Exploring the potential for progressive failure of graphite bricks in an Advanced Gas Cooled Nuclear Reactor (AGR) core during seismic excitation

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 particular issue of the potential for progressive failure of multiple graphite bricks during a seismic event as dynamic loads get redistributed around the core. To this end a model AGR core brick has been designed that will crack when a predefined force is applied to it. For practical reasons it was not appropriate to create such a brick using modelled materials, and it was also desirable that the brick 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 electromagnets powered by an adjustable current supply. During the tests the contact between the two brick halves is monitored by the electronics within the brick. When the hold force of the magnets is overcome, and any movement between the two half bricks is detected, 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 CoD bricks 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.