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DEFENCE IN DEPTH
Defence in depth: sources

LOST OR FOUND

 

This is a summary of Table 6 from the Manual. The full version can be found on page 49.

Let's start by looking at just two rows of Table 6 from the Manual, which we will use to illustrate the overall approach. You will see that the column on the left of each row describes a type of event.

 

Cat 4

Cat 3/2

Cat 1

 

Missing, subsequently recovered intact within an area under control

Found source device or package, including 'orphans'

Lost or stolen, not yet recovered

Lost or stolen, but now located in a safe location with no evidence of actual doses

 

Misdelivered transport package but receiving facility has all the radiation safety procedures

 

Misdelivered transport package but receiving facility does NOT have all the radiation safety procedures

 

 

 
 

The first line shows an event where the main safety provisions all remained available, but there is a problem with safety culture in that the administrative controls failed. For this reason, that type of event is rated at Level 1 (Basic Level 0 + uprating due to safety culture) for all types of sources.

The size is not relevant to this particular event.

On the other hand this row describes an event where there are no safety provisions remaining as we do not know, and may never know, what has happened to the source. This event is, therefore, rated at the maximum for defence in depth.

That level, of course, varies with source category and cannot be affected by safety culture issues. Hence the ratings of

  • Level 3 for a Category 1 source,
  • Level 2 for a Category 2 or 3 source, and
  • Level 1 for a Category 4 source.
Now let's look at the whole table »
 

Cat 4

Cat 3/2

Cat 1

 

Missing, subsequently recovered intact within an area under control

 

Found source device or package, inlcuding 'orphans'

 

Lost or stolen, not yet recovered

 

Lost or stolen, but now located in a safe location with no evidence of actual doses

 

Misdelivered transport package but receiving facility has all the radiation safety procedures

 

Misdelivered transport package but receiving facility does NOT have all the radiation safety procedures

 

 

The second row covers events where we have managed to find a lost source. This event is clearly worse than the event described in the first row and again shows a safety culture issue.

As Level 1 is the highest defence in depth rating for Category 4 sources, that is the rating shown here. For Category 2 or 3 sources, the table shows a choice.

See the footnote giving more guidance on page 49 of the Manual. Basically it says, if there are no safety provisions remaining, you rate the event at the maximum under defence in depth, i.e. the same as the third row. If on the other hand there are some provisions remaining, this seems too high, so there is a choice of a lower rating. Wherever possible the choice is limited to two values.

You should now be able to understand the basis of the rating for the other rows.

Type of events

Event rating depending on the source category

Cat 4

Cat 3/2

Cat 1

Missing radioactive source, device or transport package subsequently recovered intact within an area under control.

1 1 1

Found source, device (including orphan sources and devices) or transport package.

1 1 or 2a 2 or 3a

Lost or stolen radioactive source, device or transport package not yet recovered.

1 2 3

Lost or stolen radioactive source, device or transport package subsequently located with confirmation that no unplanned exposures occurred, but where a decision has been made and approved not to recover the source as it is in a safe or inaccessible location (e.g. underwater).

1 1 1

Misdelivered transport package, but receiving facility has all the radiation safety procedures required to handle the package.

0 or 1 1 1

Misdelivered transport package, but receiving facility does not have all the radiation safety procedures required to handle the package.

1 1 or 2b 2 or 3b

a - The lowest proposed rating is more appropriate where it is clear that some safety provisions have remained effective (e.g. a combination of shielding, locking devices and warning signs).

b - The lowest proposed rating is more appropriate where it is clear that some safety provisions have remained effective (e.g. a combination of shielding, locking devices and warning signs).