6. Sealed source category and the 'D' value

Source: Ionactive Resource

We cannot discuss the security of radioactive sources in significant detail for obvious reasons. Much work has been achieved over the last decade to formalise and upgrade security arrangements in the UK and around the world. Restricted UK guidance is available from NaCTSO (National Counter Terrorism Security Office). The guidance is used by the operator during an application process (for example to determine the correct permit type), and by the Counter Terrorism Security Adviser (CTSA) to assess the level of risk and therefore adequate security provisions.

Some sealed radioactive sources (or practices) are placed directly into source and security categories independent of their activity (e.g. Co-60 sealed sources used in industrial sterilization plants are category 1). The activities are usually high and will typically require:

  • A detailed site security plan
  • Ability to extend plan according to security threat level
  • Background checks for employees who have access to the sources (i.e. the plant containing the sources)
  • Various physical security measures to meet the deter, detect, and delay principles.

Sealed sources which are considered to be less of a threat will be placed into lower categories (2-4). These will also be based on practice 'type' with or without an activity constraint. Examples include:

  • Industrial radiography - category 2 sealed source (Co-60, Ir-192, Se-75) regardless of activity
  • High dose brachytherapy - category 2 sealed source (Co-60, Cs-137, Ir-192) where activity exceeds a threshold (i.e. for Ir-192 it is > 110 GBq)

All of the above will then be assigned a security group rating which will not be discussed further in this guidance.

Category 5 sources and A/D

The lowest category is 'category 5' sealed sources. These exclude all of the specific practices noted above (where the category type is determined by practice type, with or without activity constraints).

For these sources we have to apply the A/D test where:

  • 'A' is the activity of the source in GBq
  • 'D' is the 'Danger' value in GBq (obtained from the restricted guidance noted above, but also available in IAEA documents online)

For the purposes of UK permit (or equivalent) applications for category 5 sealed sources, the value of 'A' is the aggregate of all sealed sources. This means that adding additional sealed sources to a site inventory may change the source classification in terms of the security group, meaning the permit is no longer valid (and will need to be upgraded and additional security provisions implemented. UK regulators require sites with category 5 sources to declare every year that the classification remains valid (the A/D detail below will explain this).


For a sealed source to be a category 5 source, the value of A/D must be < 0.01. As noted above, this must be so for the total of A (i.e. aggregate of activity). Here are some examples D values.

  • Co-60 30 GBq
  • Cs-137 100 GBq
  • C-14 50,000 GBq
  • Po-210 60 GBq
  • Ni-63 60,000 GBq

Example (1)

You have two 10 MBq Cs-137 sealed sources. Are they category 5?

Let A = 20 MBq (total activity of both sources)

So, A/D = 0.02 GBq / 100 GBq = 0.0002. Since A/D << 0.01 you have category 5 radioactive sealed sources.

Example (2)

You are a site working with sealed Ni-63 sources. The sources each have a maximum activity of 600 MBq (0.6 GBq). How many sources can you have on site so that they all meet the definition of a category 5 sealed source?

The D value for Ni-63 is 60,000 GBq.

The value of A/D must be < 0.01 (where A is the total activity of all sources.).

Therefore A will (=) 0.01 x 60,000 GBq = 600 GBq. So number of sources would need to be less than 600 GBq / 0.6 GBq (=) 1000 sources.

It is suggested you ensure you are comfortably within A/D<0.01, so perhaps 900 600MBq Ni-63 sources is a sensible working maximum.

More information and a certificate?

If you would like more information and lots of examples, why not take the Ionactive Radioactive Substances and Waste - Environmental Compliance Course ? This would be ideal if you are a Radiation Protection Supervisor (RPS) or Radiation Protection Officer (RPO) with radioactive substances and wastes at your workplace.

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