The impact of organic compounds on iodine behaviour under conditions relating to nuclear reactor accidents

grantor: University of TorontoThe impact of organic compounds on iodine behaviour was investigated under a range of post-accident chemical conditions expected in a reactor containment structure. A bench scale apparatus installed in the irradiation chamber of a Gammacell was used to provide continuous measurement of iodine volatilization rates from 10-6 to 10 -4 CsI solutions with pH values from 5 to 9. The rate of production of volatile iodine was evaluated in the presence of 10-3 M concentrations of various alkyl halides, carbonyls, and aromatics; the three classes of organic compounds most likely present in containment. Iodo-organics and molecular iodine in the gas and liquid phases of the irradiated samples were analyzed using gas chromatography, mass spectrometry, and UV spectrophotometry. A model was developed that simulates the radiation chemistry of iodine in the presence of organic compounds and evaluated against the experimental results. The results indicated that organic compounds can be classified into groups, based on their distinct effects on iodine behaviour. Iodine volatilization increased significantly, up to two orders of magnitude, in the presence of carbonyls and alkyl chlorides, while it decreased in the presence of aromatics. Gas and liquid phase analysis indicated that chloro-iodo organics and alkyl iodides are the major types of volatile iodo-organics formed in the presence of alkyl chlorides and carbonyls, respectively, while no volatile iodo-organics are formed in the presence of aromatics. Molecular iodine measurements in the systems showed that I2 concentration increases in the presence of alkyl chlorides and decreases in the presence of carbonyls and aromatics. The kinetic-based model, containing a mechanistic description of iodine chemistry and generic semi-mechanistic reactions for various classes of organics, provided a reasonable prediction of the experimental results. The majority of the model and experimental results were in agreement within an order of magnitude. The results of this research will assist in predicting and reducing the radiological consequences of reactor accidents. In particular, the results indicate the advantage of maintaining basic pH and avoiding alkyl halide or ketone based solutions in post-accident reactor containment, in order to reduce radioactive iodine volatility, and hence, improve reactor safety.Ph.D