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DEFENCE IN DEPTH
Defence in depth: reactors

INITIATOR FREQUENCY

 

Expected

Predicted to occur once or several times during the operational life of the plant (i.e. >10-2 per year)

Possible

Less likely than ‘Expected’, but have an anticipated frequency during the plant lifetime of greater than about 1% (i.e. between about 10-2 and 10-4 per year)

Unlikely

Less likely than ‘Possible’, but considered in the design of the plant

Beyond design

Initiators of very low frequency not normally included in the conventional safety analysis of the plant but considered as part of the overall risk analysis. Systems may well be provided to mitigate risks.

The definitions are fairly clear and each plant will have a fault schedule as part of its safety justification that will identify initiators and their frequency.

Some examples based on previous designs are provided in Annex II, it's quite long so please refer to page 174 of the Manual. It is important to understand that these are only examples.

As there are different reactor types, use Annex II for a specific list of rating events that may occur for that particular reactor type.

At this point you may like to read section 5.1.1 on page 71 of the Manual to reinforce what we have learned so far about initiators.

5.1.1. Identification of initiator frequency

Four different frequency categories have been defined:

(1) Expected
This covers initiators expected to occur once or several times during the operating life of the plant (i.e. >10–2 per year).

(2) Possible
These are initiators that are not expected but have an anticipated frequency (f) during the plant lifetime of greater than about 1% (i.e. 10–4< f <10–2 per year).

(3) Unlikely
These are initiators considered in the design of the plant, which are less likely than the above (>10–4 per year).

(4) Beyond design
These are initiators of very low frequency, not normally included in the conventional safety analysis of the plant. When protection systems are introduced against these initiators, they do not necessarily include the same level of redundancy or diversity as measures against design basis initiators.

Each reactor has its own list and classification of initiators as part of its safety analysis, and these should be used in rating events. Typical examples of design basis initiators that have been used in the past for different reactor systems are given in Annex II categorized into the previous frequency categories. These may provide a guide in applying the rating process, but it is important wherever possible to use the initiators and frequencies specific to the plant where the event occurred.

Small plant perturbations that are corrected by control (as opposed to safety) systems are not included in the initiators. However, if the control systems fail to stabilize the reactor, that will eventually lead to an initiator. For these reasons, the initiator may be different from the occurrence that starts the event (see Example 36); on the other hand, a number of different event sequences can often be grouped under a single initiator.

For many events, it will be necessary to consider more than one initiator, each of which will lead to a rating. The event rating will be the highest of the ratings associated with each initiator. For example, a power excursion in a reactor could be an initiator challenging the protection function. Successful operation of the protection system would then lead to a shutdown. It would then be necessary to consider the reactor trip as an initiator challenging the fuel cooling function.