Hunterston B and Hinkley Point B Nuclear – safety concerns
nuClear News No.94 April 2017 The UK’s Office for Nuclear Regulation (ONR) has published its assessment of the Periodic Safety Review (PSR) for Hunterston B (HNB) and Hinkley Point B (HPB). ONR has also accepted EDF’s revised graphite core safety case for both sites, but has included a number of recommendations as part of this acceptance. Acceptance of the safety case is reliant on a revised inspection and monitoring strategy. (1)
To comply with a nuclear site licence, a periodic review – a comprehensive study of plant safety – is carried out every ten years to justify continued safe operations. This requirement means that the site licence company regularly reviews and reassesses safety at nuclear sites, making improvements where necessary. The four Hunterston B and Hinkley Point B advanced gascooled reactors (AGRs) started up in 1976 are scheduled to close in 2023.
Graphite The graphite core of each of the reactors is made up of around 6000 graphite bricks – 3000 of these are the graphite bricks containing fuel channels – which are all connected together. Graphite ageing is one area used to determine the lifespan of an AGR nuclear power station. EDF says greater understanding of the ageing process by sampling and modelling can lead to them operating safely for longer.
In November 2015, EDF Energy said it had found cracks in three of the graphite bricks in one of the Hunterston B reactors. Similar cracks were found in October 2014 in two of the graphite bricks of the other reactor. A recent BBC Radio Programme revealed that the
ONR was considering doubling the limit it had set on the percentage of cracked bricks it is willing to accept from 10% to 20%. This has been a particularly controversial part of this process with people living near theses reactors finding it difficult to understand why the definition of “safe” seems to be changing. ONR has now agreed to this increased limit. It says:
“Continued operation of HPB/HNB reactors is now supported by NGL’s [EDF’s] safety case NP/SC 7716 which sets an operational limit of 20% cracking in the core. The justified period of operation of each reactor at HPB/HNB is therefore dependent upon the findings from the inspections at each outage.”
The ONR is also concerned about a very specific 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. 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. 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.
John Large says that if the cracks get any worse it could jeopardise the reactor’s stability in the event of a significant disaster – such as a small earthquake – and make it impossible to lower control rods to shut the reactor down. He said: “These keyways are beginning to fracture… that means the locking together – the way that force can be transferred from one brick to another – is lost, so it becomes a very loose stack of bricks.” Allan Jeffrey of Stop Hinkley added: “This weakness in the graphite core could end up distorting the channels the fuel and the boron control rods use. In cases of emergency there are sudden changes in temperature and pressure which could all end up starting to deform these channels. If you can’t get the control rod down then you can’t control the temperature inside the reactor and you’re heading for accidents – possibly even meltdowns.” (2)
ONR said that EDF had attempted to predict the rate of KWRC. Originally the first cracks were not expected to occur until 2019, but the first KWRC was observed at Hunterston B in 2015.
Inspection will “play a crucial role in supporting the period of safe operation of the reactor in later life,” the regulator said, adding that certain improvements are necessary, such as the development of a capability to measure the condition of control rod channels. EDF Energy should develop improved inspection and monitoring technology; in particular equipment capable of performing visual inspection and dimensional measurements of control rod channels, it said. http://www.no2nuclearpower.org.uk/nuclearnews/NuClearNewsNo94.pdf
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