Fundamentals of current and future uses of nuclear graphite

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.

The importance of maintaining the structural, thermal and mechanical properties of reactor moderator graphite over lifetimes of up to a century, far beyond current experience, is paramount to the future energy economy of the U.K.. The impact of this proposed research is therefore very broad and has the potential to influence safety, economic, environmental and social factors at the highest levels and over significant timescales. Direct beneficiaries of outputs of this research, over the short to medium term (from project inception to 5 to 10 years into the future), will be: Research scientists, engineers and technologists in the varied disciplines engaged in this proposed project (see Academic Impact Summary for a detailed treatment of how and why). UK nuclear utilities (primarily, BEGL), which are responsible for ensuring AGR operating lifetimes are extended to the maximum possible value by demonstrating to the regulator (HSE(ND)) that the behaviour of the graphite components over the next safety case period is fully understood. Other UK nuclear interests, AMEC, HSE, NNL and Serco will benefit directly from this research in relation to the design, development and commissioning of future graphite moderated reactor designs (Gen4). IAEA members, in particular the Technical Working Group on Gas Cooled Reactors and the IAEA Graphite Irradiation Creep CRP, will gain from direct interaction with the ongoing research in this project, as will EU framework programmes related to VHTR technology such as RAPHAEL and Carbowaste. Policy makers will have a strong interest in this project, given the importance of ensuring that current AGRs are allowed to continue operating until the New Build reactors come on stream. If this objective is not achieved, the UK is unlikely to meet its carbon reduction targets to which it is committed within the EU. U.K. Ltd will benefit in that the project will reinvigorate the UK's historical leading role in nuclear graphite research, which might otherwise be sacrificed. The Case for Support expands on our tangible links with each of these beneficiaries, and the planned workshops (see Impact Plan) will engage these beneficiaries directly. Indirect beneficiaries over a more diffuse timescale (10-50 years) will include: UK nuclear industry, as the UK will maintain and expand its world class expertise in nuclear graphite technology. This in turn will generate a substantial contribution in terms of knowledge and expertise to the international energy research network. UK plc will benefit from the production of hydrogen using VHTR technology; a clean and efficient energy storage and transfer vector. Ultimately, energy consumers, the environment and UK security will benefit from new, passively safe, low carbon-footprint, proliferation resistant and environmentally friendly VHTR reactor designs, developed for the future hydrogen economy. International bodies involved in the development of VHTRs and NGNP, INL(USA), ORNL(USA), PBMR(South Africa), Tsinghua University INET HTR-10 and HTR-PM, JAEA (Japan) HTTR.NRG-Petten (Netherlands) VHTR irradiation programmes will also benefit from the UK's knowledge contribution. The primary mechanisms for enabling and encouraging this impact will lie with an integrated dissemination strategy. Although this is primarily an academic project, the end-users of the outputs will be integrated into the project, with a small number of workshop events being scheduled throughout, in which the academics, the industrial partners and our international collaborators can share information and plan future strategy. Indeed, this proposal emerged from one of the regular UK academic/industry meetings on nuclear graphite. Refereed journal articles and conference papers will be significant outputs, but equally important will be the transfer of knowledge and skill between academic and industrial partners. This will be given in detail in the Impact Plan.