1.1.
BACKGROUND
The application of radioactive sources in medicine, research, industry, agricultural and consumer products is a worldwide phenomenon. Consequently, many countries now have sources that need to be managed and disposed of carefully and in a safe and secure manner. These sources contain different radionuclides in highly variable quantities. In some cases, the activity of a source decays to a level below which the source is no longer suitable for its original purpose, in others the associated equipment may become obsolete, worn out, or damaged, and in others the source may develop a leak and so is no longer used. In all these circumstances, the radioactive sources are referred to as ‘disused’
or ‘spent’[1], even though their activity could still be very high 1.
The majority of sources are small in physical size (e.g. Ra-226 needles), with the only items of significant size being some industrial radiography units and commercial irradiators. Despite their predominately small physical size, radioactive sources can contain very high activities, with typical levels in the MBq (106 Bq) to PBq (1015 Bq) range [2]. Therefore, if they are not managed properly, radioactive sources can represent a significant hazard to human health and the environment, which is evident from the number of accidents that have taken place worldwide as a result of the mismanagement of such sources [3]. Some sources can be returned to their manufacturers and recycled, but for many users of sources it is impractical or uneconomical to recycle all sources, and many sources end up being stored for long periods of time [4]. Storage in a secure facility can be considered as an adequate final management option for sources containing quantities of short lived radionuclides, which decay to harmless levels within a few years. However, for most other sources a suitable disposal option is required. The IAEA has developed and published requirements and guidance on radioactive waste disposal [5], [6], [7].
Many countries have existing or proposed near surface radioactive waste disposal facilities [7].
However, the specific activity of many disused sealed radioactive sources exceeds the waste acceptance criteria for such facilities since the source constitutes a high, localized concentration in the facility and could give rise to unacceptable radiation doses in the event of human intrusion or other causes of facility disruption. Safety cases for many near surface disposal facilities assume a period of institutional control (typically a few tens to hundreds of years) during which disruption of the waste is assumed to be unlikely. However, even within these, and particularly for longer timeframes, it is possible that such control will no longer be fully in place and thus disruption of the waste cannot be ruled out. Consequently, disused sources that will not decay to negligible levels within a few tens to hundreds of years need to be disposed of in facilities that will provide higher levels of isolation than provided by surface storage or near surface facilities.
Deep geological disposal [6] offers the highest level of isolation available within disposal concepts currently actively considered. Such facilities are under consideration for the disposal of spent nuclear fuel, high level waste and intermediate level waste in a number of countries. However, they are expensive to develop and only viable for countries with extensive nuclear power programmes. It is unlikely that such an option will become available in many countries since they have no nuclear power programme requiring such resources. In particular, some countries in Africa, Asia, South America, and the former Soviet Union have limited infrastructure or administrative capability to manage or dispose radioactive waste in their country. Therefore increasing attention has been given in recent years to the disposal of disused sources in narrow diameter (a few tens of centimetres) borehole facilities with a view to providing a safe disposal option for limited amounts of highly active radioactive waste including disused sources [4], [8].
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1 According to Ref. [1], subtle differences can be noted between the terms ‘spent’ and ‘disused’. A disused source differs from a spent source in that it may still be capable of performing its function, even though it is no longer used for that purpose. To be consistent, the broader ‘disused’ term is used in this document.
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1.2.
OBJECTIVE
The objective of this TECDOC is to document a GSA for this borehole disposal concept, with the purpose of identifying the concept’s key safety features, under varying disposal system conditions, in order to support the concept design and licensing processes, and facilitate its site specific implementation.
1.3.
SCOPE
The focus of the work described in this publication is the post-closure, radiological safety assessment of the disposal of disused sealed radioactive sources. The publication considers exposure of humans due to natural processes and human intrusion, but excludes intrusion that can be considered as deliberate (i.e. intrusion by a human when the intruder knows that the facility is a radioactive waste disposal facility). Consistent with Ref. [8], the impact of deliberate human intrusion is considered to be the responsibility of those intruding and is beyond the scope of the current assessment, as are malicious acts that might arise from deliberate human intrusion.
A variety of borehole designs have been used for the disposal of radioactive waste with differing depths (a few metres to several hundred metres) and diameters (a few tens of centimetres to several metres) (see Ref. [4] for details). The design evaluated in the GSA is based on the narrow diameter (0.26 m) design developed under the IAEA’s AFRA project [9] since this design has been developed specifically for the disposal of disused sealed radioactive sources and uses borehole drilling technology that is readily available in all countries. The design can accommodate disused sources of less than 110 mm in length and 15 mm in diameter. This means that the design is applicable to a wide range of sources; Table 1 provides some examples taken from Ref. [10]. It is assumed that the sources are disposed at least 30 m from the ground surface. The geological, hydrogeological and geochemical conditions considered in this TECDOC have been selected to represent a broad spectrum of site conditions.
It is considered that the reference activity values derived are applicable to situations in which the inventory, design and site conditions fall within the envelope of assumptions and data used in the GSA. For situations falling outside the envelope defined by the GSA, the GSA could be used to guide and support the development of the site specific assessment. Furthermore, the derived generic reference activity values could be used as a benchmark against which to compare values derived from the site specific assessment.
It is recognized that, whilst radiological safety is of key importance, it is still only part of a broader range of issues that need to be considered in a safety case such as planning, financial, economic and social issues, and non-radiological safety [11]. However, these issues are not specifically covered in this TECDOC. They need to be considered as part of the wider safety case documentation to be developed to support any site specific implementation of the borehole disposal concept. Separate guidance will be developed by the IAEA on the development of safety case documentation.
1.4.
STRUCTURE
The GSA has been undertaken using an approach that is consistent with best international practice.
Specifically, the approach developed by the Coordinated Research Project of the International Atomic Energy Agency (IAEA) on Improving Long Term Safety Assessment Methodologies for Near Surface Radioactive Waste Disposal Facilities (the ISAM Safety Assessment Approach) [12] (Fig. 1) has been used, with the aim of ensuring that the assessment is undertaken and documented in a consistent, logical and transparent manner.
TABLE 1. EXAMPLE SOURCES SUITABLE FOR DISPOSAL IN THE NARROW DIAMETER BOREHOLE CONSIDERED IN THE GSA
Source Typical Dimensions Typical Activity When New
(Bq)
Wide variations Wide variations Wide range of isotopes up to 3.7E+7
Instrument calibration
The ISAM Safety Assessment Approach consists of the following key steps:
• The specification of the assessment context;
• The description of the disposal system;
• The development and justification of scenarios;
• The formulation and implementation of models; and
• The presentation and analysis of results.
These steps are presented in Sections 2 to 6. Initial guidance on the use of the GSA and its results is provided in Section 7. Finally, overall conclusions are presented in Section 8, whilst supporting information is provided in Appendices I to XIII.
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FIG. 1. ISAM safety assessment approach (reproduced from Ref. [12]) .
Specify Assessment Context
Disposal System Description
Develop and Justify Scenarios
Formulate and Implement Models for Consequence Analysis
Interpretation of Results
Acceptance Make Decision
Assess Further Information
Needs
Effective to Improve Assessment
Components
Rejection Make Decision Perform
Consequence Analysis
Compare Against Assessment Criteria
Yes
Yes
No
No
Review and Modify
Adequate Safety Case?