Office for Nuclear Regulation

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Hunterston B Reactor 3 - FAQs

Should I be worried about the restart of HNB Reactor 3?

ONR would not allow a nuclear facility to operate if it wasn’t safe to do so. Our specialist inspectors have spent the past two and a half years thoroughly examining the safety case for this next period of operation. We have required EDF to provide substantial evidence to demonstrate the safety of the reactor in a number of key areas. ONR is satisfied that the reactor is safe to operate for a further six months, after which it will be shut down and the core re-examined.

Are you confident EDF's modelling of the extent of cracking in the core is accurate?

In the past the extent of cracking was underestimated, however the reasons for that are well understood now. Predictions at that time did not account for the potential for cracks to be induced in adjacent bricks by keyway root cracks. We are satisfied that induced cracking is now included in the core state predictions. Recent inspections on the extent of cracking in other reactors have been within predicted levels.

We recognise that with any modelling there are uncertainties and that is why we require EDF to demonstrate substantial margins of safety. We are satisfied these safety margins are such that public and worker safety will not be put at risk. We are satisfied that Reactor 3 is safe to operate for up to 16.425 Terawatt days (approximately a six month operating period), and can be safely shutdown when required.

This reactor is old - how can it be safe?

Graphite cracking is a well-known phenomenon – it is expected as reactors age – and we have assessed the evidence and are satisfied that this reactor can still operate safely. We apply stringent national and international standards to our decisions which take into account ageing and degradation. We require a detailed safety case that takes into account the ageing effects of the reactor and justifies why it is safe to operate. We also take into account defence in depth which means the reactor does not rely on a single system for safety.

There are none of these reactors anywhere else in the world and this is the oldest reactor, we are in uncharted terrorory here, aren't we?

As with any reactor, it is the role of the operator, in this case EDF Energy NGL, to demonstrate to us that it can operate safely. Substantial work has been done to demonstrate that the reactor is safe to operate. It’s taken two and half years to get to the point where we are convinced that public and worker safety will be assured. During this period, we have required that EDF provide substantial evidence to demonstrate the safety of the reactor in a number of important areas. We would not allow this reactor to restart if we had any concerns at all about its safety for the permitted period of operation.

At the end of this operating period, EDF NGL will be required to shutdown the reactor for a further core examination. We will only give our permission for the reactor to re-start if there is an appropriate safety justification in place.

Does cracking still matter?

Yes it does. There was always an expectation that EDF NGL would develop its safety case further to allow for more cracking than allowed for in earlier safety cases. The important consideration for ONR is whether the cracking could impede the insertion of control rods or obstruct fuel channels within the reactor core which would affect its ability to shut down or cool the fuel. Any proposed change to those limits must be presented to ONR through a robust safety justification, which demonstrates that it is safe to adjust the limits, based on the evidence provided. We are satisfied that a robust safety case has been made and the nature of the cracks in the core of Reactor 3 would not impede the insertion of control rods or obstruct fuel channels.

Why six months?

EDF’s safety case justifies a period of operation up to a core burn up of 16.425 Terawatt days (approximately a six month operating period). ONR’s team of specialist inspectors have examined EDF’s safety case thoroughly and are satisfied that the reactor is safe to operate for that period of time.

Does the fact that its only six months mean you're concerned about its long-term safety?

We made our decision based on the safety case provided by EDF which proposed a core burn up of 16.425 Terawatt days (approximately a six month operating period). After the existing operating period, the core will be inspected and any further period of operation would require EDF to provide a new safety case which would be assessed by ONR. We would only permit further operation if it is safe to do so.

How have you reached your decision when you haven't been to site due to COVID-19?

Our assessment work has always had a significant desk-based element and has continued as normal throughout the pandemic period.  As key workers, ONR inspectors have continued to go to nuclear facilities to conduct essential regulatory business, in accordance with public health measures. We have undertaken readiness inspections at the site and are satisfied with the measures in place to return and operate the reactor safely.  Worker and public safety is our priority and we would not allow the reactor to operate if we were not satisfied that it was safe to do.

What is causing these cracks?

Cracking is a well-known phenomenon and is to be expected. Whilst the reactors are operating, the graphite bricks age which leads to their properties changing due to the interaction with radiation and the reactor coolant. This can lead to the graphite losing weight and the development of cracks in the graphite bricks. The important consideration for us is whether the cracking could impede the insertion of control rods or obstruct fuel channels within the reactor core which would affect its ability to shut down or cool the fuel.  Our assessment has focused on ensuring that we are satisfied that the cracking observed in the graphite core of the reactor will not compromise its fundamental nuclear safety requirements.

After a detailed and rigorous assessment exercise, informed by extensive scrutiny of the evidence underpinning the safety case, our team of specialist inspectors has concluded that an adequate safety case has been provided and that EDF Energy NGL has demonstrated to us that the level of cracking observed will not compromise the ability of the core to fulfil those requirements, and that substantial margins of safety exist.  We are satisfied that the reactor is safe to operate and can be safely shutdown when required.

Are you relying on the licensee for information?

ONR is an independent regulator, protecting society by securing safe nuclear operations. As well as carrying out a thorough assessment of EDF’s safety case, our specialist advisors have also analysed information to make this decision. We have also appointed independent advisors to conduct work and repeat and validate information provided to us by EDF. We have sufficient information of our own (not provided by EDF) to make this decision in the interests of public and worker safety. Further information on our Graphite Technical Advisory Committee is available on our website.

Has this been a difficult decision?

ONR has taken two and a half years to ensure it has the required evidence from EDF to demonstrate that the reactor can operate safely and be safely shut down when required. This has been a very complex and technical process, led by a team of specialist ONR inspectors.

Are you satisfied that this can be turned off if there's an earthquake tomorrow?

We are satisfied that EDF’s safety case has demonstrated that the reactor can be safely shutdown and that all control rods can enter the reactor in the case of a seismic event.  In the unlikely event that one or more of the control rods does not insert following an earthquake, there are fall-back safety systems that would shut the reactor down and ensure it remained shut down. The safety of workers at the site, local residents and the wider public is our priority and we would not allow any nuclear facility to operate if it wasn’t safe to do so.

What does this mean for other AGRs in the UK?

ONR assesses each reactor individually. Any further safety cases would be analysed thoroughly to ensure they provide an adequate justification that the reactor in question can operate safely.

What happens if there is a COVID-19 outbreak at the plant?

Like other nuclear facilities across the country, EDF has plans in place for the safe running of its plants during the COVID-19 pandemic. We are satisfied with the site’s COVID-19 safety measures. Worker and public safety is our priority and we would not allow the reactor to operate if we did not think it was safe to do.

What about the change in CEDTL? Haven't you just increased the number?

The term Currently Established Damage Tolerance Level (CEDTL) was used because it was always intended by EDF, and understood by ONR, that with further work it may be possible to justify that a higher level of cracking than the original CEDTL of 700 cracks could be demonstrated to be safe. An increase in the CEDTL has been possible because of improvements in the analysis methods. ONR is satisfied that the nature of cracks at this level would not impede the reactor’s ability to operate safely, and be safely shutdown when required.

What have you learned from the R4 period of operation last year? How has that impacted your decsion or not?

ONR assesses each reactor individually. However, much of the work that justified operation of Hunterston B Reactor 4 up to 16.025 TWd in 2019 remains relevant to the safety case provided for Hunterston B Reactor 3. Key developments since ONR agreed to the operation of Hunterston B Reactor 4 up to 16.025 TWd last year, are detailed in the Project Assessment Report (p15).

Why is this six months and not four months?

EDF’s safety case justifies a core burn up of 16.425 Terawatt days (approximately a six month operating period). ONR’s team of specialist inspectors have examined EDF’s safety case thoroughly and are satisfied that the reactor is safe to operate for that period of time.