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Cradle-to-Grave Radioactive Waste Management

Radioactive Waste Management means the whole sequence of operations starting with the generation of waste and ending with disposal.

Operational Management

The grave - Mt Walton East near-surface disposal facility, WA (photo - S. Woollett)

Typical steps in the pre-disposal "operational management phase" of radioactive waste include treatment, conditioning, short-term storage, and transport.

Treatment of radioactive waste may involve segregation, chemical adjustment, decontamination, containment, volume reduction, removal of radionuclides from the waste, and change of composition.

Conditioning may involve conversion of the waste into a solid form, cementation, vitrification, enclosure of waste in containers, and provision of an over-pack for transport.

Operational management involves "categorisation" of the waste based on its physical and chemical characteristics. The categories are loosely related to occupational risk.  "Classification" of radioactive waste for disposal is conversely related to long-term risks to members of the public and the environment.

Waste Classification

The new Australian classification scheme for disposal of radioactive waste is based fundamentally on two criteria – the radioactivity level and the half-lives of relevant radionuclides.

As the level of radioactivity rises, the need to contain the waste and to isolate it from the biosphere also increases. At the lower range of activities, below the "clearance levels", the management of the waste can be carried out without consideration of its radiological properties.  Activity limitations for a given disposal facility will in particular depend on the radiological, chemical, physical and biological properties of individual radionuclides.

Six classes of waste form the basis of Australia’s new radioactive waste classification scheme.

Exempt waste (EW) meets the criteria for exemption from regulatory control for radiation protection purposes (i.e. waste of an activity low enough so as to not require safety measures).

Very short-lived waste (VSLW) can be stored for decay over a limited period of up to a few years and subsequently cleared according to arrangements approved by the relevant regulatory authority, for uncontrolled disposal, use or discharge.  This class includes waste from industrial, medical and research operations.

Very low level waste (VLLW) needs a moderate level of containment and isolation, and therefore is suitable for disposal in a near-surface, industrial or commercial landfill-type facility with limited regulatory control.  Such landfill type facilities may also contain other hazardous waste.  Typical waste in this class includes soil and rubble with low activity concentration levels.  Concentrations of longer-lived radionuclides would generally be very limited.

Low level waste (LLW) has limited amounts of long-lived radioactivity.  It requires robust isolation and containment for periods of up to a few hundred years and is suitable for disposal in engineered near-surface disposal facilities.  This class covers a very broad range of materials that can include short-lived radionuclides at higher activity levels and long-lived radionuclides, but only at relatively low activity concentration.

Intermediate level waste (ILW) is waste that, because of its content (particularly of long-lived radionuclides) requires a greater degree of containment and isolation than is provided by near-surface disposal.  However, ILW needs little or no provision for heat dissipation during its storage and disposal.  It may contain long-lived radionuclides that will not decay to an activity concentration that is acceptable for near surface disposal during the time for which control of the disposal site can be relied upon.  Therefore ILW requires disposal at greater depths.

High level waste (HLW) has radioactivity levels high enough to generate significant quantities of heat by the radioactive decay process or has large amounts of long-lived radionuclides that need to be considered in the design of a disposal facility for such waste.  Disposal in deep, stable geological formations usually several hundred metres or more below the surface is the generally recognised option for the disposal of HLW.

Storage

Waste storage is the temporary retention of waste.

The international community has adopted the policy, through the development of the International Atomic Energy Agency’s (IAEA) waste safety standards, that storage of radioactive waste is an interim measure and that national policy should regard disposal as the ultimate solution to dealing with the waste.  A fundamental safety principle of the IAEA is that the problem of dealing with radioactive waste should not be passed to future generations.

One important use of temporary storage is to allow time for short-lived radionuclides to decay.  For some types of waste containing only short-lived radioactivity, this is all that is required for the levels of radioactivity to fall below the levels at which the waste can be cleared from regulatory control.

Disposal

Waste disposal is the emplacement of waste in an appropriate facility without the intention of retrieval at a later date.

Many types of engineered waste disposal facilities have been designed and are in operation around the world.  Broadly, they fall into three categories.

Near-surface disposal facilities:  disposal structures that are constructed above or below the ground surface up to several tens of meters in depth.

Borehole disposal:  some waste is too radioactive and too long lived for near-surface disposal, requiring higher levels of containment and isolation.  Borehole disposal provides the option for disposal at greater depths, typically depths beyond a few tens of metres and up to a few hundred metres (i.e. the depth range between near-surface disposal and geological disposal). Borehole disposal is typically used for discrete radioactive sources and small volume wastes.

Geological disposal:  the emplacement of waste in a land-based engineered facility at such a depth – typically beyond 300 m - that it is far removed from the accessible environment.  The timescale for change in the geologic formations in which it is placed is very long - tens of thousands of years - and hence the geologic formations provide the facility with a stable environment for a very long time.

Safety Case and Safety Assessment

Considerable effort has been put in to developing systematic and internationally recognised approaches for assessing the safety of radioactive waste management and disposal facilities.  This involves developing a safety case for a specific facility.  In IAEA and Nuclear Energy Agency (NEA) publications the safety case is described as “an integration of arguments and evidence that describe, quantify and substantiate the safety, and the level of confidence in the safety, of the radioactive waste facility”.

The acceptability of a disposal facility for particular waste types will depend on the arguments in the safety case about the site and the facility engineering (e.g. the engineered barrier system and the characteristics of the site), the results of safety assessment and the management arrangements to assure quality in all aspects of safety-related work.

Safety assessment is the assessment of all aspects of a practice that are relevant to protection and safety.  This includes siting, design and operation of a storage or disposal facility.  Safety assessment is the systematic process that is carried out throughout the lifetime of the facility or activity to ensure that all the relevant safety requirements are met by the proposed (or actual) design.

The primary purposes of the safety assessment are to determine whether an adequate level of safety has been achieved for a facility or activity and whether the basic safety objectives and safety criteria established by the designer, the operator and the regulatory body have been fulfilled.

A graded approach is used in determining the scope and level of detail of the safety assessment carried out for any particular facility or activity, consistent with the magnitude of the possible radiation risks arising from the facility or activity.

For a disposal facility, safety assessment entails evaluating the performance of the disposal system and quantifying its potential radiological impact on human health and the environment.  Safety assessment is one component of the safety case for a disposal facility and should consider the possible radiological impacts of the facility both during its operation and in the post-closure phase.  Radiological impacts may arise from gradual processes which may cause the facility and its components (e.g. barriers) to degrade, and from discrete events that may affect the isolation of the waste (e.g. earthquakes, tsunamis, floods, fire, inadvertent human intrusion).

Safety assessment should provide reasonable assurance that the disposal facility complies with applicable regulatory requirements.  In cases where safety assessment results indicate that impacts should be reduced (e.g. as may be the case at some of the older existing facilities), the safety case should include an appropriate improvement plan for meeting the Safety Requirements.

International Projects on Safety Assessment

Members of the ARPANSA Waste Safety Team participate in the IAEA international program SADRWMS to examine the application of safety assessment methodology to predisposal waste management practices and facilities including waste storage, and in the IAEA project ASAM (Application of Safety Assessment Methodologies for Near Surface Disposal Facilities) and its successor, PRISM.

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