Graphite bricks are used in the reactor cores of all 14 Advanced Gas-cooled Reactors (AGRs) in the U.K.

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.

Each reactor core is made up of around 6,000 bricks, 3,000 of which are graphite bricks containing fuel channels.  Each fuel channel brick is 825mm high and 460mm in diameter. The reactor core is bound in a steel restraint and sits within a three metre thick concrete pressure vessel.

Uranium fuel is inserted into the reactor in a fuel assembly through channels in the graphite core. Control rods, containing boron, are also inserted through other channels in the core to control the reaction and also used to shut down the reactor. We have more than 80 control rods in each reactor but only 12 are needed to shut it down.

During the normal operation of the reactors the structure of the graphite bricks will change, including some cracking in some of the bricks as they age or through loss of weight.  We have been working over many years to fully understand and prepare for these late life changes to the reactor core, and regular inspections at all our plants have provided a clear understanding of how the reactor cores age. 

Cracked bricks have no impact on the normal operation of the reactor and would only present a challenge in the event of a major earthquake – at a level we have never experienced in the UK. Even though the chances of this happening are extremely remote, it is right that we base our safety case on the most extreme possibility and it is therefore modelled for a one in 10,000 year event.

In such an extreme event, we need to be sure that the control rods will fall into the core and shut the reaction down.  The control rod channels and fuel channels are connected and held in place by keys which fit into keyway slots in the graphite bricks. We can demonstrate that, even if some of the keyways have cracks, there is insufficient movement in the core structure during the earthquake to prevent control rod entry.

It is important that we ensure our safety arrangements can deal with that scenario, and that we can control and shutdown the reactor safely in such an event. Taking into account the impact of any possible movement in the reactor core we have 12 specially designed articulated or super articulated control rods each reactor, which can move around any obstruction and quickly shut down the reactor.

In addition, the station has a further back-up system that would quickly inject nitrogen gas into the core and stop the nuclear reaction.

Over £100m has been spent on the graphite research programme which benefits from the expertise of our own team of graphite specialists, along with expert academics at several universities across the U.K. The range of computer and physical modelling we are employing gives us extensive knowledge and understanding of how graphite ages in our reactors.

For example, we have a drop rig that can test how effective the entry of super-articulated control rods would be with a range of distortions and our work with Bristol University includes a quarter sized physical model of a reactor like those at Hunterston which sits on a hydraulic earthquake simulator platform and is subjected to a range of seismic events to monitor how the core would be affected. Find out more about this testing method here.

The Office for Nuclear Regulation has produced some helpful background information on graphite here.

A World Nuclear News article on graphite research is available here.

Graphite facts

  • Graphite bricks are used in the reactor cores of all 14 Advanced Gas-cooled Reactors (AGRs) in the U.K.
  • 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.
  • Our 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 in a fuel assembly through channels in the graphite core. Control rods, containing boron, are also inserted through other channels in the core to control the reaction and also used to shut down the reactor. We have more than 80 control rods in each reactor but only 12 are needed to shut it down.
Watch video: Understanding graphite cores in nuclear reactors

Graphite explained

Dr Jim Reed, Graphite Chief Engineer at EDF Energy, talks through the role of the graphite core in our nuclear reactors in housing the fuel and control rods. Using a 1/4 scale model Jim illustrates how the graphite core is structured and the behaviours we expect over time.

Update from Hunterston B

In May 2018, it was decided that Hunterston B's Reactor 3 would remain offline to enable us to work with the regulator to ensure that the longer term safety case reflects the findings of the recent inspections and includes the results obtained from other analysis and modelling.

Go behind the scenes

We've prepared a number of interesting videos that show you around one of our nuclear power stations and introduce you to some of the people who power our energy.