Iodine, volatilization

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References 12-17 provide background reading on the effects of organic reactions on iodine volatility. 

The possibility that at very low iodine concentrations (less than about 10 g/1) other iodine species than I2 are responsible for iodine volatility has also been discussed by other authors. The investigations of Pelletier and Hemphill (1979), which were mainly related to iodine release from open spent fuel pools and, therefore. 

Lin (1983) also reported on a detailed investigation of the distribution of iodine volatilized from the reactor water in BWR water—steam circuits with forward-pumped heater drains. In this plant design, only the main condensate is directed to the condensate polishing system, whereas the different drains from the turbine are fed directly back to the feedwater tank. Under such plant conditions, about 75% of the iodine carried by the main steam flow is precipitated together with the condensate in the moisture separator downstream from the high-pressure part of... 

It is suspected that, besides I2 and HOI, hydroiodous acid HI can, in principle, contribute to iodine volatility in a transducer line break. HI is a strong acid which in dilute aqueous solution is highly dissociated. However, according to the equilibria... 

Hellmann et al. (1991), in their experiments mentioned above, also measured the extent of iodine volatilization from the sump remaining in the low-pressure vessel after termination of a flash release test. From these measurements, a volume-related partition coefficient of about 2 10 was calculated at a water temperature of 60 to 80 °C, resulting in an iodine volatilization of less than 0.1% within 10 hours under the conditions prevailing in a transducer compartment. Thus, iodine release from the sump formed by the escaped primary coolant can be neglected in the evaluation of the consequences of such an accident. 

In this accident sequence as well, the question arises as to the significance of undissociated HI as an additional potential volatile iodine species. Calculations taking into account the conditions in the hot primary coolant showed that under the prevailing conditions the equilibrium fraction of HI in the solution is very small and that this compound does not contribute noticeably to iodine volatility under flashing conditions. 

Bawden et al. (1983) assumed an iodine vapor separation factor in the faulted steam generator of 3 10 (best estimate value) or of 1 10 (conservative value). Thus, iodine volatility would be insignificant against 0.1% entrainment if the best estimate value is taken, but it would become comparable when the conservative value is used. Organoiodide compounds are not assumed to significantly increase total iodine volatility. 

S.4.2 Basic iodine chemistry in aqueous solution and iodine volatility... 

There is comparatively little detailed information on the kinetics of the backre-action (3), i. e. on the formation of I2 in solutions containing I and 103 (the so-called Dushman reaction) in the pH range under consideration. This question is of interest with regard to the evaluation of I2 supply from the disproportionation products in the event that the I2 fraction in the solution is diminished by volatilization. As far as is known, reaction (1) proceeds very rapidly also from the right to the left side, whereas the rate of the backreaction (3) seems to be too slow (Palmer and Lyons, 1988) to be of signiflcance as a determinant of iodine volatility, compared to the effect of radiolytic oxidation of I". Only in the case of low radiation doses would the Dushman reaction be the controlling parameter for iodine volatilization rates from aqueous solutions. 

In large volumes of sump water, HI does not contribute significantly to iodine volatility, due to its virtually complete dissociation in the solution. However, it cannot be ruled out that in the final stage of the boiling down of the sump water, non-dissociated HI will be produced in the now concentrated solution and that this compound can be volatilized with steam. Preliminary experiments reported by Furrer and Biihler-Gloor (1988) showed that during evaporation of a Csl solution to dryness, the volatilized proportion of iodide depended highly on the initial solution pH. In the presence of boric acid in the solution, up to about 15% of the... 

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). 

Ashmore, C. B., Gwyther, J. R., Sims, H. E. Some effects of pH on iodine volatility in containment. Proc. 3. Intemat. Conf. on Containment Design and Operation, Toronto, Ontario, Can., 1994, Vol. 1... 

Evans, G. J., Palson, A. S. ACE bench-scale studies of iodine volatility and interaction with epoxy painted surfaces. Proc. 3. CSNI Workshop on Iodine Chemistry in Reactor Safety, Tokai-mura, Japan, 1991 Report JAERI-M 92-012 (1992), p. 230-245 Fletcher, J. W., Miller, O. A. Radiolytic formation and reactivity of aqueous hypoiodous acid. Proc. Specialists Workshop on Iodine Chemistry in Reactor Safety, Harwell, UK, 1985 Report AERE-R-11974 (1986), p. 107-120 Fluke, R. J., Frescura, G. M., Sagert, N. H., Tennankore, K. N., Vlkis, A. C. The Canadian program on iodine chemistry in reactor safety. Proc. 3. CSNI Workshop on Iodine Chemistry in Reactor Safety, Tokai-mura, Japan, 1991 Report JAERI-M 92—012 (1992), p. 49-61... 

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