Evaluation of overall iodine behavior in the containment

In the early US Regulatory Guides 1.3. and 1.4. it was assumed that in the course of a maximum credible reactor accident 50% of the maximum iodine inventory of the reactor core would be released to the containment. Of this amount, 25% would be available for leakage, i. e. present in airborne form in the containment atmosphere as for the chemical states, 91% of the airborne iodine was assumed to be elemental I2, 5% to be in the form of particulate iodide and 4% in the form of organic iodide. Subsequent investigations, both experiments and 

Basic fission product compounds (e. g. CsOH, Cs borates) basic 

Pyrolysis and radiolysis products from organic materials acidic 

In recent years, several attempts have been made to estimate the magnitude of the airborne iodine fraction in the contaimnent in a highly simplified manner, mostly based on the results of laboratory experiments. But because of the multitude of potentially influencing parameters, no well-founded results can be expected from such a procedure. On the contrary, the relative significance of the individual parameters for the specific conditions of the accident sequence under consideration has to be evaluated in detail. 

One of the most important parameters controlling iodine volatility is sump water pH not only will the I2 hydrolysis equilibrium and the iodine partition coefficient be affected by this parameter, but the product yields of radiolytic reactions and the extent of formation of organoiodine compounds as well. Because of the lack of practical experience, the sump water pH to be expected under severe accident conditions has to be calculated on the basis of assumed concentrations of potential sump water ingredients. In Table 7.17. (according to Beahm et al., 1992) an overview of substances to be expected in the sump water, which would effect a shift in solution pH either to lower or to higher values, is given. Besides these chemical substances, radiation may also affect sump water pH irradiation of trisodium phosphate solution (5.3 kGy/h) was reported to decrease the pH from an initial value of 9.0 to about 4.0 after 60 hours of irradiation (Beahm et al., 1992). It is obvious that in such a complicated system definition of the sump water pH to be expected in a real severe reactor accident is a difficult task. Nonetheless, a model for calculation has been developed by Weber et al. (1992). 

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