Neutrons help understand cracks in nuclear reactor moderator materials

Bristol University scientist, Andreas Andriotis

Bristol University scientist, Andreas Andriotis loading the sample of graphite into ISIS instrument, Engin-X. (Credit: Dong Liu, Bristol University)
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In neutron diffraction studies, researchers investigated how the graphite material used in UK Nuclear Reactors deforms non-linearly under stress, to show how the material has some tolerance to damage.

This research adds to the understanding of the science behind engineering assessment methods, and contributes to confidence in the structural integrity of the British Advanced Gas Reactors that currently supply around 16% of the UK’s electricity.

A team from Bristol University used Engin-X to study the deformation of polygranular graphite. A rectangular graphite beam supplied by EDF Energy UK was loaded in bending, and neutron diffraction measurements were taken at six locations that experienced different levels of tensile and compressive stress.

Using Engin-X, the scientists could study the deformation of the graphite microstructure, observed at the crystal level.  X-ray diffraction experiments were conducted in parallel on the I12 beamline at the Diamond Light Source by a team from Oxford University.

“Neutrons can penetrate through the graphite and measure the strain inside the material.  Especially for in situ deformation tests, the crystal deformations measured inside the material could describe fully the strain tensors that otherwise are not be measurable on surface. This has unparalleled advantages compared to surface-limited observation techniques,” said Dr Dong Liu from Bristol University, who led the ISIS experiments.

“Nuclear graphite is a complex porous material, and characterisation of its deformation and fracture needs to involve a multi-scale approach from nano-metres to meters. Neutron diffraction provides the opportunity to bridge this gap by providing a direct link between the nano-scale crystal lattice and macro-scale bulk deformation,” Liu added.

Nuclear graphite stress is measured using Engin-X (Credit: Dong Liu)

Neutron diffraction measurements were taken at six different locations of the graphite sample, which experienced varying levels of tensile and compressive stress. (Credit: Dong Liu, Bristol University)
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Both neutron and X-ray measurements showed the tendency for graphite to deform non-linearly with applied tensile stress. With increased force, small micro-cracks formed in the microstructure of the graphite.  This enabled the material to absorb the strains that ultimately cause fracture.

“These results provide a critical input for the multi-scale computer modelling of graphite, and assist in mechanistic understanding of the origins of the damage tolerance of this material,” commented Liu.

The risk of graphite fracture increases in later life of the reactor due to the effects of progressive irradiation on the graphite’s mechanical properties. Fracture of the graphite, which is used in the reactor core, may compromise safe operation.

These experiments allow scientists to follow the progression of deformation from the crystal level to the holistic level used by engineers, and provide insight into the mechanisms of damage development.

The results of the EPSRC supported study, which is published in Carbon, show how polygranular graphite has some tolerance to mechanical damage.  The study was performed on graphite that has not experienced irradiation, and its success enables the planning of future experiments to examine graphite that has been irradiated in a nuclear reactor. 

“This is fundamental research which is supported by the industry that needs to understand the scientific basis of engineering processes,” said Prof James Marrow, lead author and Diamond user from the University of Oxford.

“It increases not only the understanding of the materials used in Advanced Nuclear Reactors but the confidence in engineering processes.”

Graphite is used as a moderator in Advanced Gas Reactors to control the rate of the fission reactions in the core of the nuclear reactor. Therefore it must maintain its mechanical and moderating properties under a radiation environment at high temperatures.

In 2016, the team aim to return to Engin-X to study the effects of high temperatures (~650° C) on polygranular graphite, equivalent to those found in an Advanced Gas Reactor and the next generation of high temperature gas-cooled nuclear reactors.

 

Emily Mobley

T J Marrows, D Liu et al.

Research date: September 2015

Further Information

The paper is available here, DOI: 10.1016/j.carbon.2015.09.058

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