CH2M HILL Technology for Waste Retrieval
CH2M HILL’s technology prowess is providing a stream of opportunities for enhanced safety, efficiency and economy in cleanup of the Hanford Site. Working with the U.S. Department of Energy’s Office of River Protection, our people are applying technological innovation with ingenious engineering to complete the hazardous and complex tasks of our mission.
Innovative technologies play a major role in tank waste retrieval projects at Hanford, equipping CH2M HILL with effective ways to conquer technical challenges, streamline retrieval processes, enhance worker safety, and cut project costs. Many technologies and innovative approaches are helping us meet TPA tank waste retrieval requirements and schedules, including:
Modified Sluicing
Sluicing is a traditional practice for many industries involved in retrieval of solid material from waste storage tanks. The technique uses high-pressure water jets to dissolve and mobilize waste in the tanks so it can be pumped out. However, for Hanford, the volume of water (up to 300 gpm) and the pressure (300 psi) were a concern. Hanford’s aging single-shell tanks are prone to leaking, and space is limited in the double-shell tanks receiving the transferred waste. CH2M HILL needed a technique that would use less water and pressure.
By modifying the sluicing system, our project engineers developed a more effective approach for our retrieval operations. The modified system involves either continuous or batch additions of low-volume liquids with concurrent waste removal by a retrieval pump. It operates at a pressure of 100 psi and a volume of 100 gpm, and it can use either raw water or recycled radioactive liquid waste known as supernatant. Using supernatant helps minimize the volume of waste to be stored in the double-shell tanks.
Remote Mobile Retrieval
CH2M HILL is demonstrating a Mobile Retrieval System (MRS) for enhanced waste retrieval. The MRS features two independent systems, an Articulated Mast System (AMS) and an In-Tank Vehicle (ITV), which work in tandem to remove tank waste. The AMS has a vacuum head at the end of a remotely controlled articulated arm and a high-performance waste conveyance system.
The AMS can reach down and around inside the tank to retrieve waste and debris. The ITV is a crawler tractor device with a plow blade on the front. It can maneuver about the floor of the tank and push waste within reach of the AMS’s vacuum head for retrieval. The ITV can access areas of the tank that are outside the reach of the AMS, and the weight of the ITV helps break up solid material to facilitate removal.
Sparging
For waste retrieval at tank S-102, operators devised a new sparging technique to free stubborn waste from the bottom of the tank. With sparging, air and other gases are injected into the waste at a force of 150 cubic feet per minute. The blast of air breaks up and softens the hardened waste so it can be dissolved and retrieved from the tank. The project team was able to double the rate of retrieval from tank S-102 using sparging.
Vacuum Retrieval
Applying innovation from another industry, CH2M HILL created a vacuum retrieval technology that is especially effective for use in tanks suspected of leaking. The vacuum retrieval system uses little or no water, and any needed water can be removed almost as quickly as it is applied.
CH2M HILL developed the technology by adapting vacuum retrieval equipment from the petroleum industry and making it safe for use in the nuclear field. Our engineering teams constructed a vacuum retrieval system at the Cold Test Facility and tested and refined it for deployment in the tank farms. Procedures were developed to ensure safety, and project teams received extensive operational training for remote control of the system in radioactive tanks.
When it was ready, the system was installed for initial use in tank C-203, a 55,000-gallon tank built in the 1940s to store plutonium processing waste. The vacuum retrieval system performed remarkably well with cleanout results surpassing DOE and regulator expectations for sludge retrieval. Using the system, tank-farm workers emptied the tank of 4,000 gallons of sludge, which met the TPA requirement for removal of 99% of the tank waste.
In a subsequent project, CH2M HILL used the vacuum retrieval system to remove 1,400 gallons of solid material from tank C-202. That project set a new record for speed of retrieval. Workers completed the C-202 retrieval operation in just six weeks, compared to the nine-month C-203 project.
Selective Dissolution
Selective dissolution is a technique for targeting saltcake. More specifically, it is useful for separating 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 salt crystals, and cesium-137. By removing the cesium-137 from liquids trapped within the salt matrix, the remaining saltcake meets Nuclear Regulatory Commission criteria for incidental waste.
Using selective dissolution, water sprayed at 100 psi and up to 100 gpm dissolves much of the saltcake and displaces the cesium-137. The liquids with the cesium are pumped out and transferred to double-shell tanks for eventual processing at the Waste Treatment Plant. The remaining saltcake can be further dissolved using water and retrieved as low-activity waste, which can be processed to immobilize the waste. CH2M HILL demonstrated the selective dissolution technique in the tank S-112 waste retrieval project.
Oxalic Acid Dissolution
Adding oxalic acid can further boost the performance of modified sluicing and was beneficial for the tank C-106 project, which was the first demonstration of the modified sluicing technology. Although modified sluicing was effective in dissolving and mobilizing the majority of material in the tank, approximately 28,000 gallons of hardened heel refused to yield. By introducing oxalic acid, the stubborn material was dissolved and pumped out. Emptying tank C-106, and especially removing the hard heel, was a significant achievement for CH2M HILL and DOE’s Office of River Protection.
Salt cake 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 at 100 psi and 100 gpm, and the mixture is allowed to soak. As the salts dissolve, they flow to a central low point where a pump removes the saturated salt mixture along with a small amount of suspended solids. An integrated system of nozzles, controls and devices helps operators introduce and manage the pressurized water. Water levels are maintained below historic operating levels to minimize the potential for tank leaks.
CH2M HILL used saltcake dissolution to empty 96% of the waste in tank S-112. The contents of the 758,000 gallon tank were almost entirely made up of a salt matrix from chemical processing waste.
Variable Height Pump
Sometimes the best solution to a problem is a creative adaptation of readily available, standard equipment. That was the case in development of the Variable Height Pump, or “Pump on a String.”
A thick layer of sludge on the bottom of the S-102 tank impeded traditional pumping methods, repeatedly plugging the pump’s inlet screen and disrupting operations. Retrieval engineers found the solution in a pump borrowed from the C tank farm. They altered the pump with a flexible hose and fashioned a cable and winch system to raise and lower the pump. By suspending the pump in the liquids above the sludge, they were able to alleviate plugging and maintain continuous pumping operations to clean out the tank. The pump performed reliably for steady operations and helped the team complete the tank S-102 project at reduced cost and with minimal worker exposure.
The Salt Mantis uses high-pressure water at a low flow rate to blast hardened material into small bits to allow for retrieval from tanks.
Salt Mantis
A high-pressure, low-volume water system is being evaluated to help complete the removal of intractable waste in the single-shell tanks. Manufactured by TMR Associates of Lakewood, Colorado, the remotely operated Salt Mantis directs a thin stream of extremely high-pressure water at hardened waste material to break it up for retrieval. At 30,000 psi and a flow rate of just 6 gpm, the technology can dislodge the waste without introducing excessive water into the tank. By comparison, a fire hose shoots a stream of water at 125 psi with a flow rate of 150 to250 gpm.
The Salt Mantis system is being demonstrated in tank S-112, which had 96% of the waste removed primarily through saltcake dissolution. At the start of the Salt Mantis demonstration project, tank S-112 held approximately 30,000 gallons of hardened material.
Real-Time Monitoring Probes
Two new monitoring probes are being deployed in tanks S-102 and S-109 to speed up waste retrieval and reduce project costs. The probes, which can detect concentrations of isotopes and other key constituents in the waste, provide real-time monitoring of waste cleanout during the transfer process.
Currently, retrieval operations must be stopped periodically to extract waste samples for laboratory analysis. Sampling is expensive and interrupts retrieval for several weeks while samples are analyzed. Using the probes, the continuous chemical composition data are available in real time so operations can proceed steadily and the retrieval process can be managed effectively.
A prototype for the In-Line Cesium Monitor probe was demonstrated during saltcake retrieval from tanks S-112 and S-102. As the retrieval process continues, the probe detects levels of isotope concentrations in the waste and helps operators determine endpoints for reduced cesium concentrations.
A second probe, the In-Line Chemical Species Monitor, accurately detects concentrations of nitrite and nitrate, sulfate, carbonate, phosphate and hydroxide during retrieval. It will be tested in the tank S-109 project supporting demonstration of the Bulk Vitrification Pilot Plant in early FY 2009.
Used in combination, these monitoring probes will aid in separating waste streams into high-level waste for feed to the Waste Treatment Plant and low-activity waste for supplemental treatment such as Bulk Vitrification.
Bulk Solids Mixer/Dryer
CH2M HILL is testing the Bulk Solids Mixer/Dryer technology to assist in treating and packaging transuranic (TRU) waste and other materials retrieved from the underground waste storage tanks. The technology is patterned after equipment used in the food and chemical processing industry and has been demonstrated as a safe and practical approach for drying commercial radioactive wastes.
For Hanford cleanup needs, a TRU unit is being readied for application in the TRU packaging strategy CH2M HILL is developing for 20 waste tanks. Another unit is being incorporated into the fabrication and testing of Bulk Vitrification equipment. Each unit weighs 48,000 pounds and is 10 feet tall and 20 feet long. The system deployed for Bulk Vitrification in FY 2009 will be the first known application of this technology to DOE tank waste.