All but one of the UK’s operating nuclear power stations are Advanced Gas-cooled Reactors (AGRs) which contain around 3,000 graphite fuel bricks forming part of the reactor core. The fuel assemblies are periodically raised and lowered through these bricks as part of the normal operation of the reactor, for example when refuelling is required. Whenever a fuel assembly is raised or lowered, the load on the fuel hoist and the height of the fuel is recorded. This information is known as a fuel grab load trace (FGLT).
As the fuel assembly is charged or discharged there is friction between the brushes and the graphite bricks. The load seen is directly influenced by the diameter of the channel and so small variations in the size of the channel bore and across interfaces between bricks are recorded as variations in load on the FGLT. This information is important as it is one of the ways EDF gathers information on the state of the reactor core in between inspections to ensure safe and efficient operation.
EDF was keen to simulate defects which might occur - but have not been identified so far - to see if and how they might be identified on a load trace.