RS-G-1.9 Categorization of Radioactive Sources

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Main 1.1. Radioactive sources are used throughout the world in medicine, industry, agriculture, research and education; they are also used in some military applications. Many are in the form of sealed sources with the radioactive materials firmly contained or bound within a suitable capsule or housing. The risks posed by these sources vary widely, depending on such factors as the radionuclides used, the physical and chemical form and the activity.
Main 1.2. Sealed sources, unless they have been breached or are leaking, present a risk of external radiation exposure only. However, breached or leaking sealed sources, as well as unsealed radioactive materials, may give rise to contamination of the environment and the intake of radioactive materials into the human body. Until the 1950s, only radionuclides of natural origin, particularly 226Ra, were generally available for use. Since then, radionuclides produced artificially in nuclear facilities and accelerators, including 60Co, 90Sr, 137Cs and 192Ir, have become widely used.
Main 1.3. The International Basic Safety Standards for Protection against Ionizing Radiation and for the Safety of Radiation Sources (BSS) [1] provide an internationally harmonized basis for ensuring the safe and secure use of sources of ionizing radiation, and the Safety Requirements for Legal and Governmental Infrastructure for Nuclear, Radiation, Radioactive Waste and Transport Safety [2] set out the essential elements of a regulatory control system.
Main 1.4. Sealed and unsealed radioactive sources are used for a variety of purposes and they incorporate a wide range of radionuclides and amounts of radioactive material. High activity sources, if not managed safely and securely, can cause severe deterministic effects to individuals in a short period of time [3–16], whereas low activity sources are unlikely to cause exposures with harmful consequences.
Main 1.5. This Safety Guide provides a risk based ranking of radioactive sources and practices in five categories. The categorization system is based on a logical and transparent method that provides the flexibility for it to be applied in a wide range of circumstances. On the basis of this categorization, risk informed decisions can be made, in a graded approach to the regulatory control of radioactive sources for the purposes of safety and security.
Main 1.6. Following an assessment of the major findings of the International Conference on the Safety of Radiation Sources and the Security of Radioactive Materials, held in Dijon, France, from 14 to 18 September 1998 [17], the IAEA undertook a number of tasks designed to improve the safety and security of radioactive sources around the world. An ensuing ‘Action Plan for the Safety of Radiation Sources and the Security of Radioactive Materials’, approved by the IAEA Board of Governors in September 1999, identified the need for a categorization of radiation sources. A publication dealing with categorization was prepared in 2000, which was subsequently improved upon and superseded by the Categorization of Radioactive Sources [18], issued in 2003.
Main 1.7. This Safety Guide provides guidance on categorizing radioactive sources and on how this categorization can be used to meet the requirements for regulatory control set out in IAEA Safety Standards Series No. GS-R-1 [2] and the BSS [1]. The categorization system is based on that of IAEA-TECDOC- 1344 [18], and was developed by considering a variety of circumstances of use and misuse of radioactive sources. An explanation of the rationale is given in Annex I.
Main 1.8. The objective of this Safety Guide is to provide a simple, logical system for ranking radioactive sources in terms of their potential to cause harm to human health, and for grouping sources and the practices in which they are used into discrete categories. This categorization can assist regulatory bodies in establishing regulatory requirements that ensure an appropriate level of control for each authorized source.
Main 1.9. The purpose of categorizing radioactive sources is to provide an internationally harmonized basis for risk informed decision making. It is envisaged that the categorization system will be used by national authorities1 in establishing the appropriate degree of regulatory control for many activities relating to the safety and security of radioactive sources. Applications of the categorization include:

  • Developing or refining national regulatory infrastructures;

  • Developing national strategies for improving control over radioactive sources;

  • Optimizing decisions about the priorities for regulation within resource constraints;

  • Optimizing security measures for radioactive sources, including measures directed against their possible malicious misuse;

  • Emergency planning and response.
Main 1.10. This Safety Guide also provides support for the international harmonization of measures for the control of radiation sources and their security, in particular for the implementation of the Code of Conduct on the Safety and Security of Radioactive Sources (Code of Conduct) [19, 20]. The same categorization system is used in the Code of Conduct for sources in Categories 1–3, and this Safety Guide provides further details of the system and of its application to sources in all categories.
Main 1.11. This Safety Guide provides a categorization system for radioactive sources, particularly those used in industry, medicine, agriculture, research and education. The principles of the categorization can also be applied, where appropriate in the national context, to sources within military or defence programmes.
Main 1.12. The categorization is not relevant to radiation generating devices such as X ray machines and particle accelerators, although it may be applied to radioactive sources produced by, or used as target material in, such devices. Nuclear material, as defined in the Convention on the Physical Protection of Nuclear Material [21] (revised in 2005), is excluded from the scope of this Safety Guide. In addition, in situations where factors other than those considered here are dominant, this categorization may not be appropriate. One example is in waste management and the consideration of disposal options for disused sources, where factors such as the specific activity, chemical properties and half-life take on different emphases [22]. This Safety Guide does not apply to packages of radioactive material in transport, for which the IAEA Transport Regulations apply [23].
Main 1.13. The categorization is concerned with sealed sources; however, the method can also be used to categorize unsealed radioactive sources, and some examples are included in Appendix I.
Main 1.14. The categorization system is set out in Section 2 and its implementation is discussed in Section 3. Further details on the recommended categories are given in Appendix I and plain language descriptions of the categories are given in Appendix II. Annexes I and II provide supporting material explaining the methods used to establish the categorization system and to categorize radioactive sources and the practices in which they are used.
Main 2.1. Table 2 in Appendix I shows examples of the wide range of radionuclides and activities in radioactive sources used for beneficial purposes around the world. In recognition of the fact that human health is of paramount importance, the categorization system is based primarily on the potential for radioactive sources to cause deterministic health effects. The categorization system is therefore based on the concept of ‘dangerous sources’ — which are quantified in terms of ‘D values’2 . The D value is the radionuclide specific activity of a source which, if not under control, could cause severe deterministic effects for a range of scenarios that include both external exposure from an unshielded source and internal exposure following dispersal of the source material (see Annex II).
Main 2.2 The activity A of the radioactive material in sources varies over many orders of magnitude (Appendix I); D values are therefore used to normalize the range of activities in order to provide a reference for comparing risks3 . The A/D values for a range of commonly used sources are given in Appendix I4 . The A/D values are used to provide an initial ranking of relative risk for sources, which are then categorized after consideration of other factors such as the physical and chemical forms, the type of shielding or containment employed, the circumstances of use and accident case histories. This consideration of other factors is necessarily subjective and is based largely on international consensus judgements, as are the boundaries between the categories.
Main 2.3. The categorization system set out in this Safety Guide has five categories. This number is considered sufficient to enable the practical application of the scheme, without unwarranted precision. Within this categorization system, sources in Category 1 are considered to be the most ‘dangerous’ because they can pose a very high risk to human health if not managed safely and securely. An exposure of only a few minutes to an unshielded Category 1 source may be fatal. At the lower end of the categorization system, sources in Category 5 are the least dangerous; however, even these sources could give rise to doses in excess of the dose limits if not properly controlled, and therefore need to be kept under appropriate regulatory control. Categories should not be subdivided as this would imply a degree of precision that is not warranted and would lead to a loss of international harmonization. A plain language description of the categories is given in Appendix II.
Main 2.4. The categorization method outlined here and described in more detail in Annex I has been used to assign sources used in common practices to one of five categories, as shown in Appendix I. Examples of commonly used sources are shown in Table 1.
Main a Factors other than A/D alone have been taken into consideration in assigning the sources to a category (see Annex I).
Main 3.1. The Safety Requirements publication No. GS-R-1 [2] and the BSS [1] place obligations on the parties involved in the use of radiation sources to ensure their safety and security. In particular, GS-R-1 requires that the regulatory body “shall define policies, safety principles and associated criteria as a basis for its regulatory actions” (para. 3.1). It also specifies that legislation “shall establish authorization and other processes (such as notification and exemption), with account taken of the potential magnitude and nature of the hazard associated with the facility or activity…” (para. 2.4 (3)); and that the extent of the control applied by the regulatory body “shall be commensurate with the potential magnitude and nature of the hazard presented” (para. 5.3). Similarly, the BSS state that: “The application of the requirements of the Standards to any practice or any source within a practice … shall be commensurate with the characteristics of the practice or source and with the magnitude and likelihood of the exposures.” (para. 2.8).
Main 3.2. The regulatory body should use the categorization system described in this Safety Guide to provide a consistent basis for implementing these requirements in different areas, including the following:

  • Regulatory measures: To provide one of the factors to be taken into account in developing a graded system for notification, registration, licensing and inspections [1, 2, 26, 27]. The categorization system also assists in ensuring that the allocation of human and financial resources to protection measures is commensurate with the degree of risk associated with the source.

  • Security measures: To provide a graded basis for assisting in the choice of security measures, while it is recognized that other factors are also important [20] (see also Ref. [28]);

  • National register of sources: To optimize decisions regarding which sources should be included and what level of detail should be used in a national register of sources, as recommended in the Code of Conduct [19] (see para. 3.7 below);

  • Import/export controls: To optimize decisions regarding which sources should be subject to import and export controls, in meeting national obligations relating to the Code of Conduct [19] and guidance on import/ export controls for Category 1 and 2 sources [29] (see para. 3.9);

  • Labelling of high activity sources: To guide decisions regarding which sources should be marked with an appropriate label (in addition to the radiation trefoil) warning of the radiation hazard, as recommended in the Code of Conduct [19];

  • Emergency preparedness and response: To ensure that emergency preparedness plans and response to accidents are commensurate with the hazards posed by the source [25];

  • Prioritization for regaining control over orphan sources: To inform decisions relating to how efforts should be focused to regain control over orphan sources [27];

  • Communication with the public: To provide a basis for explaining the relative hazards associated with events involving radioactive sources (see also Ref. [30]).
Main 3.3. The regulatory body should use the data in Table 1 and Appendix I to categorize sources. If a particular type of source is not listed in Table 1 or Appendix I, the activity ratio A/D should be calculated and compared with those given in Appendix I for similar types of source. This should be done by taking the activity A of the source (in TBq) and dividing it by the D value for the relevant radionuclide given in Annex II. The resulting ratio A/D should then be compared with the values tabulated in the right hand column of Table 1. In some situations it may be appropriate to categorize a source on the basis of A/D alone — for example, when the practice for which the source may be used is unknown or not confirmed, as may happen at the time of import or export of the source. However, when the circumstances of use of the source are known, the regulatory body may make a judgement to modify this initial categorization using other information about the source or its use. In some circumstances it may be convenient to assign a category on the basis of the practice in which the source is used.
Short half-life radionuclides and unsealed sources 3.4. In some practices, such as nuclear medicine, radionuclides with a short half-life are used in a source form that is unsealed. Examples of such applications include 99mTc in radiodiagnosis and 131I in radiotherapy. In such situations, the principles of the categorization system may be applied to determine a category for the source, but a judgement should be made in choosing the activity on the basis of which to calculate the ratio A/D. These situations should be considered on a case by case basis.
Aggregation of sources 3.5. There will be situations in which radioactive sources are in close proximity to each other, such as in manufacturing processes (e.g. in the same room or building) or in storage facilities (e.g. in the same enclosure). In such circumstances, the regulatory body may wish to aggregate the activity in the sources to determine a situation specific categorization for the purposes of implementing regulatory control measures. In such situations, the summed activity of the radionuclide should be divided by the appropriate D value and the calculated ratio A/D compared with the ratios A/D given in Table 1, thus allowing the set of sources to be categorized on the basis of activity. If sources with various radionuclides are aggregated, then the sum of the ratios A/D should be used in determining the category, in accordance with the formula:
Aggregation of sources AggregateA/D=∑n∑iAi,nDn
Aggregation of sources where
Aggregation of sources 3.6. In each case it should be recognized that other factors may need to be taken into consideration in assigning a category. For example, the aggregation of sources during their manufacture and their aggregation in use may have different safety implications.
Aggregation of sources 3.7. As recommended in the Safety Guide on Regulatory Control of Radiation Sources [26], the regulatory body should maintain a national register of radioactive sources. Furthermore, it is stated in para. 11 of the Code of Conduct [19] that the register “should, as a minimum, include Category 1 and 2 radioactive sources” and that “[f]or the purpose of introducing efficiency in the exchange of radioactive source information between States, States should endeavour to harmonize the formats of their registers.”
Aggregation of sources 3.8. In view of the fact that Category 3 sources have the potential to cause severe deterministic effects, the regulatory body may also consider including them in a national register together with the Category 1 and 2 sources. Although sources in Categories 4 and 5 are unlikely to be dangerous to a person, such sources could give rise to detrimental consequences if misused, for example, through undue exposure of persons or contamination of the local environment. National authorities should, therefore, determine whether there is a need to include sources of Categories 4 and 5 in a national register.
Aggregation of sources 3.9. The Code of Conduct [19] provides guidance on the import and export of radioactive sources in Categories 1 and 2. Paragraphs 23–25 of the Code of Conduct recommend that:
Aggregation of sources “23. Every State involved in the import or export of radioactive sources should take appropriate steps to ensure that transfers are undertaken in a manner consistent with the provisions of the Code and that transfers of radioactive sources in Categories 1 and 2 of Annex 1 of this Code take place only with the prior notification by the exporting State and, as appropriate, consent by the importing State in accordance with their respective laws and regulations.
Aggregation of sources Paragraphs 26–29 of the Code of Conduct provide further guidance on import/export — including an ‘exceptional circumstances’ clause for cases where paras 24–25 reproduced above cannot be satisfied.
Aggregation of sources 3.10. Detailed advice for States that import and/or export radioactive sources can be found in the IAEA ‘Guidance for the Import and Export of Radioactive Sources’ [29].
Aggregation of sources I.1. Table 2 gives examples of sources that have been or were in 2004 in common use (column I). The list is not exhaustive — there may be sources with a higher or lower activity than those described, and this may also change over time with technological developments. Column II identifies the radionuclide(s) typically used. Columns III–V give examples of maximum, minimum and typical activity. The D values are given in column VI and activity ratios A/D in column VII. The categorization is shown in columns VIII and IX. Column VIII shows the initial categorization based on A/D and column IX shows the recommended categorization with account taken of additional factors generally known to be associated with particular types of source. The regulatory body may modify this categorization on the basis of specific knowledge of relevant factors, such as method of construction, physical or chemical form of the source, use in remote or harsh environments, history of accidents and portability. Note that Table 2 lists only single sources; where sources are aggregated, the recommendations in para. 3.5 should be followed.
Aggregation of sources a Calibration sources are found in all categories except Category 1. They have been assigned to Table 2 in accordance with the radionuclide and activity. The regulatory body may modify the assignment on the basis of specific factors and circumstances.
Aggregation of sources (The following plain language description of the categorization of sources has been developed for purposes of public information.)
Aggregation of sources II.1. Radioactive sources are used throughout the world for a wide variety of beneficial purposes in industry, medicine, agriculture, research and education. When such sources are safely managed and securely protected, the risks to workers and the public are kept acceptably low and the benefits will outweigh any associated hazards.
Aggregation of sources II.2. If a radioactive source were to become separated from the system of control or the radioactive material from the source were to become dispersed as a result of an accident or a malevolent act, people could be exposed to radiation at dangerous levels. For the purposes of this Safety Guide, and in accordance with the Safety Requirements for emergency preparedness and response [25] and the Code of Conduct [19], a radioactive source is considered dangerous if its misuse could be life threatening or could cause a permanent injury that would reduce the quality of life of the person exposed. Possible permanent injuries include burns requiring surgery and debilitating injuries to the hands. Temporary injuries such as reddening and irritation of the skin or temporary changes to the composition of the blood are not considered dangerous. The extent of any such injuries will depend on many factors, including: the activity of the radioactive source; how close a person is to the source and for how long; whether the source is shielded; and whether or not its radioactive material has been dispersed, thus leading to contamination of the skin or inhalation or ingestion. For the purposes of categorization, any possible harm from delayed effects of radiation — such as radiation induced cancer developing later in life in any persons who are exposed — is treated as a secondary consideration to the paramount need to protect against the dangerous consequences described above.
Aggregation of sources II.3. The categorization shown in Table 3 provides a ranking of radioactive sources in terms of their potential to cause early harmful health effects if the source is not safely managed or securely protected. Sources are classified into five categories: Category 1 sources are potentially the most dangerous and Category 5 sources are the most unlikely to be dangerous. In Table 3, two types of risk are considered: the risk in handling or being close to a source, and the risk associated with radioactive material being dispersed from a source by fire or explosion. A third type of risk arises from the potential for a source tocontaminate a public water supply. It would be highly unlikely for a Category 1 source to contaminate a public water supply to dangerous levels, even if the radioactive material were highly soluble in water. It would be virtually impossible for a source in Category 2, 3, 4 or 5 to contaminate a public water supply to dangerous levels.
Aggregation of sources a The size of the area to be cleaned up would depend on many factors (including the activity, the radionuclide, how it was dispersed and the weather).

  • FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS, INTERNATIONAL ATOMIC ENERGY AGENCY, INTERNATIONAL LABOUR ORGANISATION, OECD NUCLEAR ENERGY AGENCY, PAN AMERICAN HEALTH ORGANIZATION, WORLD HEALTH ORGANIZATION, International Basic Safety Standards for Protection against Ionizing Radiation and for the Safety of Radiation Sources, Safety Series No. 115, IAEA, Vienna (1996).

  • INTERNATIONAL ATOMIC ENERGY AGENCY, Legal and Governmental Infrastructure for Nuclear, Radiation, Radioactive Waste and Transport Safety, IAEA Safety Standards Series No. GS-R-1, IAEA, Vienna (2000).

  • INTERNATIONAL ATOMIC ENERGY AGENCY, The Radiological Accident in Goiânia, IAEA, Vienna (1988).

  • INTERNATIONAL ATOMIC ENERGY AGENCY, The Radiological Accident in San Salvador, IAEA, Vienna (1990).

  • INTERNATIONAL ATOMIC ENERGY AGENCY, The Radiological Accident in Soreq, IAEA, Vienna (1993).

  • INTERNATIONAL ATOMIC ENERGY AGENCY, The Radiological Accident at the Irradiation Facility in Nesvizh, IAEA, Vienna (1996).

  • INTERNATIONAL ATOMIC ENERGY AGENCY, The Radiological Accident in Tammiku, IAEA, Vienna (1998).

  • INTERNATIONAL ATOMIC ENERGY AGENCY, The Radiological Accident in Istanbul, IAEA, Vienna (2000).

  • INTERNATIONAL ATOMIC ENERGY AGENCY, The Radiological Accident in Lilo, IAEA, Vienna (2000).

  • INTERNATIONAL ATOMIC ENERGY AGENCY, The Radiological Accident in Yanango, IAEA, Vienna (2000).

  • INTERNATIONAL ATOMIC ENERGY AGENCY, The Radiological Accident in Gilan, IAEA, Vienna (2002).

  • INTERNATIONAL ATOMIC ENERGY AGENCY, The Radiological Accident in Samut Prakarn, IAEA, Vienna (2002).

  • INTERNATIONAL ATOMIC ENERGY AGENCY, Accidental Overexposure of Radiotherapy Patients in San José, Costa Rica, IAEA, Vienna (1998).

  • INTERNATIONAL ATOMIC ENERGY AGENCY, Investigation of the Accidental Exposure of Radiotherapy Patients in Panama, IAEA, Vienna (2001).

  • INTERNATIONAL ATOMIC ENERGY AGENCY, Lessons Learned from Accidental Exposures in Radiotherapy, Safety Reports Series No. 17, IAEA, Vienna (2000).

  • INTERNATIONAL ATOMIC ENERGY AGENCY, Lessons Learned from Accidents in Industrial Radiography, Safety Reports Series No. 7, IAEA, Vienna (1998).

  • Safety of Radiation Sources and Security of Radioactive Materials (Proc. Int. Conf. Dijon, 1998), IAEA, Vienna (1999).

  • INTERNATIONAL ATOMIC ENERGY AGENCY, Categorization of Radioactive Sources, IAEA-TECDOC-1344, IAEA, Vienna (2003).

  • INTERNATIONAL ATOMIC ENERGY AGENCY, Code of Conduct on the Safety and Security of Radioactive Sources, IAEA/CODEOC/2004, IAEA, Vienna (2004).

  • INTERNATIONAL ATOMIC ENERGY AGENCY, Security of Radioactive Sources — Interim Guidance for Comment, IAEA-TECDOC-1355, IAEA, Vienna (2003).

  • INTERNATIONAL ATOMIC ENERGY AGENCY, Convention on the Physical Protection of Nuclear Material, Legal Series No. 12, IAEA, Vienna (1982).

  • INTERNATIONAL ATOMIC ENERGY AGENCY, Management of Spent High Activity Radioactive Sources (SHARS), IAEA-TECDOC-1301, IAEA, Vienna (2002).

  • INTERNATIONAL ATOMIC ENERGY AGENCY, Regulations for the Safe Transport of Radioactive Material, 1996 Edition (As Amended 2003), IAEA Safety Standards Series No. TS-R-1, IAEA, Vienna (2003).

  • INTERNATIONAL ATOMIC ENERGY AGENCY, Method for Developing Arrangements for Response to a Nuclear or Radiological Emergency: Updating IAEA-TECDOC-953, EPR-Method 2003, IAEA, Vienna (2003).

  • FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS, INTERNATIONAL ATOMIC ENERGY AGENCY, INTERNATIONAL LABOUR ORGANIZATION, OECD NUCLEAR ENERGY AGENCY, PAN AMERICAN HEALTH ORGANIZATION, UNITED NATIONS OFFICE FOR THE CO-ORDINATION OF HUMANITARIAN AFFAIRS, WORLD HEALTH ORGANIZATION, Preparedness and Response for a Nuclear or Radiological Emergency, IAEA Safety Standards Series No. GS-R-2, IAEA, Vienna (2002).

  • INTERNATIONAL ATOMIC ENERGY AGENCY, Regulatory Control of Radiation Sources, IAEA Safety Standards Series No. GS-G-1.5, IAEA, Vienna (2004).

  • INTERNATIONAL ATOMIC ENERGY AGENCY, Strengthening Control over Radioactive Sources in Authorized Use and Regaining Control over Orphan Sources: National Strategies, IAEA-TECDOC-1388, IAEA, Vienna (2004).

  • INTERNATIONAL ATOMIC ENERGY AGENCY, Prevention of the Inadvertent Movement and Illicit Trafficking of Radioactive Materials, IAEA-TECDOC-1311, IAEA, Vienna (2002).

  • INTERNATIONAL ATOMIC ENERGY AGENCY, Code of Conduct on the Safety and Security of Radioactive Sources: Guidance on the Import and Export of Radioactive Sources, IAEA/CODEOC/IMP-EXP/2005, IAEA, Vienna (2005).

  • INTERNATIONAL ATOMIC ENERGY AGENCY, The International Nuclear Event Scale (INES) User’s Manual, 2001 Edition, IAEA, Vienna (2001).
Aggregation of sources I-1. When radioactive sources are managed in accordance with safety standards in a safe and secure manner, the radiation risks to workers and the public are kept acceptably low. However, if they are not managed appropriately, as in the case of accidents, malicious use or orphan sources, high activity sources may cause a range of deterministic health effects, including erythema, tissue burns, acute radiation sickness and death.
Aggregation of sources I-2. In recognition of the fact that human health is of paramount importance, therefore, the categorization system is based on the potential for radioactive sources to cause deterministic health effects. This potential is due partly to the physical properties of the source, especially its activity, and partly to the way in which the source is used. The practice in which the source is used, the provision of any inherent shielding provided by the device containing the source, the level of supervision and other factors need to be taken into consideration, as described in paras I-13–I-14.
Aggregation of sources I-3. Certain factors are specifically excluded from the categorization criteria:

  • Socioeconomic consequences resulting from radiological accidents or malicious acts are excluded, as no methodology for quantifying and comparing these effects, especially on an international basis, has yet been developed;

  • The intentional exposure of patients for medical reasons is excluded from the categorization criteria, although the radioactive sources used for these purposes are included in the categorization system as there have been accidents involving such sources [I-1–I-3].
Aggregation of sources The stochastic effects of radiation exposure (e.g. an increased risk of cancer) are not quantified in the derivation of the D values. However, given that the risk of stochastic effects increases with exposure, higher category sources will, in general, present a higher risk of stochastic effects. Furthermore, the deterministic effects resulting from an accident or malicious act are likely in the short term to overshadow any increased risk of stochastic effects. The deliberate ingestion of radioactive material by individuals was not taken into consideration.
Collection of data I-4. Data on the radionuclides and activities used for various sources and practices [I-4, I-5, I-6] are given in Appendix I. For each practice (e.g. industrial radiography) and each radionuclide used within the practice, three activity levels are given: the maximum, minimum and typical levels (there may be exceptions that have not been included). These data are given in Columns I–V of Table 2 in Appendix I.
Normalizing factor I-5. In order to rank sources and practices numerically on a common basis, each source activity is divided by a normalizing factor — the D value, as described below. Initial consideration was given to using the A1 and A2 values from the IAEA Transport Regulations [I-7] as normalizing factors. However, although the A1 and A2 values are well established and could be used as a means of comparing risks arising from radionuclides in transport, other factors limit their application for other uses. Since A1 and A2 were derived for transport related purposes [I-8], whereas the categorization system is needed for general application, it was considered inappropriate to use A1 and A2 values as the normalizing factors.
Normalizing factor I-6. The IAEA has developed a listing of radionuclide specific activity levels for the purposes of emergency planning and response [I-9]. These levels, referred to in this Safety Guide as D values, are given in terms of an activity above which a radioactive source is considered to be a ‘dangerous source’ as it has a significant potential for causing severe deterministic effects if it is not managed safely and securely. Since the categorization is also based on the potential for sources to cause deterministic health effects, the D values were considered to be compatible normalizing factors for the purpose of generating a numerical ranking of sources and practices. A comprehensive listing of radionuclide specific D values for both external (D1 values) and internal (D2 values) exposure is given in Ref. [I-9]. For the purposes of developing the categorization, the more restrictive of the D1 and D2 values was used as the radionuclide specific normalizing factor. The D values, for the radionuclides listed in Appendix I, are given in Table II-2 of Annex II. (Note that because Table II-2 shows only the more restrictive value of D1 and D2, it cannot be used in reverse to derive doses that may arise from sources with a known activity.)
Ranking of sources I-7. For each source, its activity in TBq (column V, Table 2, Appendix I) was divided by the corresponding radionuclide specific D value in TBq (column VI) to give a dimensionless normalized ratio of A/D (column VII).
Number of categories I-8. In order to fulfil the various needs of the categorization system, the relative ranking of sources needs to be divided into a number of discrete categories. The optimum number of categories and the A/D bounding values between them require a certain degree of judgement based on professional expertise. The following factors were taken into consideration:

  • Setting too few categories might lead to categories being split at a later date to meet national or other needs. This could lead to a loss of transparency of the categorization system and a loss of international harmonization, resulting in the potential for inconsistent approaches to similar issues.

  • Setting too many categories might imply a degree of precision that is not warranted and would be difficult to justify. Having too many categories might, furthermore, lead to difficulties in the application of the categorization system and discourage its use.
Category boundaries based on radionuclide and activity I-9. Sources with an activity greater than D have the potential to cause severe deterministic effects. The activity ratio A/D = 1 was, therefore, considered to be a logical category bounding value, resulting in two categories. However, in order for the categorization system to accommodate the many and various uses, there clearly needed to be more than two categories.
Category boundaries based on radionuclide and activity I-10. In the development of the D values, it was recognized that a source activity ten times greater than D could give rise to a life threatening exposure in a relatively short period of time [I-10]. Hence a category bounding values was set at A/D = 10. This, however, left some very high activity sources (e.g.RTGs) in the same category as sources with significantly lower activity (e.g. high dose rate (HDR) brachytherapy sources). It was therefore decided to use operational experience, professional judgement and lessons learned from radiological accidents to separate these practices, which resulted in a further bounding value at A/D = 1000.
Category boundaries based on radionuclide and activity I-11. As there was a wide range of practices and sources with activity below A/D = 1, a further category bounding value was needed. Operational experience, professional judgement and lessons learned from radiological accidents were again used to draw a bounding value at A/D = 0.01, with a lower cut-off for this category set at the activity of a radionuclide that is considered ‘exempt’ from regulatory control. Radionuclide specific exemption levels are given in Schedule I of the BSS [I-11].
Category boundaries based on radionuclide and activity I-12. Taking all the above factors into consideration resulted in a five category system, as shown in Appendix I. The assignment of sources to categories was then refined by considering, where appropriate, factors other than their activity A.
Refinement of the categorization system I-13. Experience and judgement were used to review the categorization of each practice or source. The results indicated that, although the A/D ratios provided a robust and logical basis for the categorization, other risk factors may also be relevant. The categorization of each type of source and the practice in which it is used was reviewed, therefore, with consideration given to factors such as the nature of the work, the mobility of the source, experience from reported accidents, and typical and unique activities within an application. For example, some low activity RTGs could fall into Category 2 if the activity alone were considered. However, all RTGs were assigned to Category 1, since they are likely to be put to use in remote locations, unsupervised, and they may contain large amounts of plutonium or strontium. Likewise, although some 169Yb sources used for industrial radiography fall into Category 3 by virtue of their activity alone, the practice was categorized as Category 2 in recognition of the relatively large number of accidental radiation exposures that have occurred with industrial radiography sources. The final categorization for some of the most common applications is given in Table 1, Section 2, and a comparison of categories based solely on A/D with those to which practices were finally assigned is shown in columns VIII and IX of Table 2 of Appendix I.
Refinement of the categorization system I-14. It was considered undesirable for a particular practice to span two categories, where it was practicable to avoid it. However, in some cases it was necessary to divide a generic practice in this way because of the wide range of activities within it (e.g. brachytherapy was divided into HDR, low dose rate (LDR) and permanent implants). In other cases, such as for calibration sources, it was not possible to assign sources to a single category since their activity may range from a low activity to over 100 TBq. In such situations, the categorization can be considered by national authorities case by case by calculating the A/D ratio and then considering other factors as appropriate.
Refinement of the categorization system REFERENCES TO ANNEX I
Refinement of the categorization system [I-1] INTERNATIONAL ATOMIC ENERGY AGENCY, Accidental Overexposure of Radiotherapy Patients in San José, Costa Rica, IAEA, Vienna (1998).
Refinement of the categorization system II-1. This annex explains the concept of a ‘dangerous source’ and the origins of the D value that was used in the development of the categorization system. It is a brief summary only, and Refs [II-1, II-2] should be consulted for a more detailed explanation.
Refinement of the categorization system II-2. A dangerous source is defined as a source that could, if not under control, give rise to exposure sufficient to cause severe deterministic effects. A deterministic effect is defined as a health effect of radiation for which generally a threshold level of dose exists above which the severity of the effect is greater for a higher dose. Such an effect is described as a severe deterministic effect if it is fatal or life threatening or results in a permanent injury that reduces the quality of life.
Refinement of the categorization system II-3. The concept of a dangerous source has been converted into operational parameters by calculating the quantity of radioactive material that could give rise to severe deterministic effects for given exposure scenarios and for given dose criteria [II-1]. In addition to typical accident situations, these scenarios include dispersion situations that may be relevant to malevolent acts. The following exposure scenarios (and pathways) were considered:

  • An unshielded source being carried in the hand for one hour or in a pocket for 10 hours, or being in a room for days to weeks (the D1 value);

  • Dispersal of a source, for example by fire, explosion or human action, resulting in exposure due to inhalation, ingestion and/or skin contamination (the D2 value).
Refinement of the categorization system II-4. The derivation of the dangerous source values relates to the following dose criteria (see Table II-1):

  • A dose of 1 Gy to the bone marrow or 6 Gy to the lung from low linear energy transfer (low LET) radiation, received by the organ in 2 d. These are the dose levels from Table IV-I of Schedule IV of the BSS at which intervention is always justified to prevent early deaths [II-3–II-5]. It should be noted that these are limiting criteria associated with the lowest dose rates that are considered to be life threatening [II-1].

  • A dose of 25 Gy to the lung from inhalation exposure to high LET radiation in 1 a. This is the dose level at which fatalities are likely to be induced within 1.5 a owing to radiation pneumonitis and pulmonary fibrosis [II-6].

  • A dose of 5 Gy to the thyroid received by the organ in 2 d. This is the dose level from Table IV-I of Schedule IV of the BSS at which intervention is always justified to prevent hypothyroidism. Hypothyroidism is assumed to reduce the quality of life.

  • For a source in contact with tissue, a dose of more than 25 Gy at a depth of: (a) 2 cm for most parts of the body (e.g. from a source in a pocket) or (b) 1 cm for the hand. A dose of 25 Gy is the threshold for necrosis (death of tissue) [II-5, II-7]. Experience [II-8] indicates that tissue death in many parts of the body (e.g. in the thigh) from carrying a source in a pocket can be successfully treated without resulting in a reduction in the quality of life provided that the absorbed dose to tissue within about 2 cm of a source is kept below 25 Gy. However, for a source carried in the hand, the absorbed dose to tissue within about 1 cm must be kept below 25 Gy in order to prevent an injury that would reduce the quality of life.

  • For a source that is considered too big to be carried, a dose of 1 Gy to the bone marrow in 100 h from a source at a distance of 1 m.
Refinement of the categorization system a Since Table II-2 does not show which dose criteria were used, these D values should not be used in reverse to derive possible doses due to sources of known activity.
Refinement of the categorization system REFERENCES TO ANNEX II
Refinement of the categorization system [II-1] INTERNATIONAL ATOMIC ENERGY AGENCY, Method for Developing Arrangements for Response to a Nuclear or Radiological Emergency: Updating IAEA-TECDOC-953, EPR-Method 2003, IAEA, Vienna (2003).
Refinement of the categorization system accident.
Refinement of the categorization system Any unintended event, including operating errors, equipment failures, or other mishaps, the consequences or potential consequences of which are not negligible from the point of view of protection or safety.
Refinement of the categorization system dangerous source.
Refinement of the categorization system A source that could, if not under control, give rise to exposure sufficient to cause severe deterministic effects. This categorization is used for determining the need for emergency response arrangements and is not to be confused with categorizations of sources for other purposes.
Refinement of the categorization system deterministic effect.
Refinement of the categorization system A health effect of radiation for which generally a threshold level of dose exists above which the severity of the effect is greater for a higher dose. Such an effect is described as a ‘severe deterministic effect’ if it is fatal or life threatening or results in a permanent injury that reduces quality of life.
Refinement of the categorization system licence.
Refinement of the categorization system A legal document issued by the regulatory body granting authorization to perform specified activities related to a facility or activity.
Refinement of the categorization system notification.
Refinement of the categorization system A document submitted to the regulatory body by a legal person to notify an intention to carry out a practice or other use of a source.
Refinement of the categorization system orphan source.
Refinement of the categorization system A radioactive source which is not under regulatory control, either because it has never been under regulatory control, or because it has been abandoned, lost, misplaced, stolen or otherwise transferred without proper authorization.
Refinement of the categorization system practice.
Refinement of the categorization system Any human activity that introduces additional sources of exposure or exposure pathways or extends exposure to additional people or modifies the network of exposure pathways from existing sources, so as to increase the exposure or the likelihood of exposure of people or the number of people exposed.
Refinement of the categorization system registration.
Refinement of the categorization system A form of authorization for practices of low or moderate risks whereby the legal person responsible for the practice has, as appropriate, prepared and submitted a safety assessment of the facilities and equipment to the regulatory body. The practice or use is authorized with conditions or limitations as appropriate. The requirements for safety assessment and the conditions or limitations applied to the practice should be less severe than those for licensing.
Refinement of the categorization system regulatory body.
Refinement of the categorization system An authority or a system of authorities designated by the government of a State as having legal authority for conducting the regulatory process, including issuing authorizations, and thereby regulating nuclear, radiation, radioactive waste and transport safety.
Refinement of the categorization system sealed source.
Refinement of the categorization system Radioactive material that is (a) permanently sealed in a capsule, or (b) closely bonded and in a solid form.
Refinement of the categorization system security of radioactive sources.
Refinement of the categorization system Measures to prevent unauthorized access or damage to, and loss, theft or unauthorized transfer of, radioactive sources.