Appendix I Waste Treatment
Waste management covers the safe and economic collection, separation, treatment, and disposal of the products coming from the decontamination process. Two general principles govern waste management: one is to avoid creating large quantities of secondary waste during treatment that must then also be treated; the other is to be guided by the trade-off between the cost of reducing the volume of waste and the cost of disposal to choose the cost-effective solution.
Waste streams vary considerably in their level of radiation contamination; accordingly, wastes from various decontamination activities must be characterized as to whether they can be released or whether they require further concentration to reduce their volume for economical disposal. Following measurement, the appropriate separation method can be selected.
The various wastes after characterization and separation can be categorized based on options for their disposal, namely:
- released without restriction;
- released for restricted use;
- sent to a landfill;
- sent to low-level radioactive waste disposal;
- sent to high-level radioactive waste disposal;
- sent to uranium storage; and
- sent on to further treatment.
A problem at the present time is that some of these classifications are very difficult, if not impossible, to assign without clearly established release criteria.
Decontamination processes at the gaseous diffusion plants (GDPs) are likely to produce a number of waste types: gaseous, solid (from mechanical decontamination methods), and liquid (from aqueous methods). For gaseous waste streams, filtration is the major separation process used to isolate contaminants, although it can be preceded by scrubbing or cyclonic separation if there are large quantities of relatively large particles present. To remove smaller particles, the gas streams can be passed through the appropriate type of filter (e.g., bag house, electrostatic precipitator, or high efficiency filter). Any organic compounds present can be removed by combustion, catalysis, or activated carbon filters. These are all well-established technologies in current use for decontamination; detailed descriptions are given in the U.S. Department of Energy (DOE) Decommissioning Handbook (DOE, 1994).
Waste from mechanical decontamination is primarily in solid form and includes waste from scraping, scabbling, grit (or CO2) blasting and related processes. In processing this waste, great care must be taken to collect any dust generated by filter systems and immobilize it, possibly by combining it with cement and disposing of it in either a landfill or as low-level radioactive waste.
Many of the waste streams from decontamination are in aqueous form. Some, such as those from washing external surfaces, may be very lightly contaminated; others, such as those from aqueous decontamination of converter interiors, may be fairly radioactive. Therefore, different technologies must be employed to concentrate the wastes from the water-based streams. The processes used to separate materials from aqueous streams are all existing technologies (DOE, 1994). Some of these processes are listed below:
- Filtration, which might be employed on waste streams with appreciable solid matter in suspension. The removed solids, or filter cake could be sent to a landfill or low-level radioactive waste disposal, and the water recycled through the process, or released if clean enough to meet release criteria.
- Chemical precipitation, in which reagents are added to an aqueous waste to precipitate as solids the materials to be separated (e.g., uranium compounds). The solids can then be removed by filtration and the water can be recycled.
- Ion exchange, in which specific ions (hazardous or radioactive) are removed by ion exchange media from the water, which is then either recycled or released. The ions captured by the ion exchange resins may be removed by regeneration, in which case a secondary waste stream is created that must be treated. Spent ion exchange resins can also be sent to low-level radioactive waste disposal.
- Evaporation, in which the excess water either evaporates at ambient temperature (as in a holding pond) or is driven off at elevated temperature. The latter process, however, is energy intensive. The residues, in either case, might be sent to a landfill or low-level radioactive waste disposal.
Waste treatment technology is summarized in Section 8.0 of the DOE Decommissioning Handbook (DOE, 1994). The major options are listed below.
- Incineration is suitable for organic materials and mixed wastes. Some of the wastes that can be treated effectively by incineration are as follows:
solids, such as contaminated soils, absorbents, biological materials;
liquids, such as lube oils, polychlorinated biphenyls (PCBs), and solvents; and
sludges from various sources.
The Toxic Substances Control Act (TSCA) incinerator at the Oak Ridge GDP is an example of this technology, which has been used successfully to treat thousands of tons of organic wastes.
- Calcination, in which various solid salts from precipitation processes are heated to high temperatures to convert the compounds to stable oxides, such as U3O8.
- Grouting is a term for solidification and immobilization of wastes in a cement matrix. This is a technology that has been practiced at some nuclear facilities (Idaho National Engineering Laboratory, Hanford, and West Valley), and a great deal of data exists regarding suitable compositions and extraction rates (Lokken, 1978). Grouting can be used for the treatment of solid wastes from scabbling of concrete surfaces, other solids from mechanical decontamination, dusts trapped by filters, solids filtered from aqueous streams, some slurries and sludges, and similar waste materials. The grout can be poured into burial vaults or containers where it sets or hardens for disposal. It seems appropriate for disposal of low-level radioactive waste.
- Vitrification, or solution of wastes in glass, is a treatment that has been proposed for the immobilization of high-level radioactive waste and, recently, for low-level radioactive waste Hanford wastes. Trial units have been constructed at West Valley, New York, and at Fernald, Ohio. These units are scheduled to be run in 1995. Much development work has also been done at both Savannah River and Hanford (Hrma, 1994). Waste volume reductions of up to 80 percent are claimed. Much waste gas treatment is required on these units to handle the large outgassing that occurs, particularly with high water content feeds. Although the product is less leachable than grouted waste, the equipment required (melters) is more expensive and more difficult to operate, with higher operating expenses. This method could be used for the higher activity, low-level radioactive waste; however, the costs should be carefully examined.
- Compaction, or mechanical crushing, is suitable for reducing the volume of such items as ductwork, piping or electrical conduit prior to further treatment (such as melt refining) or burial.
- Melt refining may produce a form suitably compact and purified for disposal. Melt refining has been demonstrated for removing uranium deposits from iron and other metals (stainless steel, nickel, copper, aluminum, lead, tin, lead–tin alloy). The possibility of breaking down and removing organic contaminants by contact with molten steel has also been under investigation (Nagel, 1994; Aune, 1991). By extension, there is the possibility of removing both types of contaminants together; however, the technology requires further development (Joyce, 1993).
Some materials resulting from the decontamination and decommissioning of the GDPs are amenable to recycling instead of waste treatment. The large volumes of scrap metal, which offer a potential economic incentive, are a particular example. Some of the advantages and disadvantages of recycling radioactive scrap metal (RSM) are given below (Cohen and Associates, 1994). Advantages of recycling RSM are as follows:
- avoidance of disposal costs;
- resource savings from use of recycled RSM in place of virgin metals;
- an immediate solution for the disposition of RSM and avoidance of surveillance and maintenance costs; and
- may be politically preferable to land disposal if the health hazards are low.
Disadvantages associated with recycling RSM are as follows:
- Health risks to workers and the general population are possible during the recycling process;
- Markets for the recycled metal must be identified, either in the nuclear industry or in general commerce. The marketplace may not accept recycled RSM, even if it has been released for unrestricted use; and
- The cost of recycling may exceed the cost of other options.
If surface contamination is low, some materials may be released under DOE guidelines, as has been done in the past (DOE, 1993). A volumetric radiological release standard, such as exists in the United Kingdom, would permit the unrestricted use of much recycled material.
Great care must be taken to ensure that release of lightly contaminated steel does not increase the residual radioactivity already present in the nation's steel supply to some unacceptable level. With the continued recycling of scrap steel, the concentration of unwanted or "tramp" constituents can increase over time to a level that inhibits the unrestricted use of steel. In the past this has occurred with other impurities from scrap gradually building up in the steel to cause problems in properties or processing.
Some lightly contaminated steel has already been smelted and cast into shielding blocks for use in facilities that handle radioactive materials. Stainless steel could be smelted and cast into slabs that could then be rolled and fabricated in a dedicated facility, such as the one at Oak Ridge, to form waste disposal canisters or casks.
Aune, J. 1991. Cost-Efficient Solutions to Solid Waste Treatment through Metals Recovery Combined with Ultimate Thermal Destruction. ENS 91. Stavenger, Norway: Elkem Technology Environmental North Seas.
Cohen and Associates. 1994. Analysis of the Potential Recycling of DOE [U.S. Department of Energy] Radioactive Scrap Metal. McLean, Virginia: Cohen and Associates for the U.S. Environmental Protection Agency.
DOE (U.S. Department of Energy). 1993. U.S. DOE Order 5400.5, Radiation Protection of the Public and the Environment. Washington, D.C.: DOE.
DOE. 1994. Decommissioning Handbook. DOEW/EM-0142P (Section 8.0). Washington, D.C.: DOE.
Hrma, P. 1994. Towards optimization of nuclear waste glass: Constraints, property models, and waste loading. Ceramic Transactions. 45: 391–400.
Joyce, E. Jr., B. Lally, R. Fruehan and B. Otzurk. 1993. Liquid Metal Recycle and Waste Treatment: Liquid Metal Melt-Slag Technology Evaluation for MWIP. LA-UR-93-3026 TTP: AL132001. Los Alamos, New Mexico: Los Alamos National Laboratory.
Lokken, R.O. 1978. A Review of Radioactive Waste Immobilization in Concrete. PNL-2654. Richland, Washington: Pacific Northwest Laboratory.
Nagel, C. 1994. Efficacy of Melting Technologies for Decontamination. Presented to the Committee on Decontamination and Decommissioning of Uranium Enrichment Facilities at the National Academy of Sciences, Washington, D.C., June 16, 1994.