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Defence in depth: sources



Now let’s have a look at Table 7; this is a summary of the table, the full table is too large to show here. Please refer to pages 50-53 of the Manual for the complete table.

This table covers degradation of safety provisions, and can apply to sources in facilities, and to problems that arise during handling or use of sources, devices or packages. This table is divided into three main areas, which are shown here. 


Cat 4

Cat 3/2

Cat 1


No degradation
e.g. Superficial damage to shielding/packaging
Failure of industrial radiography source to return with safe recovery

Partial degradation
e.g. Failures in alarms/interlocks, degraded containment, not following some safety procedures

No safety provisions
e.g. Loss of all shielding, failure of all monitoring procedures, failure of all interlocks/alarms


For the first set of events 'no degradation' the choice between Level 0 and Level 1 depends on whether there are safety culture issues.

Look at the actual table on page 50. There are five examples of events that may occur under the 'no degradation' heading. Take note of the fourth example. For events that can be described as 'relatively frequent operational problems' the basic rating should be Level 0 if the response to the failure is carried out safely, as planned. If there are some problems with the application of procedures, this may be a justification for rating at Level 1.

Looking at the second section of the table 'partial degradation' again you will see that there are different examples of events listed. You will also note that in every case there is reference to a footnote to give further guidance on how to choose between possible ratings.

In each case the two ratings proposed are based on consideration of both the size of the source and the significance of the degraded safety provisions. One important note in footnote b is that for Category 1 sources in facilities, the guidance on rating events at facilities should be used, i.e not this simplified approach which cannot address all the issues related to safety provisions within facilities.

Finally, let’s look at the last section of the table 'no safety provisions'. Looking at the complete version in the Manual you will see that there are five types of events listed. We would expect these events to be rated at the maximum under defence in depth and generally, that is the case. While there appears to be a choice for most of the examples, in fact the lower rating only applies to some particular circumstances.

Footnote d explains that the maximum potential consequences for Category 3 sources in facilities cannot be higher than Level 2, so the Level 1 values are appropriate here. Similarly, footnote e explains that events involving provisions only related to worker exposure should be rated at Level 2. It is important to remember that these footnotes are an important part of the rating guidance.