New Graphite Examination Techniques
The strength and integrity of the graphite moderator in Advanced Gas-Cooled Reactors (AGR) is an important consideration for safe operation and plant life extension. Samples of core graphite are routinely taken for monitoring physical properties, including the Young's modulus (the amount a material stretches when pulled). In addition, reactor operators need to know the material's Poisson's ratio (the effect in other dimensions to stretching). These parameters are required for computer code used during reactor core assessments.
The UK's National Nuclear Laboratory has developed a technique which uses electronic speckle pattern interferometry (ESPI) to accurately measure the 3D displacements which occur during a 3-point bend test. This mechanical testing arrangement was used to accommodate the available size of material extracted, sampled from the AGR cores. These displacements can be used to determine both Young's modulus and Poisson's ratio over the range of loading conditions up to failure.
This technique has never been used for quantitative measurements before and NNL has spent 3 years perfecting it in partnership with EDF, as part of a project for developing new techniques. Development of the technique highlighted that assumptions normally made during such mechanical testing may be invalid when compared to the accurate three dimensional motions measured by ESPI. Extensive development was required to both understand and overcome these effects by comparing the measured movements with those of 3D finite element modelling.
The technique will ensure that more accurate measurements are taken during sampling in an efficient and effective way. This will save customers time and money and provide assurance that the data they are using to support safety cases is highly accurate