Categorization, or classification, is an approach used, mainly when the quantity of elements considered (objects or ideas) is large, to facilitate management of the elements by reducing their number. Categorization is realized by selecting the main features (criteria) and by structuring these criteria. SRSs are used for a variety of purposes and they incorporate a wide range of radionuclides and amounts of radioactive material. In this particular case, the categorization is based mainly on the features of the radiation and the risk represented by the source for human beings and the environment. Some risks associated with radioactive sources are described briefly in the Annex.
3.1. RATIONALE FOR RADIOACTIVE SOURCE CATEGORIZATION
Categorization or classification of radioactive sources may be helpful at any stage of the life cycle of a radioactive source, from its initial production and usage phase to final disposal. A source categorization system is also needed to provide a risk informed ranking and grouping of the sources and their applications, which in turn is of direct relevance to the following issues:
— Regulatory measures. To provide a logical and transparent basis for a risk informed system of notification, registration, licensing and inspections. The categorization also provides a basis for ensuring that the allocation of human and financial resources is commensurate with the risks associated with the use of the source.
— Safety measures. To provide a risk informed basis for the determination of safety measures to be applied during the life cycle of the sealed source (from in-use status until disposal as disused). To assess potential hazards associated with the various types of sources.
— Management options. To assist in planning and designing the conditioning and storage facilities and making a decision on a possible disposal route for a source.
— Security measures. To provide a risk informed basis for assisting in the determination of security measures, recognizing that other factors are important (e.g. threats against specific facilities/sources).
— National registry of sources. To optimize decisions on the categories of sources and the level of detail to be included in national reporting.
— Import/export controls. To optimize decisions regarding sources that may be subject to import and export controls.
— Labelling of sources. To optimize decisions regarding sources that may be marked with an appropriate label (in addition to the trefoil) to warn persons of the radiation hazard.
— Emergency preparedness and response. To ensure that emergency preparedness plans and response to accidents are commensurate with the category of the source.
— Prioritization for regaining control over orphan sources. To optimize decisions relating to where efforts need to be focused to regain control over orphan sources.
— Communication with the public. To provide a basis for explaining the relative hazard of events involving radioactive sources.
The purpose of categorizing radioactive sources is to provide a fundamental and internationally harmonized basis for risk informed decision making. Various source categorization systems have been developed by international organizations, but these systems have been developed for different decision making purposes. The three main international categorization systems developed for informing management strategies for disused radioactive sources are described below. Additionally, for those Member States considering disposal options, the IAEA waste classification system is described in Section 12.
Member States that have no source classification (categorization) system can develop their own system based on relevant international guidance or can adopt existing international source categorization systems to meet specific national requirements.
14
3.2. THE IAEA SOURCE CATEGORIZATION SYSTEM
Recognizing the need for a risk based approach to the regulatory control of radioactive sources, the IAEA provides a categorization scheme for radioactive sources and practices in the BSS [11] and in Safety Guide RS-G.1.9 [23], based on IAEA-TECDOC-1344 [24] .
Table 2 in Appendix I of Ref [23] 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, considering both the physical properties and the specific application of the source. 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 of Ref. [24]). For each practice and radionuclide used in the practice, the source activity in TBq is divided by the corresponding radionuclide specific
‘D’ value in TBq, resulting in the dimensionless normalized ratio of A/D.
The final categorization of specific source applications is given in Table 4 (Table 1 in Ref. [23], and a comparison of categories, based solely on the A/D ratio, to those assigned to practices is presented in Appendix I in Ref. [23]. Category 1 sources demonstrate the highest risk in managing sealed sources, while Category 5 refers to the lowest risk.
TABLE 4. RECOMMENDED CATEGORIES FOR SOURCES USED IN COMMON PRACTICES [23]
Category Categorization of common practicesa Activity ratiob
(A/D)
3 Fixed industrial gauges that incorporate high activity sources
Well logging gauges 10>A/D≥1
4 Low dose rate brachytherapy sources (except eye plaques and permanent implants) Industrial gauges that do not incorporate high activity sources
Bone densitometers Static eliminators
1>A/D≥0.01
5 LDR brachytherapy eye plaques and permanent implant sources x ray fluorescence devices
Electron capture devices Mossbauer spectrometry
Positron emission tomography (PET) check sources
0.01>A/D≥Exemptc/D
a Recognizing that factors other than A/D have been taken into consideration.
b This column can be used to determine the category of a source, based purely on A/D. This may be appropriate if, for example: the practice is not known or is not listed; sources have a short half-life and/or are unsealed; or sources are aggregated.
c Exempt quantities are given in Schedule I of the BSS [11].
If a practice involves the aggregation of sources into a single storage or use location where sources are in close proximity, such as in storage facilities, manufacturing processes, or transport conveyance, the total activity can be integrated and treated as one source for the purposes of assigning a category. Therefore, the integrated, total activity of the radionuclide can be divided by the appropriate D value, and the calculated A/D ratio compared to the
2 D values were originally derived in the context of emergency preparedness [38] to establish a reference point corresponding to a ‘dangerous source’ [37] on a scale of the risks that could arise from uncontrolled sources.
A/D ratios given in the right hand column of Table 4, thus allowing a category, based on activity, to be allocated to the practice. If sources with several radionuclides are aggregated, then the sum of the A/D ratios can be used to determine the category in accordance with the formula:
Aggregate A/D = ∑n = ∑i Ai,n/Dn where
Ai,n is the activity of each individual source i of radionuclide n;
Dn is the D value for radionuclide n.
In each case, it should be recognized that other factors might need to be taken into consideration in assigning a category. Furthermore, when considering the accumulation of sources, it is important to recognize that the practice may change, e.g. the ‘manufacture’ of a level gauge is a different practice from the ‘use’ of such a gauge.
3.3. THE IAEA SECURITY GROUPING OF SOURCES
Unsecured sources have resulted in deaths and serious injuries in many parts of the world. The IAEA has published a number of reports that describe the human health consequences, as well as the economic losses of incidents and accidents, due to uncontrolled sources.
Prior to 11 September 2001, the security of radioactive sources was addressed largely by measures protecting the sources from access by inadequately trained personnel or from attempts at theft for financial gain. This assumption has now been modified to also include the need to prevent access to certain sources by individuals deliberately and malevolently seeking to cause radiation exposure or dispersal of radioactive materials.
Ensuring the security of sources requires that measures be applied to prevent unauthorized access to radioactive sources at all stages of their life cycle, as well as loss, theft, and unauthorized transfer of sources.
To ensure the safety of radioactive sources, controlling exposure to radiation from the sources is required, both directly and as a consequence of incidents, so that the likelihood of harm attributable to such exposure is very low.
Safety and security aspects of sources are intimately linked and many of the measures to address one issue will also address the other.
The IAEA published in 2009 an Implementing Guide on the Security of Radioactive Sources [25], which takes into account the overall security approach established in that publication which some States may have used as a reference in devising their current security regimes. This Guide proposes a graded approach to security using a set of security levels, and the security functions of deterrence, detection, delay, response and security management.
The publication has been harmonized with the IAEA categories of radioactive sources [23], the requirements of the BSS [11] and the requirements of the Code of Conduct [14]. Three security levels (A, B and C) have been developed to allow specification of security system performance in a graded manner. Security level A requires the highest degree of security while the other levels are progressively lower.
Each security level has a corresponding goal. The goal defines the overall result that the security system should be capable of providing for a given security level. The following goals have been developed:
— Security level A: Prevent unauthorized removal of a source.
— Security level B: Minimize the likelihood of unauthorized removal of a source.
— Security level C: Reduce the likelihood of unauthorized removal of a source.
Malicious acts can involve either unauthorized removal of a source or sabotage. While the security goals only address unauthorized removal, achievement of the goals will reduce the likelihood of a successful act of sabotage.
Security systems that achieve the goals listed above will provide some (although limited) capability to detect and respond to an act of sabotage.
16
A proper security level should be assigned to each category of radioactive sources specified in Section 3.2.3 Category 1 sources should have security measures which meet the security objectives of Security Level A. Category 2 sources should have security measures which meet the security objectives of Security Level B. Category 3 sources should have security measures which meet the security objectives of Security Level C.
The BSS [11] include general requirements for the security of radioactive sources. This guide considers that while those control measures provide a sufficient level of security for radioactive sources in Categories 4 and 5, enhanced measures specified in this guide should be applied to radioactive sources in Categories 1, 2 and 3 in order to reduce the likelihood of malicious acts involving those sources. Furthermore, the regulatory body, taking account of its national threat, may wish to enhance the security of sources in Categories 4 and 5 sources in appropriate circumstances. The malicious use of radioactive sources may not necessarily involve sources that are ranked highest in this categorization scheme. Most Category 1 sources, for example, will be held within shielding and inside fixed devices or facilities. Efforts to remove the source would take time and may expose the adversaries to a significantly harmful level of radiation. It is, therefore, possible that adversaries will focus on sources of a lower category, more accessible, less of a hazard to handle, portable, and more easily concealed. The assignment of a radioactive source to a security group is most effectively achieved by using the outcomes of the threat assessment.
This allows most flexibility and specificity to account for the variability in threat levels and security environments within Member States. It also permits different choices of security groups for sources in the various stages of their life cycle. Alternatively, some Member States may perform countrywide threat and vulnerability assessments and accordingly make allocations of sources to security groups, based on these assessments.
Consistent with the Code of Conduct [14], each of the categories includes the radioactive material released if any of the sources in the group is leaking or broken. The categorization methodology also allows for aggregation of sources in one location. One reason for a source being categorized in a higher security group could be that the specific threat assessment may reveal that some facilities with sources or some mobile sources are more vulnerable to acquisition, even though they may not be the highest activity sources.
3.4. THE ISO SOURCE CLASSIFICATION
The International Organization for Standardization’s ISO 2919 [26] establishes a system for the classification of sealed sources based on test performance. The standard specifies general requirements, performance tests, production tests, marking and certification. Prototypes of sources are tested for temperature, external pressure, impact, vibration and puncture in classes of increasing severity [22]. Sources intended for an application are required to meet minimum criteria outlined in the standard.
The ISO classification of sources, based on the type of radiation and application, is as follows:
— Gamma sources;
— Beta sources;
— Alpha sources;
— Neutron sources;
— Other sources for special use (e.g. gamma and neutron sources for borehole logging).
It should be noted that sources generally emit mixed radiation: Radioactive decay is accompanied by radiation, and radioactive decay daughter products are often beta emitters. Additionally, the conversion of the particle (electron) energy during absorption results in Bremsstrahlung.
3 Guidance provided here in the form of ‘should’ statements, or simply in the present tense indicative, describing good practices, represents expert opinion but does not constitute international consensus recommendations on how to meet the relevant requirements.