Dr Ian Fairlie, Independent Consultant in Radioactivity in the Environment
Dr David Toke, Reader in Energy Policy at the University of Aberdeen
February 9, 2019
In our view, Hunterston B nuclear power station needs to be closed for safety reasons, but this should not be lamented because
- there is presently a surplus of electricity generation in Scotland, and more is in the pipeline. Indeed there is so much renewable energy capacity being built that Scottish electricity exports to England and Wales will continue to increase,
- there will be no significant job losses in Scotland, and
- Scottish energy security will be improved as Hunterston B’s operation results in many Scottish wind farms being turned off at certain times and periods.
The two reactors (460 MW net each) at Hunterston B nuclear power station situated near Ardrossan are 43 years old – among the oldest reactors in Europe. Their operating lifetimes have already twice been extended at the request of the operators EDF Energy – a UK subsidiary of EDF in France. They are now scheduled to close down for good in 2023.
Since March 2018 (R3) and October 2018 (R4), the two reactors have been off-line for safety reasons. The decision as to whether these reactors will ever be allowed to restart is for the Office of Nuclear Regulation (ONR). It is currently awaiting a revised safety case for this from EDF Energy.
In recent months, EDF Energy, the trade union GMB, some politicians and newspaper editors have been pressing for the station to be allowed to restart. Quite a few incorrect media statements and misrepresentations have been made: this briefing sets out to put the record straight.
Need for Electricity?
Scotland has a surplus of electricity generating capacity, and this surplus will increase over the next decade regardless of whether Hunterston B, and even Torness, were both to close. According to the Scottish Government (SG), renewable electricity supplied 70 per cent of Scottish electricity consumption in 2017 and will supply around 87% in 2020.
On top of this, sufficient wind power has already been given planning consent to supply the equivalent of a further two-thirds of Scottish electricity consumption. Much of this electricity will be sold to England and Wales. As renewable energy costs continue to plummet, even more wind supplies will be forthcoming and Scotland will become the main source of low carbon energy for the whole of the UK, helping to meet both UK and SG targets for the reduction of greenhouse gas emissions.
Moreover, closing down Hunterston B will allow more wind power from Scottish wind farms to be supplied to the grid, thus improving Scottish energy balance and security. The grid is currently overloaded partly because the Scottish nuclear power stations (Hunterston B and Torness) will not/cannot turn down their power to accommodate renewable energy, so wind farms have to be switched off regularly. This wastes their capacity and causes newspaper controversies over so-called ‘constraint payments’ paid to wind farm operators to compensate their lost income.
It would be ironic if Hunterston B were to be allowed to return to service, because the electricity it produces is considerably more expensive than that produced by the renewables. This is mainly because the cost of nuclear fuel is high, whereas the fuel cost of solar and wind is effectively zero.
Some exaggerated claims have been made by a trade union as to the ‘thousands’ of jobs that would be lost if Hunterston B were closed. The reality is that about 370 employees work there, and none would lose their jobs for at least two to three years after closure.
The reason is that when a reactor is shut off, radioactive decay heat is still produced in the nuclear fuel – this heat cannot be switched off. This means the cooling circuits have to be operated day and night for at least two to three years until radioactive decay rates have declined significantly. After that, the many decommissioning jobs envisaged will employ most of the existing workers, just as occurred at the Dounreay nuclear site in North Scotland. In fact, it is likely there will be more decommissioning jobs at Hunterston B than there were operating jobs.
The most important matter is, of course, safety. Would it be safe to turn the Hunterston B reactors back on? In our view, the answer is, on balance, no. The vital issue is that the current unstable state of the graphite moderator cores increases the possibility of a major nuclear accident. Although the probability of such an accident remains low, the consequences could be so severe, ie the radioactive contamination and evacuation of both Glasgow and Edinburgh, that the risk should not be taken.
The main problem is a serious generic fault in AGR reactors. This is called keyway root-cracking which arises in the graphite moderator cores of the reactors. These cores consist of about 3,100 graphite barrels stacked ~10 m high by ~10 m wide and ~10 m long. These are normally locked together rather like a three-dimensional jigsaw, by means of shims inserted into keyway slots. The difficult problem is that over 100 barrels have been observed to be split from top to bottom because of cracks near the keyway slots. And that is with only 28% of the barrels being examined. EDF has estimated that about 370 barrels are cracked, ie about 12% of the total number of barrels.
This is a serious matter because if an untoward incident were to occur – for example an earth tremor, gas excursion, steam surge, sudden outage, or sudden depressurisation, the barrels could become dislodged and/or misaligned. According to John Large, the late independent nuclear engineer, this could in turn result in the following happening:
- 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).
- coolant gas channels could become partly blocked by misaligned barrels, and
- 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
These events could in turn lead to large emissions of radioactive gases. Further, if hot spots were to occur and if nuclear fuel were to react with the graphite moderator they could lead to explosions inside the reactor core. In the very worst casethe hot graphite core could become exposed to air and ignite leading to radioactive contamination of large areas of central Scotland, including the metropolitan areas of Glasgow and Edinburgh.
This does not bear thinking about. In our view, it should not be allowed to occur.
In more detail, the decision as to whether to allow the Hunterston B reactors to restart is a matter for the ONR to decide. This is a difficult and complex decision as Probabilistic Risk Analyses are necessary and these require much research effort and are time-consuming. Hundreds of computer programs and dozens of researchers are involved. Barry Marsden, a professor of nuclear graphite technology at the University of Manchester has stated “The thing which will close (these reactors) down in the end will be the cost of ensuring safety. It is possible to make a safety case for a significant amount of cracked bricks but it takes time and costs money.” https://uk.reuters.com/article/uk-britain-nuclear-edf-analysis/cracks-in-british-nuclear-reactor-ring-power-alarm-bells-idUKKBN1IA2P0
But even this may be an optimistic view. Under the ONR’s rules (see references a,b,c below), in the case of high consequence, low frequency events of beyond design basis, operators of nuclear power stations are required to conduct Probabilistic Safety Analyses (PSAs). These are expected to look hard at low frequency events. In judging the adequacy of safety cases, inspectors are required to especially consider the effects of very low frequency events, eg seismic events.
At the end of the day, EDF’s forthcoming revised safety case will have to demonstrate to ONR that its computer models indicate there is a less than one in ten million chance (ie 1 in 10,000,000 or 10-7) of such an accident occurring given its estimated number of graphite barrel cracks. Will EDF’s models be able to show this? One needs to keep in mind that EDF’s computer models so far have not accurately predicted the numbers and locations of the cracked barrels at reactor 3.
And if the EDF models were to show this and if the ONR were to accept it, should the Scottish public necessarily accept it? Given that the reactors are very old, well past their sell-by dates, are not needed, and that closure would not threaten jobs, the public may well ask why they should be exposed to such an existential threat even if the chances of it occurring were very low. Put another way, since the station is to close anyway in 2023, do we need to run the risk?
ONR (Office for Nuclear Regulation) (2018a) Safety Systems, Nuclear Safety Technical Assessment Guide, NS-TAST-GD-003 Revision 8, Bootle: ONR, http://www.onr.org.uk/operational/tech_asst_guides/ns-tast-gd-003.pdf
ONR (2018b) Guidance on the Demonstration of ALARP (As Low As Reasonably Practicable), Technical Advice Guidance 5, (NS-TAST-GD-005 Revision 9) Bootle: ONR, http://www.onr.org.uk/operational/tech_asst_guides/ns-tast-gd-005.pdf
ONR (2018c) ONR Guide External Hazards, Technical Advice Guidance 13, NS-TAST-GD-013 Revision 7, Bootle: ONRhttp://www.onr.org.uk/operational/tech_asst_guides/ns-tast-gd-013.pdf