Studies on Chemical Speciation of Sodium Aerosols Produced in Sodium Fire

In the context of safety analysis of fast reactors, information on chemical speciation of sodium aerosol formed due to sodium fire is important. Clough and Garland studied theoretically the formation of NaOH and Na₂CO₃. Hofmann et al. and Cherdron and Jordan reported their experimental results on chemical speciation of sodium aerosols after certain periods of exposure to atmosphere based on wet chemical analysis. It is difficult to obtain quantitative information on chemical species present in dilute solutions by conventional chemical analysis. Appropriate chemical instrumentation is needed for this purpose, the development of which, along with the methodology adopted for chemical speciation, is discussed in this paper. The present technique provides rapid information on the composition of species as a function of time following a sodium fire. Experiments were conducted in the Aerosol Test Facility (ATF) in which sodium aerosols were generated, collected on filter papers, dissolved in water, and subjected to chemical characterization using a laboratory-developed high-resolution conductometric titration facility. The titration plots revealed the presence of NaOH and Na₂CO₃ as the two major components in the dissolved aqueous solutions. The concentrations of these species were derived with due consideration of the hydrolysis of Na₂CO₃ in water. It was possible to relate these concentrations to the compositions of the samples on the filter paper residues through a separate series of measurements on the dissolved solutions of the synthetic mixtures of NaOH and Na₂CO₃ of known compositions in the solid phase. It has been confirmed by our experimental results that for the initial mass concentration of the aerosol at ~2 gm^-3 and in a confined environment of 1 m³, at a relative humidity of 50 to 65%, the entire species of sodium compound aerosol becomes sodium carbonate within 500 s from the onset of fire.