Volatile iodine species

The data suggest that iodine will be released, predominantly, as cesium iodide under most postulated light water reactor accident conditions. However, formation of more volatile iodine species (e.g., elemental iodine and organic iodines) is not impossible under certain accident conditions. 

A volatile iodine species, neither elemental nor organic, which has been found in steam/air atmospheres, has been identified as hypoiodous acid (HOI) (Cartan et al., 1968). In water-cooled power reactors, any fission products released from fuel will pass into hot alkaline water and thence to a steam-air mixture. These conditions are thought to favour the formation of HOI (Keller et al., 1970), but the evidence is indirect. For example, tests for elemental iodine or iodine with an oxidation state higher than that of HOI gave negative results. 

Cartan, F.O., Beard, H.R., Duce, F.H. Keller, J.H. (1968) Evidence for the existence of hypoiodous acid as a volatile iodine species produced in water/air mixtures. Proceedings 10th AEC Air Cleaning Conference. CONF 680821. Springfield VA NTIS. 

The atmospheric deposition of ozone to the sea surface can cause the release of volatile iodine compounds to the atmosphere. There is also evidence that methyl iodide can be produced (as well as destroyed) by photochemical processes in surface sea waters. The release of these volatile iodine species from the sea surface or from atmospheric aqueous phases (aerosols) by these processes may act as a control on the level of ozone in the marine troposphere via iodine-catalyzed ozone destruction. 

The volatile iodine species in water and air are normally determined by GC combined with an ECD or ICP-MS. 

The chemical forms of gaseous iodine in the containment atmosphere that are of interest are molecular iodine and organic iodides whether these species reached the containment as a result of iodine release from the fuel, partitioning of volatile iodine species from water or as a result of combustion processes. Another volatile form of iodine, HI, is very hygroscopic and will rapidly dissociate in contact with water to form solvated T. There can be some conversion of molecular iodine, hig), into organic iodides, RI, in the radiation field of the containment atmosphere ... 

Linden, E., Turner, D. J. Carry-over of volatile iodine species in some Swedish and American BWRs. Proc. 3. BNES Conf. Water Chemistry of Nuclear Reactor Systems, Bournemouth 1983, Vol. 1, p. 111-119... 

In sum, the reactions that can occur on the way from the sump water to the containment free volume (and vice versa) are assumed to result in a significant reduction of the amount of airborne radionuclides, in particular of volatile iodine species. Ignoring these reactions, therefore, would lead to an overestimation of the amount of volatile radionuclides and, as a consequence, to a conservative assumption regarding a potential release. 

The investigations reported on by Clinton and Simmons (1987) also showed that the presence of dissolved oxygen in the solution results in an iodine partition coefficient lower by a factor of about 10, apparently due to the formation of volatile iodine species. This effect might be of interest in a steam generator tube rupture accident which happens shortly before a planned shutdown of the plant, after hydrogen had been removed from the coolant and/or H2O2 had been added to reduce the primary circuit contamination and radiation dose rates. In order to cover such conditions as well, in the US Guidelines a total iodine partition coefficient of 2 10 has been specified for tube rupture accidents. 

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. 

In spite of its high thermodynamic stability, Csl transported in the steam flow can be decomposed by the action of third partners. Several reactions can effect this decomposition, which results in the formation of highly volatile iodine species. In general, according to the overall equilibrium equation... 

Due to its different chemical forms present in the containment atmosphere, fission product iodine shows a complex behavior in these areas as well. Csl carried by water droplets or by aerosol particles will behave in the same manner as do the other aerosols. The volatile iodine species I2 and CHjI will be distributed between the newly formed sump water and the atmosphere of the annuli, as is schematically shown in Fig. 7.45. Experiments have demonstrated that the extent of I2 plate-out depends on the degree of steam condensation, amounting to about 90% at a condensation temperature of about 40 °C (see Section 7.3.3.4.2.). The newly formed sumps in the annuli and in the auxiliary building show a pH of about 7 (since they consist solely of condensed steam), and temperatures between 50 and 80 °C this means that I2 plated out into these liquid phases is not only instantaneously hydrolyzed to I and HOI, but will also disproportionate rather quickly under formation of lOs . As can be seen from Fig. 7.24., under such conditions the disproportionation equilibrium will be reached within a comparatively short time. If one assumes very conservatively an initial I2 concentration in the sump water of about 1 mg/1 and a temperature of 100 C, then it can be derived from Fig. 7.24. that the... 

Volatile iodine species are not efficiently retained by the steel fiber filter. For this reason, an additional filter was proposed containing a silver-impregnated... 

---