Experimental evaluation of re-entrainment from wet scrubber filtered containment venting systems

摘要

Wet scrubber-based filtered containment venting systems (FCVSs) are commonly deployed at nuclear power plants, owing to the high degree of aerosol, iodine, and organic iodine retention that is possible through the pool scrubbing action that takes place during their operation. However, after a FCVS has been operating during a severe accident, the pool water will become extremely contaminated with previously-captured radionuclides, and the bubbling action of gases passing through the pool, could result in the re-entrainment of contaminated liquid droplets into the gas stream. This would produce a secondary source term during the late phases of an accident. As such, a set of experiments have been conducted in order to evaluate the aerosol re-entrainment rates and size distribution from a system prototypical of a pool-type FCVS. Entrainment rates were measured by tracking the carry-over of a NaCl tracer into a set of liquid traps, while the size of the re-entrained droplets was measured with a phase Doppler anemometer. The experiment employed a prototypical venturi nozzle and gas injection conditions to examine the influence of gas flow rate, liquid level, and pool water temperature. Entrainment factors between 5 x 10(-5) and 1 x 10(-3) were measured, and the water droplet count median diameters were between 3 and 13 mu m, and fairly wide particle size distributions, with mass mean diameters were on the order of 40-140 mu m. These experimental results help the assessment of the potential magnitude of the late phase source term hazards posed by the fission products retained in pool-type FCVS (secondary release), and the potential loading on the secondary dry filter stage (metal fiber filters) employed in FCVSs after the primary pool scrubbing stage. The secondary metal fiber filters should have a high efficiency in capturing water droplets produced through this mechanism, given the droplet's relatively large size.