11 Jul 14

Information on graphite weight loss levels for Advanced Gas-cooled Reactors (AGRs)

An article in today’s Times looked at the issue of graphite weight loss in Advanced Gas-cooled Reactors at nuclear power stations across the UK. Here is some background information on the subject.

Seven of the UK’s nuclear power stations have Advanced Gas-cooled Reactors and they are all operated by EDF Energy. They contain thousands of interconnected graphite bricks which are known as “the moderator” as it enables nuclear reaction to occur. They also contain circular channels into which fuel and control rods are inserted and removed.

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).

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.

Safety limits approved by the ONR do not define a boundary between plant being safe or unsafe. Beyond these limits, there is a wide margin before the plant would be deemed to pose an unacceptable safety risk. In the highly unlikely event of a safety issue arising, the most extreme effect would be the reactor safely shutting down and being put out of service.

EDF Energy would always take a nuclear power station out of service long before this moment was possible, regardless of commercial considerations.

The current limits for graphite weight loss and the current estimated average weight loss at each power station are as follows:

Current Limit

Estimated Average Weight Loss

Hunterston B & Hinkley Point B



Torness & Heysham 2



Dungeness B






Heysham 1



The difference in limits between stations reflects the difference in the design of the plant. The limits are approved during formal safety reviews which happen at least every three years.

EDF Energy has a continuous programme of monitoring, sampling and modelling graphite behaviour in conjunction with leading academic institutions in the UK.

In addition to multiple back up safety systems on site, EDF Energy has also invested in further safety modifications at its nuclear plants to give enhanced protection for AGR reactors. For example, it has made modifications to gas plant control systems to improve reactor chemistry control.

Brian Cowell, Director of Nuclear Operations, EDF Energy, says:

“The safety of the public and our employees is our overriding priority. EDF Energy takes a cautious approach, works closely with the regulator and operates with very conservative safety margins. We have stringent safety requirements. We carry out regular inspections, including of the graphite bricks that are part of the cores of the reactors.

We work continuously with scientists and UK universities to understand how materials in our nuclear reactors change over time and how that will affect the stations’ operations. We regularly refine our own safety assessments as we uncover new information. All our findings and new research feed into mathematical models based on pessimistic assumptions, and maximum safety margins.

We share our findings with the regulator and with the wider scientific community. We also share them with the local community around each of our stations.

Nuclear power provides the UK with almost 20% of its electricity, and EDF Energy is making big investments to safely extend the lives of its 15 nuclear reactors.”



AGRs - Advanced Gas-Cooled Reactors are the second generation of British gas-cooled reactors. They use graphite as a moderator and are cooled primarily with carbon dioxide. Nuclear generation does not however emit any CO2. The first AGR in commercial operation was Hinkley Point B in 1976.

Moderator - in nuclear engineering a neutron moderator is a medium that reduces the speed of the nuclear fission reaction or in other words, slows down the neutrons to improve the efficiency of the nuclear reaction

Graphite Bricks - The core of the AGR is made up of graphite bricks with channels that contain the fuel elements and control rods.

Fuel elements - AGR fuel consists of stainless steel pins. These pins are made up of small pellets containing uranium, which are built into a graphite sleeve. Seven or eight smaller fuel elements are fixed together vertically to form one large fuel element.

Control Rods – These control the reactor power. The graphite core contains channels for the boron steel control rods, which can be raised and lowered to control the reactor power

For more information contact:

Sue Fletcher
Senior Media Manager (Generation)
01452 653267
07736 727964

Notes to editors

EDF Energy

EDF Energy is one of the UK’s largest energy companies and the largest producer of low-carbon electricity, producing around one-fifth of the nation's electricity from its nuclear power stations, wind farms, coal and gas power stations and combined heat and power plants. The company supplies gas and electricity to 6 million business and residential customer accounts and is the biggest supplier of electricity by volume in Great Britain.

EDF Energy’s safe and secure operation of its eight existing nuclear power stations at sites across the country makes it the UK’s largest generator of low carbon electricity. EDF Energy is also leading the UK's nuclear renaissance and has published plans to build four new nuclear plants, subject to the right investment framework.

These new plants could generate enough low carbon electricity for about 40% of Britain’s homes. They would make an important contribution to the UK’s future needs for clean, secure and affordable energy. The project is already creating business and job opportunities for British companies and workers.

Through Our Better Energy Ambitions, EDF Energy has developed one of the biggest environmental and social programmes of any British energy company.

In 2014 EDF Energy received seven ‘Big Ticks’ in the Business in the Community (BITC) Responsible Business Awards. In 2013 EDF Energy received the Environmental Leadership for Behavioural Change Award in the national Environment and Energy Awards and was highly commended in the first ever pan European Corporate Social Responsibility Awards scheme for its Sustainable Schools programme – the Pod.

EDF Energy is part of EDF Group, one of Europe’s largest power companies. The company employs around 15,000 people at locations across the UK.

To find out more about the UK's energy challenges look at www.edfenergy.com/energyfuture/