Iodine partition coefficient

The equilibrium partitioning of iodine between the sump liquid and the eontainment atmosphere is examined for the extreme additive concentrations determined in Seetion ni.f.a.(2), in combination with the range of temperatures possible in the containment atmosphere and the sump solution. The reviewer should eonsider all known sources and sinks of acids and bases (e.g. alkaline earth and alkali metal oxides, nitric acid generated by radiolysis of nitrogen and water, alkaline salts or lye additives) in a post-accident containment environment. The minimum iodine partition coefficient determined for these eonditions forms the basis of the ultimate iodine decontamination factor in the staff s analysis described in subsection III.4.d. 

Pelletier, C. A., Hemphill, R. T. Nuclear power plant related iodine partition coefficients. Report EPRI NP-1271 (1979)... 

The final partitioning of fission product iodine between the sump water and the atmosphere of the containment is determined by the equilibrium partition coefficient as will be discussed in more detail in Section 7.4.3.1., this figure depends on several parameters, such as iodine concentration and sump water pH and, to a lesser extent, temperature. As can be seen from Fig. 4.13., the iodine partition coefficient increases with decreasing iodine concentration in the solution this behavior is the consequence of the shift in the h hydrolysis reaction towards a lower I2 equilibrium fraction at lower total iodine concentration. Below a concentration of about 10" g/1, however, the partition coefficient remains virtually constant, an effect which has been attributed to the volatility of the hydrolysis product HOI (Lin, 1981). This constant value is about l(f at pH 7 and 25 °C it applies as well for the concentration range to be expected in a loss-of-coolant accident. Concerning the HOI partition coefficient at 100 °C, several measurements yielding quite different values have been reported. The lowest one is 240 given by Lin (1981) Lemire et al. (1981) reported values between 100 and 1(1, while Wren and Sanipelli (1984) measured values between 7 10 and 3 10. Since the HOI compound was... 

When calculating the h equilibrium concentration in the containment atmosphere from these data, one has to remember that the partition coefficients usually were determined using pure I2 solutions. The fission product iodine in the sump water, however, mainly consists of non-volatile iodide with smaller fractions of I2 on the order of 10% and less. The airborne fraction of I2, therefore, has to be calculated from the dissolved I2 concentration only. The excess 1 present in the solution will act to shift the I2 hydrolysis equilibrium towards a higher I2 fraction at the low total iodine concentrations present, however, this effect is not expected to result in a significant decrease in the total iodine partition coefficient. 

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. 

Detailed measurements of iodine partition coefficients from PWR primary coolant at 285 °C and 6.9 MPa (i. e. under secondary-side conditions) were performed in laboratory experiments by Clinton and Simmons (1987), using pure iodide solutions (6 10 " mol/1) in boric acid solutions of various pH values between 4 and 9. The results showed a gross iodine partition coefficient that decreases with... 

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. 

Clinton, S. D., Simmons, C. M. Iodine partition coefficient measurements at simulated steam generator conditions. Report NUREG/CR-4817 (1987)... 

The formation of non-dissociated HOI in the vapor phase was claimed to be possible only if the steam contained liquid water, i. e. in saturated steam (Lemire et al., 1981). On the other hand, from measurements of iodine partition coefficients it was concluded that HOI is the predominant neutral iodine species in steam at low concentrations and high temperatures (see Section 4.3.4.2.). In any case, the possible appearance of HOI under the prevailing conditions is of little significance for the chemical and transport behavior of fission product iodine, because of the low I2 fractions present and of the large excess of possible reaction partners. 

Hence, the most important parameter for the magnitude of the integral iodine partition coefficient is iodate formation and, consequently, as a result of the rather slow rate of HOI disproportionation in the pH range 5 to 7, a time-dependent value of the partition coefficient can be expected. This effect, quantitatively predicted by the kinetic calculations e. g., of Bell et al. (1982 b), has been confirmed by numerous measurements. As an example, the experimental investigations of Beahm and Shockley (1983) shall be mentioned, which show that at a total iodine concentra-... 

Figure 7.25. Iodine partition coefficient as function of solution pH and total iodine concentration (no HOI volatility assumed)...

Figure 7.26. Iodine partition coefficient of I2 - I" mixtures (Richter and Neeb, 1985 by courtesy of AEA Technology pic)...

Systematic investigations of integral iodine partition coefficients in l -l2 mixtures at pH 5.3 and 100 °C were reported on by Richter et al. (1985 a) as well as by Richter and Neeb (1985). These results demonstrated that, as expected, the partition coefficient increases with decreasing I2 fraction in the solution as can be seen from Fig. 7.26., at high excess I a pronounced increase in the partition coefficient is observed, reaching a value of about Ky at a mixture of 98% I" +2% I2 (total iodine concentration 10 g/1). These data, which were obtained after short... 

The comparatively high values of the integral iodine partition coefficient measured in the large-scale CSE tests (see Section 7.3.3.4.S.) at temperatures between 80 and 120 C, which were about 2 l(y, are presumably due to the presence of considerable fractions of iodide in the solution, which had been formed in the feed line from the I2 originally added. The influence on the partition coeffident of reducing constituents in the solution will be treated in more detail in Section 7.3.3.4.3. 

The discussion of iodine partition coefficients in this section relates solely to a gas phase consisting of saturated steam which is in thermal equilibrium with liquid water. With increasing steam temperatures and pressures, dissociated compounds such as Csl also exhibit an increasing volatility with steam according to the measurements reported by Styrikovich and Martynova (1963), NaCl shows a partition coefficient of 10 to 10 at a steam pressure of 7 MPa. Beyond the critical temper-... 

In the preceding section, iodine chemistry and the iodine partition coefficient in a pure iodine solution have been discussed. The containment sump water which is to be expected in a severe reactor accident, however, may contain a number of other substances that have a potential impact on the chemical reactions and on the resulting reaction products. It is not possible to give detailed and trustworthy information on the nature and the concentrations of all these substances, which in addition might be different in different accident sequences and in different plants therefore, only some of them can be treated here exemplarily. The particular case of the presence of an excess of 1 in the I2—H2O system and its implications for the iodine species distribution and for the partition coefficient have been discussed in the preceding section. 

Figure 7.30. Iodine partition coefficient as a function of the redox potential in terms of O2 concentration of the solution...

I2 and HOI present in the sump water can be reduced rather quickly by reducing substances such as organic impurities, hydrazine and metallic trace impurities like Fe " and Cu" ". Dissolved iodate is only slowly reduced by these substances. Addition of 100 ppm Na2S203 to a solution containing 0.05 g I2/I rises the partition coefficient from 900 to 3 10 addition of 100 ppm hydrazine results in a partition coefficient of about 3 10 (Postma, 1980). On the other hand, it was concluded from comparative measurements in pure water and in spent fuel pool water at comparatively low temperatures that hydrazine concentrations up to 60 ppm do not significantly affect the iodine partition coefficient (Pelletier and Hemphill, 1979). 

Sump water pH is the most important parameter for controlling the iodine partition coefficient, as was discussed in the preceding section. When the sump water is merely composed of the primary coolant and the emergency core coolant solutions (boric acid solution), a pH on the order of 5.5 has to be expected. Addi-... 

Tests with an alkaline system (pH 8 to 9) yielded low overall volatility of iodine, giving partition coefficients in the range of 10 to 10 . Simultaneously, low levels of organic iodine species were formed (on the order of 0.1%), even when relatively large amounts of epoxy paint materials were present further, low iodine deposition onto the surfaces of the vessel took place. Under acidic conditions (pH 4 to 6), the total iodine partition coefficient was lower by a factor of about 100 and the organic iodide level was higher by about a factor of 10 (but still remained a small fraction of the total iodine) than under alkaline conditions. Moreover, wider acidic conditions an appreciable iodine buildup on vessel surfaces is to be expected which may constitute a significant delayed source of volatile iodine if the system equilibrium is disturbed (Ritzman et al., 1991). 

This reaction is assumed to be of little importance since the intermediately produced radicals CH3- and I- are preferentially trapped by O2, as well as by H2O radiolysis products. Methanol in an irradiated iodide solution of low pH was found to suppress radiation-induced iodide oxidation and, thus, to keep the iodine partition coefficient high, with the responsible mechanism probably being the effective scavenging of the primary OH- radicals by CH3OH (Lutz and Kelly, 1988). 

Addition of organic impurities such as methyl ethyl keton (MEK), a common paint solvent, to an aqueous I solution in the absence of radiation proved not to result in a measurable organoiodide formation. Under the influence of radiation, however, addition of MEK leads to an increase in the total airborne iodine concentration by more than an order of magnitude over a period of several hours, corresponding to a decrease of the steady-state iodine partition coefficient from 10 to 10 in the absence of MEK to about 10 after addition of this compound (Fluke et al., 1991). 

Figure 7.41. Iodine partition coefficient with and without contact with steel surface (Total iodine concentration 10 mg/ml, mass ratio I to total iodine 0.S8, boric acid solution pH...

After termination of the experiment, the containment atmosphere contained only a comparatively small fraction of the fission products iodine and cesium. From these data, iodine partition coefficients between sump water and gas phase in the range 10 to 10 were calculated, which are comparatively high values when one considers that the sump water consisted solely of condensed steam and, consequently, had a pH of about 7. Obviously, a large fraction of the initial h was... 

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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