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Defence in depth: reactors



Here is the definition of safety function operability. 

Safety function operability is the capacity of the system to perform the required function in the required manner. It is the operability of the safety function of the overall that is considered,
NOT the operability of an individual component of the system

It is very important to note the middle section of the definition, "It is the operability of the safety function that is considered."
If the safety function is post trip cooling of fuel then this could be provided by:

  • Electrically driven pumps, and
  • Turbine driven pumps.

Each is a safety system, but together they provide the same function.

For example, an door with two bolts is successfully locked when either is used; either lock stops entry.

Together the safety system provides one safety function.

We will see this in an example shortly. But first let’s look at the definition of the four levels of operability.

  • Inadequate
  • Adequate
  • Minimum required by operational limits and conditions
  • Full

We’ll start from the lowest level and work up.