Iodine oxides in large-scale THAI tests

The conversion of gaseous molecular iodine into iodine oxide aerosols has significant relevance in the understanding of the fission product iodine volatility in a LWR containment during severe accidents. In containment, the high radiation field caused by fission products released from the reactor core induces radiolytic oxidation into iodine oxides. To study the characteristics and the behaviour of iodine oxides in large scale, two THAI tests Iod-13 and Iod-14 were performed, simulating radiolytic oxidation of molecular iodine by reaction of iodine with ozone, with ozone injected from an ozone generator. The observed iodine oxides form submicron particles with mean volume-related diameters of about 0.35 μm and show low deposition rates in the THAI tests performed in the absence of other nuclear aerosols. Formation of iodine aerosols from gaseous precursors iodine and ozone is fast as compared to their chemical interaction. The current approach in empirical iodine containment behaviour models in severe accidents, including the radiolytic production of I2-oxidizing agents followed by the I2 oxidation itself, is confirmed by these THAI tests.

► Iodine oxide particles were produced from gaseous iodine and ozone.
► Ozone replaced the effect of ionizing radiation in the large-scale THAI facility.
► The mean diameter of the iodine oxide particles was about 0.35 μm.
► Particle formation was faster than the chemical reaction between iodine and ozone.
► Deposition of iodine oxide particles was slow in the absence of other aerosols.