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DEFENCE IN DEPTH
Defence in depth: sources

METHODOLOGY FOR DEFENCE IN DEPTH RATING

 

So far we have addressed the first aspect of our process for rating under defence in depth - 'how severe'.
We looked at the 'wider issue of safety culture' in the overview module, but have yet to deal with the aspect of 'how close' we were to an accident.

The 'how close' aspect considers how many of the safety provisions were affected and how much their effectiveness was impaired.

Depends on source category

Shielded containment
Vaults
Interlocks
Source housing
Safety/warming devices
Administrative controls
Operational procedures

Depends on safety culture issues

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Ratings are provided for a range of possible events in a series of tables.

These tables incorporate all three steps in the rating process:

How severe,
How close,
Wider issues,

There are three separate tables for different 'families' of events:

Table 6: For lost or found sources, devices and transport packages
Table 7: For degraded safety provisions
Table 8: For other safety related events

Let's start with Table 6, events involving lost and found radiation sources.

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).

The full table is too large to show here, please refer to pages 50-53 of the Manual for the table.

Type of events Event rating depending on the source category
Cat 4 Cat 3/2 Cat 1

Member of the public receiving a dose from a single event in excess of annual statutory dose limits.

1 1 1

Workers or members of the public receiving cumulative doses in excess of annual statutory dose limits.

1 1 1

Absence of or serious deficiency in records such as source inventories, breakdowns in dosimetry arrangements.

1 1 1

Discharges to the environment in excess of authorized limits

1 1 1

Non-compliance with licence conditions for transport.

1 1 1

Inadequate radiological survey of transport.

0 or 1a 0 or 1a 0 or 1a

Contamination on packages/conveyance where the resulting contamination is of little or no radiological significance.

0 or 1 0 or 1 0 or 1

Contamination on packages/conveyance where a number of measurements reveal excessive contamination above the applicable limits, and there is potential for the public to be contaminated.

1 1 1

Shipping documents, package labels or vehicle placards incorrect or absent. Marking of packages incorrect or absent.

0 or 1 0 or 1 0 or 1

Radioactive material in a supposedly empty package.

1 1 or 2b 1, 2 or 3b

Radioactive material in the wrong type or an inappropriate packaging.

0 or 1c 1 or 2c 2 or 3c

a - The rating should take into account the degree of inadequacy of the surveys as well as any safety culture implications.

b - The choice of rating should take into account the safety provisions that might still be in place even though the package was supposed to be empty.

c - The higher rating in each category reflects situations where the wrong or inappropriate packaging could reasonably result in inadvertent exposures.