There are about 3,000 graphite bricks in the core of each reactor. The bricks are roughly cylindrical in shape and are hollow; the inner bore is where the nuclear fuel is placed (see image). The bricks are held together by a number of keys and keyways to maintain the overall integrity of the core.
As the reactors have aged, some of the bricks (up to about 10%) have shown signs of cracking, but, to date, these cracks have been initiated at the bore and moved towards the outside of the brick. It was always predicted that later in the reactors’ lives a different sort of crack would be seen that starts at the keyways on the outside of the brick and moves towards the bore. These cracks are called keyway root cracks and two such cracks were found for the first time at Hunterston B.
The properties of the graphite bricks vary according to details of the original manufacturing process. The keyway root cracks were found in so-called ‘high shrinkage’ bricks; these are bricks that are ageing faster than most of the bricks in the reactor. It was predicted that these bricks would be the first to experience keyway root cracking.
For ten years Quintessa has provided support to the operators of the Advanced Gas Cooled Reactors (AGRs) to ensure that it is safe to continue to run the power stations with the observed level of cracking in the graphite bricks. It is necessary to have confidence that the cracking does not significantly affect the overall core integrity, so that, for example, nuclear fuel can still be moved into and out of the reactor core as required. The work undertaken by Quintessa includes statistical modelling of brick cracking and developing models to help understand the evolution of the properties of the graphite as it ages.
The current fleet of AGRs provide a valuable contribution to the UK’s energy supply and it is important that they continue to produce electricity for as long as this can be done safely whilst a new generation of nuclear power stations is planned and built. It is not just the electricity itself that is important, but the fact that it is produced without the emission of carbon dioxide.
Quintessa is proud to be able to contribute in this way to the implementation of the UK’s energy and environment policies.
Images courtesy of EDF Energy Nuclear Generation Limited