Chapter 50: Predisposal of Radioactive Waste – Processing | A Book: Nuclear Energy

This chapter was published on “Inuitech – Intuitech Technologies for Sustainability” on April 22, 2013.

Waste that contains or is contaminated with radionuclides arises from a number of activities involving the use of radioactive material. Such activities include the operation and decommissioning of nuclear facilities; the use of radionuclides in medicine, industry, agriculture, research and education; the remediation of sites affected by radioactive residues from operations of various types or from accidents; and the processing of raw material containing naturally occurring radionuclides. The nature of this radioactive waste is likely to be such that radiation safety considerations must be taken into account for its safe management. The importance of the safe management of radioactive waste for the protection of human health and the environment has long been recognized, and considerable experience has been gained in this field.A global nuclear safety regime is in place and is being continuously improved. The International Atomic Energy Agency (IAEA) safety standards, which support the implementation of binding international instruments and national safety infrastructures, are a cornerstone of this global regime. The IAEA safety standards constitute a useful tool for contracting parties to assess their performance under these international conventions.

The established safety objective and the fundamental safety principles apply to all facilities and activities in which radioactive waste is generated or managed, for the entire lifetime of facilities. This includes the associated transportation of radioactive material and the management of radioactive waste. To meet the safety objective, in considering options for the management of radioactive waste, due consideration has to be given to the protection of workers, the public (including future generations) and the environment.

1. THE PRINCIPLES OF RADIOACTIVE WASTE MANAGEMENT:

Radioactive waste occurs in a variety of forms with very different physical and chemical characteristics, such as the concentrations and half-lives of the radionuclides. This waste may occur:

In gaseous form, such as ventilation exhausts from facilities handling radioactive materials;
In liquid form, ranging from scintillation liquids from research facilities to high level liquid waste from the reprocessing of spent fuel; or
In solid form, ranging from contaminated trash and glassware from hospitals, medical research facilities and radiopharmaceutical laboratories to vitrified reprocessing waste or spent fuel from nuclear power plants when it is considered a waste.

Such wastes may range from the slightly radioactive, such as in those generated in medical diagnostic procedures, to the highly radioactive, such as those in vitrified reprocessing waste or in spent radiation sources used in radiography, radiotherapy or other applications. Radioactive waste may be very small in volume, such as a spent sealed radiation source, or very large and diffuse, such as tailings from the mining and milling of uranium ores and waste from environmental restoration. Basic principles for radioactive waste management have been developed even though there are large differences in the origin and characteristics of radioactive waste, for example, concentration, volume, half-life and radiotoxicity. Although the principles are generally applicable, their implementation will vary depending on the types of radioactive waste and their associated facilities.

Radioactive waste, as a source of ionizing radiation, has long been recognized as a potential hazard to human health. National regulations and internationally recommended standards and guidelines dealing with radiation protection and radioactive waste management have been developed, based on a substantial body of scientific knowledge.

It has been a feature of radioactive waste management that special attention has been given to the protection of future generations. Considerations related to future generations may include potential radiation exposure, economic consequences and the possible need for surveillance or maintenance. Radioactive waste may also contain chemically or biologically hazardous substances and it is important that hazards associated with these substances are adequately considered in radioactive waste management.

The following presents radioactive waste management principles that apply to radioactive material, as defined to be radioactive waste by the appropriate national authorities, and to the facilities used for the management of this waste from generation through disposal. These principles apply to all aspects of radioactive waste management except where an activity is the specific subject of an IAEA document outside the Radioactive Waste Safety Standards (RADWASS) series or an international instrument, for example, the transportation of radioactive material and exports and imports of nuclear material. The principles also apply in the management of radioactive waste containing, for example, chemically or biologically hazardous substances, even though other specific requirements may also be applicable.

Responsible radioactive waste management requires the implementation of measures that will afford protection of human health and the environment since improperly managed radioactive waste could result in adverse effects to human health or the environment now and in the future.

The timely creation of an effective national legal framework and an associated organizational infrastructure provides the basis for appropriate management of radioactive waste. The individual steps in radioactive waste management may be dependent on each other, and thus require co-ordination. Taking this interdependence into account will help to ensure safety in all radioactive waste management steps. Observance of the principles of radioactive waste management will ensure that the above considerations are addressed, and thus contribute to achieving the objective of radioactive waste management.

a) Principle 1: Protection of Human Health: Radioactive waste shall be managed in such a way as to secure an acceptable level of protection for human health;

b) Principle 2: Protection of the Environment: Radioactive waste shall be managed in such a way as to provide an acceptable level of protection of the environment;

c) Principle 3: Protection beyond National Borders: Radioactive waste shall be managed in such a way as to assure that possible effects on human health and the environment beyond national borders will be taken into account;

d) Principle 4: Protection of Future Generations: Radioactive waste shall be managed in such a way that predicted impacts on the health of future generations will not be greater than relevant levels of impact that are acceptable today;

e) Principle 5: Burdens on Future Generations: Radioactive waste shall be managed in such a way that will not impose undue burdens on future generations;

f) Principle 6: National Legal Framework: Radioactive waste shall be managed within an appropriate national legal framework including clear allocation of responsibilities and provision for independent regulatory functions;

g) Principle 7: Control of Radioactive Waste Generation: Generation of radioactive waste shall be kept to the minimum practicable;

h) Principle 8: Radioactive Waste Generation and Management Interdependencies: Interdependencies among all steps in radioactive waste generation and management shall be appropriately taken into account; and

i) Principle 9: Safety of Facilities: The safety of facilities for radioactive waste management shall be appropriately assured during their lifetime.

2. RADIOACTIVE WASTE FORM:

2.1 Unacceptable waste and those to be prepared and made safe:

Solid waste containing liquid shall contain as little free standing and non-corrosive liquid as is reasonable achievable, but in no case shall the liquid exceeds 1 percent of the volume. In addition, the non-aqueous content of any liquid in the waste must be treated so that no visible oil or grease will be released by leaching of an un-compactable waste form. When liquid oils are intended to be sent to disposal, the details will be cleared in advance with the operator of the repository.

Waste containing ion exchange material must be stabilized to ensure retention of its radioactivity content. For treatment and/or conditioning methods to be applied, details must be cleared in advance with the repository operator.

Waste containing hazardous, biological, pathogenic, or infectious material must be treated to reduce to the maximum extent practicable the potential hazard from the non-radiological materials. Waste containing putrescible material (such as food, vegetables and animal remains) will be excluded.

Waste containing or being capable of generating fire or explosion hazard must either be excluded or made safe. For methods that are to be employed to make waste safe, the details must have been cleared in advance with the repository operator. Such waste includes:

Combustible metals, such as lithium, potassium, sodium (including sodium vapors lamps), calcium, magnesium, zinc and other metals in finely divided form;
Pyrophoric material;
Phosphorous;
Hydrides of boron and other chemical compounds representing a high fire hazard;
Other materials which react with water to evolve heat and flammable gases, for example hydrides, nitrides and carbides; and
Strong oxidizing, acidic or alkaline compounds.

Waste must not specifically comprise complexing agents that may have effects on radionuclide migration from the site. Where such materials are reduced to the maximum extent reasonably practicable, waste containing complexing agents (e.g. EDTA, NTA, gluconate, oxalate, citrate, pthtalate, acetate) will be sent to disposal only with a prior written agreement of the repository operator.

Waste must not contain, or be capable of generating, quantities of toxic gases, vapours, or fumes harmful to persons, while operations such as transporting, handling or disposing are executed on site.

Waste covered by special requirements within the framework of this specification or any other waste intended to be sent for disposal only based on a written consent of the repository operator (e.g. agreement, letter of acceptance or any other formula proposed by the operator).

3. PROTECTION OF HUMAN HEALTH AND THE ENVIRONMENT:

The objective of predisposal management of low and intermediate level radioactive waste is to ensure that the waste is managed in a manner that protects human health and the environment, both now and in the future, without imposing undue burdens on future generations. As well as controlling impacts on future human health, the aim should be to limit reliance on actions to be undertaken by future generations. As far as practicable, the predisposal management of waste should not rely on complex long term institutional arrangements or actions as a necessary safety feature.

Radioactive waste should be treated and conditioned to enhance its long term safety and security. If there is an established disposal facility, the waste should be treated and conditioned to meet the waste acceptance criteria of the disposal facility. If there is no established disposal facility, undertaking irreversible treatments that might limit future options should be avoided, unless there are clear safety and security benefits.

Where the predisposal management of radioactive waste has the potential for release or migration of radionuclides beyond jurisdictional boundaries, the effects beyond the boundaries should be discussed by the operator and/or the regulatory authority with the relevant regulatory authority in the neighbouring jurisdiction.

4. CHARACTERIZATION OF RADIOACTIVE WASTE:

Characterization of radioactive waste is an important aspect at every stage of predisposal management. By knowing the characteristics, it is possible to establish further adjustment, treatment, conditioning, or suitability for further handling, processing, storage or disposal. Waste characterization involves determination of the physical, chemical and radiological properties of the waste. Waste characterization is defined as:

“The determination of the physical, chemical and radiological properties of the waste to establish the need for further adjustment, treatment, conditioning, or its suitability for further handling, processing, storage or disposal.”

Radiological waste characterization involves detecting the presence of individual radionuclides and quantifying their inventories in the waste. This can be done by a variety of techniques, depending on the waste form, radionuclides involved and level of detail/accuracy required. For example, a simple radiation dose rate measurement will give an indication of the total quantity of gamma emitting radionuclides in a waste package, but will not identify individual radionuclides or their concentrations. Gamma spectroscopy will identify the individual radionuclides and, when properly calibrated, their quantities as well. Other techniques, such as active/passive neutron interrogation, alpha spectroscopy, and liquid scintillation counting are used for other classes of radionuclides. The preferred methods are often referred to as “non-destructive” or “non-invasive”, since they do not involve opening a waste package to take samples. The terms most frequently used are NDA (non-destructive assay), NDE (non-destructive examination) and NDT (non-destructive testing).

Over the past several decades, there has been an increasing emphasis on the importance of producing a ‘quality end product’ for all disposed radioactive waste. This is achievable only by obtaining a thorough and accurate assessment of the physical, chemical and radiological characteristics of the waste, a process which is referred to as ‘waste characterization.’ This must be accomplished or verified at the point of generation, during waste conditioning, and upon disposal, with a clear demonstration that the waste meets the performance objectives established by the disposal acceptance criteria. Moreover, it must be accomplished in a systematic manner using proven methodologies, technologies and techniques with an overriding emphasis on quality assurance and quality control.

The nuclear industry has found the following:

When characterization is conducted at the point of waste generation or production, it can be carried out more simply and accurately without having to make assumptions that need to be validated;
Characterization will be less expensive when performed at the earliest practical stage in the waste life cycle. Early waste characterization also has safety benefits: the waste is likely to be handled less, samples are likely to be representative, and waste characteristics are likely to be more accurate; and
Incomplete waste characterization often leads to reliance on overly conservative assumptions, which are costly. Early waste characterization provides greater accuracy for determining waste characteristics and, thereby, better utilizes waste storage and disposal facilities.

Characterization actions are currently being performed during various stages of the radioactive waste life cycle: during generation (including waste retrieval activities), processing (treatment, conditioning), and storage/disposal. The type and extent of the characterization efforts depend on many factors, e.g.:

The type of waste or waste form;
The disposal concept;
The regulatory regime;
The amount of process knowledge that is available; and
The kind of characteristics to be measured.

It is consequently impossible to define or recommend a single characterization procedure for even similar waste forms or packages. In order to elaborate a strategy for the characterization actions in any given case, the following should be critically understood and assessed:

The waste acceptance criteria (and the rationale behind it); and
The safety assessment for the disposal concept (e.g. assumptions made, information needed).

It is of importance that waste characterization is considered as part of the larger waste management strategy, and interactions should be occurring among the various working groups involved. One of the outputs of this is that the minimum accuracy required and detection limits of the characterization efforts may be better identified.

Defining the organizational scheme into which the characterization efforts fit will take into account the relationships between the various bodies involved: those responsible for waste management, the disposal organization, the regulator, the various waste producers and operators, and the independent laboratory. A basic quality principle relates to the independence of certain characterization facilities and activities:

The generator of the radioactive waste and the operators (treatment and conditioning, storage and disposal) perform characterization programmes for various reasons;
These activities need to relate with and report to the waste management staff. The independent laboratory must respond and report to the regulator; and
The application of an additional quality control (independent laboratory) is a basis of quality assurance and will strongly contribute to the public confidence in the back-end of the nuclear fuel cycle. This additional quality control needs to be carefully balanced to provide the maximum added value to the other characterization programmes, assuming the financial impacts remain reasonable.

Waste characterization samples will range from liquids to slurries and solids to final waste forms, such as waste glass or drummed waste. The type of waste sample, depending upon the consistency and complexity of the waste properties, will influence the methods of sampling. Another important consideration that will affect sampling will be the occupational dose uptake to plant operators and laboratory personnel, which must satisfy ALARA principles. Sampling events, quantities and amount of analytes measured will also depend on the data need and usage, e.g. for primary waste flux characterization, process control, process monitoring, contractual compliance, environmental compliance, safety basis compliance, technical investigations, and commissioning/decommissioning.

For these reasons, an integrated sampling and analysis publication should be developed early in the process and agreed upon in conjunction with the waste generator, treatment and conditioning plant owner and operators, storage and disposal personnel, and regulators. Waste compliance and regulatory requirements would be included in this publication. Determining the time that samples will be gathered and analyzed, including all quality assurance and control data, is important because it will have a direct impact on the output of the treatment or conditioning process. Based upon negotiations between participants, sampling and analysis by an independent laboratory may also need to be included in the plan. This type of development approach leads to increased communication and agreement between waste generators, flow sheet developers, plant and laboratory designers, owner/operators, and regulatory and governmental agencies.

5. RADIOACTIVE WASTE MANAGEMENT:

5.1 Predisposal Radioactive Waste Management:

Radioactive waste, depending on the origin, can occur in different physical states like solid, liquid, or gas. It can have a variety of characteristics such as activity levels and half-lives of the radionuclides present in the waste. In the life cycle of radioactive waste, disposal is the final step. Before final disposal, the waste usually goes through a number of steps which are classified as predisposal activities. Predisposal management of radioactive waste encompasses all the activities from waste generation up to final disposal including characterization of waste.The principal approaches to the predisposal management of radioactive waste are commonly termed “delay and decay”, “concentrate and contain” and “dilute and disperse”. “Delay and decay” involves holding the waste in storage until the desired reduction in activity has occurred through radioactive decay of the radionuclides contained in the waste. “Concentrate and contain” means reduction of volume and confinement of the radionuclide content by means of a conditioning process to prevent or substantially reduce dispersion in the environment. “‘Dilute and disperse” means discharging effluent to the environment in such a way that environmental conditions and processes ensure that the concentrations of the radionuclides are reduced to such levels in the environment that the radiological impacts of the released material are acceptable.

The approaches “delay and decay” and “concentrate and contain” often involve the holding of waste in a storage facility or the emplacement of waste in a disposal facility. Radioactive waste therefore has to be processed, as necessary, in such a way that it can be safely placed and held in a storage facility or a disposal facility.

The approach “dilute and disperse” is a legitimate practice in the management of radioactive waste, but only when carried out within authorized limitations established by the regulatory body.

Various factors, including the nature and the amount of radioactive waste, occupational and public exposures, environmental effects, and human health, safety, and social and economic factors, are to be considered when deciding between options in the predisposal management of radioactive waste. However, the preferred option, as far as is reasonably practicable, is to concentrate and contain the waste and to isolate it from the biosphere.

In the predisposal management of radioactive waste, decisions often have to be made at a time when no disposal facility is available and the waste acceptance criteria for disposal are unknown. A similar situation would arise if radioactive waste were to be stored over long periods of time for reasons of safety or for other reasons. In both cases, consideration has to be given to whether, for the purposes of safety, the radioactive waste will be stored in a raw, a treated or a conditioned form. The anticipated needs for any future steps in radioactive waste management have to be taken into account as far as possible in making decisions on the processing of the waste.

5.1.1 Processing:

The main purpose of processing radioactive waste is to enhance safety by producing a waste form, packaged or unpackaged, that fulfills the acceptance criteria for safe processing, transport, storage and disposal of the waste. Waste has to be rendered into a safe and passive form for storage or disposal as soon as possible. The processing of radioactive waste can yield effluent that is suitable for authorized discharge or material that is suitable for authorized use or clearance from regulatory control.Waste has to be processed in such a way that safety is appropriately ensured during normal operation, that measures are taken to prevent the occurrence of incidents or accidents, and that provisions are made to mitigate the consequences if accidents occur. The processing has to be consistent with the type of waste, the possible need for its storage, the anticipated disposal option, and the limits, conditions and controls established in the safety case and in the assessment of environmental impacts.

Various methods are applied for processing radioactive waste of different types. Consideration has to be given to identifying suitable options and to assessing the appropriateness of their application. Decisions have to be taken within the overall approach to the predisposal management of radioactive waste on the extent to which the waste has to be processed, with account taken of the quantities, activity and physical and/or chemical nature of the radioactive waste to be treated, the technologies available, the storage capacity and the availability of a disposal facility.

Radioactive waste has to be processed in such a way that the resulting waste form can be safely stored and retrieved from the storage facility up until its ultimate disposal.

Provisions have to be established by the operator for identifying, assessing and dealing with waste and/or waste packages that do not meet process specifications and requirements for its and/or their safe handling, transport, storage and/or disposal.

Consideration has to be given to the consequences of dealing with any secondary waste (both radioactive and non-radioactive) that is created during processing.

5.1.1.1 Pretreatment or Segregation:

Pretreatment is defined as an activity where types of waste or material (Radioactive or Exempt) are separated or are kept separate on the basis of radiological, chemical and/or physical properties, to facilitate waste handling and/or processing. The main objectives of pretreatment are:

To segregate waste into active and non-active streams;
To facilitate transport, treatment, conditioning and packaging by separating active streams into components or converting the waste into a form suitable for such operations; and
To recover products for recycling.

A number of factors need to be considered when selecting a pretreatment method including:

Radiological protection standards and objectives;
Waste minimization;
Availability of pre-treatment technologies;
Economic aspects; and
Requirements for the further treatment, conditioning, storage, off-site transport and final disposal of the waste.

Pretreatment results in improved safety, lowered radiation exposure and significantly lower costs in subsequent waste management operations. These benefits need to be balanced with radiation exposure and pre-treatment costs.

Pretreatment of radioactive waste includes any operations prior to waste treatment, to allow selection of technologies that will be further used in processing of waste, such as:

a) Collection/Segregation: Collection involves the receipt of waste from the waste-generating processes. Collection and segregation lead to additional exposure for personnel. Segregation is where waste or materials (radioactive or inactive) are separated or are kept separate according to radiological, chemical, biological and/or physical properties, which facilitate waste handling and/or processing. The main factors considered in segregation are:

Physical and chemical characteristics of the waste;
Type and half-lives of radionuclides;
Concentration of radionuclides; and
Specifications or requirements to be fulfilled for further waste processing.

5.1.1.2 Treatment:

Operations intended to benefit safety and/or economy by changing the characteristics of the waste. Three basic treatment objectives are:

To reduce volume;
To remove radionuclides from the waste (or reduction); and
To change the composition.

Treatment may result in an appropriate waste form. If treatment does not result in an appropriate waste form, the waste may be immobilized.

Operations intended to benefit safety and/or economy by changing the characteristics of the waste:

Volume reduction through techniques such as size reduction, super compaction, incineration or high temperature processes etc.;
Removal or reduction of radionuclide inventories through decontamination, ion exchange, filtration/segregation, evaporation etc.; and
Change of composition through techniques such as high temperature processes, incineration, precipitation, flocculation etc.

a. Volume Reduction:

Size Reduction: Size reduction of waste items can be used effectively to optimize package loading and increase waste density. There are many available techniques including cold cutting, hot cutting, shearing, cropping, sawing, laser cutting, nibbling etc. Many of these techniques can be applied remotely if required.
Supercompaction: Volume Reduction by Supercompaction. Typically a 1000 to 2000 Tonne hydraulic press is used to vertically compress a 220L drum into a ‘puck’. Volume reduction factors of up to 5 are typical.
Incineration: Incineration is a very effective method for achieving volume reduction factors of up to 20. In keeping with the waste hierarchy it also offers the possibility of energy recovery from waste. Resultant ashes can be supercompacted or immobilized by solidification. Abatement systems are critical to control and limit discharges to the environment and these may result in secondary wastes.
Metal Melting:
- Specific to metals;
- Results in recycling of up to 95% of original material;
- Radioactivity is concentrated in furnace slag and usually returned to the originating site as radioactive waste (<5% of original volume); and
- Recovered metals are conventionally recycled.

b. Radioactivity Reduction:

Decontamination: There are many different forms of decontamination technique and many processes utilize more than one technique. Selection of the right techniques depends on the substrate and the type of contamination encountered.

Decontamination techniques generally fall into either ‘wet’ or ‘dry’ classifications. They include; concrete scabbling, shaving, planning, diamond wire slicing, high pressure and ultra-high pressure water jetting, dry ice blasting, abrasive blasting, bead blasting, ultrasonic bath, chemical treatment (acidic and/or chelating agents are common), strippable coatings (spray on or paint on), ice pigging, laser scabbling.

Decontamination generally results in the production of a secondary waste and consideration of downstream treatment and conditioning of this waste must be a key consideration in selecting a technique.

Decay Storage: Generally beneficial where short half-life radionuclides dominate the inventory of a waste type (e.g. 60Co activation products in steel alloys).

Employed extensively to support deferred decommissioning programmes (e.g. Magnox ‘Safestore’ philosophy in UK).

This technique results in a genuine reduction in radioactive inventories Radioactivity Reduction – Decay Storage and is entirely consistent with the philosophy of waste hierarchy.

Waste can be stored in-situ (i.e. safestore) or in appropriate shielded containers (e.g. Mosaik® casks).

c. Waste Conversion: Waste conversion can beneficially alter the composition of wastes and may also result in immobilization/passivation, meaning that further conditioning is not required. Techniques include wet oxidation, plasma, vitrification, cold crucible melting, steam reforming, precipitation, flocculation, dissolution etc. These techniques are particularly suited to wastes that are not directly suitable for conditioning (for example ion exchange and organic materials).

5.1.1.3 Conditioning:

Conditioning of radioactive waste is defined as those operations that produce a waste package suitable for handling, transport, storage and/or disposal. Conditioning may include the conversion of the waste to a solid waste form, enclosure of the waste in containers and, if necessary, provision of an overpack.

Encapsulation: Encapsulation is the process of immobilizing solid waste by application of a fluidic matrix (often cement based) that fully penetrates the waste and cures to form a solid monolith.
Embedding: Embedding (or entombment) is the process of immobilizing solid waste by placing items into a container and filling the void with a fluidic matrix (again cement is common) that cures to form a solid monolith.

Generally used for immobilizing containers or larger waste items.

Solidification: Solidification is the process of immobilizing ‘mobile’ waste by adding a quantity of waste to a container and mixing with, typically, cement, fillers and water. A ‘lost paddle’ is used to homogenize the mix and it cures in a similar manner to encapsulation to form a solid monolith.
Vitrification: Vitrification is the process of immobilizing liquid (typically HLW) waste by melting a quantity of waste with glass formers to produce a very high integrity solid monolith. High integrity stainless steel bottles are used for storage and ultimate disposal.
Robust Container: In some instances it is possible to validate waste packaging without the use of immobilization, generally where a very robust, thick walled container is utilized and all potential waste degradation mechanisms have been mitigated (e.g. by drying of sludges) to achieve a passive state.
Overpacking: Overpack: A secondary (or additional) outer container for one or more waste packages, used for handling, transport, storage and/or disposal.

Overpacking is commonly used for the packaging of spent nuclear fuel but can also be used for interim management of conditioned waste packages.

It may be that not all processing steps are necessary. The type of processing necessary depends on the particular waste, its form and characteristics, and the overall strategy for waste management. Where appropriate, waste or material resulting from processing can be reused or recycled, or released from regulatory control.

Resources:

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