Shaking table testing to explore progressive failure of graphite bricks in an AGR core

Advanced Gas Cooled Nuclear Reactor (AGR) cores comprise of many graphite components whose geometry and mechanical properties change under prolonged exposure to neutron irradiation. The changes in the mechanical properties of the graphite have the potential to result in cracking of the graphite bricks later in the operational life of the core. This could result in disruption to the core geometry with possible negative implications on fuel cooling and/or control rod insertion. This component ageing issue needs addressing in both the computational and the physical models employed in the seismic resilience assessments. This paper looks at the development of methods to measure the dynamic loads in the model AGR bricks during physical seismic testing at Bristol University. The paper then explores the issue of the potential for progressive failure of multiple graphite bricks during a seismic event as dynamic loads get redistributed around the core. A variety of model AGR core bricks have been designed to measure forces in the array as well as some bricks that will ‘crack’ when a predefined force is applied to them. For practical reasons it was not appropriate to create such a brick using modelled materials, and it was also desirable that the bricks could be repeatedly reset and allowed to crack again in subsequent tests. These "Crack on Demand" CoD bricks are therefore manufactured from Acetal in two halves. At the start of a seismic test the brick halves are held together using high strength electromagnets which connect the brick halves via a load cell. During the tests the normal and shear loads between the two brick halves are monitored by the electronics within the brick. When the applied loads exceed the ‘breaking force’ of the brick, the electromagnets are automatically switched off and the two halves of the brick are allowed to move freely. This paper describes the design and calibration of the various bricks incorporating load measurements and outlines the results from initial testing of an AGR model including these bricks. An initial assessment of the potential for progressive failure of graphite bricks in an AGR Core during seismic excitation is also discussed.