Experimental studies on morphological properties of sodium combustion, fission product, structural material and mixed aerosols in a closed chamber towards fast reactor safety

Abstract

In the event of Core Disruptive Accident (CDA) condition in a sodium cooled fast reactor (SFR), the environmental source term (i.e. quantity of radioactivity released into the environment surrounding to the reactor) is determined by the activity released from the containment building which in-turn depend on the concentration of radioactive aerosols remain suspended in the containment. The aerosol concentration depletes with progress of time inside the containment. Various models were used to predict the time evolution of aerosol concentration inside the containment. It is observed from the literature that there exists significant difference in the model prediction with simulated experimental condition. One of the reasons is the aerosol characteristics used as an input in the models is not accurate. In particular, the sodium aerosol characteristics (generated by the combustion of sodium) get changed both by physical and chemical processes. The other aerosols like fission products, fuel and structural materials form aggregates with sodium aerosols and their morphological properties are also get changed with progress of time. Hence an experimental study is carried out to determine the morphological properties viz. particle diameters, particle density, effective density, shape factors of sodium combustion aerosols, fission product aerosols (Cerium Oxide and Strontium oxide), structural aerosols (D9 steel) and mixed aerosols of sodium and fission products; and reported in this paper. Since sodium aerosols are hygroscopic, experiments are conducted at 3 different humidity conditions.In all the cases, the aerosol density is found to be lesser than the material density, and it changes with time towards the steady state. In case of sodium aerosols for mass concentration of 2 g/m3, the average particle densities and the standard relative uncertainties arrived at 75%, 55% and 30% RH are 1.67 ± 0.41, 1.43 ± 0.36 and 1.2 ± 0.31 respectively. Sodium aerosols are mostly spherical at humidities of 55% and 75% RH, where as they are deviated from spherical at 30% RH. Particle densities of D9 steel, CeO2 and SrO2 aerosols are 3.92 ± 0.58, 4.27 ± 0.54 and 3.57 ± 0.48 g/cc respectively. The CeO2 and SrO2 aerosols are found to be nearly spherical whereas the structural steel aerosols are found to be chain-like structures.