In case of a hypothetical severe accident in a nuclear LWR (light water reactor), the high radiation fields reached in the reactor containment building due to the release of fission products from the reactor core could induce air radiolysis. The air radiolysis products could, in turn, oxidise gaseous molecular iodine into aerosol–borne iodine–oxygen–nitrogencompounds. Thereby, this reaction involves a change of iodine speciation and a decrease of iodine volatility in the reactor containment atmosphere. Kinetic data were produced within the PARIS project on the air radiolysis products formation and destruction, and on their reaction with molecular iodine, with the objective of developing and validating existing kinetic models.
The Program on Air Radiolysis and Iodine adsorption on Surfaces (PARIS) was therefore initiated in 2002 by IRSN in collaboration with AREVA NP, as part of the research programs performed to improve severe accident modelling and evaluation of subsequent fission product release into the environment with specific objective of measuring:
• the rate and amount of ARP production and destruction,
• rate and extent of radiolytic oxidation of molecular iodine into iodine oxides,
• the effect of the containment structural surfaces, namely decontamination coating (“paint”) and stainless steel, on radiolytic oxidation of I2,
• the effect of silver, representing silver-containing aerosol particles, on radiolytic oxidation of I2.
Important new features of the PARIS project were: (1) more realistic low iodine concentrations, (2) surface to volume ratios of paint, steel and silver surface area to containment volume ratio representative of LWR or PHEBUS containments, (3)higher steam fractions and (4) representative dose rates. The PARIS database, containing about 400 tests, was intended to provide data to develop and validate empirical models, and finally to derive a simplified model for ASTEC Code and other severe accident iodine codes.
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