Nuclear reactor graphite cores cracking: Hinkley Point B and Hunterston B

NuClear News No 90 26 Nov 16   Radio Four’s Costing the Earth has been investigating whether it is safe to keep reactors running long past their expected lifespan of about 30 years. Five of Britain’s seven AGRs are already older (Torness and Heysham 2 are only 27 years old). Hinkley Point B and Hunterston B are already 40 years old but EDF energy wants them to continue operating for at least another 7 years.

In 2005 the Nuclear Installations Inspectorate (now the Office for Nuclear Regulation -ONR) expressed concern about the structure of the reactor core. The core is made up of 6,000 graphite blocks. Around half of these are 1 metre tall with a bore or channel running through each block. Around 200 of these channels contain rods of nuclear fuel. If anything goes wrong control rods are inserted between the channels to dampen the nuclear reaction and shut down the reactor.

Nuclear Engineering consultant John Large explains that graphite is not elastic, it doesn’t bend, and it is not particularly strong. And now the graphite bricks are cracking. The core is an assembly of several thousand bricks, loosely stacked together and the expectation was that the core would never fail, so there was no facility to replace any individual blocks if they did become damaged. But now there are physical changes occurring in the core, in the individual bricks – cracking and fracturing – that must result in some loss of strength – not only of the individual bricks, but of the core as a whole.

The BBC used a Freedom of Information request to obtain a number of documents. One paper from ONR reveals that one third of the channels inspected at Hinkley B and Hunterston B contain what they describe as significant cracks. EDF says the cracks were anticipated at this stage in the reactors’ life and it is safe to operate for years to come. It says evidence suggests that its predictions about cracking are accurate.

Brian Cowell, director of nuclear operations, says: “in fact we are looking to extend life further (than 2023) if we can.” The analysis suggests that we can have more than 1,000 axial cracked bricks and still operate with massive margins of safety. 1,000 cracked bricks would exceed the current safety limit set by ONR, but the regulator is considering changing that limit.

Mark Foy – Deputy Chief Nuclear Inspector says the percentage of cracked bricks ONR is currently happy to accept is 10%, but they are considering increasing that to 20%. Foy says that the original safety case provided by EDF was on the basis of 10% cracking. As experience is gained and analysis and research is undertaken it allows EDF and ONR to gain a more informed and accurate view of what is acceptable and what isn’t.

EDF has now provided ONR with a safety case for allowing 20% cracking. This is based on the analysis EDF has undertaken; samples they’ve taken and the inspections they’ve undertaken. The focus has been to look at the likelihood of core disruption after an earthquake which could prevent the control rods being inserted. ONR is considering the new safety case.

Keyway Route Cracking

The ONR is also investigating a very specific and more concerning form of cracking. The keyway is a slot that holds each brick to the adjacent brick, the bricks underneath and the bricks on top. These keyways, which are acknowledged to be the limiting factor in the life of these reactors, are beginning to fracture. John Large points out that this will make the graphite blocks a very loose set of bricks.

Prof Paul Bowen of Birmingham University sits on the graphite technical advisory committee for ONR. He says the keyway cracks could potentially prevent the entry of the control rods. If the core distorts too much, it’s easy to see how trying to feed anything in could become very difficult

Seven of the keyways have been discovered to have cracks at Hunterston B. John Large believes the presence of keyway cracks casts doubt on the safety of the reactor in the event of an emergency like an earthquake. We have a cracked and deteriorating core that’s lost its residual strength and we don’t know by how much. Some of the design case accidents will test the core – one of these would be a seismic shake where the whole core is wobbled. If the core becomes misaligned, and the fuel modules get stuck in the core, the fuel temperature will get raised and could undergo a melt. If the radioactivity gets into the gas stream and the reactor is venting because it’s over pressurised then you have a release to the atmosphere and you have dispersion and a contamination problem.

ONR agrees keyway cracks could compromise safety. One of the documents the BBC obtained said the discovery of keyway route cracks at Hunterston invalidates the previous safety case. EDF had to consider what information to present to ONR to satisfy them that the reactor was still safe to operate. EDF brought in articulated control rods and nitrogen injection systems to address the extra risks posed by the keyway route cracking. The new rods are bendy making them easier to insert into a distorted core and an injection of nitrogen could buy several hours of invaluable time in the event of an accident.

However, concern remains because we can’t be certain how many keyway route cracks there are. John Large explains that to examine where the cracks are you have to take the fuel out of the reactor and put a camera down to inspect the inside of the bore, but these keyway cracks are on the outside of the bricks so you can’t actually see them.

It’s very hard to inspect the channels in which the fuel sits. Around 10% are inspected each time the reactor is shutdown. So there may be keyway route cracks that have never been seen at Hunterston and Hinkley. In the absence of a full visual inspection a mathematical model is used to work out the likelihood of cracks in particular parts of the reactor. The trouble is the model has already been shown to be flawed.

Paul Bowen says they haven’t been able to get the exact timing of the cracks right. The industry argued that cracks would appear first in layers 4 and 5, but they actually appeared in level 6. John Large says the model relied upon by ONR is not working, so they can’t predict the strength of the core. More to the point they can’t work out where to put their investigative probes to see where cracking is taking place. So they’re in the dark.

If the ONR gives the go-ahead for an increase in the number of cracked bricks from 10 to 20%, it might be difficult for people living near theses reactors to understand why the definition of “safe” seems to be changing.


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