Over time during reactor operation, the graphite bricks age and their properties change due to interaction with the radiation environment and the reactor coolant. This can lead to the graphite losing weight and the development of cracks in the graphite bricks, both of which are well-known phenomena and have been the subject of significant interest by the industry, academics and the regulators for many decades.
We require the licensee, EDF Energy, to demonstrate through their safety case that they have adequate understanding of the graphite behaviour, to justify safe operation of the core in a clear, evidence-based manner. We require EDF Energy to clearly define conservative limits of operation based on the extent and adequacy of their understanding of graphite core ageing.
The last operating Magnox reactor ceased operation at the end of 2015, but the UK's fleet of AGRs will continue to account for between 10-15% of the UK electricity supply for a number of years to come. The graphite core of these reactors cannot be replaced and ageing mechanisms such as weight loss and cracking can change the mass, dimensions and material properties within the core. As such, they pose unique challenges to the operator, EDF Energy and to us as the regulator.
As well as moderation, the fundamental safety requirements of an AGR core include allowing free movement of control rods, free movement of fuel and directing the flow of coolant gas to ensure adequate cooling of the fuel and core structure. Essentially, significant weight-loss and cracking may compromise these safety requirements.
During operation, the graphite slowly loses weight due to oxidation caused by the reactor's carbon dioxide coolant gas. Loss of weight affects both the mechanical properties of the graphite brick, and reduces its effectiveness as a moderator. Weight-loss is potentially a life limiting condition for the reactors, although we believe that most of the AGRs will have their life limited by the progression of cracking.
Three types of cracking have been observed in graphite bricks:
It is the responsibility of the licensee, EDF Energy to demonstrate to ONR that the graphite core can continue to operate safely as it ages, and it undertakes an extensive programme of testing and analysis to support its safety case for operation. Furthermore, EDF Energy periodically shutdowns each reactor as it is required to carry out inspections and remove samples of the graphite to determine the level of weight-loss and cracking.
Within its operational safety cases EDF Energy sets safety limits on weight-loss and cracking for each reactor core based on extensive research and regular surveillance and analysis of the graphite behaviour.
It is a requirement of the nuclear site licence for the licensee to determine and justify the operating limits of its nuclear plant. This means that the limits can change depending on the licensee's understanding of the graphite behaviour. They may obtain information through surveillance and associated research, to revise the limits for weight loss and cracking stated within the safety cases for the reactors.
However, any proposed change to those limits must be presented to ONR through a robust safety justification, which demonstrates that it is safe to adjust the limits, based on the evidence provided.
Following our own detailed assessment, which can include discussions with academic experts, if we are satisfied with the evidence and the safety justification, we will agree to operation with the revised limits.
We require EDF Energy to demonstrate through their safety case that they have adequate understanding of the material changes to the graphite and their rate of progression, to justify safe operation of the core in a clear, evidence based manner. We also require that there are clear safety margins between the worst case core condition predicted during a period of operation and that which has been shown to be safe, with no “cliff edge” change of behaviour. They are required to conduct regular inspections to understand the changes in the core, also remove samples of the graphite to conduct suitable examinations or experiments. Subsequent analysis of these results is used as the basis of the case for continued operation.
At the end of each statutory reactor outage, ONR inspectors conduct detailed assessments of the licensee's safety case that supports the proposed return to service, following the inspections of the graphite and supporting research, considering all of the evidence provided. If there are any safety concerns, we would not permit the return to service of the reactor until they have been satisfactorily addressed.