calculate exactly the structures that will result from neutrons colliding with carbon atoms by solving the equations of motion of the electrons that hold atoms together. The comparison between the length of a carbon-carbon bond, which is about one seventh of a nanometre, and the length of a typical graphite component (about a metre) is unbelievably large: 7,000,000,000! So we must use different theories for different length scales so that we can combine our understanding from measurements and simulation at every scale in between. Thus we use a multiscale approach to calculate the shape, strength and rigidity of the graphite components taking into account what the neutrons do to individual atoms, to the layers they reside in, to the crystallites and then to the component as a whole.The result will give predictive power to the nuclear utilities and to the designers of the next generation of inherently safe and efficient very high temperature reactors.
 
Description This grant formed part of the Fundamentals of graphite research consortium. The overall objective was to bring our understanding of the effects of fast neutron scattering on nuclear graphite up to date. We contributed to several areas of this work. Firstly we demonstrated the use of Small Angle Neutron Scattering to describe the fractal distribution of Mrozowski Cracks in nuclear graphite, their connection to external atmosphere and the temperature at which they are filled due to thermal expansion (Z.Mileeva et al Carbon). Secondly, we developed the use of Molecular Dynamics to simulate the effects of fast neutron irradiation in graphites. This work was done in collaboration with Nigel Marks, Curtin University, Perth, Australia. The calculations reproduced the number of defects produced in a reactor fast neutron spectrum and the distribution of different types of defect produced. Coherent inelastic scattering from polycrystalline graphite was measured, showing the effects of defects on the inelastic scattering
Exploitation Route Further EPSRC consortium grant proposal has been submitted (SAFEGUARD) and the results are being used by EDF which is concerned that uncertainties in radiation damage in reactor graphites might force the shutdown of AGR reactors.
Sectors Energy

 
Description Understanding on the porosity of graphites and how this changes due to irradiation has been provided to EDF re operation of the remaining AGR reactors.
First Year Of Impact 2014
Sector Energy
Impact Types Economic

 
Description Collaboration on fitting and software development 
Organisation Curtin University
Country Australia 
Sector Academic/University 
PI Contribution This Salford - led collaboration was responsible for the building, development and testing of the SCATTER and PREFIT software packages. Salford team designed, wrote and developed both software packages (in collaboration with partners), and applied the software to the development of the poly-CINS spectroscopic method.
Collaborator Contribution Curtin - 1 PhD student (and time provided by collaborating academic J. D. Gale) on fitting and model construction and development for the magnesium hydride system (and backplane methods developed to make this system accessible) RGU - 1 PhD student (and time provided by collaborating academic H. Gonzalez-Velez) on large scale parallelisation (multi-core, GPU and cloud enabled) of the SCATTER code and GULP's internal eigensolve routines.
Impact Multi-disciplinary (computational chemistry, materials science, physics (neutron) and computer science (advanced parallelisation and visualisation)
 
Description Collaboration on fitting and software development 
Organisation Robert Gordon University
Country United Kingdom 
Sector Academic/University 
PI Contribution This Salford - led collaboration was responsible for the building, development and testing of the SCATTER and PREFIT software packages. Salford team designed, wrote and developed both software packages (in collaboration with partners), and applied the software to the development of the poly-CINS spectroscopic method.
Collaborator Contribution Curtin - 1 PhD student (and time provided by collaborating academic J. D. Gale) on fitting and model construction and development for the magnesium hydride system (and backplane methods developed to make this system accessible) RGU - 1 PhD student (and time provided by collaborating academic H. Gonzalez-Velez) on large scale parallelisation (multi-core, GPU and cloud enabled) of the SCATTER code and GULP's internal eigensolve routines.
Impact Multi-disciplinary (computational chemistry, materials science, physics (neutron) and computer science (advanced parallelisation and visualisation)
 
Title PreFiT - A poly-CINS analysis and workflow toolbox 
Description PreFiT (written by Garba and Roach) was designed to provide labour saving automation of the time-consuming processes used in poly-CINS. Using PreFiT, a user may view and compare experimental and theoretical neutron scattering data, perform edge detection, build fitting files for use in GULP and many other powerful and time-saving activities (including edge detection, signal processing and intensity filtering functionality). Months of manual analysis can now be accomplished in hours - all packaged in a user friendly and intuitive visual interface. 
Type Of Technology Software 
Year Produced 2014 
Impact None yet. 
 
Description I'm a Scientist - Get me out of here (STFC Nuclear Zone) 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? Yes
Geographic Reach National
Primary Audience Schools
Results and Impact School children got the opportunity to field broad scientific questions to practicing research scientists. Questions were broad, but could be very detailed and enquiring. The outcomes suggested children are interested in science and the participants hoped they had stimulated the desire for careers in science for some of those participating children!

Generally, the feedback from the organisers suggest that this kind of outreach activity is very strong at promoting the understanding of what researchers do in their careers, and that it opens up the prospect (to kids from more challenged socio-economic backgrounds) of earning a living doing science!
Year(s) Of Engagement Activity 2014
URL http://imascientist.org.uk/