On the 8th anniversary of the Fukushima disaster there are events taking place by campaigners in the UK and around the world to Remember Fukushima. It seems however that lessons have not been learned by the regulators and by the nuclear industry. The precautionary principle which should apply with solid brass bells on, to the nuclear industry, is as nebulous and unreliable as Scotch Mist. In Scotland the nuclear industry (EDF) wants to restart Hunterston B despite the hundreds of cracks in the graphite moderators of the nuclear reactors. With Dr Ian Fairlie‘s kind permission we have published here the:
Technical Note on Safety Problems at Reactors 3 and 4 at Hunterston B Nuclear Power Station near Ardrossan, Scotland
By Dr IAN FAIRLIE January 29, 2019
Hunterston B nuclear power station was commissioned in 1976 and was designed with an operating life of approximately 35 years, ie for closure to occur in 2011. The closure date has twice been extended at the request of the station owners EDF Energy (a wholly-owned subsidiary of Électricité de France), first to 2016 and more recently to 2023. At present, the station is 43 years old and by 2023 when EDF Energy plans to close the station, 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 number of cracks in the graphite moderators of its two reactors, R3 and R4. (See below for more detail on graphite moderators.)
Because of this problem, the two reactors have been closed since March 2018 (R3) and October 2018 (R4) pending further investigation
The cracking issue has been known about since at least 2006 when Hunterston B’s previous owners, British Energy, confirmed that cracks in graphite bricks were a symptom of neutron bombardment during fission over many years: these cracks are associated with the age of the reactors. 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 barrels – was found at Hunterston B (see photo 3 below). This keyway root-cracking had been previously theorized but not observed. The existence of this type of crack is a serious matter as the cracks act to split each graphite barrel from top to bottom in two places. If the number of cracked barrels were to exceed a certain threshold the reactor would be need to be shut down permanently. For more details on why root-cracks are a serious safety matter, see below.
In 2017, 77 such cracks were observed in R3. In 2017, Dr I R Bramwell, an independent advisor to the Office for Nuclear Regulation (ONR) and a member of the ONR’s Health and Safety Specialist Group, stated that the threshold would be ~350 cracks, 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 Reactor 3 than predicted by its computer models. Consequently in May 2018, EDF announced that R3’s present shutdown would be extended for further investigation and revised modelling.
At the end of December 2018, EDF stated “We have now observed around 100 keyway root cracks in Reactor 3. This is from inspecting just over a quarter of the reactor. Using modelling to project the number of cracks across the whole reactor our best estimate of the current number of cracks is around 370. This takes the core over the operational limit of 350 contained in the existing safety case for that period of operation…” https://www.edfenergy.com/sites/default/files/hunterston_december_update_final.pdf
The fact that the estimated number of root cracks is higher than EDF’s safety case is a vital matter as EDF’s argument for restarting its reactors is mainly based on the accuracy of its computer modelling. The discrepancy also means that, at present, EDF does not have a full understanding of the ageing mechanisms inside the reactors’ graphite cores.
R4 was also shut down from mid-October 2018 although the number of cracks at R4 is understood to be smaller than at R3.
C. 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 Root Cracks, and that it would be safe to restart the two reactors. However, in March 2017, Dr. Bramwell of the ONR’s Health and Safety Specialist Group, stated in response to an oral question that the ONR did not agree that 1,000 KRC cracks 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.
Keyway root-cracks considerably weaken the integrity and stability of the reactor core 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.
EDF are pressing the Office for Nuclear Regulation (ONR) for the reactors to be restarted in March 2019 and April 2019.
The Graphite Moderator of the AGR
Each AGR reactor has approximately 3,000 graphite “bricks” making up about 300 fuel channels and 80 control rod channels. See photo 1 of a graphite brick below. Each is about one meter high and one meter in diameter. Therefore a better word than “brick” is “barrel”.
Since the graphite barrels are locked together by graphite shims (which are inserted into the keyways), an intact massive graphite core provides an important degree of solidity and stability to the reactor. See photo 2 below.
This stability protects against adverse effects from unexpected incidents such as earth tremors, gas excursions, steam surges, sudden outages, and sudden depressurisations. This is an important consideration in the eyes of the ONR as it provides a vital safety margin to the operation of AGRs. Unfortunately, keyway root cracking is now been estimated to exist in about 12% (ie >370/3000) of the graphite barrels. This cracking is illustrated by the red zigzags in the photo 3 below.
These keyway root cracks invariably form in pairs directly opposite each other (as shown above), which therefore split the barrels in two – as shown in photo 4 below. Keyway root- cracking originates from the outside of the barrel and, when they are seen by remote cameras inside the fuel channel, they mean there is a complete fissure of the barrel rather than just a surface crack.
WHY ARE KEYWAY ROOT-CRACKS A SERIOUS SAFETY MATTER?
During the normal operation of AGRs, untoward events sometimes occur such as earth tremors, gas excursions, steam surges, sudden outages, and sudden depressurisations. They are infrequent but do occur. In the situation where a large number of graphite barrels have root-cracks, ie are split in two from top to bottom, such untoward events could result in these barrels being dislodged and/or misaligned. According to John Large, the late independent nuclear engineer, this could result in the following scenarios:
(a) control rods could be blocked from dropping into the reactor core by the resulting displaced graphite barrels. (Only 12 of the 81 control rods in R3 are articulated).
(b) coolant gas channels could become partly blocked by misaligned barrels, and (c) fuel assemblies could become stuck and not be able to be withdrawn. Large
explicitly mentioned this in the BBC programme “Costing the Earth” at the end of 2017. https://www.bbc.co.uk/programmes/b080t880
Normally when all the graphite barrels are intact and locked in place, barrel displacements or misalignments and the above scenarios would not occur. This is why the ONR and Large have placed much emphasis on the structural integrity and stability of the moderator core.
After any of the above scenarios, pressure/temperature transients and fuel assembly ruptures could occur in the fuel channels. This was also explicitly mentioned by Large in the above BBC programme. https://www.bbc.co.uk/programmes/b080t880
(i) Gaseous Releases
These scenarios could lead to radioactive gas releases escaping via the emergency vents and stacks of the reactor building. AGRs reactors, unlike PWR reactors, do not have biological containments preventing the escape of gases in the event of an accident. (The external concrete structure seen at an AGR station is the reactor building is not an airtight containment structure.) This is explained in the report “Nuclear Reactor Hazards” by Helmut Hirsch, Oda Becker, Mycle Schneider, Antony Froggatt for Greenpeace Europe in 2005 https://www.greenpeace.org/archive-international/Global/international/planet- 2/report/2006/8/nuclearreactorhazards.pdf
The volume of gas released would depend on the initial untoward event. It would form a cigar-shaped plume of radioactive gases travelling in downwind direction. The prevailing wind direction at Hunterston B is towards the East North-East (ENE), ie towards Glasgow, Central Scotland and Edinburgh. See map below.
Hunterston B: prevailing wind – towards ENE
(ii) More Serious Explosions
However, following an untoward event, the reactor operators might be unable to shut down the reactor using control rods because of the misalignments of graphite barrels. In this case an emergency shutdown mechanism exists, viz nitrogen gas purges. But this system has never been tested in actual practice, so we don’t know if it would work. If it did not work, H2 explosions and/or nuclear explosions could result as occurred during the Chernobyl disaster in 1986. Plus a 10-day graphite fire if the graphite were to ignite.
This situation would be very unlikely but if, heaven forbid, the worst were to occur, the consequences could be very severe, including the possible radioactive contaminations and evacuations of Glasgow and/or Edinburgh.