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Safety Concerns at Hunterston B with Dr Ian Fairlie

Technical Note of Safety Problems at Reactors 3 and 4 at Hunterston B Nuclear Power Station near Ardrossan, Scotland by Dr Ian Fairlie - 11th January 2019

Background

Hunterston B nuclear power station was commissioned in 1976 and was designed with an operating life of approximately 45 years i.e for closure in 2011. The closure date has twice been extended at the request of the station owners Electricite de France (EDF): first to 2016 and more recently to 2023. At present, the station is 43 years old and by 2023 it will be 47 years old. Hunterston B, along with its sister Hinkley B in Somerset, are the oldest operating nuclear power stations in Europe.

Hunterston B has a long history of technical problems but the most serious is the continuing rise in the the number of cracks in the graphite moderators of its two reactors, R3 and R4.

Because of this problem the two reactors have been closed since March 2018 (R3) and October 2018 (R4) pending further investigation. EDF are pressing the Office for Nuclear Regulation (ONR) for the reactors to be restarted in March and April 2019 respectively.

The cracking issue has been known about since 2006 when Hunterston B's previous owners, British Energy, confirmed that cracks in the graphite bricks were a symptom of neutron bombardment during fission over many years. Graphite moderator cracks cannot be repaired: if their number exceeds a certain limit, the reactor must be closed.

In October 2014, a new type of crack - arising in the keyway slots of the graphite moderator bricks - was found at Hunterston B. This Keyway Root Cracking (KRC) had been previously theorized but not observed. The existence of this type of crack is a very serious matter: if the number of cracks were to exceed a certain threshold the reactor would need to be shut down. In 2017, 77 such cracks were observed in R3. In 2017, Dr I R Bramwell, an independent advisor to the ONR and a member of the ONR's Health and Safety Specialist Group, stated that the threshold would be ~350 cracks, as this was the limit cited in EDF's previous safety case for Reactor 3's continued operation.

In early 2018, during a scheduled outage, EDF discovered a higher number of keyway root cracks in R3 than predicted by its computer models. Consequently in May 2018, EDF announced that R3's present shutdown would be extended for further investigation and new modelling.

As of December 2018, over 370 KRC's had been observed - a higher number than EDF's previous safety case had cited and larged than that predicted by its computer models. This is a vital matter as the EDF's arguments for restarting its reactors are mainly based on the accuracy of its computer modelling. The discrepancy means that, at present, EDF does not have a good understanding of the ageing mechanisms inside the reactor's graphite cores.

Even more serious is the recent relevation that only ~28% of the R3's fuel channels have actually been inspected by camera, so that the number of KRCs could exceed 1,000. R4 was also shut down from mid-October 2018 although the number of cracks at R4 is understood to be smaller than at R3.

Present situation:

As of the end of 2018, the ONR is awaiting EDF's new safety case which is understood will claim that it is safe to operate R3 with up to 1,000 keyway cracks, and that it would be safe to restart the two reactors. However it is understood that, in March 2017, Dr Bramwell of the ONR's Health and Safety specialist group, had stated in response to an oral question that the ONR did not agree that 1000 KRC was a safe number. EDF's new safety case will have to satisfy the ONR that EDF's engineers understand the cracking process in order for EDF to gain consent to restart its reactors.

Additional notes:

1. Each AGR reactor has approximately 3,000 graphite 'bricks' making up about 300 fuel channels and 80 control rol channels. Each is about 1m high and 1m in diametre. Therefore a better word than brick is barrel. Since the graphite barrels are locked together by graphite shims (which are inserted into the keyways), and intact massive graphite core provides an important degree of solidity and stability to the reactor. 

2. This stability protects against adverse effects from unexpected incidents such as Earth tremors, gas excursions, steam surges, sudden outages, and sudden depressurisation. This is an important consideration in the eyes of the ONR as it provides a vital safety margin to the operation of the AGRs. Unfortunately, keyway root cracking has now been observed in more than 12% i.e. 370/3000 of the graphite barrels.

3. These keyway crack invariably form in pairs opposite each other which therefore split the barrels in two. Keyway root cracking originates from the outside of the barrel and, when see by cameras inside the fuel channel, it means there is a complete crack through the barrel rather than just a surface crack.

4. Keyway cracking considerably weakens the integrity and stability of the reactor rendering the reactor potentially unstable and therefore unsafe. The question of whether reactors in such a state are too unsafe to operate is presently the subject of negotiations between ONR and EDF.