Fuels, Reactors and Reprocessing

Graphite makes up the moderator core of Advanced Gas Cooled Reactors (AGRs) and previous Magnox Reactors.

PIE allows the licensee and the regulators to build up knowledge of graphite material properties and predict future behaviour of graphite, ensuring safe operation of the UK’s ageing fleet of AGRs.

A variety of PIE techniques are used to measure material properties including density, Young’s modulus, gas diffusivity, electrical resistivity, thermal expansion, and fracture strength.

NNL is the sole provider for Graphite PIE in the UK, with more than 40 years’ experience analysing over 19,000 samples of AGR and Magnox graphite.

Testing and characterisation of trepanned graphite cores is carried out in the unique world-class Windscale facilities based at Sellafield.

Customers: Magnox, EDF Energy, University of Manchester, SL, NDA, DSRL, Innovate-UK

Skills: NNL is the sole provider for Graphite PIE in the UK, with more than 40 years’ of experience analysing over 19,000 samples of AGR and Magnox graphite. Testing and characterisation of trepanned graphite cores is carried out in the unique world-class Windscale facility based at Sellafield.

A variety of PIE techniques are used to measure material properties including density, Young’s modulus, gas diffusivity, electrical resistivity, thermal expansion, and fracture strength, as detailed below;

• Bespoke cutting equipment is used to prepare graphite samples for further analytical techniques.
• Density of samples can be measured using two techniques, by measuring dimensions with length and diameter contact rigs, and by using a laser mensuration rig.
• Dynamic Young’s modulus of samples can be found using a rig which passes sound waves through samples of known density.
• The diffusivity ratio of gasses through the samples can be measured using a gas diffusivity rig, which allows Oxygen and Carbon Dioxide to diffuse through a sample in the absence of pressure. This then allows the porosity of the sample to be evaluated.
• Open and closed porosity of samples can be measured with a gas pycnometer which uses gas displacement to measure the volume of samples.
• Electrical resistivity of graphite core samples is measured by measuring the voltage through samples with a constant current passed through them, which is used to validate the customers’ in-core eddy current inspections.
• The coefficient of thermal expansion of samples is measured in cell by monitoring sample temperature and length during heating and cooling cycles in a furnace.
• Samples can be exposed to a high intensity flash, and the temperature rise on the other side of the samples is measured to determine the thermal conductivity of graphite samples in cell.
• Fracture strength of samples can be measured using a 3-point bend testing rig, measuring the extension of samples when a force is applied.

Facilities:

• A large number of rigs for materials properties measurements and characterisation
• Remotely operated measurement rigs in Windscale AHF and fume hood rigs in Central lab
• Analysis tools developed for this work

Collaborations: NNL works closely with the universities of: Manchester, Liverpool, Loughborough, Oxford and Bristol.

NNL is in collaboration with EDF Energy and The University of Manchester on Innovate UK projects and New Techniques research.

New techniques are constantly being developed, these include electronic speckle pattern interferometry (ESPI), work of fracture, and optical, polarised and fluorescent microscopy.

NNL chairs the ASTM committee D02.F on Manufactured Carbons and Graphite.