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Defence in depth: reactors



In this example what are the initiators? 

During on-load testing of turbo-alternator, a faulty microswitch initiated spurious valve closure

The reduction in steam flow caused a reactor transient beyond the capability of the control system

A reactor trip occured

What is the initiator?

The start of the problem is a faulty micro switch; the result was closure of a valve but neither of these are an initiating event. 

A reactor transient caused by reduced steam flow is the initiating event and the reactor protection system has been designed to protect against such faults and shut the reactor down.

The reactor trip is another initiator as it requires all the post trip systems to work and cool the reactor.

The initiator is not the failure that starts the incident, it is the event that challenges the safety systems.