EDF Energy said on 7 August that it was once again extending the outages at the UK’s Dungeness and Hunterston NPPs. Dungeness B Reactor 22 was taken offline for pre-planned outages in August 2018 followed by Reactor 21 the following month. During during maintenance work, on the two 520 MWe AGR reactors, problems with the main steam line and with corrosion were discovered and later issues involving the site's boilers. Inspection revealed that several systems providing a safety function were “corroded to an unacceptable condition”. The corrosion has been rated level 2 on the International Nuclear and Radiological Events Scale (INES).
The two units at Dungeness B began operation in 1983 and 1985 and had been upgraded for continued operation until 2018. Unit B-21 was initially scheduled to restart in April 2019, which was then extended until 30 September 2019 and then until September 2020. Unit B-22 was also originally scheduled to restart in April 2019, but this was extended until October 2019 and then until September 2020. The new restart dates are now 20 December for Reactor 21 and 10 December for Reactor 22.
In 2015, EDF announced a ten-year life extension for Dungeness B to 2028 with the aim of keeping UK NPPs in operation until at least 2023 when Hinkley Point C was due to be commissioned. The Hinkley commissioning date has since been moved to 2025.
According to Dungeness station director John Benn: "The team at Dungeness continues to work tirelessly to prepare the site for a return to operations. The issues identified early in this outage have been addressed, as part of our significant investment programme over the last two years. As more work has been done, further issues have surfaced that require further detailed analysis and engineering work. To enable this work to be done, and to give the Office for Nuclear Regulation (ONR) sufficient time to consider its response, the decision has been made to push back our restart dates until mid-December."
In Scotland, the Hunterston Reactor 3 (also known as B7) was taken offline in March 2018 and Hunterston Reactor 4 (B8) the following October when cracks in their graphite cores were found during routine inspections. The reactor cores of the UK’s 14 AGRs comprise graphite bricks with channels for nuclear fuel, and control rods. This graphite is known to change over time. Each core contains 6000 graphite bricks, 3000 of which contain the fuel channels.
The 475MWe B7 began operations in 1976 and the 485MWe B8 in 1977. They were originally scheduled to shut in 2011, but their operating life was extended to 2016, and following a technical and economic evaluation, EDF Energy later said they could operate until 2023. EDF said in 2015 that the core is designed to contain many redundant bricks meaning a "very large number of bricks" would have to crack before there were any significant safety concerns. However, in May 2018 EDF admitted the cracks were expanding more quickly than anticipated and in December extended the outages for further work, with restart dates of 30 April 2019 for B7 and 31 March for B8.
In April 2019 the outages were extended again to 30 June (B7) and 30 May (B8) while ONR assessed whether it was safe for them to restart. Further extensions were announced in May 2019 to 31 July (B7) and 24 June (B8). Further extensions followed. In August 2019, ONR agreed to B8 to operate for four months after finding that the reactor cores can still fulfil their fundamental safety requirements, despite the cracks in its graphite bricks. EDF said it had spent more than GBP125m ($163m) completing the “most extensive investigation of the reactor core that has ever been undertaken” to prove the plant’s safety. Following the approval of a safety case with the ONR, B8 was briefly returned to service from 25 August to10 December 2019.
However, B7, which was found to have some 377 hairline cracks in its graphite core, remained closed with a provisional restart date of November postponed to 15 January 2020. This was again deferred, with EDF saying it aimed to restart both reactors in April 2020, pending ONR approval.
Following B8’s four months of operation, ONR posted five detailed technical reports online. One on “structural integrity” noted that some of the graphite bricks in reactors three and four had begun to disintegrate. Inspections of B7 found that “crack opening of fuel bricks was causing other fuel bricks to crack”, the ONR report said. Inspections showed bricks with one or two “full-height” cracks and additional “partial-height” cracks.
ONR warned that there was “the potential for bricks to crack into three or more vertical parts” and become “multiply-cracked bricks” which could cause the uranium fuel to become snagged. The extent of the cracking exceeded EDF’s predictions, leading to a revision of its safety case. “Brick cracking was also found in some instances to be generating graphite debris, i.e. small pieces of graphite separating from the brick,” according to ONR. EDF inspections of a quarter of B7 and 10% of B8 had observed “seven pieces of debris and 51 fragments”.
With both reactors remaining closed, EDF said it hoped to bring the two units back on line in August 2020. B7 had been due to restart on 20 August, but that has now changed to 30 August. The restart date for B8 is set at 17 September.
"The ONR is continuing its assessment into the Hunterston B return to service case, using the information EDF has provided after extensive rounds of modelling and analysis,” an EDF spokesman said. “In the last few years, EDF has invested over GBP200 million in understanding the likely impacts on the graphite reactor under a range of worse case scenarios, including up to a 1 in 10,000 year seismic event, much larger than the UK has ever experienced. We remain confident that we would be able to shut down the reactor in all such scenarios."