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Defence in depth: reactors



Let's look at an example...

Reactor coolant leak appeared - within capability of make-up system - operators isolated pipework before leak got worse

Step 1:
Assume structure failed or action not taken

Step 2:
Identify what the INES rating would have been

Step 3:
Reduce rating based on likelihood of assumption in step 1

The size of the pipe containing the defect would have led to a small LOCA had it failed completely. This is a 'possible' initiator and at the time all the required safety systems were fully available.

If we look at Table 9 on page 75, you will see, from box A2 that the basic rating is Level 1.


Now we need to apply step 3 and reduce this rating depending on the likelihood that the leak could have reached its maximum rate. In this case, we will assume that the advice was that it was a fairly stable small leak not likely to result in rapid failure. Hence, we will reduce the basic rating to Level 0.