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DEFENCE IN DEPTH
Defence in depth: sources

WORKED EXAMPLE: RADIOACTIVE SOURCE STOLEN DURING TRANSPORT

 

Summary:

  • Missing 1.85 GBq 60Co source
  • Dose rate of 0.5 mSv/h at 1 metre
  • Failure to comply with transport regulations

Scenario

When a package of a sealed 1.85 GBq 60Co source was delivered by the shipper, it was found to be empty. The source was found seven hours later in a delivery truck. The package had been intentionally opened. An activity of 1.85 GBq of 60Co delivers a dose rate of 0.5 mSv/h at a distance of one metre. It appeared that the event was a direct result of failure to comply with the regulations for the transport of radioactive materials:

  • The security seal required by the regulations was not affixed to the package,
  • The shipping declaration had not been completed, and
  • The 'radioactive' label did not appear to have been fixed to the container (although this was never clearly established).

INES procedure (click the highlighted flowchart items to follow the procedure)

Figure 8, page 149 of the Manual

Calculate A/D and determine category

Here is the calculation:

1.85 GBq 60Co = 0.00185 TBq 60Co
3.E-02 = 0.03
0.00185(60Co)/0.03 = 0.061666

Check the category and maximum defence in depth rating here

A/D ratio

Source category

Rating for maximum
potential consequences

Maximum rating under
defence-in-depth

0.01 ≤ A/D < 1

4

1 ≤ A/D < 10

3

10 ≤ A/D < 1000

2

1000 ≤ A/D

1

 

Return to flowchart

Answer

Here is the rating for a Category 4 source with inadequate packaging, where no safety provisions remained. This time we use Table 7 to calculate the rating

The radioactive source stolen during transport was rated at Level 1