In September 2000, the Oak Ridge Reservation (ORR) in Tennessee took a giant step toward tank closure with the successful completion of the Gunite and Associated Tanks (GAAT) Remediation Project. With key technology development provided by the Tanks Focus Area (TFA) and the Robotics Crosscutting Program (RBX), partners at Oak Ridge National Laboratory (ORNL) concluded a three-year project that cleaned out eight underground gunite tanks, removing more than 423,000 gallons of radioactive sludge and liquid containing approximately 82,000 curies of radiation.

The GAAT cleanup is not only a major milestone on ORR’s tank closure path but also a noteworthy achievement in lowering risks to human health and the environment. The use of robotics helped ORNL reduce the overall GAAT remediation baseline schedule by about 12 years and avoid approximately $135 million in costs.

In the beginning

The Gunite and Associated Tanks (GAATs) were constructed in the 1940s as underground tanks to collect, neutralize, store, and transfer liquid radioactive and/or hazardous waste generated by routine facility operations. Using traditional high-pressure sluicing methods, the bulk of the liquid waste was removed from the tanks in the 1980s. This operation left behind a dense sludge layer on the bottom of the tanks. To protect workers, the public, and the environment from risks posed by the remaining waste, ORNL initiated the GAAT Remediation Project in the early 1990s. The objective of this project consisted of removing the waste from eight gunite tanks and cleaning the tanks to the level necessary to allow closure actions to proceed.

Based on the need for identifying safe and cost-effective options for retrieving the GAAT waste, ORR requested the assistance of TFA. In close coordination with ORNL users, TFA teamed with RBX and the Accelerated Site Technology Deployment initiative to deliver an integrated suite of technologies to mix, mobilize, retrieve, and transfer the gunite tank waste.

Tackling sludge retrieval and transfer with robotics

TFA, RBX, and the ORNL Environmental Restoration Program developed an innovative waste retrieval strategy by marrying the strengths of two different retrieval methods—arm-based and vehicle-based—to apply a highly effective retrieval system in the GAATs at ORNL.

Collectively referred to by the project team as the Radioactive Tank Cleaning System, the technologies include the following:

Modified Light-Duty Utility Arm (MLDUA, Tech ID 40)—This mobile, multiaxis, robotic arm is equipped with a computer-based system for controlling and monitoring in-tank activities and a gripper for holding various tools.

Houdini (Tech ID 98 and 2085)—A remotely operated vehicle, this robotic crawler folds out on the tank floor to provide a working platform. It includes a manipulator arm for grasping tools and positioning them in remote areas of the tank, as well as a plowblade for pushing dislodged materials to a jet pump.

A number of pipes were used to deliver waste and transfer it among the gunite tanks. To seal off these internal tank pipes and isolate the tanks, the Tanks Focus Area teamed with laboratory partners to develop a Pipe Cutting and Isolation System (Tech ID 2093). Adapted from inexpensive commercial tools, this technology was deployed on the MLDUA in Tanks W-6, W-7, W-8, W-9, and W-10.

Confined Sluicing End Effector (CSEE, Tech ID 12)—Gripped by either the MLDUA or Houdini, the CSEE uses rotating, high-pressure water jets to cut part and slurry the sludge waste. It is coupled to a jet pump and hose system for conveying the waste up and out of the tank.

Hose Management Arm (HMA)—A companion arm to the MLDUA, this mechanical follower “swinging arm” is the waste conveyance system that starts at the CSEE. A jet pump located inside the HMA deployment mast sucks waste from the CSEE and pumps it to the end of the HMA and into the flow control box at the surface.

Beginning in June 1997, these technologies worked together to mobilize and retrieve the sludge waste and transfer it to GAAT W-9. Here, the waste was consolidated for conditioning prior to transfer to the stainless steel Melton Valley Storage Tanks (MVSTs). Additional technologies were deployed in Tank W-9 to mix the waste and separate dense sludge particles from the lighter waste fraction.

Waste Conditioning System (Tech ID 2385)—Designed to keep the waste within transfer specifications, the system consists of an enclosure for the primary conditioning system components (samplers and a waste classifier) and a separate Solids Monitoring Test Loop enclosure containing the solids monitoring instrumentation.

Pulsed-Air Mixer (Tech ID 1510)—An integral part of the Waste Conditioning System, this mixer delivers pulses of air from an array of steel discs at the bottom of the tank. These air pulses create large bubbles that rise to the waste surface, creating a mixing action among the sludge and slurry waste. By varying the air pulses, the degree of mixing is controlled to limit solids entrainment to within transfer specifications.

There’s always one “special case” in every story, and this is no exception. In early 1998, remote inspections of Tank W-5 indicated that the interior walls had significantly deteriorated. Chunks of gunite were falling from the walls, exposing the wire mesh used in the construction process. Rather than risking damage to the robotic retrieval equipment, the GAAT team deployed two commercially available Flygt Mixers (Tech ID 2232) to mix and mobilize the waste in Tank W-5 into a transportable slurry. At the completion of retrieval operations in August 1998 using the Flygt Mixers, more than 60% of the residual contamination was removed, leaving a waste heel of less than 2% of the tank volume. As a result of the success in Tank W-5, one of the Flygt Mixers was also deployed in the consolidation tank, W-9, to facilitate mobilization and mixing of sludge waste.

Heavy Waste Retrieval System (HWRS, Tech ID 2194)—To transfer the final dense sludge out of Tank W-9, the HWRS uses a positive displacement pump in combination with a surge tank and adjacent supernatant reservoir. Together, this equipment provides a means of maintaining continuous flow to the transfer line and keeps solids from settling in the line.

By July 2000, more than 60,000 gallons of sludge was retrieved from seven gunite tanks, resulting in the transfer of 483,000 gallons of sludge and process water to the MVSTs in a series of 18 transfers. The project team then focused on removing the remaining dense sludge layer from the consolidation tank, W-9. The high concentration of solids in this tank presented a very real potential for transfer line plugging during waste delivery to the MVSTs. The team decided to first transfer the waste from Tank W-9 to the site’s Bethel Valley Evaporator Service Tanks. Here, the waste was mixed with additional supernatant and sludge, then transferred to the MVSTs when an acceptable solids concentration was reached. By September 2000, approximately 53,000 gallons of sludge and supernatant waste was successfully retrieved from Tank W-9, signaling completion of the majority of the GAAT retrieval efforts.

What a load of waste!

At the start of GAAT retrieval in June 1997, more than 300,000 gallons of supernatant and about 96,000 gallons of sludge waste were stored in the eight gunite tanks. In just over three years, the GAAT Remediation Team cleaned all eight tanks to a level the Tennessee Department of Environment and Conservation deemed acceptable for closure activities to commence. By September 2000, only 7,500 gallons of combined sludge and supernatant remained in the tanks, reducing the curie count from 85,838 curies to just under 4,200 curies. The table shown summarizes the “before and after” tank waste volumes and the associated curie levels.

Completion of GAAT retrieval represents a significant site milestone toward completing the GAAT Remediation Project. The success of the GAAT project represents a key accomplishment for University of Tennessee-Battelle, Bechtel-Jacobs, TFA, RBX, and other contractor contributors that developed the equipment and supported the retrieval operations. In addition to successfully accelerating cleanup efforts at ORNL, results of the GAAT retrieval activities provide valuable lessons learned and information for other DOE tank sites currently evaluating retrieval options and developing plans for their retrieval projects.

Lessons learned

As with any large project—especially one involving limited-access radioactive waste tanks—only so much advance planning can be done. Over the course of the multiyear remediation project, the project team encountered numerous instances where the in-tank situation was not what they expected. This meant regrouping and figuring out how to approach the problem, while keeping the project schedule in mind. The team was able to manage these difficulties through operational efficiencies gained in each successive tank.

The team also benefited from extensive cold-testing conducted in a nearby test facility. All equipment and modifications were demonstrated in a nonradioactive environment before deployment in the gunite tanks to enable the team to check for any operational glitches before contaminating the equipment. One such case involved the MLDUA.

Initial testing of the robotic arm showed that its position control mechanism was highly sensitive to deviations from the commanded position, as might occur when trying to push against a fixed obstacle such as a tank wall. The team devised a method of obtaining samples from the wall surfaces using a spring-loaded scrape tool held by the MLDUA. The spring mechanism kept the tool pushed against the wall, while the arm moved straight up, safely following its commanded positions.

The team also modified the original Houdini vehicle design based on operational experience in three gunite tanks from June 1997 to August 1998. During initial deployment, the team encountered significant problems with operation of the onboard manipulator arm. A new and improved Houdini-II began in-tank sludge mobilization work in January 1999. The versatility of the Houdini vehicle enabled new tools to be quickly integrated into the system, based on emerging situations encountered during the retrieval campaigns.

Originally designed to cut and cap abandoned pipes, the Pipe Cutting and Isolation System (Tech ID 2093) also proved useful for clearing in-tank obstructions to facilitate wall scarification efforts. In addition, an unexpected benefit of the pipe-capping tool was an improvement in the tank vacuum level, reducing the potential for release of airborne contamination.

Project closeout and preparing for tank closure

TFA assisted GAAT closure planning efforts early in the project through the assessment of tank closure options and development of a reducing grout formulation for tank stabilization. One promising option, called MultiPoint Grout Injection™ (Tech ID 2368), from Ground Environmental Services was selected on the basis of its superior ability to mix tank waste with specifically tailored stabilizing grouts. This technology was cold-tested and proposed for deployment if significant quantities of waste and radioactivity remained in the tanks after retrieval. Due to the efficiency of the waste retrieval operations and the minimal quantity of waste remaining in the GAATs, the enhanced mixing ability of the MPI™ process was deemed unnecessary for closure of the GAATs.

The site must now determine a path forward for disposition of the equipment used in the GAAT Remediation Project. In 2001, TFA is assisting with these disposition efforts by supporting transfer of the technologies for application at other DOE sites, including readiness preparations and equipment transport, as well as expert technical assistance from ORNL GAAT project staff. So far, the Savannah River Site plans to take the Houdini-II vehicle as a backup for its current sludge retrieval technology, and the Savannah River Technology Center is interested in the MLDUA for small tank cleanup at its research facilities. In addition, the Hanford Site is interested in using Houdini-I to support a facilities deactivation and decommissioning project.

The success of the GAAT Remediation Project exemplifies the results that can be obtained by leveraging the resources of multiple organizations, in both funding and technical expertise. Working together through all phases of development and deployment, the site users and technology developers integrated numerous technologies into the GAAT Remediation Project, shaving 12 years off the project schedule and providing significant cost savings over the planned life of the project. Another invaluable contribution of this effort is the extensive retrieval experience that can be shared to help other DOE sites with larger and more complex tank retrieval challenges. Combined with the lessons learned, knowledge gained from the GAAT project is applicable to the many planned and ongoing retrieval efforts under way at DOE high-level waste sites across the country.

For more information, contact Jacquie Noble-Dial, TFA Site Representative at DOE Oak Ridge Operations Office, (865) 241-6184, nobledialjr@oro.doe.gov; Pete Gibbons, Numatec Hanford Company, TFA’s Retrieval Technology Integration Manager, (509) 372-4926, peter_w_gibbons@rl.gov; or Barry Burks, Providence Group, Technical Lead for the Robotics Crosscutting Program, (865) 671-1434, blburks@providence-group.com.