Design and performance of a full-scale spray calciner for nonradioactive high-level-waste-vitrification studies

Abstract

Since 1977, the Pacific Northwest Laboratory has tested a large-scale spray calciner for the solidification of simulated high-level neutralized and acid radioactive wastes. This is a report of valuable operating experience gained during 1830 h of processing time in which ~390,000 L of simulated wastes were solidified.Experience from spray calciner operations has demonstrated that atomizing-nozzle flow conditions that produce 100-~ median-volume-diameter or smaller droplets are required for large-scale spray calciners. Both internal-and external-mix nozzles have been tested. Due to its lower airflow requirements and the fewer large droplets produced, the internal-mix nozzle has been chosen for primary development in the spray calciner program at PNL. Atomizing nozzle air caps made of reaction-bonded or hot-pressed silicon nitride should provide satisfactory wear resistance and thermal shock resistance for long-term spray calciner operation.While several alternative feed system components exist, spray calciner feed system experience at PNL has been primarily with a centrifugal pump, control valve, and magnetic flowmeter. This system has performed satisfactorily, and problems have been limited to the following:• occasional plugging in the magnetic flowmeter by large foreign particles. (This problem was solved by reducing the size of the strainer-screen mesh to remove particles larger than the flowmeter orifice and by installing a larger flowmeter that had a 0.64-cm orifice instead of a 0.40-cm opening.)• erosion of the pump impeller housing, pipe elbows and ball valve in the feed tank's 304L stainless steel recirculation line resulting from feeds containing a high percentage of solids. (Solutions to this problem include use of a more erosion-resistant piping material, long-radius elbows or piping bends, ball valves with port diameters at least as large as the pipe's inside diameter, and lower fluid velocities.)• minor settling of solids around the outside edges on the bottom of the flat-bottomed tank. (Use of tank with a conical or dished bottom would be a solution to this problem.)The spray calciner has been operated at chamber wall temperatures from 400° to 900°C, depending on the type of waste feed and the feedrate. Calcine sintering, corrosion, and thermal stresses limit the maximum temperature. Rates up to 400 L/h have been demonstrated.Total furnace power requirements (not including heat losses) can be predicted by assuming that the waste feed is water and calculating the sensible and latent heat required to raise the inlet feed and atomizing air to the desired offgas outlet temperature. Furnace power consumption is not axially uniform. Data from actual calciner operation indicate that 60% of the power input is supplied by the lower half of the furnace. This is likely due to convective flow patterns inside the spray chamber.

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

“…The vertical-cantilevered centrifugal pump system was used extensively during the full-scale spray calciner development program (Miller 1981) over a period of four years. During this time, 43 processing runs were conducted, logging -1830 hours of feeding time.…”