Mechanical Property Measurements on AGR Core Graphite Using Electronic Speckle Pattern Interferometry

Abstract

The continued strength and integrity of the graphite moderator in Advanced Gas-cooled Reactors (AGRs) is an important consideration for safe operation and plant life extension. Accordingly, samples of core graphite are routinely taken for monitoring of its physical properties, including the elastic modulus. Elastic modulus is currently measured using an ultrasonics technique that requires knowledge of the Poisson's ratio. However, although the Poisson's ratio of virgin graphite has been measured, there is little information available on the Poisson's ratio of irradiated (and oxidised) graphite. A new non-contacting technique has been developed, which uses Electronic Speckle Pattern Interferometry (ESPI) to accurately measure the 3-dimensional displacements which occur during a 3-point bend test. These displacements can be used to determine both Young's modulus and Poisson's ratio over the range of loading conditions up to failure. Development of the technique revealed that assumptions normally made during such mechanical testing were not necessarily valid when compared to the actual 3-D motions measured by ESPI. Extensive development was required both to understand and overcome these effects by comparing the measured movements with those predicted by 3-D finite element modelling. A new method is described here which can successfully measure both Young's modulus and Poisson's ratio using the small irradiated Gilsocarbon samples extracted from the AGR reactors.