The release of sodium aerosols intothe open environment from a Sodium Cooled Fast Reactor (SFR) is envisaged in two scenarios; (i) In theevent of Core Disruptive Accident (CDA), Reactor Containment Building (RCB) isbottled-up with sodium, fuel and fission products aerosols and there existsleast possibility of these aerosols getting released into the open environment(Indira et al., 2006), and (ii) In thesecondary heat transfer loop in the Steam Generation Building (SGB), theleakage of sodium from the fluid carrying pipes (due to cracks developed in thepipes) leads to the generation of dense sodium aerosols formed due to sodiumfire (Jorden et al., 1988) and it occurs in three ways viz. (i) Spray fire (Pinholes or jets on the pipe leads to spray fire), (ii) Column Fire (results oflarge dimension of jet where sodium flows down as a column) and (iii) Pool fire(results of large leakage where sodium flows down to the lowest part of groundand forms a pool). The sodium fire in SGB is classified into 4categories.
The category -1 is confined within the loop, while in category 2and 3, the sodium is collected in the leak collection tray (mounted beneath thefluid carrying pipe) and drained back into the drain tank. In category- 4leakage, the hot sodium is over-flownand spread over the entire floor area of SGB and continues to burn as pool fire(Chardon et al., 1988). The quantity of sodium leak in the event of spray andcolumn fires is small when compared to pool fire (Category -4), in which largequantity of release of aerosols envisaged.
The released aerosols enter theenvironment through multiple path ways (direct release or through exhaustsystems). The released aerosols are transported by the wind, dispersed byatmospheric turbulence and deposited over the ground by both dry and wetdeposition processes. The aerosols are mainly sodium oxide (Na2O) orhigher oxides of sodium (Na2O2 and NaO2)depending upon the ratio of sodium to oxygen available during the onset offire. Since these oxides are highly reactive, they are further converted tosodium hydroxide and sodium carbonate and then sodium bi-carbonate uponreaction with water vapour and carbon dioxide present in the environment(Clough et al., 1982). Of these, Sodium hydroxide is highly corrosive and it isresponsible for material damage and restricts the functionality of variousdevices in SGB. Sodium hydroxide causes severe burns on contact with the skinand also damages the eyes.
The Threshold Limiting Value (TLV), for theatmospheric concentration of soda (NaOH) is 2 mg m-3 (ACGIH – AmericanConference of Governmental Industrial Hygienists) (Annexure- I) for humans.However, no such stringent limit exists for sodium carbonate. If sodiumaerosols in hydroxide form get in contact with humans, it is hazardous, but ifit is in carbonate form, the concern is reduced to a great extent. Thus, studies related to physical and chemicalcharacterization of sodium aerosols help in evaluation of sodium fire hazardsin SFR and the associated research facilities with respect to sodium fire anddispersion of sodium aerosols. Hence, astudy on sodium aerosols characteristics has been initiated in RadiologicalSafety Division (RSD), Indira Gandhi Centre for Atomic Research (IGCAR). Underthis study, an Aerosol Test Facility (ATF) has been commissioned and severalstudies on sodium aerosol characteristics have been conducted and published(Baskaran et. al.
, 2004 and 2006; Subramanian et al., 2007, 2008, 2009 and 2011; Gilardi et al., 2013 Misra et al., 2013 andAnandanarayan et al., 2015). In the above studies, one of the important studiesis the chemical characterization of sodium combustion aerosols with time, whichwas carried out in collaboration with CEA, France. This study primarily focuses experimentaldetermination chemical species of sodium aerosols followed by theoreticalmodeling of carbonation of single aerosol particle upon diffusion of CO2in a confined environment (Gilardi et al.
, 2013).As a follow up, studies on sodiumaerosol characteristics in open environment for impact assessment is carriedout and it formed important contribution to the safety aspects of SFR. Theimportant aspect to be considered in sodium aerosol dispersion in openenvironment and its environmental impact is (i) it deals with particulatedispersion and (ii) the relative humidity (RH) present in the atmosphere willplay a major role in the conversion of oxides into hydroxide and further conversionto carbonate and bicarbonate depending on the availability of moisture, oxygenand carbon-di-oxide in the atmosphere while the aerosol plume is dispersed. Atthe tropical coastal site Kalpakkam, the RH% varies as 50-90% in variousseasons and wind velocity changes from 1 m s-1 (during calm landbreeze condition) to 8 m s-1 (during sea breeze condition) (Bagavatsigh etal., 2015) Hence, in order to haveenvironmental impact assessment of sodium aerosols, a series of small scaleexperiments are conducted by releasing sodium fire aerosols in the openenvironment and by measuring the ground level concentration, chemicalspeciation and total quantity of fallout aerosols upto 700 m in the downwinddirection. The experiments provided large data to validate the codes used fordispersion calculation and a validated model is evolved to predict the sodiumaerosols parameters for a given sodium fire event and its environmental impactviz. ground level concentration, chemical speciation, desposits etc. Twodispersion models namely Gaussian Plume model (GPM) and FLEXPART have been usedto model the aerosol dispersion and its characteristics.
The detailed experimentalprocedure, theoretical prediction and results are presented in this paper.