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Office of Nuclear Regulation Meeting re: Hunterston B

This is an update from Dr Ian Fairlie about the meeting that he and Sean Morris of Nuclear Free Local Authorities have had with the Office for Nuclear Regulation about Hunterston Nuclear Power Station in Ayrshire. Radiation Risks working group within Scottish CND have been doing some work around this topic and have recently released a short report which can be found on our website and printed copies available in the office.

Agreed Note of ONR Meeting held at ONR’s London offices on Tuesday, July 23 2019.


1.       In attendance were Dr Ian Fairlie, Sean Morris NFLA, Mr Donald Urquhart, Deputy Chief Inspector ONR and Mr Steve Harrison, ONR Head of Assessment in Operating Facilities Division. The meeting was friendly, informative and lasted 2 hours and 15 mins.

2.       On Reactor 4, ONR stated that it had not yet reached a decision on the return to service. An issue had recently been identified concerning the need for further analysis of the significance of crack openings greater than 1.2 cm wide (ie ~½ inch), a small number of which are predicted to occur in the next period of operation. ONR was pursuing further evidence and was now in correspondence with EDF about such cracks. 

3.       In answer to the query as to when a decision on R4 might be made, ONR stated that no firm date could be given but it could be 3 to 4 weeks from now. However ONR stressed that any date would depend on its acceptance or otherwise of further evidence from EDF and that ONR would take as long as is necessary to complete its assessment and reach a decision.

4.       If a restart were allowed by ONR, it would be for a period of 4 months. If EDF has not made a case for a longer period of operation during this period, which would need ONR’s agreement, the reactor would then have to be stopped, some fuel removed and more inspections made to examine the core for further cracks, widened cracks and multiple-cracked bricks.

5.       On R3, ONR has now received the Safety Case to support its return to service. The main report was >200 pages long plus many technical references. ONR had started assessment of this report and it was expected to take a number ofmonths to complete its examination.

6.       EDF is developing an approach to defining ‘End of Life’ criteria for the AGR reactors, in other words, the maximum levels of core degradation that can be demonstrated to be safe, whilst leaving a significant margin of safety.  Such criteria would need to be agreed by ONR. These included not only the number of cracked bricks (see para 7 below), but also their sizes (ie how wide they were) and also the numbers and locations of multiply-cracked barrels (ie barrels with 3 or 4 cracks or more stretching from top to bottom). In ONR’s view, these latter matters were likely to be more important than the numbers of cracked barrels. The most important factor was for EDF to demonstrate to ONR that all 81 control rods could be inserted unimpeded,  even in a 1 in 10,000 year seismic event.

7.       As regards the numbers of bricks with cracks, the latest Best Estimates were 377 in R3, and 209 in R4. The existing operational allowance was 350 cracked bricks, although safe operation had been demonstrated with 700 cracked bricks. The reason that R3 had exceeded the operational allowance was due to induced cracking which had not been expected by EDF. ONR’s view was now that for every pair of predicted keyway root cracks, another one should be added (ie a 50% increase) to take into account induced cracking.

8.       ONR explained that the 99.9 % (ie 1 in 1,000) calculational confidence level used for the predicted core state  was based upon a  1 in 10,000 year seismic event (10-4) and a cut-off of  1 in 10 million (10-7) per annum for fault sequences considered to be within the design basis. In effect this meant that there was no increase in the risk of a major accident due to not being able to shutdown the reactor due in turn to control rods not inserting because of cracks in the graphite core.

9.       A lengthy discussion on REPPIR then followed. ONR were not responsible for issuing guidance on emergency procedures, evacuation zones or pre-distribution of iodine tablets. However it took a keen interest in these matters. It accepted that the inhalation of gaseous I-131 was the main path of intake for radioactive iodine.

10.   ONR undertook to inform NFLA if any planned changes were forthcoming re R3 and R4.



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