Three-dimensional study of irradiation creep and elastic deformation of nuclear graphite

Professor James Marrow

Graphite is a critical material that moderates the neutron spectrum to achieve fission of the fuel of some nuclear reactors. Irradiation creep is the dimensional change that occurs when graphite is irradiated by fast neutrons whilst under load. This creep is very important for the relaxation of the stresses that develop in graphite components due to thermal gradients and neutron dose gradients. Without irradiation creep at a sufficient rate, this stress could cause the graphite to fracture.

Experimental data for irradiation creep, used to predict its effect on stress evolution in reactor cores, are quite limited. Graphites have complex microstructures and the potential effects of graphite microstructure on their creep behaviour need to be understood to improve the design the new graphites that are required for future high temperature gas-cooled reactors.

A recent study applied digital volume correlation (DVC) to high resolution X-ray computed tomography (µXCT) from the ‘ACCENT’ irradiation creep experiment, which was conducted by NRG Petten for EDF Energy. DVC is an image analysis technique that obtains three-dimensional displacements from tomographs (e.g. https://doi.org/10.1016/j.carbon.2015.09.058. This preliminary study measured creep strains within the microstructure constituents for the first time.

We now aim to utilise this unique dataset further, which will require developments in the processing and analysis of large data sets and the image-based modelling. We also aim to apply non-destructive in situ µXCT and DVC in a new way to measure the static elastic properties (shear modulus, bulk modulus, Poisson ratio) of neutron-irradiated graphites. This project will develop and test image-based microstructure models (e.g. https://doi.org/10.1016/j.carbon.2017.06.031) for the static elastic properties, creep and dimensional change of nuclear graphites.

The project is suitable for graduates with an engineering, mathematical or physical sciences background.

Displacements due to Creep within nuclear graphite, measured by digital volume correlation of X-ray computed tomographs (unpublished work)

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