Modelling of the chemical behaviour of gaseous iodine releases in the atmosphere

Doctorant: Julien TRINCAL (début de thèse octobre 2012)

The computer codes used by the IRSN in case of accidental release of radioactive products do not consider the dispersion and drawdown effects; the source term is chemically inert. However, the reactivity of iodine in the atmosphere is well known, and this hypothesis has to be reassessed.
During the Fukushima accident, the simulations have defined quantities of cesium relatively close to those measured in the field, this is not the case for iodine. This discrepancy could be explained by the chemical reactivity of iodine in the atmosphere, which is not implemented in the codes.
The purpose of this thesis is to couple a kinetic mechanism of iodine chemistry with an atmospheric dispersion code, and to evaluate the potential impact of this reactivity on the results of a representative case, which has to be defined.
During the first year of this thesis, a large kinetic mechanism of iodine chemistry in the atmosphere has been worked out, through a bibliographic study. The mechanism includes 153 iodinated reactions, in the elemental system { I – H – O – N – Br – Cl – C }. In the second year, these reactions will be added to atmospheric chemical mechanisms such as RACM [1], which is focused on pollutant (O3, NOX) and organic species (VOC). RACM groups species by categories, according to their reactivity (alkanes, alcohols, aldehydes, …). The 153 reactions will be reduced to tie in this categories, and sensibility studies will be carried out, to finally obtain a main mechanism of iodine chemistry in the atmosphere.
After a validation step on atmospheric measurements, the mechanism will be implemented in the Polyphémus platform [2], and simulations will evaluate the impact of iodine reactivity on deposits for a given accident scenario.

[1] RACM : Regional Atmospheric Chemistry Model, Stockwell et al. (1998)
[ 2 ] Polyphémus : global air quality simulation’s system, CEREA (Centre d’Enseignement et de Recherche en Environnement Atmosphérique)

Financement: IRSN

Laboratory: PC2A and IRSN

Responsibles: Valérie Fèvre-Nollet (PC2A); Laurent Cantrel (IRSN)