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4.3.3 Planning approach: Removal of source term

Contents
4.3.3.1 Excavation
4.3.3.2 Contamination control methods for decreasing the potential for exposures
4.3.3.3 Excavation techniques
4.3.3.4 Immobilisation and solidification (ex-situ)

4.3.3.1 Excavation

It should be noted that in general any method relying on the removal of contaminated soil is likely to require the substitution of the removed material with clean soil. Therefore, in addition to considerations with respect to technical feasibility, an economic source of clean soil will be required to make this option viable. Conversely, a precondition for any removal option is the availability of a suitable disposal site for the excavated materials, whether they are left untreated or whether they are conditioned before emplacement.

Retrieval consists of excavating and removing buried wastes or subsurface contaminated soil or sediments. For buried wastes, retrieval could entail removal of overburden soil, interstitial soil and possibly impacted underlying soil as well. Retrieving low level radioactive and hazardous soil and buried wastes from a site is a proven and reliable approach. However, retrieval and waste management techniques for transuranic wastes have not been proven to the same extent and may require site specific and innovative design elements to ensure protection of human health and the environment. In addition, problems with the acceptance of wastes in disposal facilities might arise.

The removal of wastes from a site allows them to be treated to reduce the toxicity and mobility of the various contaminants with a view to making the wastes suitable for disposal in a licensed engineered facility. Retrieval removes or greatly reduces the risks associated with the site if the retrieved wastes are disposed of off-site or isolated from the environment. This typically results in significantly reduced long term site monitoring and maintenance requirements. Furthermore, with a complete removal of the contaminants, the site can be released for unrestricted use. However, it has to be borne in mind that the disposal site may now need such monitoring and maintenance. Nevertheless, some advantage would be gained by concentrating contaminants at a smaller number of sites requiring supervision.

The retrieval and disposal of waste materials is time consuming and expensive. One of the greatest concerns in retrieving buried radioactive and hazardous wastes and contaminated soil is increased potential for worker exposure, contamination spread and off-site release.

Two categories of technology are usually implemented during retrieval of contaminated materials and wastes from sites with mixed contamination:

  1. Excavation: various associated equipment are available on the market, including conventional heavy earth moving equipment, standard construction equipment with appropriate modifications (e.g., sealed and pressurized cabins with filtered intakes and extracts or supplied air) and remotely controlled equipment. Most equipment used for excavation of soil and buried wastes is standard heavy construction equipment proven for use at hazardous waste sites around the world. If the hazards at a site are particularly severe, remotely operated equipment and hermetically (airtight) sealed equipment with filtered or supplied air can also be used.
  2. Contamination control minimizes the spread of contamination and controls the source. Depending on site-specific conditions and materials present (e.g., soils, bulk debris, process sludge’s and containerized liquids), various controls may be used.

Most of the required equipment and techniques for excavation or retrieval have been proven in highly contaminated environments. For example, remote excavators have been proven successful in waste retrieval simulations and have been used throughout facilities for decontamination and demolition work. In addition, shielded excavators and hermetically sealed vehicles have been used successfully. In general, hermetically sealed retrieval equipment is less expensive, needs less maintenance, is capable of more precise digging and can be operated faster than remote equipment. In some environments, shielding (e.g., LexanTM windows) of equipment is required to protect workers from potential explosions and radiation. Filtered or supplied air can be added to equipment to protect operators, as has been proven at many sites. Additional information can be found in [IAEA-2006b].

It should be noted that where a medium, typically water, is being used, secondary mixed wastes or wastewater may arise that require treatment and disposal.

4.3.3.2 Contamination control methods for decreasing the potential for exposures

In general, controls are grouped into two categories – those used before retrieval and those used during retrieval. Both types can be effective at controlling contamination, thus decreasing the potential for exposure, the costs of operation and maintenance of equipment and the cost of decontamination. Process options for contamination control include the following [IAEA-2006b]:

  • Confinement: enclosures constructed from plastic, metal, fibreglass or other materials are used to prevent the spread of airborne contaminants by enclosure of a piece of equipment, work area or an entire site. Enclosures may be relatively lightweight and portable or they may be more substantial, sturdier and less portable. Enclosures are typically double walled to minimize the potential for contaminant releases.
  • Ventilation and vacuum systems: ventilation systems use laminar airflow at the dig-face of an excavation and within enclosures to direct dust to high efficiency particulate air (HEPA) filter units. Vacuum systems are used to remove loose particles from equipment and structures and draw in dust and debris generated during excavation activities.
  • Foams, sprays, misters, fixatives and washes: their application is intended to control odors, volatile organic compounds (VOC), dust and other emissions, creating a barrier between the work surface and the atmosphere, or to aid settling airborne particulates. They are also used in the decontamination of personnel and equipment. The materials selected are non-toxic, non-hazardous, non-flammable and bio-degradable.
  • Electrostatics: electrically charged plastics and electrostatic curtains can be used to minimize the spread of contamination from enclosed areas. Curtains can be used upstream of emission filtering systems to neutralize charged dust particles.
  • In-situ stabilization: in-situ stabilization can be performed before initiating excavation operations to control contamination in the soil and waste matrix. Grout, resins or polymers may be injected into wastes or soil to solidify material or sprayed onto the surface to suppress dust generation. Stabilization can also be achieved by in-situ vitrification or ground freezing technologies.

4.3.3.3 Excavation techniques

A number of hand-held tools of specialized designs have been developed to facilitate the retrieval of various waste forms. Designs include grappling devices for waste containers and debris, as well as water jets, magnets and vacuum systems. A summary of potentially available hand-held tools is presented in Table 4.4.
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Technology Description

Remote excavators

Remotely controlled demolition robots Remotely controlled excavators with a fully articulated telescopic arm. Available with several different end-tools that can be used for hammering, cutting and scooping wastes. The largest varieties can reach approximately 4 m below the ground surface.

Remotely controlled confined space demolition equipment Remotely controlled excavators with a telescopic boom capable of moving in three dimensions. Available with several end-tools. The largest Keibler Thompson machine can reach approximately 5 m below the ground surface.

Remotely operated excavator The excavator is mounted on a wheeled undercarriage that was developed to retrieve unexploded ordnance. A television set provides images for remote control. The only such excavator in existence is currently used at an air force base.

T-Rex® front shovel excavators that require modification for use Tele-operated, heavy lift, long reach excavators designed to retrieve boxes, drums and containers with a front shovel excavator. Controls can be operated from distances of up to 380 m (1250 ft) from the excavator.

Front end loaders with a bucket of 2 m3 volume Front end loaders developed for use by remote control. They provide a three dimensional colour video/audio feedback and can be controlled from distances of up to 500 m. These systems can be modified for use on excavators.

Tele-operated excavators using T-Rex® remote control kits Remotely controlled excavators (bucket and thumb) adapted for hazardous environments, such as unexploded ordnance (UXO), through sensors, controllers and hydraulic components.

Remotely controlled excavator vehicle system experimental platforms based on excavators

Remotely controlled, tethered platforms for excavators. Attachments can grasp objects, sift soil and make an excavator act as a bulldozer. A clam shell and air jet vacuum system can also be attached.
Automated ordnance excavators Remotely controlled excavators with extended reach capability, developed for unexploded ordnance (UXO) removal. Can grasp objects such as drums and boxes.

Small emplacement excavators Military tractors with a front end loader and backhoe remote operation for retrieving buried wastes and soil. Systems can be controlled from distances of up to 800 m.

Remotely controlled excavators, Hitachi excavators, innovative end-effectors and self-guided transport vehicles.
Standard excavators with end-effectors (such as buckets, rippers and breakers) used for buried waste retrieval.
Systems can be controlled from inside a cab, via a remote tether or from distances of up to 750 m.
Modified Bobcats®.

Remotely controlled skid steer loaders with a Bobcat® vehicle base with barrel grapple, sweeper and bucket attachments. Modified for use in hazardous environments, remote kit for other excavators.
Standard construction equipment with modifications

Sealed and pressurized cabins, with filtered air intakes and extractors Standard construction equipment with modifications made to the cabins. The sealed and pressurized cabins use filtered air (through high efficiency particulate air filtration).

Sealed and pressurized cabins, with supplied air Standard construction equipment with modifications made to the cabins. The sealed and pressurized cabins use supplied air.

Remote end-tools

Safe excavators High pressure probes dislodge compacted soil, other hardened materials using an air jet/vacuum end-effector system. Vacuums up soil.

Two armed, tethered, hydraulically powered interstitial conveyance systems Crane deployed with two excavators and vacuums designed for low level radiation fields. Maximum pick-up load of 320 kg.

Highly manipulative tentacles Tele-operated manipulators and bellows actuators.

Schilling Tital II® Manipulators deployed by cranes for selective retrieval. Basic components include a hydraulic system, positioning system, electronics module and mechanical interface.

Mineclaw® Manipulators with a strong electromagnet to pick up barrels. Custom grapple with a payload of several hundred kilograms and an electromagnet to retrieve metals.

Confined sluicing end effectors Water jets designed for waste tank clean-up. Use high pressure water jets to cut material into small pieces and evacuate with a vacuum jet pump. Captures slurry water.

Soil skimmers Skimmers remove soil overburden, for example in 8, 10 or 15 cm increments. Adjustable depth controls determine the depth of cut without disturbing soil underneath.

Innovative end-effectors These consist of three assemblies: a thumb, an attachable/detachable integrated transfer module and a shovel assembly capable of soil retrieval and dust-free waste dumping.

Quick-change couplers These are available in manual and hydraulic versions. They are used on various buckets, rakes, clamps, rippers and other end-effectors.

Vacuum systems Nuclear grade vacuum systems for contamination control and retrieval of soil with high efficiency particulate air (HEPA) filtration and waste containers safe from criticality.

Table 4.4 Description of retrieval equipment.
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As indicated in Table 4.5 individual criteria of a remediation technique may be evaluated, if data for the technique are available.

Criteria   Quality  
Effectiveness in remediating the contamination  
Ease of implementation  
Cost associated with the remediation programme  
Occupational safety and health risks associated with the technology  
Potential secondary environmental impacts (collateral damage)  
Prior experience with the application of the technology  
Socio-economic considerations  

Legend: x low; xx medium; xxx high.

Table 4.5 Evaluation of remediation criteria for given technique.

4.3.3.4 Immobilisation and solidification (ex-situ)

A solidification of excavated materials comes usually after excavation. Often the objective is not only to immobilize the contaminants but to add value to the waste material by converting it into a useful product, for example for construction purposes. Use in general construction as a substitute for valuable raw materials requires special testing and licensing procedures to ensure environmental compatibility and compliance with quality criteria such as compressive strength, freeze-thaw cycle stability, leachability, etc. Solidified wastes may also be used in the construction of cappings, etc., for (hazardous waste) landfills. In cases where no further use is envisaged, minimization of the volume increase by the solidification agents is desirable to save valuable raw materials and repository space. If only small volumes arise, the material may be combined with material from other waste streams requiring a similar immobilization treatment. Combining waste streams can make the process more economically viable, as products in marketable quantities are produced [IAEA-2004b].

The treatment may be undertaken on or off the site at dedicated facilities. In the case of off-site treatment, the material has to comply with the applicable transport regulations and must meet the appropriate safety criteria while being handled. The additional risk from transporting material must be worked into the respective safety and cost-benefit analyses.

The main conclusion of a recent report on the European perspective of naturally occurring radioactive material (NORM) waste treatment was that immobilization is not widely used or accepted as a treatment. Many companies regard this type of technology as less feasible for naturally occurring radioactive material (NORM) waste material and hence have not pursued the development of immobilization techniques as a waste treatment process. However, for treating the radioactive remainder of a separation step, immobilization is widely seen as a treatment with a high potential [IAEA-2004b].

Into this latter classification would also fall ground freezing as a temporary measure to prevent the dispersal of contaminants. Either an impermeable screen around a contamination can be established or the contaminated material itself can be frozen in order to facilitate its handling. In either case, it is unlikely that in the present context of low level dispersed contamination this method would find a field of application [IAEA-2004b].

The removal of a contaminated topsoil layer is, of course, the most effective measure, but generates large quantities of waste and is only applicable to small areas of land. Moreover, the most fertile layer of the soil is removed in the process. The overall efficiency of such a measure depends very much on the operating conditions and on the distribution of the contamination in the vicinity of a critical group. In Belarus, the Russian Federation and Ukraine the removal of contaminated topsoil was recommended for all settlements where the 137Cs activity exceeded 555 kBq/m2 and for 25 – 33 % of settlements where the 137Cs activity was in the range of 370 to 555 kBq/m2. It was estimated that it would incur costs of about €325 per inhabitant [IAEA-2004b].

Containment technologies aim to prevent exposures by isolating contaminants at the site and obstructing migration to surrounding soils and groundwater. Containment technologies are considered when contaminated materials are to be permanently disposed at a site or as a temporary control measure to prevent the spread of contamination. Containment options are considered when extensive subsurface contamination precludes treatment or excavation of the waste [IAEA-1999]. In general, containment technologies are applicable to all forms and types of waste.