Design and performance of feed delivery systems for simulated radioactive waste slurries

Abstract

Processes for vitrifying simulated high-level radioactive waste have been developed at the Pacific Northwest Laboratory (PNL) over the last several years. Paralleling this effort, several feed systems used to deliver the simulated waste slurry to the melter have been tested. Because there had been little industrial experience in delivering abrasive slurries at feed rates of less than 10 L/min, early experience helped direct the design of more dependable systems. Also, as feed delivery requirements changed, the feed system was modified to meet these new requirements. The various feed systems discussed in this document are part of this evolutionary process, so they have not been ranked against each other. The first system was a cantilevered, centrifugal-pump system which circulated slurry through primary and secondary recirculation loops. Feed was supplied to a large-scale spray calciner through a feed line attached to the primary recirculation loop. During 1830 hours of experimental use, the feed system supplied-390,000 L of simulated wastes to 'the calciner. The system worked well for transferring the relatively dilute waste slurries. However, because of high velocities in the pump housing and line, pipe and equipment wear was unacceptably high. Harder construction materials would wear better. The idea of using an airlift to move the slurry into the melter was tested next. Two designs were used. The first airlift was a feed slurry metering device only, the head pot being supplied with slurry by a pump and recirculation loop system. The second airlift system was a complex, four-stage system that transferred slurry completely by airlift. While both airlifts effectively lifted and metered feed into the melters, feed traveling through the overflow return lines and horizontal sections was prone to drying and settling. Modifying the design should improve the operation of any airlift systems. The third type of feed system was a pressurized recirculation loop system that utilized a positive displacement diaphragm pump. By controlling the v pressure within the recirculation loop, the amount of slurry entering an attached feed line could be controlled. Because the flow velocity within the recirculation loop was reduced, no significant equipment wear occurred, even though the slurries contained-350 gIL of glass frit. This system performed well during five separate experiments totaling-550 hours, in which more than 42,000 L of simulated wastes and glass formers were fed to the vitrification equipment. The last feed system tested was a variable speed, positive-displacement pump system that delivered feed directly to a melter, eliminating the need for a recirculation loop. This system was reliable in pumping slurries at rates >1.0 kg/min. However, at rates <1.0 kg/min, or with slurries containing very fast-settling solids, plugging occurred in the flowmeter. Many of the plugs were caused by foreign material such as wood splinters or small stones which entered the feed system.