EDF Energy has found cracks in three of the graphite bricks in unit 3 of its Hunterston B nuclear power plant in North Ayrshire, Scotland. Similar cracks were found in October last year in two of the graphite bricks of unit 4. In both cases, the company said the cracks have no safety implications.
EDF Energy said today that, as part of routine inspections, engineers had looked at part of unit 3’s graphite core.
“Three graphite bricks were found to be cracked. This is known as keyway root cracking and was predicted to start happening at this point in the station’s lifetime,” the company said. “It does not affect the operation of the reactor and the findings have no safety implications and are well within any limits for safe operation.”
Planned statutory outages are carried out at each of the two reactors every three years. Reactor 4’s latest such outage was between early August and early November 2014. Reactor 3’s current outage started on 2 October and is expected to be completed by early December.
EDF Energy said it is publicising its findings as part of its commitment to openness and transparency. Further inspections will look at more of the graphite core and if any additional cracks are found, the company said it will share this information.
“Nuclear safety drives everything we do,” Hunterston B station director Colin Weir said. “This means we work within very large safety margins. This applies to graphite bricks too. The level of cracking which is considered reasonable is far below anything which would affect the reactor’s safe operation. It is accepted by our regulators and materials experts that cracks will occur in some of the bricks and that the core will lose some of its mass as part of the normal ageing process.”
The observations were anticipated, Weir said, and the company’s view of the “best estimate lifetime planning date” of 2023 for both units has not changed.
Weir told World Nuclear News today: “We don’t need to replace cracked graphite bricks and in fact this wouldn’t be possible. What we are focused on is monitoring and modelling the graphite to ensure the expected cracks remain well below a level which doesn’t allow distortion of the core and hence continues to allow insertion of the control rods unimpeded.”
EDF Energy said it would always take a nuclear power plant out of service “long before an issue became unsafe, regardless of commercial considerations”.
Hunterston B power plant is a nuclear licensed site operating two Advanced Gas-cooled Reactors – units 3 and 4. The graphite core of each of the reactors is made up of around 6000 graphite bricks – 3000 of these are the graphite bricks containing fuel channels – which are all connected together.
“The structure is designed to contain many redundant bricks meaning a very large number of bricks would have to crack before there were any significant safety concerns,” EDF Energy said.
“Over time, graphite slowly loses weight as part of the normal ageing process. This is a well-known phenomenon which was fully considered as part of the stations’ design and is factored into safety limits approved by the independent regulator, the Office for Nuclear Regulation (ONR),” it said.
Graphite ageing is one area used to determine the lifespan of an AGR nuclear power station. Greater understanding of the ageing process by sampling and modelling can lead to them operating safely for longer, giving the UK secure and reliable low-carbon electricity, the company said.
Jim Reed, EDF Energy’s graphite core safety case group head, said that a quarter-scale model of an AGR unit the company has built with the help of Bristol University, demonstrates that such a reactor can continue to operate normally and safely with cracks in up to 30% of the graphite bricks.
“Over time we expect cracks to appear in fuel bricks. The safety is really quite clear because there’s a large margin between the control rods and the distortion you would get in the reactor,” Reed said in an information video posted on the company’s website today. “And even at 30% cracking there is still plenty of margin to get the control rods into these channels,” he said.
Source: World Nuclear News