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Defence in depth: reactors


  • During routine operation, reactor coolant leaked. The leak was within the capability of the makeup system and Technical Specification allowed continued operation while the leak was investigated.
  • Operators found a small hole in a pipe and isolated it from the primary circuit.

  • The reactor was safely shutdown.

Was there an initiator?
Well there is certainly some loss of coolant, but none of the safety systems were required. Remember the definition from earlier;

An initiator is an occurence that challenges the safety systems and requires them to function

And you read earlier in the course 

If you think you have identified an initiator, ask yourself the question—what safety systems did it require to function. If you can’t answer that, you may not have identified an initiator!

The truth is, there nearly was an initiator.

Remembering back to our five layers of defence in depth, the first layer failed

but the second layer worked

and there was no actual initiator

Before we look at how to address this type of event, take a look at some other typical examples of events.