Maintaining nuclear safety as a priority
We work within very large safety margins and this applies to graphite bricks too. The level of cracking or weight loss which is considered reasonable is far below anything which would affect the reactor’s safe operation.
The safety margins approved by the regulator don’t define a boundary between safe and unsafe. There’s a massive margin before the plant would be considered to create an unacceptable risk, and our plants would never be in an unsafe position because we would close them down long before the graphite reached a state that was anywhere near unsafe. We are required to demonstrate operational limits which ensure we operate with large safety margins. As our knowledge improves, then our process requires us to agree changes to the operational limits with the external regulator.
For graphite cracking, significant safety margins have been demonstrated. These are based on our understanding of graphite behaviour, and extensive research and development programme.
In addition to this, we have invested in modifications to provide further margins to safety. In extreme and unprecedented circumstances, such as a seismic event, each reactor has several super-articulated control rods which would be inserted into the reactors to safely shut it down. In the highly unlikely event that the number of control rods inserted into the reactor was unable to stop the nuclear reaction, our stations are equipped with plant that would quickly inject nitrogen gas into the core and stop the nuclear reaction.