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DEFENCE IN DEPTH
Defence in depth: facilities

WORST POSSIBLE ACCIDENT

 

Remember the importance of defining exactly what we mean by "the worst possible accident, should all provisions fail"?
Let's look again...

Now we must define more carefully exactly what we mean by

An event of the maximum potential consequences, should
all the provisions fail.

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We are not talking about
accidents that are considered likely to happen
or even
the scenarios considered in the safety justification of the facility.

We are talking about
an accident that the designer or operator may consider incredible.

 

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It is the maximum potential consequences that could occur if all safety provisions failed and a situation develops as severely as physically possible taking into account

  • The radioactive inventory that could potentially be involved in the event,
  • The physical and chemical properties of the material involved, and
  • The mechanisms by which that activity could be dispersed.

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The maximum potential consequences are not specific to the event, but apply to a set of operations at a facility. However, any one site may contain a number of facilities with a range of tasks carried out at each facility. The maximum potential rating should be specific to the type of facility where the event occurred and the type of operations at the time of the event.

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When considering consequences related to worker exposure, the maximum potential consequences should generally be based on exposure of a single individual as it is highly unlikely that several workers would all be exposed at the maximum credible level. For facilities, the assessment of the event of the maximum potential consequences will come in part from the safety justification for the facility and in part from expert judgement. For radioactive sources, it is based directly on the source category using D values.

If you are not familiar with this concept, click here.

Defining source size

We talk of big sources and small sources but we clearly need a more scientific way of calculating source size. The approach used is based on D values, a concept we met in the earlier module on radioactive release. The definition is shown below and the D value for a range of isotopes is contained in Table 22 of the Manual.

Based on D value for the isotope:

an activity above which a source is considered to be 'a dangerous source' and has a significant potential to cause severe deterministic effects if not managed safely and securely

Severe deterministic effects:
 - fatal or life threatening,
 - reduces quality of life

Reference (IAEA RS-G-1.9 Categorization of Radioactive Sources)