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4.6.3 Implementing remediation actions

Contents
4.6.3.1 Special considerations
4.6.3.2 Remediation of areas of extensive surface contamination
4.6.3.2.1 Agricultural and forested zones
4.6.3.2.2 Urban zones
4.6.3.3 Remediation of areas of localised contamination
4.6.3.4 Remediation of radioactively contaminated sites from extraction and processing of ores

The implementation of remediation actions should include: procurement of the selected technology; preparation of the site; development of a health and safety plan; development of operations procedures; staff selection and training; completion of site clean-up; verification; waste disposal; and release of the site for any future use.

4.6.3.1 Special considerations

The general approach to remediation of radioactively contaminated sites may require special adaptation to address sites covering very large surface areas, or those which are deep and difficult to access. Small localized sites may benefit from removal or isolation approaches which are not feasible for very large sites. In addition, rigorous quality assurance techniques may be very important to demonstrating success of these projects as remediation criteria approach environmental background values. Each of these special considerations is addressed in the sections below [IAEA-1999].

4.6.3.2 Remediation of areas of extensive surface contamination

Radioactive contamination of the environment, such as caused by nuclear explosives testing, or nuclear accidents resulting in environmental dispersion, can cover surface areas of hundreds of square kilometres. These areas may include urban areas (roofs, walls, streets, yards), agricultural and open areas (crop lands, grasslands, parks) and forested regions (undeveloped, forest product areas).

Although contamination for such events is largely spread over a large area, the radionuclides can be redistributed both laterally and vertically with time. For example, rainfall may assist in moving the contaminants into deeper sections of the soil and potentially into the groundwater. Runoff or flooding can also redistribute the contaminants thus contaminating river flood plains, or causing accumulation of radionuclides behind engineered structures such as dams. Wind may also spread contamination.

The clean-up associated with this type of contamination can itself result in secondary radioactive waste streams which may be difficult or impractical to recover and process further. For example, the following waste types requiring further management and disposal may be generated during remediation or by other activities occurring in the contaminated zone: radioactively contaminated municipal sanitary wastes; sludge arising from waste water treatment; radioactively contaminated ash from domestic heating facilities that use radioactively contaminated firewood and peat; and radioactively contaminated dredged soils.

The selection of the methods to be used to clean-up an area should consider site specific factors such as the type of contamination, how it was deposited, soil types, value of the land, alternative land use, population distribution, size of the affected area, and the equipment available. Many techniques and types of equipment may be required. The methods selected should prevent contaminants from entering the food chain and should have minimal ecological impact. In addition, the methods should be safe, practical and cost effective because of the logistic problems and huge costs associated with the clean-up of large areas and the subsequent need to dispose of the wastes [IAEA-1999].

4.6.3.2.1 Agricultural and forested zones

For radioactively contaminated agricultural areas, selected technology should provide in-situ, effective and economical remediation, as well as ecological safety and respect of the environment. In some cases, they should allow the utilization of the remediated areas for agricultural production. Some technologies such as in-situ bio-remediation and land farming have already been demonstrated but need further development and improvements for optimal application. Past experience in remediation of forests includes the decontamination of wood cuttings, as well as measures to preserve the forest while radionuclide decay occurs (e.g., protecting the forests from pests and diseases; improving fire-protection capabilities; and so on).
The clean-up of land can be carried out by selectively separating the radionuclides from the soil matrix, by deep ploughing to remove the contamination from the surface and the root zone or by removing the vegetation and/or top layer of soil containing the contaminants. The volume of wastes arising from the clean-up would be smallest for deep ploughing and largest for layer removal. The volume of wastes from the separation technique would depend on how well the separation could be done. The cost of storing, transporting, additional treatment and/or disposal of radioactively contaminated soils and vegetation is an important factor in selecting the proper method [IAEA-1999].

4.6.3.2.2 Urban zones

In urban zones, consideration should be given to people occupying the areas as well as to their personal health and safety. The nature of land uses, structures and utility systems present should also be considerations.
A large variety of decontamination techniques and chemical mixtures have been developed over the years to assist in removing contamination from various surfaces. These were developed in association with nuclear facility decommissioning or for facilities used in support of environmental remediation. A decontamination process must be selected on the basis of site specific considerations taking into account a wide variety of parameters such as the following [IAEA-1999]:

  • Type of material: metal, asphalt, concrete, soil, wood, etc.;
  • Type of surface: rough, porous, coated (paint, plastic, etc.);
  • The method of deposition: the distribution of the contaminant and its adherence to the surface; can depend on whether the deposition was wet or dry;
  • Nature of the contaminant: activation or fission products, actinides, etc.;
  • Chemical and physical form of the contaminant: solubility, aerosol, flocculent particles, complex compound with other materials, etc.; for many decontamination processes, the smaller the particle, the more difficult is to remove it from a surface;
  • Specification of clean-up standards;
  • Potential future re-use for decontaminated materials, and
  • The proven efficiency of the process.

Other factors which are important in selecting the method and equipment include the following [IAEA-1999]:

  • Availability, cost and complexity of the decontamination equipment;
  • The need to condition the secondary waste generated;
  • Occupational and public doses resulting from decontamination;
  • Other safety, environmental and social issues;
  • Availability of trained staff; and
  • The amount of work involved and the difficulty in decontaminating the equipment used for the clean-up if it is to be reused.

4.6.3.3 Remediation of areas of localised contamination

Localized accidental spills and intentional dumping have resulted in contamination in soils to extensive depths, in groundwater, and within surface waters. Waste forms can be in both liquid (surface and groundwater) and solid (solid wastes and radioactively contaminated soils) form. For example, in the past, liquid radioactive effluent has been directly disposed to the soil, injected directly into the groundwater, or disposed to natural surface drainage. Some holding tanks for high-level radioactive wastes have leaked into the soil. Solid wastes from nuclear weapons processing or medical applications were commonly buried directly into soil trenches, without sufficient packaging. Moving plumes of contamination underground, which may be many metres below the surface, are difficult to detect, monitor and access, in order to conduct remedial operations.

Although the remediation of these sites is probably more complicated and more expensive on a per unit volume basis than for the sites considered previously in this section, the approach and the process leading to a decision are not fundamentally different. Nevertheless, one must consider the importance of the cost factor during the evaluation of the necessity for remediation [IAEA-1999].

4.6.3.4 Remediation of radioactively contaminated sites from extraction and processing of ores

Another potentially significant area of radioactive remediation activities is found in the mining field. Natural radionuclides may be contained in non-radioactive ores and, depending on the chemical and physical properties of the elements in the ore, may be enriched during the smelting process and later found in products or in residues (slag and other). In these residues, the radionuclides of the decay chain are frequently not in radioactive equilibrium, because the daughter products have shorter half-lives relative to the parent products. Also, flue dust and other air-borne smelting residues found in exhaust air can contain decay products like 210Po and 210Pb.

Site remediation at mining and associated nuclear materials sites include the mines themselves, on-site plants and structures, tailings impoundments, and facilities where mine products are processed, stored or used. The scale of such remedial projects can be large [IAEA-1999]. The methods and technologies used in the remediation and decommissioning of uranium mining and related facilities are dealt with in detail in the relevant IAEA reports.