15 Mar 2021

Graphite programme assures regulator of reactor safety

Graphics blocks in the core of an advanced gas-cooled reactor. Image courtesy of EDF
Graphics blocks in the core of an advanced gas-cooled reactor © EDF

Graphite bricks are used in the core of all of the UK's Advanced Gas-Cooled Reactors (AGRs).

Each reactor core is made up of around 3,000 fuel bricks measuring 825mm high and 460mm external diameter which are all connected together. The core has hundreds of vertical channels for fuel rods and control rods to be inserted.

Two main changes to the graphite can be expected as it ages: 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 the reactor operator, EDF, needs 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.
Weight loss happens over a long period of time and can affect the ability of the graphite to act as a moderator.

Jacobs is deeply engaged in the programme of work to increase understanding of the cores’ tolerance to damage as they age. This includes stress analysis modelling and damage tolerance modelling, as well as validating the results of these models using a range of bespoke test rigs installed at Jacobs’ UK nuclear laboratories in Birchwood Park, Warrington.

Thanks to this work and to continual monitoring and regular inspections, EDF has been able to show conclusively that the reactors can be shut down safely during normal operation and in a highly unlikely earthquake.

This video provides an insight into the Dynamic Control Rod Drop Test Rig, one of the rigs built and operated by Jacobs in partnership with EDF.

The graphite programme and other seismic work supports the UK fleet’s graphite safety cases and gives assurance to the regulator and other stakeholders of the continued safety of the stations.

To learn more: mark.proctor@jacobs.com


Marc Proctor