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DEFENCE IN DEPTH
Defence in depth: sources

WORKED EXAMPLE: MELTING OF AN ORPHAN SOURCE

 

Summary:

  • Melted orphan source of 1 TBq 137Cs
  • 50 non-radiation workers received doses of 0.3 mSv

Scenario

An orphan source of 1 TBq of 137Cs, inadvertently included in scrap metal, was melted in a steel factory. Fifty employees at the factory received an estimated dose of 0.3 mSv each.

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 for the category:

1.E-01 = 0.1
1(137Cs)/0.1 = 10

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

 

Based on the A/D calculation, the category for this event would be Category 2.

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Answer

Here is the rating for a Category 2 orphan source, if the source was melted and no safety provisions remained.

The melted orphan source would, therefore, be rated at Level 2.