What is a category 5 radioactive sealed source (UK)?

Source: Ionactive Resource

The category of a sealed radioactive source is designated from 1-5 as defined by IAEA internationally - see Categorization of Radioactive Sources - IAEA Safety Standards Series No. RS-G-1.9 (PDF download). This is essentially a security / hazard rating for sealed radioactive sources, where a category 1 source is the most hazardous (and needs rigorous security protocols) and a category 5 source is the least hazardous (and only needs a minimum level of security). The ratings overall are based on a combination of activity (GBq) and type of radionuclide (noting for example that the ionising radiation emissions from Po-210 are very different to those from Co-60).

  • Example of a category 1 sealed radioactive source - 370 TBq Co-60 source used in industrial sterilisation.
  • Example of a category 5 sealed radioactive source - 10 MBq Cs-137 calibration source used in a laboratory.

An uncontrolled and unshielded category 1 source is capable of delivering deterministic levels of whole body or extremity exposure in the 10's - 100's of Gy range (and above) - leading to radiation injury and potential death.

An unshielded category 5 radioactive sealed source should not be directly handled, but is not capable of delivering deterministic levels of exposure. Bad practice with such a source is still capable of delivering radiation exposures which are not ALARP, but this would would be stochastic in nature (and possibly in breach of health and safety legislation).

UK Category 5 Source

In the UK, source categorisation is specified in the NaCTSO 'Security Requirements for Radioactive Sources' (this is a restricted document since it specifies the expected level of security for each source category in terms of Security Groups A-D).

A category 5 radioactive sealed source is where the A/D ratio is < 0.01 (see below for explanation). This designation excludes certain practices regardless of A/D where, for example, the practice involves brachytherapy, static eliminators (e.g. Po-210), gauges (e.g. Cs-137), industrial radiography (e.g. Ir-192), irradiators and medical teletherapy units (e.g. Co-60). However, it is not likely that a radioactive source which meets the < 0.01 ratio would be used in the above noted practices.


The A/D is the ratio of activity of the source (s) of interest (A GBq) over the D (Danger) value (D GBq). The D values are published in the above noted NaCTSO document (restricted) but are also available via the IAEA website (IAEA D values, note radioactivity in this document is expressed in TBq).

When determining the category of a radioactive source in the UK, particularly when applying for a permit / authorisation/ registration, the value A is the aggregate value of all radioactive sources. Therefore, whilst a single source of activity 'A' might meet the definition of a category 5 source, a number of such sources taken together might exceed the required ratio and move the overall source category up (i.e. to 4).

Category 5 source examples

Consider the following D values for some radionuclides (see above reference for other D values).

  • Am-241 (60 GBq)
  • Cs-137 (100 GBq)
  • Ir-192 (80 GBq)

Note that for a category 5 source the A/D value must be < 0.01

  • A single Am-241 source of < 0.6 G Bq (< 600 MBq) is a category 5 source.
  • A site with two sources of Cs-137 at 0.49 GBq (490 MBq) = 980 MBq (0.98 GBq) has two category 5 sources individually, and in aggregate.
  • A site with one 0.9 GBq Ir-192 tries to apply for a category 5 source standard conditions permit in England. The application fails since A/D > 0.01 [0.9 / 80 = 0.01125], this being > 0.01.

As noted above, do not forget that NaCTSO (etc) specifies practices in the UK that might require a category higher than 5 (i.e. 1-4) regardless of the A/D ratio.

Aside - note that the above mentioned D values are also used as the threshold values for where a radioactive source becomes a HASS source. If you wish you can read more about HASS sources by visiting our environmental legislation guidance on HASS : High Activity Sealed Sources (HASS).

Need more information?

If you need more information, or would like to discuss this issue more formally, then please visit our Radioactive Waste Adviser (RWA) page and contact Ionactive. Whilst an RWA is required to provide advice on radioactive substances and radioactive waste accumulation and disposal, the RWA also provides advice on environmental radiation safety and this will include sealed radioactive sources and HASS (High Activity Sealed Sources) - including the matters in this resource article.

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