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DEFENCE IN DEPTH
Defence in depth: reactors

DEFENCE IN DEPTH

 

There are five levels of defence in depth, following guidance produced by INSAG (International Nuclear Safety Advisory Group). 

The first level is the quality of design, construction, operation and maintenance. This is the important first barrier aimed at preventing faults from occurring.

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The second level is control systems that seek to bring minor perturbations back to normal operation.

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The third level is a set of safety systems, specifically designed to safely shutdown and cool the reactor.

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The fourth level are accident management arrangements, put in place should the safety provisions fail.

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The fifth and final barrier are the emergency arrangements designed to manage the accident and implement protective actions.

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For each of the levels, the 'barrier' shows an example of what is provided for a pressurized water reactor (PWR) to provide defence against loss of primary coolant. What we want to assess, in rating events, is how much of this defence in depth failed in an event and more importantly, how much is left.