Treatment and Conditioning of Nuclear Wastes
(Updated 30 September 2016)
- Before disposal, nuclear wastes need to be in solid form and resistant to leaching.
- Packaging should be appropriate to the wastes and their disposal.
- High-activity wastes require shielding.
Treatment and conditioning processes are used to convert a wide variety of radioactive waste materials into forms that are suitable for their subsequent management, including transportation, storage and final disposal. The principal aims are to:
- Minimise the volume of waste requiring management via treatment processes.
- Reduce the potential hazard of the waste by conditioning it into a stable solid form that immobilises it and provides containment to ensure that the waste can be safely handled during transportation, storage and final disposal.
It is important to note that, while treatment processes such as compaction and incineration reduce the volume of waste, the amount of radioactivity remains the same. As such, the radioactivity of the waste will become more concentrated as the volume is reduced.
Conditioning processes such as cementation and vitrification are used to convert waste into a stable solid form that is insoluble and will prevent dispersion to the surrounding environment. A systematic approach typically incorporates:
- Identifying a suitable matrix material – such as cement, bitumen, polymers or borosilicate glass – that will ensure stability of the radioactive materials for the period necessary. The type of waste being conditioned determines the choice of matrix material and packaging.
- Immobilising the waste through mixing with the matrix material.
- Packaging the immobilised waste in, for example, metal drums, metal or concrete boxes or containers, or copper canisters.
A more sophisticated approach is incorporating the particular wastes into the crystal structure of natural minerals which are geochemically stable.
The choice of process(es) used is dependent on the level of activity and the type (classification) of waste. Each country's nuclear waste management policy and its national regulations also influence the approach taken.
High-level wastes (HLW) are the main focus of attention, though they comprise only about one percent of all radioactive wastes. The main scope for volume reduction is low-level wastes (LLW), and intermediate-level wastes (ILW) are often cemented. Both ILW and HLW require shielding, so the handling and conditioning may be in hot cells of various kinds to provide that.
Incineration and compaction
Incinerationa of combustible wastes can be applied to both radioactive and other wastes. In the case of radioactive waste, it has been used for the treatment of low-level waste from nuclear power plants, fuel production facilities, research centres (such as biomedical research), medical sector and waste treatment facilities.
Following the separation of non-combustible constituents, the waste is incinerated in a specially engineered kiln up to around 1000oC. The gases and fumes produced during incineration are treated and filtered prior to emission into the atmosphere and emissions must conform to international standards and national regulations. After incineration, the resulting ash, which contains the radionuclides, may require further conditioning prior to disposal such as cementation or bituminisation. Compaction may also be used to further reduce the volume, if this is cost-effective. Overall volume reduction factors of up to around 100 are achieved, depending on the density of the waste.
The incineration of many kinds of hazardous waste (e.g. waste oils, solvents) and non-hazardous waste (municipal waste, biomass, tyres, sewage sludge) is practised in many countries, subject to credible application of emission limits.
Compactionb is straightforward means of reducing the volume of wastes and is used for processing mainly solid industrial low-level waste (LLW). Some countries (Germany, UK and USA) also use the technology for the volume reduction of intermediate-level and transuranic wastes. Compactors can range from low-force compaction systems (~5 tonnes or more) through to presses with a compaction force over 1000 tonnes, referred to as supercompactors. Volume reduction factors are typically between 3 and 10, depending on the waste material being treated.
Low-force compaction is typically applied to the compression of bags of rubbish, in order to facilitate packaging for transport either to a waste treatment facility, where further compaction might be carried out, or to a storage/disposal facility. In the case of supercompactors, in some applications, waste is sorted into combustible and non-combustible materials. Combustible waste is then incinerated whilst non-combustible waste is supercompacted. In certain cases, incinerator ashes are also supercompacted in order to achieve the maximum volume reduction before secure containment.
Low-force compaction utilises a hydraulic or pneumatic press to compress waste into a suitable container, such as a 200-litre drum for transport either to a waste treatment facility, where further compaction might be carried out, or to a storage/disposal facility. Other wastes including incinerator ash may be supercompacted before storage or disposal. Here a large hydraulic press crushes a drum or other receptacle containing various forms of solid low- or intermediate-level waste (LLW or ILW). The resulting pellets are then sealed inside an overpack container for interim storage and/or final disposal.
A supercompaction system may be mobile or stationary and range up to an elaborate computer controlled system which selects drums to be processed, measures weight and radiation levels, compresses the drums, places the crushed drums in overpack containers, seals the overpacks, and records the drums' and overpacks' content via a computerised storage system.
Every year worldwide tens of thousands of drums are volume-reduced and stored, with wastes generally being reduced in volume by up to a factor of 5.
Cementationc through the use of specially formulated grouts provides the means to immobilise radioactive material that is in various forms of sludges and precipitates/gels (flocks) or activated materials, as well as fragmented solids.
In general the solid wastes are placed into containers. The grout is then added into this container and allowed to set. Sludges and flocks are placed in a container and the grouting mix, in powder form, is added. The two are mixed and left to set. In each case the container with the now monolithic block of concreted waste is then suitable for storage and disposal.
This process has been used for example in small oil drums and 500-litre containers for intermediate-level wastes and has been extended to ISO shipping containers for low-level waste materials.
The technology is being used in the immobilisation of many toxic and hazardous wastes that arise outside the nuclear industry and has the potential to be used in many more cases. In some cases bitumen is the matrix material.
The immobilisation of high-level waste (HLW) requires the formation of an insoluble, solid waste form that will remain stable for many thousands of years. In general borosilicate glass has been chosen as the medium for dealing with separated HLW. The stability of ancient glass for thousands of years highlights the suitability of borosilicate glass as a matrix material.
This type of process, referred to as vitrificationd, has also been extended for lower level wastes where the type of waste or the economics have been appropriate.
Most high-level wastes other than spent fuel itself, arise in a liquid form from the reprocessing of spent fuel. This HLW comprises highly-radioactive fission products and some transuranic elements with long-lived radioactivity. To allow incorporation into the glass matrix this waste is initially calcined (dried) to a granular powder. This product is then incorporated into molten glass, poured into a robust stainless steel canister about 1.3 metres high and allowed to cool, giving a solid matrix. The containers are then welded closed and are ready for storage and final disposal.
This process is currently being used in France, Japan, Russia, UK and USA and is seen as a suitable and adequate process for management of separated HLW arising from reprocessing. The capacity of western European vitrification plants is about 2,500 canisters (1000 t) a year, and some have been operating for three decades.
In-situ vitrification has also been investigated as a means of 'fixing' activity in contaminated ground as well as creating a barrier to prevent further spread of contamination.1
Synroc and composite wasteforms
Synroc is basically a ceramic made from several natural minerals which together incorporate into their crystal structures nearly all of the elements present in high-level radioactive wastes, providing a very stable and enduring material for disposal.
The original form was intended mainly for the immobilisation of liquid HLW arising from the reprocessing of light water reactor fuel. However, by 1980 those reprocessing used fuel had chosen borosilicate glass as the medium for immobilisation because it was the most technically mature technology. Over the past few years, different forms of Synroc have been developed to deal with wastes for which there is no current disposition route, particularly military radioactive wastes.
Synroc wasteforms are tailored to suit the characteristics of the particular nuclear wastes to be immobilised, rather than adopting a single one-size-fits-all approach.
The Synroc process involves hot isostatic pressing at over 1200°C and 150 MPa. In a hot cell, the radioactive liquid waste is mixed with additives that create a slurry which is then dried to produce a free-flowing powder. The granular powder is dispensed into cans where it is sealed. Each can is placed inside a furnace contained within a hot isostatic press (HIP), where heat and pressure are applied to lock in the radionuclides – the powdered mixture fuses together to form a solid without releasing any emissions. The compressed can is a cylindrical shape about two-thirds the original volume.
The thin-walled vessel (left) is filled with a granulated waste form powder that is subsequently compressed into a solid mass by hot isostatic pressing. The can is designed to collapse into a cylindrical shaped vessel (right) for storage. The centre of the image shows a cross-section of the dense solid waste form that is encapsulated within the vessel. (Image: Ansto)
At the ANSTO Nuclear Medicine Project in Sydney, the final storage volume with Synroc will be 1% of the same wastes cemented for disposal.
Further information in Radioactive Waste Management Appendix 1: Synroc.
Glass-ceramic composites which combine the process and chemical flexibility of glass with the superior chemical durability of ceramics have been developed and have achieved very high waste loadings (50-80%). They can be tailored for the particular wastes and applied to those which are intractable due to their complex and heterogeneous chemistry.
Where used fuel is not reprocessed to recycle its useful constituents, after long storage and before disposal it is encapsulated. The fuel itself comprises stable ceramic fuel pellets inside tubes. But before disposal these are put into large metal canisters about five metres long to provide additional containment. Sweden and Finland are using copper canisters with a cast iron or boron steel internal structure, each holding about 12 fuel assemblies.
a. More detailed information can be found in the brochure Incineration of Radioactive Waste, NUKEM Technologies GmbH (2007). [Back]
b. See brochure Compaction of Radioactive Waste, NUKEM Technologies GmbH (2007). [Back]
c. See brochure Cementation of Radioactive Waste, NUKEM Technologies GmbH (2007). [Back]
d. See Handbook: Vitrification Technologies for Treatment of Hazardous and Radioactive Waste, United States Environmental Protection Agency, EPA/625/R-92/002 (May 1992) available from the EPA's National Service Center for Environmental Publications (www.epa.gov/nscep). [Back]
1. An Overview of In Situ Waste Treatment Technologies, S. Walker, R. A. Hyde, R. B. Piper, M. W. Roy, Idaho National Engineering Laboratory, presented at the Spectrum '92 Conference, Boise, Idaho (August 1992). [Back]