STUDIES ON THE CHEMICAL BEHAVIOUR OF SODIUM FIRE
AEROSOLS DURING ATMOSPHERIC DISPERSION
A. PLANTAMP, T. GILARDI, C. PERRAIS
CEA, DEN, Cadarache, DTN/SMTA/LIPC
13108 Saint-Paul-Lez-Durance - France
H. MUHR
LRGP CNRS UMR 7274, ENSIC/Université de Lorraine
1 rue Grandville, BP 20451, 54 001 NANCY - France
EXTENDED ABSTRACT
The use of liquid sodium, as a coolant in Sodium cooled Fast Reactors (SFR) circuits, requires
to study the consequences of a sodium fire for safety analysis, and particularly the toxicological
impact of sodium fire aerosols, which can be potentially released in the atmosphere. More
particularly, the carbonation of sodium fire aerosols from sodium hydroxide (NaOH) to sodium
carbonate (Na2CO3) is investigated.
The sodium fire aerosols initially composed of sodium oxides (Na2O and Na2O2) are quickly
converted into sodium hydroxide by hydration with air moisture, and then into sodium carbonate
with atmospheric CO2. A kinetic model based on a shrinking core description is developed to
calculate the chemical conversion from NaOH to Na2CO3 during atmospheric dispersion and
validated with several experiments for small particles (mean diameter around one micron) and
relative humidity higher than 20% [1]. This first model takes into account the internal diffusion of
CO2 through the product layer as the limiting step of the carbonation kinetics. The impact of the
particle expansion is also added, even if the layer porosity is not considered.
Different improvements of this model are developed. The first one deals with the hydration
equilibrium of sodium hydroxide particles with moisture. We take into account the influence of
relative humidity on the size and density of NaOH initial particles, due to the hygroscopic and
deliquescent properties of this compound. The calculation is made considering thermodynamic
equilibrium of H2O content between gas phase and the particle surface [2] and using, in
particular, the water activity coefficient in an electrolyte solution of NaOH, determined
experimentally [3-4].
Secondly, the integration into the model of the external transport as a limitation of the CO 2
transfer enables to take into account a possible change of kinetic regime depending on the
dispersion conditions and the aerosol characteristics. It is shown that the surface reaction is not
a limiting step, by calculating the Thiele modulus (greater than 100 for particle diameter greater
than 1 micron) from kinetic data in liquid phase [5].
Finally, the aerosol porosity is incorporated in the model, within the sodium carbonate density
and the CO2 effective diffusivity in the sodium carbonate layer. The evaluation of this last
parameter is made from a theoretical model based on the molecular diffusion of CO2 in air and
on the influence of the porous layer microstructure. Indeed, as a first approach, it is assumed
that the pores of the sodium carbonate layer are filled with air.
This kinetic model, is adapted to the experimental results [6-8], using an adjusted value of the
effective coefficient De of 2.10-10 m²/s and a porosity of 50% to fit with these experimental data.
Figure 1 shows the comparison graph of the kinetic model results with the experimental results
of Cherdron et al. [6]. For the kinetic model calculations, the value of the non-hydrated initial
diameter of the particles is taken at 0.65 micron. The values of the kinetic model are consistent
with those experiments, for relative humidities greater than 20%. Indeed, the kinetic model is
available when hydrated NaOH is a liquid phase, i.e. for relative humidity greater than 7 %
(corresponding to the solubility limit of NaOH in water at 20 °C).
Conversion rate XNaOH
1
0.8
Model RH = 3%
Cherdron RH < 3%
Model RH = 20%
Cherdron RH = 20 %
Model RH = 50 %
Cherdron RH > 50 %
0.6
0.4
0.2
0
0
100
200
300
400
500
600
Time (s)
Figure 1. Comparison of the kinetic model with the experimental data of Cherdon et al. [6]
It is necessary to improve the model through a better estimation of the aerosol porosity and the
effective diffusion coefficient, since so far, there is no direct measurement of these aerosol
properties. Therefore, a sensitivity analysis on these two parameters is shown below, and the
experimental program described below includes aerosol characterization and measurement of
the effective diffusion coefficient De.
The sensitivity study on the two major model parameters (porosity and effective diffusivity)
shows that porosity has a moderate influence on the full conversion time, ttotal while the influence
of the effective diffusivity is more significant considering the possible range of variation. Table 1
presents the data of the full conversion time calculated for values of effective diffusivity between
5.10-6 and 1.10-10 m²/s, and by setting the values of relative humidity (50%), porosity (50%) and
tortuosity (2) :
De (m²/s)
5.10-6 1.10-6 5.10-7 1.10-7 5.10-8 1.10-8 5.10-9 1.10-9 1.10-10
t total (s)
0,048 0,241 0,482
2,41
4,82
24,1
48,2 240,8
2408
Table 1. Influence of the effective diffusion coefficient De on full conversion time of NaOH aerosols
Therefore, the value of the effective diffusivity (currently based on the molecular diffusivity of
CO2 in air) must be confirmed. The adjusted value for the kinetic model of the effective
coefficient De (2.10-10 m²/s) corresponds to a range of usual diffusivity in liquid phase.
But, the validation of kinetic model is only partial: the experimental results validate the model
only for small particles diameter (<1 micron), while particles of several microns are observed in
some cases.
An experimental study is investigated to obtain the missing data of the kinetic model namely: the
microstructure of aerosols from a sodium fire (especially the porosity) and the direct
measurement of the effective diffusion coefficient of CO2 in air through a porous sodium
carbonate, representative of aerosol microstructure in a dedicated experiment. The material
considered to represent sodium aerosols is the product of sodium carbonation using a treatment
process for waste containing sodium [9].
References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
Gilardi, T., Chassery, A., Baskaran, R., Subramanian, V., Latgé, C., Perrais, C., (2013).
Modeling of the chemical behavior of sodium fire aerosols during atmospheric dispersion.
International Conference on Fast Reactors and Related Fuel Cycles: Safe Technologies
and Sustainable Scenarios (FR13), Paris - IAEA CN 199/196.
Colle, S. (2006). Etude thermodynamique et cinétique de l’absorption du dioxyde de
soufre dans des solutions d’acide sulfurique de moyennes et fortes concentrations
contenant du peroxyde d’hydrogène, Ph. D. thesis (in French), Chapter 6, Faculté
polytechnique de Mons, Belgium.
Robinson, R. A., Stokes R. H. (1970). Electrolyte solutions. Butterworths, London.
Hamer, W.J., Wu, Y-C, Osmotic coefficients and mean activity coefficients of uni-univalent
electrolytes in water at 25 °C, J. Phys. Chem. Ref. Data, 1 (1972), 1047-1099
Pohorecki, R., Moniuk, W., Kinetics of reaction between carbon dioxide and hydroxyl ions
in aqueous electrolyte solutions, Chem. Eng. Sci., 43 (1988), 1677-1684
Cherdron, W., Jordan, S., Linder, W., Sodium fire aerosols – chemical transformation and
properties, Proc. Conf. Liquid Metal Engineering Technology, BNES, London (1984), 287290
Hofmann, C., Jordan, S., Linder, W., Chemical reactions of sodium fire aerosols in free
atmosphere, The annual conference of the GAF (1978), 191-192
Subramanian, V., Sahoo, P., Malathi, N., Ananthanarayanan, R., Baskaran, R., Saha, B.,
Studies on chemical speciation of sodium aerosols produces in sodium fire, Nuclear
Technology, 165 (2009), 257-269
Baqué, F., Liger, K., Rodriguez, G., Boucherat, T., Ollivier, J., Joulia, E., French sodium
waste storage rules, IAEA-TECDOC-1633, Vienne (2009), 77-93