The graphite moderator of the UK’s gas-cooled reactors fulfils key safety functions. For the advanced gas-cooled reactors operating in the UK, its continued use presents a challenge for both the nuclear industry and the regulator.
Laurence Poulter, a nuclear inspector at the Office for Nuclear Regulation (ONR), explains why it’s vital to monitor the continued use of graphite as reactors reach the end of their planned operation.
In the UK, there are 14 operating AGRs, two at each station, operated by EDF Energy Nuclear Generation Ltd (NGL). There is also one remaining operational Magnox reactor. All of these reactors use graphite as their moderator. This means the core is constructed from thousands of graphite components which, besides sustaining the nuclear reaction to enable safe electricity generation, form channels in the reactor core. These channels contain the nuclear fuel, reactor control and shutdown equipment and allow the passage of carbon dioxide coolant gas to remove heat from the reactor fuel and core.
During reactor operation, the graphite components oxidise and age due to interaction with the radiation environment. This can lead to a reduction in graphite component strength and the potential for development of cracks in some of the graphite components. It is a licensee’s job to demonstrate to ONR that the graphite reactor core can continue to be used safely as it ages. ONR inspectors like me are involved in detailed assessment of the licensee’s safety cases and related inspections of their reactor examination programmes.
Looking specifically at NGL, it carries out a very extensive programme of testing and analysis to support its current safety case for operation. This programme includes extensive in-reactor examinations and testing of graphite samples removed from reactors.
Although the prime responsibility for nuclear safety rests with the licensee, the lack of comparable experience outside the UK led ONR to conclude that there was a need to maintain national expertise in graphite technology that was independent from the licensees.
For this reason, ONR supports research groups at several universities including Manchester and Birmingham who carry out work on our behalf and are able to advise us. As part of this programme, an independent advisory committee was formed in 2004, consisting of senior academics and other internationally recognised experts in nuclear graphite technology. This committee continues to provide authoritative advice to ONR and enables the regulator to provide a detailed and constructive technical challenge to the licensee’s own analysis.
ONR is also engaged with NGL on its ‘Lifetime Management Programme’, which proposes extensions to the operational lifetimes of their existing fleet of nuclear power reactors. This has included reviews of the plants to establish where improvements could be made, in order that ONR can be confident that the operator is taking adequate account of safety. A key aspect of this activity is work on the graphite core safety case. ONR inspectors have been considering the NGL proposals and discussing the technical challenges that need to be addressed with its staff.
ONR keeps under review the safe operation of nuclear power stations at those sites it regulates, this includes consideration of: the validity of the extant periodic safety review; satisfactory safety case and operation within that safety case; satisfactory maintenance, inspection, examination and test results; and compliance with nuclear site licence conditions. If at any stage ONR is not satisfied that the nuclear power station can be operated safely, it will not allow it to continue.