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 Energy 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 fully 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 the lifetime of each reactor. 

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 prove 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 properly in a severe earthquake. 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 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 slivers of graphite come loose during the ageing process that they would not be a challenge to safe operation.


Safety has always been and remains our number one priority. Nuclear safety drives everything that we do. 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.

Neither EDF Energy nor the ONR, the UK nuclear safety regulator, would ever allow any of our reactors to operate unless completely satisfied that it is safe to do so.

We also have 12 specially designed super articulated control rods in each reactor, these have additional joints which can deal with channel distortion 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 from the channels and lower down specialist measuring equipment and cameras to record the data.  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 aging 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. We expect it to generate low carbon electricity until 2023.

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 an extensive inspection programme 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, even an earthquake larger than the UK as ever experienced. The industry regulator, the ONR, is now considering a 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 make electricity for a period of around four months. Since returning to service, the reactor has operated safely and efficiently. We are now approaching the end of the four month operational period and intend to take the unit back offline in December to carry out more graphite inspections.

We have carried out significant modelling and analysis work to predict the future state of the graphite cores at Hunterston B and we are bringing all of this work together to produce a further safety case which will take the reactors through to the end of generation.

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 Energy 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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