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DEFENCE IN DEPTH
Defence in depth: reactors

HOW TO DETERMINE SIGNIFICANCE

 

Initiator occured?

Safety components found degraded?

Significance depends mainly on response of safety systems

'Operability of Safety Functions'

Significance depends on extent of degradation and likelihood of systems being needed

'Initiator frequency and Operability of Safety Functions'

If an initiator has occurred, the significance will depend mainly on the availability of safety systems and whether they work as intended. This is what is meant by the term 'operability'. It’s not a yes/no question. There could be some parts of a system still working and/or there could be a second system available.

If safety components are found to be degraded, a key part of the rating will be how likely it was that an initiator, requiring that equipment to work, might have occurred.

You have now seen two key terms in the methodology for rating reactor at power events under defence in depth.

  • 'Operability of safety systems'
  • 'Initiator frequency'

For the former we define four levels of operability; for the latter four ranges of initiator frequency. You will see the definitions on the following pages.