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Iodine Chemistry in the reactor coolant system of a nuclear power plant in case of a severe accident – Study of CsI/MoO3 mixtures under steam

Marion LACOUE-NEGRE, doctorate of University of Lille, 262 p., defended on the 6th december 2010

Document type > *Mémoire/HDR/Thesis

Keywords > iodine, ionic microscopy, radioactive aerosols, MFPR code

Research Unit > IRSN/DPAM/SERCI/L2EC

Authors > LACOUE-NEGRE Marion

Publication Date > 06/12/2010

Summary

The fraction of volatile radioiodine in the containment of a pressurized water reactor (PWR) following a severe accident is one of the major issues of the nuclear safety analysis. The simulations performed with computer codes cannot account totally for the amount of gaseous iodine observed in the experimental PHEBUS-FP tests. A likely explanation is that gaseous iodine would come directly from the reactor coolant system (RCS) where chemistry interactions between fission products (FP) and control rod materials are complex. In particular, those interactions lead to the formation of large amounts of aerosols whose chemical composition is still poorly known. A better knowledge of the aerosol speciation would provide valuable information on reaction schemes and also a better estimation of the aerosol reactivity towards gaseous iodine. An experimental setup was developed to study the chemical behavior of CsI/MoO3 mixtures at 1600°C under steam and then during the steam cooling to 150 °C. These hydrothermal conditions are representative of the primary circuit of PWR. The analyses using ICP-MS, powder XRD, MEBE-EDX and Raman microspectrometry identify submicrometric aerosol particles as CsI, MoO3.xH2O and Cs2MonO3n+1  n=1, 2, 3, 5, 7) according to the starting CsI/MoO3 ratio. The formation of Cs2MonO3n+1 induces the generation of gaseous iodine. The simulations of vapor  phase chemistry and aerosol phenomena of the {I, Cs, Mo, O,H} system in the experimental setup were carried out using the SOPHAEROS code based on the thermodynamic chemical equilibriums. Some discrepancies were observed between experimental and simulated results, particularly for Mo rich particles and the volatile iodine species release.

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Hervé Mutelle, the thesis supervisor

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