Letter to Hunterston SSG - 4 February 2019
Dear SSG Member,
As I have been regularly updating you on the progress of our graphite work I thought it timely to give you an update on where we are presently on our progress to returning our reactors to service.
As you are aware, on 2 November we submitted the safety case for Reactor 4 to the ONR for independent assessment.
You will also recall that I wrote on 18 December to tell you that we had changed the return to service dates for both units; to the end of March for Reactor 4 and the end of April for Reactor 3.
In that letter I told you that our strategy was progressing well and we were engaging positively with the Office for Nuclear Regulation on what is a complex programme of work.
This is still the case and I thought you might like to have some insight into some of the specific areas we have been engaging with the ONR on as they independently review the Reactor 4 safety case. I hope this will reassure you of the robust approach of both ourselves and the regulator and help explain the time being taken to bring the reactors back on line.
There are two main areas I would like to highlight. The first is the buildings motion assessments which consider how the buildings and structures at the power station would behave during a severe earthquake (modelled to 7.2 on the Richter scale). As I have explained and demonstrated at previous SSG meetings our existing models were extremely conservative and didn’t take into account numerous factors such as the rock backfill under the pressure vessel itself. We completed all our analysis for our submission and we now await the ONR who will conduct their own assessment into this aspect.
Secondly, as part of our ongoing analysis and modelling of graphite cracking and aging (which has been ongoing for decades) we are now modelling the possibility of having more complex crack morphologies (termed multiply cracked bricks or MCBs). Our work so far shows that the inherent stability of the core during normal operations would mean that were we to discover a brick cracked in this way it would not present a challenge to the safety nor operation of the reactor.
As we have done with other crack propagations (predominantly singly and doubly cracked bricks) we are also modelling how MCBs behave in the unlikely event of an earthquake stronger than anything ever recorded in Scotland. The strength in depth that we have in our stations would ensure that the reactor could be safely shut down and controlled in even the most extreme situations.
It is important to be clear that the vast majority of cracked bricks observed to date have one keyway root crack and a very small number are doubly cracked. I want to emphasise that we have not seen any bricks with more than two full height cracks during the extensive investigation work that we have undertaken so far; but as a responsible operator we continue to look at every possibility, underlining our determination to operate with safety as our overriding priority.
I appreciate the role the Hunterston SSG plays in feeding back what we are doing at the power station to the wider community but I am conscious that as our work progresses people in the area may have questions that they want to ask me directly.
For that reason we are planning to hold a number of open evenings at the Visitor Centre to allow people to come and find out more about the graphite issue and the work we are carrying out to ensure the station can continue to operate safely and make a positive contribution to the North Ayrshire economy. I will be in touch shortly with more information about these sessions.
At the 6 December meeting I was also able to demonstrate the app we have had developed to help people understand the graphite issue and I committed to let you know when you would be able to download the app. We are still working out some administrative issues with the release of this but you have my assurance that I will contact you with a link to the full app as soon as possible. In the meantime you can see an animation of the app on our website at Graphite blocks in nuclear power stations.
Please feel free to contact me if you have any concerns or questions.
Dr Roddy Angus
Acting Station Director