PERFORMANCE ASSESSMENT OF THE WASTE DISLODGING AND CONVEYANCE SYSTEM DURING THE GUNITE AND ASSOCIATED TANKS REMEDIATION PROJECT

摘要

The Waste Dislodging and Conveyance System (WD&CS) and other components of the Tank Waste Retrieval System (TWRS) were developed to address the need for removal of hazardous wastes from underground storage tanks (USTs) in which radiation levels and access limitations make traditional waste retrieval methods impractical. Specifically, these systems were developed for cleanup of the Gunite and Associated Tanks (GAAT) Operable Unit (OU) at the Oak Ridge National Laboratory (ORNL). The WD&CS is comprised of a number of different components. The three primary hardware subsystems are the Hose Management System (HMS), the Confined Sluicing End-Effector (CSEE), and the Flow Control Equipment and Containment Box (FCE/CB). In addition, a Decontamination Spray Ring (DSR) and a control system were developed for the system. The WD&CS is not a stand-alone system; rather, it is designed for deployment with either a long-reach manipulator like the Modified Light Duty Utility Arm (MLDUA) or a remotely operated vehicle system such as the Houdini. The HMS was designed to act as a pipeline for the transfer of dislodged waste; as a hose-positioning and tether-management system; and as a housing for process equipment such as the water-powered jet pump that provides the necessary suction to vacuum slurried waste from the UST. The HMS was designed to facilitate positioning of an end-effector at any point within the 25-ft- or 50-ft-diameter USTs in the GAAT OU. The CSEE is equipped with three rotating cutting jets mounted 120° apart. The jets, capable of delivering water at pressures of up to 10,000 psig, nearly converge at a point about 5 in. below the conveyance line intake on the end-effector. As the jets rotate, hard waste is dislodged and vacuumed up through the center of the CSEE and into a 2-in.-inside-diameter hose by the suction provided by the jet pump mounted in the mast of the HMS. A higher-pressure (30,000 psig) variation of this end-effector, the Gunite Scarifying End-Effector (GSEE), was also developed to scour the tank walls after the bulk sludge was removed from the tank floor. The FCE/CB attaches to the HMS and is equipped with process piping, instrumentation, and equipment that includes valves for flow control, flushing, and automatic sampling of the waste stream. Instrumentation in the FCE/CB was incorporated to facilitate monitoring of the waste stream flow rate and density. The FCE/CB also provides secondary containment for the waste stream and the interface to the destination tank and/or distant process piping and equipment. After undergoing extensive cold testing in the ORNL Tanks Technology Cold Test Facility, the WD&CS system was placed in service in the GAAT OU North Tank Farm (NTF) in July 1997. After completing the cleanup of two 25-ft-diameter waste storage tanks in the NTF, the WD&CS was re-deployed to the South Tank Farm (STF) where it was subsequently used with other components of the TWRS to complete cleanup of five additional 50-ft-diameter USTs. The initial waste inventory of the tanks was estimated at approximately 88,000 gal of radioactive sludge and solids and about 250,000 gal of supernatant. Sludge radioactivity was estimated at 63,000 Curies (Ci) of various isotopes. The supernatant and tank walls were estimated to contain an additional 15,000 Ci. In approximately 900 hr of operation since its deployment in 1997, the WD&CS has proven to be reliable and effective. It has been used successfully as part of the TWRS to remove approximately 95% of the curie content and 99% of the sludge waste from these tanks. This paper addresses the performance of the WD&CS during its deployments in the GAAT OU. Overall, the system performed quite reliably; system strengths as well as weaknesses will be discussed. In addition, a number of innovations and improvements, which were made by the GAAT team as it gained experience with the system, will also be presented.