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MOOSE Development at Three Mile Island

Robotics Decontamination Keeps Operators Clear of Danger

Article by George Harris; Feature image by Z22, licensed under CC BY-SA 3.0 via Wikimedia Commons

During the March 1979 accident at the Three Mile Island Nuclear Power Station Unit 2 near Harrisburg, Pennsylvania, significant quantities of contaminated water were discharged to the Reactor Building and the Auxiliary and Fuel-Handling Building. This subjected the concrete floors and walls to various concentrations of contaminants.

Most of the floors and wall areas were protected by special epoxy coatings. On examination, contamination appeared to have migrated into the coatings and, to some extent, into the concrete substrate. In areas where the concrete was unpainted or the protective coatings failed, the penetration of contamination into the concrete was significant.

The radioactive contaminants released into the facility were principally Cs-137 and Sr-90. Once embedded in the concrete floors, they produced an unacceptably high radiation field for continuous recovery operations by human workers. At TMI-2, it was confirmed that up to one-third of the background radiation exposure to workers in the reactor building resulted from radiation sources in the concrete floors. Most other facilities share the same data. Therefore, to permit greater personnel access to these areas, it was decided that the surfaces be remotely decontaminated to achieve the lowest possible radiation field.

Three Mile Island, 1979

“Three Mile Island (color)-2” by United States Department of Energy – Licensed under Public domain via Wikimedia Commons

Various wash methods, wet and dry abrasive blasting techniques and surface scarification processes have been applied for concrete decontamination. When chemical washing methods are utilized, the solvent (or foam) must be capable of putting the contaminants into solution. However. this can drive the contaminants further into the concrete, increasing the physical removal efforts which must eventually be undertaken. Further, for deeply penetrated contamination, the surface concrete may test clean after chemical washing, yet a few weeks later it can become re-contaminated owing to migration of highly concentrated contaminants towards the surface.

Much of the fixed contamination can be efficiently and effectively removed using a surface scarification process. This technique physically abrades the surface down to the depth of sound, uncontaminated concrete. At the time this method was being further developed by Pentek for use at TMI-2, it was considered a radical approach to remediate contaminated concrete owing to inefficient dust collection and poor performance of scarifying tools. However, Pentek’s MOOSE® scarifying robot and other dustless decontamination tools are now setting the standards in concrete remediation.

Early Development and Goals

Before design began on the first MOOSE® decontamination robot, a list of goals was developed by both Pentek and the Electric Power Research Institute (EPRI) to be applied at TMI-2. Those goals were to:

  1. Increase the scabbling productivity above the early rates of 100 ft²/hr using manually operated scabblers
  2. Decrease operating personnel radiation exposure by reducing the pneumatically powered scabbling head and number of operators required, and by providing distance between the operator and the area being scabbled
  3. Decrease operator fatigue and heat exhaustion associated with the jack-hammer type reaction forces, which must be physically resisted by the operator, as he/she tries to guide the scabbler over the contaminated concrete surface.
  4. Maintain a positive control over attitude, direction and depth of cut, a situation which, at that Lime, was difficult to achieve with a manual scabbler
  5. Develop a self-contained, on-board vacuum system capable of removing and collecting the dust and debris generated during the scabbling process

MOOSE wheeled floor scabbler performing trails during development at Three Mile IslandThe various critical factors for consideration during the final MOOSE® design were: power/utility requirements, on-board vacuum system, maintaining the design integrity of the mobile chassis, consideration of decontaminability and radiation resistance, and operating control system logic. This last critical factor posed some formidable challenges almost ten years ago.

The systems available for controlling the vehicle are wireless (e.g. radio control) or tethered (hardwired). With the wireless system, the vehicle must have its own on-board power supply, such as batteries. This extra weight would decrease the on-board debris-carrying capacity. Since the tethering of a compressed-air line-hose was already required for the vacuum system, it was decided to use a tethered control system. In addition. the tethered system provides the positive reliability associated with signal transmission over a hard wire.

For guidance of the vehicle, it was decided to use direct visual contact. Although closed circuit television cameras can be mounted on the vehicle for high exposure areas, the additional cost could not be justified for its initial application.

It was recognized that various interlocks and safety controls would be required. Examples include logic which ensures that the scabbler:

  1. Cannot be engaged, unless the operator is capable of keeping the vehicle moving; this is to minimize the probability of the hits inadvertently digging into the surface, causing undesirable irregularities
  2. Cannot be engaged, unless the vacuum is operating; this is to prevent the possibility of generating uncontrollable airborne contamination:
  3. Must be interlocked with translator, so that both start at the same time; this is to prevent inadvertent grooving.

MOOSE wheeled floor scabbler performing trails during development at Three Mile IslandGauges and indicators would be needed in the system to inform and warn the operator of vehicle, scabbler, and vacuum status; administrative controls rather than autonomous controls were selected for this application to limit cost and complexity.

The final prototype represented a useful piece of equipment to aid nuclear facilities in the clean-up of contaminated areas of plants. Several significant developments emerged during the design:

  1. The MOOSE® was required to tether an external air-hose into the work area, Tethered power supplies are essential to the conduct of energy-intensive work applications for compact remote vehicles.
  2. The vacuum pick-up nozzle was designed to provide effective removal of dust and debris over the entire range of anticipated variations in air-supply pressure rates.
  3. A unique yoke assembly and vibration isolation system was designed to harness the scabbling process, resulting in positive control with no perceptible vibration transmitted to the mobile chassis.
  4. A unique scabbling and piston seal design was developed to eliminate blow-by, oil leakage. and crud build-up in the vicinity of the cutting bits.
  5. All attachments to the mobile chassis should conform to the design-dimensional envelope of the original chassis, be modular to enhance maintenance and expedite changeover to other applications (e.g. reconnaissance modules), and not violate the leak-tight integrity of the mobile chassis.
  6. The line-of-sight control philosophy adopted for this application proved to be very economical for most applications.
First Commercial Experiences: The West Valley Demonstration Project

West Valley Demonstration Project (WVDP) is the site of a former commercial nuclear fuel-reprocessing facility near Buffalo, New York. During the years of operation between 1968 and 1974, normal activities at the facility required decontamination of the floors. Conventional practice was to perform a thorough decontamination and apply a coating of a paint to fix residual alpha and beta radiation, and to shield personnel from exposure to these sources. This continues to be conventional practice at most operating nuclear power-plants and other nuclear facilities. A small test in a clean area was conducted in a storage building, providing a single lane test strip, approximately five-feet long. The 1.5 foot width of the translating scabbling head therefore covered approximately 7.5 ft² per pass. After some preliminary adjustments and safety checks, scabbling commenced, with the primary purpose of demonstrating the effect of scabbling on the surface of concrete, as well as overall vacuum system performance.

The MOOSE® performed per design, and scabbled the test strip to a depth of one-sixteenth of an inch in four passes. There was no identifiable contribution to airborne dust; all surface dirt, sealers, and laitance were removed. Only inconsequential non-airborne concrete particles remained on the test area. Later analysis of material scabbled during the contaminated demonstration showed that the vacuum system removes over 99 per cent of the scabbled material from the floor, resulting in a very clean operation.

Present Applications

Since the design of the original MOOSE, several modifications have been done to provide a remotely-operated decontamination system with low-to-none down-time. The collection rates for dust and debris have increased to 99.5 per cent, specially designed, drawer-mounted 21-gallon drums have been installed, and production rates have increased to over 300ft²/hr. Although still very active in nuclear decontamination to an international list of clients, it also has applications for the safe and efficient collection of PCBs, chemical residuals, cyanide, heavy metals and other hazardous materials from concrete floors. Our most recent application of the latest generation MOOSE is the FMC Superfund site in Yakima, Washington.

Concern of Cross-contamination Brings MOOSE to FMC Superfund Site

In the latest phase of remediation, the FMC Superfund site in Yakima, Washington, employed the services of Pentek’s MOOSE® remotely operated floor-scabbler. In 1982, the 58,000 ft² Farm Machinery Corporation’s (FMC) pesticides-manufacturing facility was placed on the US Environmental Protection Agency’s (EPA) National Priorities List, based on the high levels of contaminants in the facility’s on-site waste-pit. It was not until 1987, when the EPA required a Remedial Investigation, that the contractor discovered hot spots of pesticides-contaminated concrete within the actual facility buildings. Pentek’s MOOSE pneumatically driven scabbler successfully removed the top layers of contaminated concrete to a depth of three-sixteenth of an inch from the hot spots.

Historical Background

The plant was used as a pesticides formulation and distribution centre for the Pacific North-west agricultural industry. Operations began in 1951 and continued until 1986. During the almost four decades of pesticides manufacturing, chemical residuals which were used in the formulation, but were not part of the final product, were placed in an on-site disposal pit. Many of these disposed compounds are listed as hazardous wastes, as defined by the Resource Conservation and Recovery Act (RCRA). Since the site has been targeted for remediation, the Farm Machinery Corporation has taken responsibility for its facility and has worked co-operatively with the Yakima community and EPA to rectify the situation.

Scarification of the Concrete Surfaces

Pesticides residuals were found in parts of the chemical formulation buildings, warehouses, and some of the exterior concrete slabs. It was determined that the contaminants had bonded with the concrete and thus their health effects were through dermal contact. Because of FMC’s concern over future liability, it was decided to decontaminate the buildings.

During typical concrete removal procedures, the surface is either grit-blasted or demolished. Engineering controls, like containment structures and ventilation schemes, are required to prevent cross-contamination to surrounding areas. However, constructing containments can be very time-consuming and costly. The MOOSE, with its integrated scabbling head and High Efficiency Particulate Air (HEPA) vacuum system, is an effective tool for concrete remediation, while reducing airborne exposure. Gary Della Vecchia, the service contractor’s Project Manager, described the effectiveness of the MOOSE: “The MOOSE (scabbling) robot, with its on-board HEPA vacuum system, completely controlled the dust. Airborne contamination was negligible and no containment structure was needed.”

Although surface hardeners were used in the concrete to protect against deterioration, the MOOSE was still able to remove the top three-sixteenths of an inch of concrete over a 10,000 ft² area. The on-board HEPA vacuum and waste-packaging unit deposited all contaminated concrete dust and debris into 21-gallon drums situated just below the collection unit. The extra-wide 18-inch MOOSE® scabbling head contains seven tungsten carbide bits, which pulverize the top layers of concrete and prepare the surface for new floor systems. Since no chemical, water, or abrasive media are used, absolute minimum waste is generated. At the FMC Superfund site, only 1,500 gallons of dry dust and debris were collected during the operation, which will be incinerated on-site. FMC is glad to be rid of any future liability.

Conclusion

Since the MOOSE’s inception, refined improvements have made the MOOSE® a “work-horse” for the effective and efficient removal of all contaminants from concrete flooring. Before the MOOSE®, removing contaminants was very hazardous, causing cross-contamination and worker health issues.

With the development of the present generation MOOSE® decontamination robot, which can simultaneously remove and safely package hazardous material, the remediation and hazardous waste management industry has been afforded a proven method (over 500,000 m² of concrete decontamination now complete) for reducing worker exposure and fatigue and eliminating airborne recontamination.

For more information please email us, see our contact page or visit our vacuum tool and robotics product pages.

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