Thermo-mechanical property measurement of nuclear graphites at elevated temperatures

The thermo-mechanical properties of nuclear graphites at elevated temperatures (up to 850 Celsius), and the influence of mechanical strain on their thermal expansion behaviour, are relevant to current and future nuclear systems. Current understanding suggests that these structure-determined properties are linked, and both influence the highly significant, but poorly understood phenomenon of irradiation creep (ie irradiation-induced dimensional change under stress). In this PhD the properties of existing and future graphites in the non-irradiated conditions will be investigated, with the objective of identifying how irradiated graphites could be tested at elevated temperatures and what knowledge would be obtained by such tests. These graphites will include Gilsocarbon (ie AGR graphite used in current reactors in the UK) and fine-grained graphite grades such as may be used to encapsulate the fuel in some designs of high temperature reactor and those in small modular reactors.

The key experimental studies will investigate the relationships between applied total strains (tensile and compressive, measured by high-precision image correlation in 2-D and 3-D) and the elastic strains in the graphite crystals (measured by synchrotron X-ray and neutron diffraction). These novel data will further improve understanding of how mechanical damage is accommodated in the graphite microstructure. Oxford's previous studies of room temperature Gilsocarbon have shown that damage changes the elastic modulus, affecting the accumulation of mechanical strain energy. These are significant factors in the fracture tolerance of graphite components and their sensitivity to stress concentrations causing a reduction in graphite component strength. High resolution methods, such as correlative Focussed-Ion Beam (FIB) tomography, electron microscopy and Raman spectroscopy, will be used to examine microstructural straining, while the evolution of local properties of the graphite filler and matrix will be measured by micromechanical testing, such as nano-indentation and micro-cantilever tests, at ambient and elevated temperatures. By determining how existing 'accommodation porosity' and damage mechanisms absorb deformations, the effect on the coefficient of thermal expansion and how strained microstructures would respond to the effects of irradiation, i.e. irradiation creep, will be explored.

The study will also characterise graphites at the bulk scale by thermo-mechanical testing (up to 250 Celsius) with in situ laboratory X-ray tomography or optical imaging and digital image correlation (DIC) analyses. Critical experiments at higher temperatures are planned using neutrons at Engin-X and IMAT (both ISIS, UK), and X-rays at the Diamond Light Source (DLS).
The obtained data will provide the inputs and supporting information for non-linear finite element modelling of the behaviour of graphites at elevated temperatures, with the potential to simulate the behaviour of engineering components. To provide the foundations for future work on irradiated graphites, this modelling approach will include micro-mechanistic models for graphite deformation. During a secondment at NNL, the student could engage with NNL's modelling work and/or property measurement of irradiated graphites to help deepen their understanding of this area.

This project falls within the EPSRC Energy Research Theme (Nuclear Power)

Case student with company NNL