This intervention was carried out to allow inspection of the work that the licensee, EDF Energy Nuclear Generation (NGL), has been carrying out on the graphite in the core of Hunterston B Reactor 4 (HNB R4) during the 2014 statutory outage.
Office for Nuclear Regulation (ONR) inspectors conduct inspections during statutory outages in a number of technical areas to support the decision as to whether to grant consent to allow the reactor to restart after the outage. This intervention record details activities that were carried out primarily on work relating to the integrity of the graphite core. At the time of my visit, the licensee's work was still in progress; they had completed about two-thirds of the graphite channel inspections they had planned. I was therefore able to inspect the work as it was taking place. However, as their work has not been completed, I do not draw any final conclusions in this intervention record, as to whether there is any reason to recommend that consent to restart should or should not be granted. I will write an assessment report in due course, after the licensee has finished their planned activities. This report will contain such conclusions.
Hunterston B is the second oldest Advanced Gas-cooled Reactor (AGR) in terms of core irradiation and is usually considered together with Hinkley Point B, the oldest AGR. Both NGL and ONR therefore pay particular attention to the graphite integrity.
At this outage, NGL has for the first time identified a crack in a graphite moderator brick that has been characterised as a Keyway Root Crack (KRC). Unlike previous cracks that have been found, that are all believed to have originated at the bore of the bricks, KRC is believed to have started at the outside and propagated inwards. I explain in the main report why KRCs are of importance to the overall safety case, but it is primarily that the number of cracks of this type are eventually expected to be more numerous.
That a single such crack should be found is not very surprising to me, although it was not predicted particularly for this outage. Hunterston B Reactor 4 has a lower core irradiation than the other three reactors at Hinkley Point B and Hunterston B. This would therefore mean that the first discovery of KRC at this reactor was surprising, were it not for the fact that there is a known, small population of high shrinkage bricks in Reactor 4. The cracked brick is part of this population and has been known to be shrinking at a faster rate, since it was first inspected in 2005. This particular crack may not therefore indicate very much about the general condition of the core, although obviously it is a matter of considerable interest to both licensee and ONR.
The discovery of a KRC in this reactor is a significant milestone in the ageing of the AGRs, but in my view, it does not represent a barrier to further safe operation. The possibility of discovery of KRCs is already covered by the graphite safety case. From the number of channel inspections performed during outages, it is possible to estimate the degree of brick cracking in the whole core. In the case of all the reactors at Hinkley Point B and Hunterston B, there are likely to be less than 2% of the graphite bricks cracked over their full axial height. This is less than the current safety case limit of 10%.
I consider it unlikely that the rate of KRC could be such that the 10% limit could be breached before the next inspections of this graphite core, which will occur in less than two years after the prospective restart. I am though aware that NGL is considering whether there is any need to change the date of the next planned inspections and I may subsequently advance a view on this matter.
Consideration of the discovery of a KRC dominated the activities I performed, whilst at Hunterston B, but I was also able to form the view that the licensee's inspection and analysis work needed on the graphite core during this outage were being performed to an adequate standard.
NGL has not yet completed the programme of activities on the graphite core that they have committed to perform during the outage. From what I inspected during my visit, the activities are being performed to an adequate standard. I have allocated an IIS rating of 3 i.e. 'adequate' against Licence Condition (LC) 28 for this reason.
I discuss the significance of the first discovery of a keyway root crack above and in the main body of this report. It is premature to draw conclusions about the significance, but I do not consider that there is any immediate challenge to the safe operation of this or other reactors. I will be paying close attention to NGL's safety case documents prepared to support their request to return to service. Together with other ONR inspectors, I will be engaging with the licensee to understand more fully the significance of this finding, which may prove to be an early, but isolated or limited, instance of graphite cracking.