on the Tanks Focus Area

More than 300 million liters of radioactive waste are stored in 280 large radioactive waste storage tanks and seven calcine storage facilities at DOE sites across our nation. Many of these tanks have exceeded their design life and, if left in place without stabilization, represent significant occupational and public risks. Although baseline tank remediation processes are planned or operating, science and technology investments are needed to address the associated high costs, technical risks, and operational inefficiencies. Using an integrated approach that brings together users, technology developers, and producers, the Tanks Focus Area mission is to facilitate the development and delivery of technical solutions that enable tank waste remediation to be successful throughout the DOE complex.

Each of the DOE tank sites is at a different stage in its tank remediation efforts. The Oak Ridge Reservation and West Valley Demonstration Project have retrieved the majority of the bulk wastes and are focused on residuals removal and tank closure. The Savannah River Site is continuing sludge and heel retrieval to feed its vitrification facility and to continue tank closure. The Hanford Site has accelerated its single-shell tank interim stabilization efforts and is preparing for waste retrieval to support feed delivery to a private treatment vendor, while the Idaho National Engineering and Environmental Laboratory is focused on interim stabilization for liquid tank waste removal while developing a permanent baseline for site treatment. This article presents highlights of recent TFA technical solutions for the various tank remediation phases.

Safe waste storage
Most sites require improvements in monitoring tank integrity, preventing tank corrosion, ventilating tanks, and characterizing tank waste. TFA is also implementing waste minimization technologies to reduce the volume of waste being added to the tanks.

Electrochemical Noise Corrosion Probe
Long-term integrity of storage tanks is critical to maintaining safe storage of radioactive waste. Corrosion control methods for Hanford Site waste tanks involve the addition of a corrosion inhibitor (a caustic sodium hydroxide solution) to maintain a high pH level and protect the tank walls. While the hydroxide is eventually destroyed, the sodium adds to waste volumes requiring treatment and disposal. Fine-tuning corrosion inhibitor additions through improved monitoring can enable additional waste volumes and associated processing costs to be minimized. (OST/TMS ID 1985)

TFA and partners at the Hanford Site worked together to develop and deploy corrosion probes using electrochemical noise (EN) monitoring capability to combat corrosion in double-shell tank walls. The EN technology measures noise generated as a result of electrochemical reactions occurring at corroding surfaces—in this case, on parts of the probe constructed of the same material used to build the original tanks. Development and deployment of a prototype EN corrosion probe was completed in 1996. Two more probes were then designed and deployed in 1997 and 1998, each one enhanced with progressively more complex instrumentation and software. Using integrated data analysis software, the probe notifies operators when corrosion is occurring, allowing them to take quick and effective actions.

Based on results from the previous probes installed in Hanford tanks, hardware and controls for the latest upgraded multifunction probe were delivered to tank farm operations in August for deployment into double-shell tank AN-105 in January 2000. In addition to the eight channels of corrosion-monitoring electrodes included on previous probes, the new system is also fitted with 22 thermocouples, a tank waste high-level detector, ports for pressure/gas sampling, and a set of strain gauges to monitor the effects of tank operations on the downhole instrumentation. These new features add greater functionality to the probe, providing a better understanding of the relationship between corrosion and other tank operating parameters and optimizing the use of the tank riser that houses the probe.

Waste mobilization and retrieval
Residual sludge and hard heels, limited riser access, and the necessity of using remote equipment present difficult waste retrieval challenges. TFA continues to investigate improved mixing and pumping technologies, including developments in the United Kingdom and Russia (see article on the International Program).

Gunite Tank Cleaning System
In the 1990s, sluicing operations at Oak Ridge National Laboratory removed the bulk of the waste in the gunite tanks. However, residual sludge remained at the bottom of the tanks, hindering closure activities. TFA and its partners developed a suite of technologies that work in tandem to retrieve sludge from the gunite tanks.

The major components of the Gunite Tank Cleaning System include a Modified Light Duty Utility Arm (MLDUA), a Confined Sluicing End Effector, and a Houdini vehicle. The sluicing end effector (OST/TMS ID 812) uses rotating, pressurized water jets to cut apart and slurry the sludge so it can be pumped from the tank. This end effector is much more effective at removing residual waste and introduces less water to the tank than traditional sluicing methods. It is easily positioned using the MLDUA (OST/TMS ID 40) or the Houdini vehicle (OST/TMS ID 98 and 2085), which also is used to plow waste and provide a work platform inside the tank.

The system began operating in the gunite tanks in 1998 and has completed sludge retrieval in five tanks (W-3, W-4, W-6, W-7, W-10), meeting regulatory requirements for acceptable retrieval levels. The system has since been relocated to another gunite tank (W-8) in preparation for retrieval activities there.

Waste conditioning and transfer
TFA is investigating waste settling, reprecipitation, solids formation, and waste transfer line plugging. This work includes monitors that report the condition of the waste, equipment to maintain proper waste conditioning, and adapting and testing systems that unblock plugged pipes. Laboratory studies on the thermodynamic and kinetic properties of waste chemistry are providing data to refine models that benefit retrieval, transfer, and pretreatment operations.

Pulsed-Air Mixer
Sludge waste retrieved from the gunite tanks at Oak Ridge National Laboratory is sluiced into a waste consolidation tank (Tank W-9) prior to transfer to the new Melton Valley Capacity Increase Tanks for storage. To keep the waste in Tank W-9 agitated and ensure its successful transfer, TFA and its partners teamed with site users to develop a Waste Conditioning System. A key component of the system is the Pulsed-Air Mixer. (OST/TMS ID 1510)

The mixer delivers pulses of compressed air from an array of plates near the bottom of the tank. The resulting air bubbles rise through the waste and pop, creating a vigorous mixing action. This effectively suspends the light waste fraction from the heavier particles, maintaining the lighter portion near the waste surface. This lighter waste can then be safely pumped through waste transfer pipelines.

Initial operations of the Pulsed-Air Mixer in Tank W-9 occurred in December 1998 to evaluate the mixer’s compatibility with the gunite tank waste. Waste transfers began in May 1999, with the Pulsed-Air Mixer operating to maintain the solids concentration of the wastes in Tank W-9 between ~3.4 and 3.8 percent prior to transfer. As of January, the system had completed a dozen transfers, with more than 1 million liters of sludge and 40,753 curies safely transferred to the new Melton Valley tanks.

Waste pretreatment
TFA is developing and implementing solutions that efficiently separate solids, extract radionuclides from wastes, and process sludge. Technical assistance provided by TFA is also helping sites ensure that the best treatment processes and flowsheets are selected.

Crossflow Filtration/Out of Tank Evaporator/Cesium Removal System
Supernatant waste in the Melton Valley Storage Tanks (MVST) at the Oak Ridge Reservation contains cesium and solid particles that can damage waste pretreatment equipment, leading to costly and time-consuming delays, as well as potential difficulties in the final waste form. In addition, the liquid waste is taking up valuable tank space. TFA teamed with ORR to deploy three technologies that work in series to filter the solids, remove the cesium, and evaporate the liquids.

A modular solid-liquid separation system using crossflow filtration technology (OST/TMS ID 350) was developed to filter the MVST waste. The system segregates undissolved solids from supernatant, supporting downstream processing operations for both liquids and solids and preventing carryover of radionuclides. In June 1999, the solid-liquid separation system began filtering MVST waste using crossflow filtration technology. After about nine hours of recycle operations, the filtrate was determined to be acceptable for further processing through the Cesium Removal System (OST/TMS ID 21) and the Out of Tank Evaporator. (OST/TMS ID 20)

The Cesium Removal System, developed and demonstrated by TFA in 1996, is a modular, transportable ion-exchange system. Made like a home water softener, it works by flowing liquid tank waste through a column packed with crystalline silicotitanate, a sorbent that selectively adsorbs cesium and lets other materials flow through. The Out of Tank Evaporator also began operating in 1996 and is used routinely at the Oak Ridge Reservation to reduce waste volume. The evaporator operates at a reduced pressure (less than normal atmospheric pressure), which reduces the liquid’s boiling point and enables more water to be evaporated at a lower temperature.

The solid-liquid separation system continues to feed the Cesium Removal System and the Out of Tank Evaporator at 1.5 gpm to process MVST waste. These combined technologies will continue to be used at ORR through 2001 or until waste consolidation operations are complete.

Waste immobilization
This wide-ranging work includes studies and tests on glass formulations, waste product performance, feed preparation, improved melter designs, and more efficient and productive melter operations. TFA work provides information to DOE to help improve vitrification operations and needed product acceptance criteria that can be used to evaluate immobilized waste forms delivered from privatization contractors.

Melter knife edge demonstrations
At the Savannah River Site, normal vitrification operations at the Defense Waste Processing Facility (DWPF) result in small deposits of glass accumulating on the melter’s pour spout and knife-edge insert. These deposits must be periodically removed, an operation that impedes the vitrification process and increases wear to the melter pour spout. In cooperation with university partners, TFA is conducting research on the dynamics of glass pouring to improve the vitrification process at SRS. (OST/TMS ID 2092)

Partners at Florida International University conducted early bench-scale work to evaluate and document glass flow mechanics. Armed with this information and new pour spout designs supplied by DWPF, TFA worked with Clemson University to design and install a DWPF prototypic pour spout with the dual knife-edge configuration in November 1998. Then, in June 1999, a demonstration of the DWPF replacement melter knife edge was completed in the large stirred melter at Clemson University’s Environmental Technology Laboratory. Modifications to the superheater pour valve were successful in reducing hydraulic resistance, while joule heating in the main pot and glass flow through a prototypic pour spout were demonstrated.

Continuing TFA and university efforts on melter pour spout improvements will increase efficiency during vitrification operations, saving both time and money for treatment and disposal of high-level waste. TFA provided a prototypic test unit to validate significant design improvements prior to radioactive operation in DWPF. Design recommendations for the new spout will be completed in FY00.

Tank closure
To assist sites in stabilizing and closing their tanks, TFA is investing in characterization of waste residuals and improving grout formulation to stabilize tanks and provide technical bases for tank closure.

Heel Sampling End Effector
To meet recent consent order requirements, the Idaho National Engineering and Environmental Laboratory (INEEL) is accelerating efforts to close a high-level waste tank. Currently, INEEL plans to close at least two tanks by grouting the heel in place. Development of a grout that will mix with the heel, solidify, and remain stable requires the availability of heel samples to characterize the chemistry of the waste. Support from TFA resulted in the development of a Heel Sampling End Effector (OST/TMS ID 2386) to provide these samples.

The sampling end effector, developed at INEEL, contains a light source, camera (with 0- to 50-ft viewing range), and a radiation detector (with 0–1000 rad/h range). A 2-inch-diameter capture tube can be immersed in up to 16 inches of liquid or soft slurry waste to pull up to 800 cubic centimeters of sample into the evacuated sample chamber.

The Light Duty Utility Arm (LDUA) was used to deploy the Heel Sampling End Effector into tanks WM-188 and WM-182 in February and November 1999, respectively. Heel samples were collected from various locations in the tanks and sent to the site’s Remote Analytical Laboratory for analysis. Solid and liquid phases of each heel sample underwent analysis for 29 elements, 18 radionuclides, and 13 organic compounds and were screened for an additional 75 compounds. Data from the heel samples will be analyzed to confirm the historical data presently used to estimate the chemical and corrosive characteristics of the tank heel and to support development of a grout formulation for eventual closure of tanks at the Idaho Nuclear Technology Engineering Center. The sample data will also help define requirements for further waste retrieval efforts, if necessary.

For additional information on solutions developed through TFA, see the TFA Web site at http://www.pnl.gov/tfa.