What does graphite do in our reactors?
The reactor cores of all 14 Advanced Gas-cooled Reactors (AGRs) in the UK are made up of graphite bricks. Channels run through these bricks for nuclear fuel, and also for control rods which can stop the nuclear reaction if needed. This graphite was always expected to change over time. How it ages is one factor which will determine how long Britain's AGRs will operate.
Since March 2018 we have been engaged in the most extensive graphite investigation programme ever undertaken which has told us a lot about the condition of the graphite cores in both Hunterston reactors.
A typical graphite core is 10 metres high – equivalent to three double decker buses - and weighs 1400 tonnes.
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The stresses inside graphite bricks change over time. This is a well-known phenomenon and 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. Our graphite research programme benefits from the expertise of our specialists, along with expert academics at many leading UK universities and companies. We have spent more than £100m in the last five years and invested more than 1000 person years into research.
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Maintaining nuclear safety as our overriding priority is at the heart of all of our operations. We build strength in depth into our operations, providing back-up systems that ensure our operational safety is never compromised.
During normal operations the numbers of cracks in the core have no impact on the safe operation of our reactors. The principal nuclear safety risk is associated with the very unlikely occurrence of major seismic (earthquake) activity close to the station. In such a scenario it could be possible for the core to distort and hamper the insertion of the reactor’s control rods necessary shut down our operations.
We operate with very large safety margins, including those relating to core distortion, and are very confident that in such an unlikely scenario as this that we would still be able to quickly shut the reactor down.
Each reactor has 81 boron control rods that are used to manage the power in the reactor. Half of them are used to control the temperature and day to day operation of the reactor, the others would be used if we needed to shut the reactor down. We actually only need 12 to shut down the reactor, the others provide strength in depth.
We also have 12 specially designed super articulated control rods in each reactor, these have additional joints which can deal with channel distortion and quickly shut the reactor down. Each station has a further back-up system that would quickly inject nitrogen gas into the core and stop the nuclear reaction.
We know that the graphite at our gas cooled reactors will deteriorate over time. This was fully considered as part of the stations’ design and is factored into operational safety cases approved by the independent regulator, the Office for Nuclear Regulation (ONR). 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 core ages.
To monitor the condition of the reactors, more frequent graphite inspections are carried out at our two longest operating stations, Hunterston B and Hinkley Point B. Similar inspections are carried out at our other AGR stations during their statutory outages which take place every three years. The other AGRs will undergo more frequent inspections as they age too.
We remove the fuel from the channels and then lower down specialist measuring equipment and cameras to record the data. Each time we monitor we inspect enough channels to give us a good understanding of the state of the core. During statutory outages – which are similar to carrying out an MOT - we perform further inspections of the reactor core by extracting a sample of the graphite core that is then sent away for detailed analysis to confirm the level of weight loss.
The results of these inspections allow us to add to our understanding of graphite behaviour, and confirm that our reactors are ageing as expected. The main purpose of the inspections is to confirm that there is no significant movement of the graphite bricks. They also confirm our assumptions on how the core is aging and enable us to demonstrate that, even in the event of a major earthquake, there is no significant impact on the core in terms of distortion, and would not present a challenge to the operation of the control rods or other shut-down systems. They also ensure that the weight loss we are finding remains within the limits agreed with the regulator.
There are no issues at Sizewell B, or for the new reactors under construction at Hinkley Point C, as they are water cooled reactors and do not have a graphite core.
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.
In March 2018 Hunterston Reactor 3 came offline to carry out routine inspections of the graphite core. As part of the normal ageing process we expect to see cracks occurring in some of the graphite bricks that make up the reactor core. This is something that is well understood and is recognised in our operational safety cases which are agreed with the UK nuclear safety regulator, the ONR. The inspections confirmed the expected presence of new keyway root cracks in the core and also identified these happening at a slightly higher rate than modelled.
On 2 May 2018, we agreed that the reactor 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.
This decision underlined our commitment to nuclear safety. Hunterston is, and will continue to be, operated with very large safety margins. The longer term safety case will build on existing analysis and will demonstrate that these margins will be maintained both now and for the projected reactor lifetime.
Graphite Inspection at Hunterston Reactor 3
In March Hunterston Reactor 3 came offline to carry out routine inspections of the graphite core. The inspections confirmed the expected presence of new keyway root cracks and also identified these happening at a higher rate than modelled. We subsequently agreed with the ONR to work with them to ensure that the longer term safety case reflects the findings of the recent inspections and results obtained from other analysis and modelling.
On November 2 we submitted a safety case to the ONR seeking approval for return to service of Reactor 4 and we are about to submit a similar case for Reactor 3.
The safety cases must demonstrate that, for the next period of operation, the reactor will operate safely and shut down safely in an earthquake or other extreme event.
The ONR will, completely independently, rigorously review the case before deciding whether or not to approve for return to service. It is not possible for us to confirm with certainty how long this process will take; our top priority is that it’s completed with appropriate rigour.
Nonetheless, energy market rules require us to provide our most likely view of return to service dates and, for this purpose, we have said January 14 2019 for Reactor 4 and February 21 2019 for Reactor 3.