Emptying Old Tanks
There are 149 aging single-shell tanks at Hanford containing a mixture of hazardous wastes. Many of these date back to World War II and have shown signs of deterioration over the years. The goal of Washington River Protection Solutions (WRPS) 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. There are numerous physical restrictions that must be dealt with in order to successfully retrieve the waste. The tanks:
- Are buried underground beneath 7 to 10 feet of soil
- Have limited access via pipes that extend from the top of the tank to the ground surface
- Contain lethal radioactive and chemical waste, requiring all work to be performed remotely
- Contain a variety of waste requiring multiple retrieval technologies
For this reason, WRPS is constantly developing and testing new techniques and technologies that aim to speed waste retrieval and reduce cost. The most current methods include:
- Modified Sluicing
- Enhanced Reach Sluicing System
- Mobile Arm Retrieval System
- Chemical Dissolution
Modified sluicing uses high-pressure water jets to dissolve and mobilize waste so it can be pumped out of Hanford’s tanks. Traditional sluicing uses a lot of water, which can quickly reduce the amount of available tank space and can cause issues when considered for use in Hanford’s leak-prone tanks. Engineers have developed a sluicing method where operating pressure and water use are reduced by two-thirds that significantly reduces impact on tank space and risk for leaks. This method can also use recycled liquid waste in place of water, further reducing the overall impact on waste volume.
The Enhanced Reach Sluicing System (ERSS) consists of a sluicer on the end of a hydraulically driven, retractable boom that extends down into the tank to attack the waste closer to its surface. The sluicer dissolves crystalized salt and sludge waste using high-pressure water or recycled liquid waste and directs the waste to a pump for transfer into a double-shell receiving tank. The device has been further modified recently to include two additional high-pressure nozzles that improve the effectiveness and efficiency of removing waste from the tank.
WRPS has worked with a commercial vendor to design and construct a robotic arm known as the Mobile Arm Retrieval System (MARS) that offers the potential to increase the efficiency of waste removal from many of Hanford’s single-shell tanks. The system includes the robotic arm, a central pump and equipment to transfer waste from tank to tank. The arm rotates 360 degrees, extends all the way to the tank walls and uses powerful sluicing tools to drive waste toward a centralized pump for removal.
A second MARS unit is being developed that uses a vacuum system. It uses much less liquid, which makes it ideal for use in tanks that are known or suspected to have leaked.
For tanks containing difficult-to-remove waste solids, chemical dissolution provides a method for dissolving and dislodging much of the hardened material so it can be pumped out. With this method, sodium hydroxide is added to the tank and the waste is allowed to soak. This softens the stuck-on material and prepares the tank for a final water rinse before the waste is pumped out.
Another waste retrieval tool is the Foldtrack®. Resembling a small bulldozer, Foldtrack is a remotely operated, track-mounted system that uses a plow blade to move radioactive waste from the floor of an underground storage tank. In addition to the front-facing blade, the unit is also armed with two onboard water jet systems, three high-pressure turbo nozzles, and a sluicing cannon for sweeping the tank floor.
A very nimble device, the Foldtrack stretches itself out to fit down a 12-inch-diameter pipe then folds back onto itself to become a robust waste-removal tool. The system is operated from a remote desktop control station located up to 800 feet away.
Cold Test Facility
Many of these technologies are tested at Hanford’s Cold Test Facility (CTF), a full-scale mockup of a radioactive waste storage where new waste retrieval and other technologies can be evaluated in a safe, non-radioactive environment. CTF is also used 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.