In the media last week it was widely reported that Reactor 4 at Hunterston B power station has been given permission to restart by the Office for Nuclear Regulation (ONR), following a long shutdown period. This is excellent news for low carbon generation of electricity, but what was the issue that EDF Energy discovered, why is it important and what did they do about it?
A nuclear power station requires a safety case to operate, in short, a demonstration that risks are understood, tolerable and minimised. In the EDF Energy Advanced Gas Cooled Reactors (AGRs), the core is made up of several thousand bricks of graphite, graphite being a suitable material for neutron moderation to promote nuclear fission.
When manufactured, the graphite was machined, with holes in the middle for fuel or control rods to be inserted. The periphery of the bricks have notches, or what are called keyways to enable the bricks to slot together, a bit like Lego bricks. There are also graphite keys that sit horizontally between bricks to help with structural integrity.
As the bricks are subjected to irradiation from neutrons (from nuclear fission in the fuel), changes take place at the atomic level which manifest in stresses that progressively build up. These stresses are modelled in computer codes to crucially predict future behaviour. It was always predicted by EDF Energy engineers that the bricks would begin to crack later in life, initially at the centre of the brick and later at the periphery.
Cracks at the centre of the brick are of no concern, but cracking at the periphery can start to affect the geometry of the core, and thus, careful modelling and inspection of the core is required to:
a) justify a safe level of cracking that still leaves adequate margin to ensure unimpeded movement of control rods and fuel,
b) understand the current core state to predict future levels of cracking over the next period of operation.
What’s been done
When Reactor 4 was shutdown last year, the level of cracking was found to be close to the operational limit, but still well within the limits set in the safety case to ensure nuclear safety. To preserve the margins in the safety case, EDF in conjunction with leading consultancies and academics at universities across the UK conducted an extensive research programme to justify a higher tolerance to cracking, and as a result have demonstrated through a extensively peer reviewed safety case that even in the most extreme conditions, control of the reactor and all control rods is assured.
EDF will continue to monitor the core and so will perform frequent graphite inspections, enabling them to continue to demonstrate that the reactor operates with large margins to ensure safe operation at all times.