French nuclear power plant operator EDF has released inspection footage of cracks developing in the graphite bricks of the reactor cores at the two-unit Hunterston B plant in Scotland, explaining the cracks, although growing faster than anticipated, were well within safety standards.
EDF has been aware of the cracks developing in the bricks since November 2015, when the company said five bricks – two in Unit 4 and three in Unit 3, had been discovered. However, there are 6,000 graphite bricks that make up the core, which is considered an abundantly redundant number as an added safety feature. By May 2018, however, the utility said the cracks were expanding faster than expected.
“Cracks are around 1mm when first observed, and they are opening very slowly. The average crack size is currently approx. 2mm and we have demonstrated in our safety cases that cracks up to a width of 10mm on the inside of the fuel channel (i.e. the bore) and 18mm on the outside of the fuel channel are acceptable,” said acting station director Roddy Angus in a letter to the public.
“We are sharing footage from two channels, one in each reactor,” he said. “They are representative of the kind of cracking we have observed."
Angus also explained that the camera’s field of vision is 80 degrees, meaning viewers only see a small portion of a fuel channel in the video. A graphite fuel channel is approximately 10m high with an internal circumference of 825mm. The clip allows a view of about 180mm out of the 825mm internal wall of the channel.
The Hunterston B reactors have been operational since 1976 and 1977, respectively. They are licensed to operate through 2023. EDF said they planned to have the Unit 4 restarted on March 31 and Unit 3 restarted on April 30.
Here's the clip:
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