When the operations manager at the Fernald, Ohio Super Fund Site Remedial Action Project needed a dredge to remediate sludge from one of four on-site waste (raffinate) pits, they required more than the usual dredging standards. The dredging operation was complicated with the possibility of unknown radon emissions, chemical contamination, and radiation that could be unleashed as material was dredged.
The Super Fund nuclear waste project will require removal of sludge from on-site waste pits and from a facility for chemical stabilization and solidification of the low-level radioactive sludge. The project is funded up to $1 billion and scheduled for completion by the year 2001. Cleanup is being conducted by the Morrison Knudsen Corp. of St. Charles, MO, in cooperation with Jacobs Engineering.
The pits contain low-level radioactive 'raffinate' debris and sludge. It was important that the contaminated sludge be kept wet to keep radon particles from becoming airborne, and to minimize inhalation or ingestion of radon. By using underwater dredging equipment and pumping the sludge through hoses to the processing operation, the mixture could remain wet and the contamination enclosed throughout the operation.
The solution to the Fernald requirements lay in the implementation of a remote controlled dredge. Shore control would eliminate operator exposure since the dredge could traverse the pit using a system of cables and winches keeping the operators a safe distance from the pit. The dredge would also have to be equipped for on-board operation.
Morrison Knudsen made the decision to purchase a dredge from Wisconsin-based Liquid Waste Technology LLC The company designs and builds the Pit Hog™ line of dredging and pumping equipment for removal of sludge, sediment, and residuals. The company’s floating dredges have been successfully used on automated sludge removal systems at wastewater treatment plants, power plants, petroleum refineries and other commercial and municipal facilities that require lagoon pumping.
Two months later, a Pit Hog™ remote control dredging system was launched. The dredge implemented at Fernald utilizes an Allen Bradley programmable logic controller (PLC) to direct its traversing movements, depth and rate of pumping. Engineers at Liquid Waste Technology programmed the PLC so that it arrived 'custom tailored to suit our needs and meet our requirements,' said Bruce Fox, project operations engineer.
There are a total of four on-site waste pits scheduled for cleanup. The Pit Hog™ dredge can be transported from pit to pit as needed. The initial dredging operation was performed in raffinate pit 3.
On-site testing was performed over a period of three weeks to evaluate the Pit Hog's™ effectiveness. The engineers were surprised by this particular dredge’s performance. Design engineers had estimated the range of percent solids delivered in the discharge slurry to between 5% and 20% solids. The Pit Hog™ dredge exceeded those expectations.
According to Fox, the dredge pumped between 16% and 22.6% solids rather than the expected 15% solids slurry. On average, a 20.1% solids content slurry with an average unit weight of 71.8 lbs/cu.ft. was pumped. Flow rates were estimated at 450 gpm slurry flow through 300’ of 4-in connecting hose with aluminum floats.
On-site disposal will be accomplished by transforming the sludge into grout, which will subsequently be used to fill voids in debris from destroyed uranium refining buildings. There are 40 'disposal cells' at the 220 acre site which are filled with rubble.
The liquid grout made from the sludge will be poured into cells, surrounding the rubble and preventing the settling of debris in the cells while at the same time disposing of the dredge waste. According to John Enger, project manager, 'waste pit dredging is a preferred method when waste is treated and disposed of on-site.' The treatment, called chemical stabilization and solidification mixes the sludge with 20% to 25% solid including flash (calcium oxide ash) and cement for a grout consisting of about 20% to 25% solids.