Graphite blocks in nuclear power stations

Graphite bricks are used in the core of all of the UK's Advanced Gas-Cooled Reactors (AGRs). They act as a moderator, helping to keep the nuclear reaction going, and perform an important safety function.


What does graphite do in Advanced Gas-cooled Reactors?


The graphite bricks act as a moderator. They reduce the speed of neutrons and allow a nuclear reaction to be sustained. They also perform an important safety function by providing the structure through which CO2 gas flows to remove heat from the nuclear fuel and the control rods used to shut down the reactor are inserted.

This graphite was always expected to change over time. How it ages is one factor that will determine how long Britain’s AGRs will operate.

EDF has a fleet of 14 Advanced Gas-Cooled Reactors which play a significant part in the UK’s energy production, generating around a sixth of the UK’s electricity and helping achieve the country’s net zero carbon ambitions. 

Sizewell B and the new Hinkley Point C are water cooled reactors and do not have graphite cores.


Advanced Gas-cooled Reactors are HUGE structures


Each one is 10 metres high, has a diameter of 10 metres and weighs 1400 tonnes – equal to 110 double decker buses.

Each reactor core is made up of around 3,000 fuel bricks measuring 825mm high and 460mm external diameter which are all connected together, bound by a steel restraint and contained within a concrete pressure vessel which is over three metres thick.

Uranium fuel is inserted into the reactor through channels in the graphite core. Control rods, containing boron, are also inserted through other channels to control the reaction and to shut down the reactor. We have around 80 control rods in each reactor but we only need 12 to shut it down.

Graphite - what is cracking and weight loss?


We have always known the graphite that makes up the cores of these reactors would change over time. We cannot replace or repair the graphite so we have been working over many years to understand and prepare for these changes.

There are seven stations in the Advanced Gas-cooled Reactor fleet, each with two reactors. The operating lives of our stations have been different. Some have fewer “miles on the clock” meaning that the amount of power they have generated over their lives is less and some have differently designed bricks. A combination of these things will determine how long each of these reactors generates electricity for.

There are two main changes to the graphite that we expect to see as it ages; these are cracking and weight loss.

Cracking happens when the stresses in the graphite bricks changes over time. On their own, cracks do not make a reactor unsafe but we need to be able to show that they will not change the shape of the channels where the fuel sits in a way that will stop the reactor from shutting down in an earthquake larger than the UK has ever experienced. We also need to be sure that any fragments that come loose after cracks form do not affect the temperature of the fuel or stop us removing it from the reactor.  

Weight loss happens over a long period of time and can affect the ability of the graphite to act as a moderator.

We have a good understanding of both of these developments and they are recognised in our operational safety cases, which are agreed with the UK nuclear safety regulator, the ONR. 

Through continual monitoring and regular inspections we have been able to show conclusively the safe shutdown of our reactors during normal operation and in a highly unlikely earthquake. Work is also underway to prove that if any fragments of graphite come loose during the ageing process that they would not be a challenge to continued operation.


Safety has always been and remains our number one priority. We work within very large safety margins which means we would always stop operations long before anything happens which would affect the reactors’ safety.

Each power station needs a set of approved safety cases to be able to operate. These cover all areas of the plant including graphite. A safety case is a set of documents that outlines all the evidence we have for our safe operation that we then pass to the regulator, the ONR, for approval.

EDF and the ONR, the UK nuclear safety regulator, would only ever allow any of our reactors to operate if completely satisfied that it is safe to do so.

Our reactors have lots of safety features built-in. Each reactor has around 80 control rods which are used to control the amount of power it generates and to shut the reactor down. The reactors only need 12 of these rods to effectively shut down, so each reactor has 12 specially designed super-articulated control rods, which have additional joints which would be able to deal with channel distortion caused by an earthquake and quickly shut the reactor down. Each station has a further back-up system that would quickly inject nitrogen gas into the core and stop the nuclear reaction.


To monitor the condition of the reactors, we carry out more frequent graphite inspections at our two longest-operating stations, Hunterston B and Hinkley Point B. Similar inspections are carried out at our other AGR stations during their statutory outages which take place every three years. We will do more frequent inspections of the other AGRs as they age too.

We remove the fuel and lower down specialist measuring equipment and cameras to film the inside of the channels. This allows us to see any cracks that have formed and, if they have been observed before, see if they have changed. Each time we monitor we inspect enough channels to give us a good understanding of the state of the core. We also remove samples of graphite, which we send for detailed analysis to confirm the level of weight loss.

The results of these inspections allow us to add to our understanding of graphite behaviour, and confirm that our reactors are ageing as expected.  The main purpose of the inspections is to confirm that there is no significant movement of the graphite bricks. They also confirm our assumptions on how the core is ageing and enable us to demonstrate that, even in the event of a major earthquake, there is no significant impact on the core in terms of distortion, and would not present a challenge to the operation of the control rods or other shut-down systems. They also ensure that the weight loss we are finding remains within the limits agreed with the regulator.

Case study: Hunterston B

Hunterston B, in North Ayrshire, started generating electricity in 1976 and when both units are operating is capable of making enough electricity to power almost 2 million homes a year. The station will move into the next phase of its operating life by 2023. At this point the reactors will stop generating electricity and we will start to remove all the fuel in a process called “defuelling”.

We first identified cracking related to the age of the graphite in 2014 and regular inspections have allowed close monitoring of its progression since then. 

Reactor 3 has been offline since March 2018, when we identified a slightly higher rate of cracking than expected. Since then we have carried out three additional rounds of inspection, the most extensive on this type of reactor, and used these results along with other updated analysis and modelling to prepare a safety case that shows the reactor can operate and shutdown in all circumstances. This includes a 1 in 10,000 year earthquake, larger than the UK as ever experienced. The industry regulator, the ONR, is now considering the safety case that would allow operation for six months. 

In October 2018 we also decided to carry out graphite inspections on reactor 4. The results showed that the graphite was ageing as expected but we kept the unit offline while we refreshed the safety case. In August 2019, the ONR gave approval for the unit to return to service for a period of around 4 months, allowing it to reach the same power output as reactor 3. As agreed, it was taken back offline in December, following a safe, continuous operating run. We have submitted a new safety case to the regulator for its assessment.

All of our work on graphite at Hunterston B has shown a cautionary approach and commitment to nuclear safety that we will adopt across the rest of the AGR fleet as graphite ageing develops.

For all the latest information on Hunterston B, you can visit the station news page.

Research into graphite


EDF graphite experts

We have teams of specialists who are experts in graphite properties and over the past six years, we have invested more than £125m and more than 1000 person-years into research with investment ongoing.

Our own expert team has also been working with specialist academics across the UK including at Strathclyde, Glasgow, Bristol, Manchester, Oxford, Sussex, Nottingham and Durham Universities as well as with leading UK companies such as AMEC Foster Wheeler, WS Atkins and Fraser-Nash.

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Visit Hunterston B Power Station

Drop into our visitor centre or book a station tour to see for yourself how we generate electricity in a nuclear power station.