EDF Energy said inspections confirmed “the expected presence of new keyway root cracks in the reactor core and also identified these happening at a slightly higher rate than modelled”.
The company said it is working with the regulator to ensure that the longer-term safety case reflects the findings of the recent inspections and includes the results obtained from other analysis and modelling. The operation of other reactors operated by EDF Energy, including Hunterston B-2, is not affected.
We have been working over many years to fully understand and prepare for these late life changes to the reactor core and regular inspections at all our plants have provided a clear understanding of how the reactor cores age.
Hunterston B has two advanced gas-cooled reactor units (AGRs). Hunterston B-1 began commercial operation in February 1976 and Hunterston B-2 in March 1977
The graphite core of both units is made up of around 6,000 graphite bricks – 3,000 of which contain fuel channels – which are all connected. The structure 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.
The longer-term safety case will build on work already completed and EDF Energy expects that this will demonstrate that there are large safety margins both now and for the projected reactor lifetime.
“Over £100m has been spent on the graphite research programme which benefits from the expertise of our own team of specialists as well as academics at several leading UK universities,” EDF Energy said.
Over time, graphite slowly loses weight as part of the normal ageing process. EDF Energy said this is a well-known phenomenon which was fully considered as part of the stations’ design and is factored into safety limits approved by the regulator.
In 2014, when similar cracks were first discovered, EDF Energy said graphite ageing is one area used to determine the lifespan of an AGR nuclear power station.
The company expects the unit to return to service before the end of 2018, resulting in a reduction in the plant’s 2018 nuclear output forecast of up to 3TWh.