Further reactions of iodine in the containment atmosphere

Similar to the situation in the reactor primary system (see Section 7.3.2.), interactions between iodine vapor and bulk material aerosols may occur in the containment atmosphere, although at a lower temperature level, and may have significant influence on iodine speciation and transport. Csl usually appears in the condensed form in the containment and is transported as aerosol particles. In rare cases, conditions may occur (e. g. in a BWR drywell) in which a significant fraction of the Csl present would exist in gaseous form, which can then migrate to cooler regions to condense on structural surfaces or on aerosols. However, in such a case normal condensation without additional chemical reactions is to be expected. 

Greater significance has to be attributed to the deposition of I2 vapor onto aerosol surfaces. This kind of reaction of gaseous iodine species with aerosols in the containment atmosphere will mainly result in a reduction in the concentrations of volatile species. These processes may be written as 

Deposition of volatile species onto aerosol surfaces includes mass transport to the surface, resulting in adsorption this step will be followed either by revaporization from the surface or by chemical reaction of the adsorbed species with the surface material. These mechanisms can be written as 

Cc = chemisorbed iodine (reaction with surface material). 

The constants ki and k2 represent mass transfer coefficients to and from the surface ks and lu denote chemical reaction rate constants (Weber et al., 1992). In most cases, the chemical reaction step is essentially irreversible, resulting in lu = 0. Since mass transfer, among others, depends on the geometry and the dimensions of the system, care has to be taken in extrapolating results which were obtained in laboratory facilities to the situation of real reactor containments. Adsorption reactions will depend on the concentrations and the composition of the aerosols and, finally, chemical interactions between the two species are highly dependent on the chemical nature of both the volatiles and the aerosols, in particular of their surfaces. 

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