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Getting to grips with graphite

By Richard Bradfield | Posted August 27, 2020

Stepping into the role as EDF’s Chief Technical Officer for the UK’s operating nuclear power stations - after many years working on them - was a great honour for me and one that continues to bring many challenges.

There is no escaping that, as our power stations age, we find ourselves managing a growing number of technical issues. These stations have been great assets to the country, for several decades now, and we have to actively manage the next phase of their lives, including moving them progressively into decommissioning.

We also need to understand what more needs to be done to ensure they maximise their contribution to helping Britain achieve net zero, until other low carbon sources come online like wind, nuclear and solar. 

An issue which keeps us focused is graphite. Understanding how graphite ages has had a profound impact on our business over the past few years and, if we are honest, we did not grasp the full extent of its significance to the operation of the AGR fleet until the last two years working on Hunterston B.

After many years of work by our teams, collaborating with specialist engineering firms and universities - and working closely with our regulator the ONR – our understanding has increased significantly.  An investment of over £200 million has been made on research and development, to really get to grips with graphite, not just at Hunterston but all our AGR stations. 

This knowledge has allowed us to confidently restart one of the two reactors at Hunterston B power station.  Close observers of our business will remember we took this reactor offline in March 2018 for planned inspections.

We always knew, since the 1960s, that the graphite cores of the UK’s seven Advanced Gas-Cooled Reactor (AGRs) power stations would exhibit some form of cracking towards the end of their generating lifetimes.  

We are a deeply risk-averse and conservative industry and that is absolutely right. Part of this means writing ultra-cautious safety cases. This is true of the first graphite safety cases back in the 1970’s, which detailed the ‘acceptable conditions’ for operation based on the information available at the time.

It is entirely normal across many industries for safety cases to change as new evidence becomes available and, some 40 years on, we have much more sophisticated inspection and modelling techniques.

Through the work we have done, we know that having a keyway root crack in every brick would still not compromise the safe operation and shut down of the reactor under normal circumstances. What we set out to prove in our new safety case is that in the event of a major earthquake – predicted to happen once every 10,000 years and far larger than any seen or expected in the UK – we can still safely turn off the reactor. We have now been able to prove this.

This evidence-based approach has given us the confidence to present, and the regulator to accept, a safety case which is still conservative but details new ‘acceptable conditions’ for operation. 

I appreciate that some people may see this as a “moving of the goalposts”. Why, they will ask, have safety limits been changed? But this isn’t a relaxation, it’s a reassessment based on evidence. And these are not safety ‘limits’; the case approved by the regulator shows additional safety margin well beyond what we know are acceptable operating conditions.

So now we are ready to let Hunterston B do what it was designed to do – produce low carbon electricity safely and reliably. The ONR is currently assessing a safety case for the return to service of the second reactor at the station. It’s right, though, that we are clear about the station’s remaining generating life; for our people, for the community and for our partners in business. 

In 2012 we extended the operation of the station to March 2023 – knowing that could be plus or minus two years. Today, we have updated employees and external stakeholders that we intend the site to move into defuelling by January 2022 at the latest, subject to regulatory agreement after the next set of graphite inspections in six months. 

Hunterston B started generating 44 years ago and in that time has produced enough electricity to meet all of Scotland’s demands for eight years. 

It has made an outstanding contribution to low carbon generation and our focus now will be safely delivering the last period of generation from this site before a smooth transition into decommissioning. 

I would encourage you to review some of the articles and case studies available on our graphite page