Iodine behavior

Weher, C.F., Beahm, E.C. and Kress, TS. (1992). Models of Iodine Behavior in Reactor Containments, ORNL/TM-12202. 

Postma, A. K., Tam, P. S. Iodine behavior in a PWR cooling system following a postulated steam generator tube rupture accident. Report Nureg-0409 (1978)... 

Stainless steel surfaces which are in contact with I2 solution (50 pM) at 100 °C effect a slow reduction of I2, while concrete surfaces, due to the alkaline nature of this material, give rise to I2 disproportionation. Surfaces which are coated with epoxy phenolic paint are very reactive towards I2 in solution, especially at elevated temperatures (100 °C), causing reduction of I2 to I, but these surfaces are also able to remove up to 50% of the initial I2 inventory of the solution (Bums et al., 1988). The results of these laboratory experiments clearly demonstrate that dirt effects may have a marked influence on iodine behavior, in particular at such low concentrations. 

From the iodine fractions measured in the liquid and in the gaseous phases of the blowdown suppression tank, an iodine partition coefficient of 77 at a pH of the water phase of 4.8 was calculated. This figure agrees reasonably well with the experimental data obtained, for example, by Richter et al. (1985) at a pH of 5.3 and a water temperature of 100 to 200 °C however, because of the low pH of the water phase, its applicability in the modelling of iodine behavior in the containment seems questionable. Moreover, the particular geometry of the blowdown suppression tank, in which the volume ratio of the gas to the liquid phase was considerably smaller than in a real reactor containment, presumably influenced the iodine partition coefficient. 

These experiments concentrated on iodine behavior under primary coolant conditions and did not take into account the mixing of the flashed primary coolant with the steam generator secondary-side water. Thus, it seeems highly unlikely that... 

Weber, C. F., Beahm, E. C., Kress, T. S. Models of iodine behavior in reactor containments. Report ORN17TM-12202 0992)... 

In general, it can be assumed that the reaction between silver and iodine species in the gas phase, as well as the reaction of iodine vapor with silver aerosol or with silver deposited on the primary circuit surfaces, is only of minor significance for iodine behavior in the course of a severe accident. The main reasons are the rather short residence time of the silver aerosols in the gas phase, the fact that iodine and silver volatilization from the reactor core may differ considerably over time and, finally, the small proportion of elemental I2 and of HI (compared with the Csl fraction) assumed to be present in the gas phase during transport through the primary circuit. In contrast, Agl formation is expected to proceed to a significant extent later on in the containment sump water (see Section 7.3.3.3.3.). 

Minato, K. Thermodynamic analysis of cesium and iodine behavior in severe light water reactor accidents. J. Nucl. Materials 185, 154-158 (1991)... 

The data on the relative concentrations of the different iodine species in the hydrolysis solution which were given above apply only when the equilibrium state has been completely established. However, if important reactions proceed only slowly, compared to the time scale of the accident, the distribution of species will be different from that to be expected for the equilibrium state. Thus, for the evaluation of fission product iodine behavior, the rates of the reactions leading to the equilibrium state also have to be taken into account. 

CSE experiment a large iodine fraction had been deposited onto the vertical surfaces of the vessel. The reason for this difference in iodine behavior is assumed to be the comparatively small input of cesium in the CSE test (1.1 g cesium and 85.5 g iodine), compared to the ACE test (266 g cesium and 101 g iodine). As a consequence, there was insufficient cesium aerosol in the CSE test to adsorb most of the h vapor and iodine removal in this test was controlled by boundary-layer vapor diffusion and sorption onto internal surfaces. In the ACE experiment, the vapor - aerosol interaction process probably was complete within the time scale of the atmosphere mixing process (a few minutes). The estimated rate constants for I2 wall deposition were about 2.7 times those for aerosol settling this difference is consistent with the measured differences in the early-stage halftimes of iodine removal in the CSE and ACE experiments. 

According to Beahm et al. 0992), the iodine behavior in the containment can be divided up, timewise, into three categories ... 

Long after the onset of a severe accident (more than about 3 weeks), gas-phase iodine is expected to be dominated by organic iodide, with a small contribution from I2 the conclusions drawn from the TMI-2 accident are highly consistent with these results of model calculations. Iodine behavior and distribution, in the long run, are expected to have little relationship to the chemical forms or amounts released into the containment, because the iodine will have had enough time to deposit onto surfaces or in water pools, so that the environmental conditions in the containment will prevail in determining the chemical forms. 

The code Iode has been developed to calculate iodine behavior in a reactor containment and the auxiliary building it is part of the French computer system Escadre which, similar to the US Source Term Code Package, describes the whole sequence of a severe reactor accident. The general philosophy of Iode is to model the main phenomena which may influence the behavior of iodine in the reactor containment IS chemical reactions are modelled, concerning both the water phase and the gas phase (Gauvain et al., 1991). Similar to Impair, radiolysis is not described in detail but is taken into account over its global effect on iodine species. The kinetic data of the reactions were taken from the literature as far as inorganic iodine thermal reactions are concerned other kinetic data were compiled from the elaboration of the Impair 2 code. 

The ORNL containment iodine behavior model Trends (Beahm et al., 1988) lies somewhere in-between the empirical and mechanistic models it includes five different iodine species, each of them existing in different phases (see Table 7.19.). Subsequent interconversions between different species are modelled, which transfer I (aq) to CH3l(aq) and l2(aq) Rs Well as l2(g) to CH3l(g), also taking the associated... 

Vikis, A. C., Torgerson, D. F. Gas-phase radiation chemistry of iodine in post-accident environments. Proc. Specialists Workshop on Iodine Chemistry in Reactor Safety, Harwell, England, 1985 Report AERE R 11974 (1986), p. 185-192 Weber, C. F., Beahm, E. C., Kress, T. S. Iodine chemical forms in LWR severe accidents. Proc. 3. CSNI Workshop on Iodine Chemistry in Reactor Safety, Tokai-mura, Japan, 1991 Report JAERI-M 92-012 (1992), p. 414-430 Weber, C. F., Beahm, E. C., Kress, T. S. Models of iodine behavior in reactor containments. Report ORNL/TM-12202 (1992)... 

Because of insufficient experimental data, many Of the physical and biologic characteristics of must be based on observations of other iodine isotopes. For example, studies of short-term iodine behavior have utilized fallout radioiodine from nuclear explosions, principally in particulate form, and releases of elemental vapor forms of in field experiments or associated with nuclear power generation. However, little information is available concerning the changes in bioenvironmental concentration processes that might result from alterations in form and availability of in the long term. Analyses of behavior and potential accumulation of released to the environment must therefore be based on studies of 1 as a model for shortterm behavior, and stable iodine for long-term behavior. 

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