Technology
Emptying Old Tanks
There are 149 aging single-shell radioactive and chemical waste storage tanks at Hanford, many of which date back to the early days of the Manhattan Project of World War II. The tanks are located underground near the center of the 586-square-mile Hanford Site. The tanks have been used to store radioactive and chemical waste that was a by-product of the production of plutonium for the nation’s defense.
The first tanks went into service in 1944 to receive waste from the nearby T Plant which was Hanford’s first plutonium processing facility. All of the tanks are made of a carbon steel liner surrounded by thick concrete walls. Up to 67 of the tanks are known or suspected to have leaked as much as one million gallons of waste to the surrounding soil.
The goal of Washington River Protection Solutions is to reduce the risk posed by these aging tanks by removing the waste and transferring it to Hanford’s 28 newer, safer double-shell tanks. Retrieval of the waste, however, is one of the most challenging environmental restoration projects in the Department of Energy complex. The tanks were designed to put waste into them, not take it out, so there are numerous physical restrictions that must be dealt with in order to successfully retrieve the waste. In addition, the waste is in a variety of forms including saltcake, sludge, and a hardened “heel” on the bottom. Most pumpable liquids were removed from the tanks years ago but the solids remain. The tanks are all buried beneath 10’ of soil, and access is via pipes that extend from the top of the tank to the surface. The lethal radioactive and chemical environment inside the tanks requires that all work performed inside the tanks be done by remote control. And because of the variety of waste forms that can be in a single tank, a variety of retrieval technologies have been developed to speed retrieval and reduce the cost.
By the end of 2008 waste from a total of seven tanks had been retrieved. Six of the seven meet the criteria for retrieval that has been established by the Tri-Party Agreement and the other is under review. In addition, waste from four other tanks has been partially retrieved. To date more than 1.5 million gallons of waste has been retrieved from the single-shell tanks and transferred to newer, safer double-shell tanks where it will remain until it is prepared for disposal.
Cold Test Facility
The Cold Test Facility is located just north of Richland, Washington. It is a full-scale mockup of a radioactive waste storage tank designed to test new waste retrieval and other technologies in a non-radioactive environment. In addition, it allows us to train operational teams and work through various options to refine operating methods, streamline transfer processes and identify ways to enhance productivity which can be applied to actual retrieval operations. The testing and training improves safety, helps alleviate potential problems, and increases the quality of cleanup upon deployment of new systems in the tanks.
Mobile Arm Retrieval System
Washington River Protection Solutions is working with a commercial vender to design and build a robotic arm known as the ) Mobile Arm Retrieval System (MARS) that offers the potential to increase the efficiency of waste removal from Hanford’s aging single shell tanks. The system includes the robotic arm as well as the ancillary equipment to transfer the tank waste to newer and safer double-shell tanks. The arm will include a wide range of motion and also include a telescoping capability to reach all parts of the tank. The system will also include the capability to change out the working end of the arm to enable the arm to use different tools to remove the different waste forms inside the tanks. Development of new and innovative technologies to safely speed the retrieval of tank waste and make retrieval operations more efficient and cost effective is a cornerstone of Washington River Protection Solutions company goals.
Modified Sluicing
Sluicing is widely used by many industries to clean waste from storage tanks. It uses high-pressure water jets to dissolve and mobilize waste so it can be pumped out. However, for Hanford, the volume of water, up to 300 gallons per minute (gpm) and the pressure, 300 pounds per square inch (psi) were a concern. Hanford’s aging tanks are prone to leaking and space in double-shell tanks is limited, so new techniques were required that would use less pressure and less water.
By modifying the sluicing system our engineers developed an effective approach for retrieval that reduced the impact on the aging tanks and reduced waste storage requirements. Operating pressure was reduced to 100 psi and water volume was reduced to 100 gpm. It can also use recycled radioactive liquid waste in place of water, further reducing the overall waste volume that has to be stored.
Remote Water Lance
A high-pressure, low-volume water system was used in single-shell tank S-112 to help complete removal of hardened waste at the bottom of the tank. Up to 96 percent of the tank’s waste had been removed using modified sluicing technology, but the remaining 4 percent would not yield. A search of the private sector for technologies yielded a remotely-operated device dubbed the Salt Mantis. This clever, yet simple device, directs a thin stream of extremely high-pressure water (30,000 psi and just 6 gpm) to break up and mobilize the hardened material. In the S-112 demonstration project the Salt Mantis helped remove approximately 30,000 gallons of hardened material that would not yield to conventional techniques. It performed beyond even the most optimistic expectations.
Saltcake Dissolution
For tanks containing saltcake, the saltcake dissolution technology provides a method for dissolving and dislodging much of the hardened material so the waste can be collected and pumped out.
With saltcake dissolution water is sprayed into the tank and the mixture is allowed to soak. As the salts dissolve they flow to a central low point in the tank where the pump removes the mixture. Water levels are maintained below historic operating levels to minimize the potential for tank leaks.
Vacuum Retrieval
This technology uses little or no water to literally vacuum waste out of the tanks and is ideal for use in tanks that are suspected of leaking. When water is needed it can be vacuumed from the tank almost as quickly as it is applied. The equipment was adapted from the petroleum industry, tested at our Cold Test Facility and was used to remove waste from four of Hanford’s 55,000 gallon tanks. The equipment performed remarkably well with results surpassing expectations.
Sparging
To remove stubborn waste from the bottom of tank S-102, engineers devised a new sparging technique which forced air into the waste at a force of 150 cubic feet per minute. The blast of air broke up and mixed the hardened waste so it could be dissolved and retrieved from the tank. The retrieval rate doubled compared to previous techniques.
Selective Dissolution
Selective dissolution is used to target saltcake. More specifically, it separates soluble radionuclides from saltcake in the tanks. At least 27 single-shell tanks at Hanford contain saltcake, with mixtures of sodium nitrate, sodium carbonate, sodium phosphate, and Cesium-137. By removing the Cesium-137 from within the salt matrix the remaining saltcake meets the specifications for treatment and disposal as low-activity waste.
Oxalic Acid Dissolution
Adding oxalic acid can further boost the performance of modified sluicing and was beneficial for retrieval of waste from tank C-106, which was the first demonstration of the modified sluicing and was the first tank to be emptied under the Tri-Party Agreement which governs Hanford cleanup.
Modified sluicing was effective in dissolving and mobilizing the majority of the waste in the tank, but approximately 28,000 gallons of hardened heel refused to yield. By introducing oxalic acid, the stubborn material was dissolved and pumped out.
High-Pressure Water Mixer
A new tool called the Rotary Viper has proven effective in mobilizing hardened waste for efficient removal from the single-shell tanks. It provides high-pressure water at the end of a long pipe, which allows focused control in directing the water spray precisely where it is needed to break up and mobilize the waste so it can be pumped from the tank. It can be attached to a variable height shaft to??? and its spray can be controlled for optimum effectiveness. It can also be lowered beside a pump head to clean the pump’s screen and help prevent clogging.
Variable Height Pump
Sometimes the best solution to a problem is a creative adaptation of readily available, standard equipment. That was the case in the approach used to remove liquids from single-shell tank S-102. A thick layer of sludge on the bottom of the tank impeded traditional pumping methods by constantly clogging the pump inlet screen. To solve this problem a pump head was attached to a flexible hose and lowered into the liquids from above. The pump performed reliably for steady operations.
Foldtrack
Hanford’s latest waste retrieval tool is the FOLDTRACK®, a remotely-operated, track-mounted system that uses a wide blade to move radioactive waste from the floor of an underground storage tank to a central pump so waste can be removed. The unit is also fitted with two on-board water jetting systems to aid in moving waste and cleaning tank walls. The innovative device can stretch itself out to fit down a 12” diameter pipe, reconfigure itself inside the tank, then right itself on the tank floor to do its work. The fully-hydraulic unit uses no electrical equipment or onboard instrumentation, making it simple and reliable. It is operated from a remote desktop control station that can be located up to 800 feet away.
The Alligator
The “Alligator,” an inexpensive and user-friendly device, has proved effective in obtaining samples of thick tank waste sludge by using a hinged jaw to capture a “mouthful” of waste and hold it securely. The Alligator is small, weighing less than two pounds, and is much less expensive to fabricate, deploy and operate inside the waste tanks compared to other sampling devices. It is made from off-the-shelf components with a small electric motor powered by a battery pack with eight standard AA batteries. The Alligator collects larger samples for laboratory analysis faster and more efficiently than other methods. Testing in the Cold Test Facility proved that by adding serrated teeth to the jaws, rather than a smooth edge, gave the Alligator the flexibility to sample diverse waste forms.
