ATOMIC ENERGY F F S L'energie ATOMIQUE of CANADA UMITED T

AECL-6f)2 ATOMIC ENERGY ffS L'ENERGIE ATOMIQUE OF CANADA UMITED Tif T DU CANADA UMITEE

ASSESSMENT OF IODINE BEHAVIOUR IN REACTOR CONTAINMENT BUILDINGS FROM A CHEMICAL PERSPECTIVE

EVALUATION DU COMPORTEMENT CHIMIQUE DE L'lODE DANS L'ENCEINTE DE PROTECTION D'UN REACTEUR

Robert J. Lemire', Jean Paquette, David F. Torgerson , David J. Wren1, J. Wallace Fletcher2

Whiteshell Nuclear Research Establishment Etablissement de Recherches Nucle'aires de Whiteshell

* Chalk River Nuclear Laboratories Laboratoires Nucleaires de Chalk River

Whiteshell Nuclear Research Etablissement de Recherches Establishment Nucle'aires de Whiteshell Pinawa, Manitoba ROE 1L0 June T981 juin ATOMIC ENERGY OF CANADA LIMITED

ASSESSMENT OF IODINE BEHAVIOUR IN REACTOR CONTAINMENT BUILDINGS FROM A CHEMICAL PERSPECTIVt

Robert J. Lemire , Jean Paquette , David F. Torgersor) , 1 2 David J. Wren and J. Wallace Fletcher

Research Chemistry Branch Whiteshell Nuclear Research Establishment Physical Chemistry Branch Chalk River Nuclear Laboratories

Whiteshell Nuclear Research Establishment Pinawa, Manitoba ROE 1L0 1981 June

AECL-6812 EVALUATION DU COMPORTEMENT CHIMIQUE DE L'IODE DANS L'ENCEINTE DE PROTECTION D'UN REACTEUR

par

Robert J. Lemire , Jean Paquet te , David F. ïorgerson , 1 2 David J. Wren et J. Wallace Fletcher

RESUME

Les paramètres thermodynamiques pour les espèces aqueuses, solides et gazeuses de l'iode à 25°C ont été obtenus à partir d'un examen de la littérature chimique. Ces données ont été rendues com- patibles avec la compilation du groupe CODATA. En utilisant les données thermodynamiques à 25°C, les valeurs de l'énergie libre de formation ont été évaluées en fonction de la température, jusqu'à 150°C, et ce pour toutes les espèces de l'iode. Les résultats sont présentés sous forme de diagrammes tension-pH, de diagrammes cîe distribution d'espèces et de coefficient de partage phase liquide/phase gazeuse. On discute aussi de la chimie de l'iode à l'intérieur même du combustible nucléaire, dans le circuit de refroidissement primaire et dans l'atmosphère de l'enceinte de protection. Cet examen du comportement chimique de l'iode démontre clairement qu'il est possible de I.xmiter la concentration de l'iode dans l'atmosphère de l'enceinte de protection, iode pouvant être libéré à la suite d'une perte accidentelle de fluide refroidisseur durant laquelle le combustible nucléaire serait endommagé.

Service de la Recherche Chimique Etablissement de Recherches Nucléaires de Whiteshell

Service de la Chimie-Physique Laboratoires Nucléaires de Chalk River

L'Energie Atomique du Canada Limitée Etablissement de Recherches Nucléaires de Whiteshell Pinawa, Manitoba ROE 1L0 1981 juin

AI :L-6812 ASSESSMENT OF IODINE BEHAVIOUR IN REACTOR CONTAINMENT BUILDINGS FROM A CHEMICAL PERSPECTIVE

Robert J. Lemire , Jean Paquette , David F, Torgerson , 1 2 David J. Wren and J. Wallace Fletcher

ABSTRACT

Thermodynamic parameters for aqueous and gaseous iodine species at 25°C have been obtained from the literature and a data base has been constructed that is consistent with CODATA values. Using the 25°C data base, Gibbs energies for the iodine species have been calculated as a function of temperature to 150°C. Results are presented in terms of potential/pH diagrams, species distribution diagrams, and liquid/gas partition-coefficient plots. Iodine chemistry in the fuel, in the primary coolant system, and in the containment building atmosphere is also discussed. This assessment of iodine behaviour clearly shows that there is considerable scope for limiting the concentration of airborne iodine in reactor containment buildings following a loss-of-coolant accident in which fuel failure occurs.

Research Chemistry Branch Whiteshell Nuclear Research Establishment

Physical Chemistry Branch Chalk River Nuclear Laboratories

Atomic Energy of Cpnada Limited Whiteshell Nuclear Research Establishment Pinawa, Manitoba ROE 1L0 1981 June

AECL-6812 CONTENTS

Page

1. INTRODUCTION

2. IODINE CHEMISTRY IN THE FUEL

3. IODINE CHEMISTRY IN THE PRIMARY SYSTEM 3

3.1 DISCUSSION 3 3.2 SUMMARY 5

IODINE SOLUTION CHEMISTRY IN THE CONTAINMENT

4.1 INTRODUCTION 5 4.2 SOLUTION SPECIES 6 4.3 IODIKE VOLATILITY 11 4.4 DISTRIBUTION DIAGRAMS 12 4.5 GAS-LIQUID PARTITION COEFFICIENTS 13 4.6 KINETIC FACTORS INFLUENCING IODINE SPECIATION IN AQUEOUS SOLUTION 15 4.7 HYDRAZINE REACTIONS 18 4.8 SUMMARY 20

5. GAS PHASE BEHAVIOUR 21

.5.1 INTRODUCTION 21 5.2 GAS PHASE REACTIONS 22 5.2.1 Inorganic Iodine 22 5.2.2 Methyl Iodide 23 5.3 IODINE SURFACE ADSORPTION AND DESORPTION 24 5.4 SUMMARY 26

6. CONCLUSIONS 26

REFERENCES 28

.../cont. CONTENTS, concluded

Page

TABLES 33

FIGURES 36

APPENDIX A SUMMARY OF LARGE-SCALE TESTS - IODINE IN CONTAINMENT 47

APPENDIX B THERMODYNAMIC DATA 57

APPENDIX C FACTORS AFFECTING SURFACE ADSORPTION AND DESORPTION OF IODINE 70 1. INTRODUCTION

The objective of this report is to assess iodine behaviour in nuclear reactor containment buildings and to identify the key areas for further research that could lead to improvements in the analysis of iodine emissions. For postulated loss-of-coolant accidents (LOCA) where fuel failure occurs, it has usually been assumed that a large fraction of the iodine released from fuel becomes airborne in the containment building and is, therefore, available for release to the environment. However, iti the Three Mile Island accident, even though a high percentage of the iodine inventory in the reactor core was released, airborne iodine concentrations were small . This emphasizes the complex behaviour of this element but, more importantly, it shows that we have an opportunity to identify processes that could be effective for iodine abatement in CANDU systems.

Figure 1 traces possible iodine behaviour if it is released from the fuel as Csl into a reducing steam environment. The broken line represents the interface between the primary coolant system and containment. The boxes represent the various chemical and physical forms of iodine. In general, iodine can change from one form to another as indicated by the arrows. If Csl encounters different conditions in the primary system, the iodine species released to the containment will change and their subsequent behaviour will change accordingly.

An important observation from Figure 1 is that both the gas phase and the solution phase are important in iodine behaviour. There are a number of "sinks" in both phases that could tie up the radioiodine until it decays, and these are marked in the upper right-hand corner of the appropriate boxes. Previously, the solution "sinks" have not been effectively assessed in accident analysis and it has been assumed that a large fraction of the iodine is available for release from the containment. Therefore, in this work, we have concentrated on a better - 2 -

characterization of the behaviour of iodine in aqueous solutions under LOCA conditions.

Because of the complex behaviour of iodine, we have used predictive methods based on thermodynamic and chemical kinetic information to analyze the overall problem and to evaluate the sensitivity of various parameters. In this way, a fundamental understanding of a relatively few key parameters has led to an improved assessment of the overall system and allowed us to focus on the important measurements.

2. IODINE CHEMISTRY IN THE FUEL

The chemistry of iodine within the fuel is controlled by thermodynamic equilibria. At fuel operating temperatures, rates of (2 3) solid-state reactions are limited by diffusion ' . Based on experimentally measured fission-product releases, the assumption that the fission products are produced isotropically within the fuel appears justified, permitting equilibrium calculations. On this basis, therroo- dynamic equilibrium calculations have been made by Pobereskin et al. (3) and Besmann and Lindemer . The controlling parameter of the chemistry

in the fuel is the oxidation potential (uQ ). Cubicciotti et al. have calculated that, at the normal oxidation potentials in fuel, cesium uranates are formed, and Besmann and Lindemer have shown that

UQ is primarily controlled by the reaction between UO and Cs?UO . CANLUB graphite lubricant inside the fuel sheath may act as a reducing agent to maintain a low oxidation potential, but this has not been investigated in thermodynamic calculations. The large excess of cesium over iodine (Cs:I -v. 10:1) and the high stability of Csl result in Csl being the most stable form of iodine at low fuel oxidation potentials • . The existence of iodine as Csl in fuel has been demon- strated experimentally by Lorenz et al. - 3 -

While Csl is the major form of iodine, Zrl, and Zrl,, both volatile at fuel temperatures, may exist at concentrations several (3) orders of magnitude lower than Csl . The high radiation fields will disturb the thermodynamic equilibria and very small steady-state concentrations of I, and of I atoms will also be present. There is no direct evidence that organic iodides exist in measureable quantities in the fuel.

3. IODINE CHEMISTRY IN THE PRIMARY SYSTEM

3.1 DISCUSSION

The chemistry of iodine within the primary coolant system is extremely complex, owing to the high temperatures, radiation fields, and a variety of oxidation potential conditions. Reaction rates will be rapid although thermodynamic equilibrium may not be attained in cases where residence times are short.

Iodine is probably released to the primary system as vapour phase Csl and subsequent reactions depend on the conditions encountered. As summarized in Figure 1, the following conditions or any combination thereof may prevail:

1. Reducing or mildly oxidizing steam. Csl is stable to at least 1600°C under these conditions . Below ^ 1300°C, Csl will plate out on surfaces a"d will only be removed if oxygen or liquid H,0 is present. If the steam/CsI mixture is condensed, iodine will be converted to I (aq).

2. Steam/air. Under oxidizing conditions, Csl may rapidly react

with 0, to form CS2O and I2. Cso0 and I^O will further react to form CsOH. Csl may be oxidized to I- either in the gas phase or on surfaces, but there are no experimental data for the high temperatures characteristic of post-accident primary systems.

3. High-temperature water. The chemical forms of iodine will depend on the temperature, the pH and the oxidation potential of the solution. Unfortunately, we do not currently have methods to extend the data base from 25°C to the high post- accident temperatures of the primary system. However, under conditions ranging from mildly oxidizing to reducing, and at neutral to alkaline pH, I will likely be the dominant species. Therefore, Csl will simply dissolve to form Cs' and I~. At more acidic and oxidizing conditions, 1^. HIO, IO^ and HIO^ could be formed depending on the total iodine concentration and the reaction rates. Strong radiation fields are reported to oxidize low concentrations of I rapidly, and essentially quantitatively, to 10.,, which is the thermodynamically stable form of iodine under oxidizing, neutral conditions '

Under conditions where Csl is converted to I , some zirconium and uranium iodides may be formed. Iodine reacts with zirconium metal to form Zrl,, Zrl, and Zrl,, which have appreciable vapour pressures at (11) (12) accident temperatures . Nakashima and Tachikawa have reported release of volatile UI, from irradiated U0 that was converted to V 0 . Large releases of both species appear unlikely since the oxidizing conditions required to produce I will oxidize the zirconium and uranium, and ZrCL and U0 are more stable than the iodides. Zrl and UI formed £2, x x in the pressure tubes will condense on cooler sections of the primary system and subsequently be dissolved by coolant water to release I (aq) to the reactor vault.

Higii radiation fields and temperatures will result in the formation of free radicals, such as OH, that could affect the iodine chemistry. However, the relevant kinetic data are not available for analyzing these reactions. - 5 -

3.2 SUMMARY

The chemical forms of iodine released to the containment will depend on the wide variety of chemical environments possible in the primary system. For a "dry" accident where the primary system contains only steam and hydrogen, Csl is the species most likely to be released. In this ca

4. IODINE SOLUTION CHEMISTRY IN THE CONTAINMENT

4.1 INTRODUCTION

Ghai has reviewed CANDU containment conditions following a The maximum total iodine inventory in a CANDU 600 MW reactor 129 is •*> 4.5 kg. Of this, approximately 85% is either long-lived I or stable I. The total volume of primary coolant, dousing water and emergency core-cooling water is 'v 2.5 x 10 kg. Therefore, if the iodine losses to containment surfaces and to the gas phase are neglected, the maximum iodine concentration following an accident will be —5 —3 < 1.4 x 10 " mol-dm . Containment gas temperatures are expected to peak at 70 to 80°C and rapidly decrease after a few minutes. Water temperatures will be in the range 30 to 60°C. At these temperatures, hydrogen wo.il probably not be an effective reducing agent and the oxidation equilibrium for solution species will most likely be controlled - 6 -

by dissolved oxygen or by chemical additives such as hydrazine. If 50 ppm (2 x 10 mol'dm ) hydrazxne are added to the emergency core- cooling and dousing systems, the pH will be 'v* 9.6. However, dissolved CO may raise the acidity somewhat. This will mix wi*-h a small volume of primary coolant water (pH ^ 10.6). Therefore, water entering containment during a LOCA will likely have a moderate buffer capacity and a pH between 5 and 10.

Following a LOCA, iodine released to containment will be par- titioned between the liquid and gas phases. In the absence of sprays, this may lead to temporary high iodine concentrations. However, large- scale tests (Appendix A) show that sprays rapidly remove inorganic iodine from the gas phase so that the volatilities of iodine species in solution determine the airborne concentrations.

Therefore, the first task is to identify what iodine species are present in solution and then to evaluate their volatilities. These can be calculated reliably using the principles of chemical equilibrium providing thermodynamic information is available and reaction rates are not too slow. We have assembled a self—consistent thermodynamic data base for iodine species at 25°C and have ueed extrapolation procedures to extend the data base to 150°C. Details of the construction of this data base, and error limits, are given in Appendix B. This information was then used to predict iodine chemical species and gas/liquid parti- tioning. It must be emphasized Chat any conclusions drawn from this analysis are dependent on the accuracy of the thermodynamic values used and on the assumption of chemical equilibrium.

4.2 SOLUTION SPECIES

Iodine exists in a number of oxidation states' which are repre- sented by such species as iodide, iodine, hypoiodite, iodate and periodate. Iodine dissolved into an aqueous phase ^nder ors a series of complex ;actions. In dilute solution, it is rapidly hydrolyzed: — 7 —

- + I2(aq) + H20 ; HlO(aq) + I + IT (1)

Iodine also reacts with the iodide ion to form the triiodide ion:

I2(aq) + I" ^ I3 (2)

The HIO tends tj disproportionate when the pH is raised, as shown in equation (3):

3HI0(aq) ^ I0~ + 21 + 3H+ (3)

The various species can also be involved in acid-base reactions such as:

HlO(aq) + 10" (4)

H I0+ — HlO(aq) (5) H HIO (aq) *" + 10" (6) ^—

Finally, under strongly oxidizing conditions, iodine can exist in solution as periodate and complex hydrated iodate species.

The distribution of iodine between the different aqueous species is governed by the various equilibrium constants. As an example, for equation (1):

+ v _ (HI0(aq))(r)(H ) K U) l ~ (I2(aq))(H2O) where the parentheses represent the activity of a given species. In dilute solution these acl' ties can be equated to concentrations - 8 -

without too much error. The equilibrium constant, K, is given by the thermodynamic relation:

AG° = -RT In K (8)

where AG° is the standard Gibbs energj of reaction and is given by:

AG° = A, AG° - ZJ AG° (9) rx products reactants

where AG° is the standard Gibbs energy of formation for a given species.

It follows that, in order to calculate the equilibrium partition of iodine among its various species, a knowledge of their Gibbs energies of formation is necessary.

Thermodynamic parameters are available in the literature for the iodine/water system at 25°C. The recent data for aqueous iodine species are generally accurate and quite reliable. HlO(aq) and its protonated and deprotonated forms, H 10 and 10 , are exceptions. These three species have not been well characterized and the available data need confirmation. Also, the standard Gibbs energy of formation for HIO in the gas phase is poorly known. The only experimental value available is the Gibbs energy of vaporization of HIO reported by Lin (14)

Whereas relatively accurate thermodynamic data are available on iodine/water systems for 25°C, the only reliable thermodynamic measurements that have been reported above 25°C are the solubility of

I2(c), which is known up to 114°C, and its vapour pressure, which has been determined over the same temperature range.

When no direct experiments have been done, Gibbs energy values at elevated temperatures can be accurately calculated from 25°C values if the entropy at 25°C, S°, and the heat capacity, C°, as a function of temperature are known, by using the exact thermodynamic relation: - 9 -

T T T 2 c. 2 ~ T2 J T"

The entropy at 25°C and the heat capacity as a function of

temperature are accurately known for I.(g), I?(c) and I (1). Thus, the use of equation (10) and the solubility and vapour pressure measurements

mentioned above provide reliable values of AG° for I.(c), I?(l), I (aq) and I,(g) in the 25 to 150°C temperature range. For ionic species, very few heat capacity values are known and Gibbs energy values must be extrapolated using semi-empirical methods, like the Criss-Cobble ' treatment, which rely on entrqMfc, correlations. No C° values are avail- J%? P able for the neutral species, HI0(aq), HI0(g) and HjIO^Caq), but these can be estimated by comparison with analogous chloride compounds or by C°- entropy correlations. Equation (10) then provides a rough estimate of their Gibbs energy values at temperatures above 25°C. The "best" literature values for the standard Gibbs energy of formation of iodine species at 25°C ?.re grouped in Table 1 together with values for higher temperatures obtained from the available experime. tal measurements (e.g., for IjCc), ^(aq), ^(g)). from estimation (HXO(aq), HIO(g), H 10,(aq)) or from a modified Criss-Cobble extrapolation procedure, for the ionic species. Because high-temperature experimental data are available for only a limited number of species, the Gibbs energy values for temperatures above 25°C are generally less reliable than those for 25°C. The high-temperature values are espe-ially crude for HI0(aq) and HIO(g) since the 25°C values are already uncertain. The sources and analysis of the data presented in Table 1 are detailed in Appendix B.

One of the best ways to represent the behaviour of aqueous systems under various pH and potential conditions, compactly and in pictorial form, is by the use of potential/pH diagrams. With the values - 10 -

from Table 1 and by using techniques described in reference (18), potential-pH diagrams have been calculated for different temperatures and total dissolved iodine concentrations. These diagrams show che predominant species in aqueous media f^r various potential and pH conditions. Figure 2 shows diagrams for temperatures of 25°C and 100°C and for total dissolved iodine concentrations of 10 and 10 mol-din

-3 -3 At low concentrations (< 10 mol-dm ), the solubility of iodine is not controlled by any solid in the iodine/water system. Cal- culations have been done considering metal ions that might be present in the aqueous phase following accidental release of radioactive iodine and that might form sparingly soluble compounds with iodine. The likely candidates are uranium and zirconium from the fuel and its cladding, and iron, nickel and cobalt from stainless steel components within the containment building. Zirconium metal reacts with iodine under certain conditions to form Zrl.(s) . However, this compound is unstable in water and releases iodide ions according to the reaction:

+ ZrI4(s) + 2H2O ^ ZrO2(s) + 4r + 4H (11)

Uranium iodides, UI (s) and UI (s), are similarly very soluble in water

and so are FeI2(s), NiI2(s) and CoI2(s). Thus, the total dissolved concentration of iodine depends only on the amount of iodine available following the release.

An examination of Figure 2 shows that, except under relatively oxidizing conditions, I is the predominant species in aqueous iodine solutions above pH=3 and that this is true at high and low temperatures, and at high and low concentrations of iodine.

Under relatively oxidizing conditions (air-saturatec water), 10, becomes the predominant species above pH=3. This is consistent with the behaviour of iodine in sea water where it has been shown that 10, -Il-

ls present near the surface and I is the dominant species at greater depth where less oxygen is available *•'.

4.3 IODIKE VOLATILITY

The data in Table 1 can be used to calculate the volatility of iodine species from equations like

X (aq) 2

and equation (8). The results of these calculations are reported in Table 2 for I. and HIO. An examination of Table 2 shows that HIO is less volatile than I- and that the volatility of both species increases with temperature, as expected. However, as mentioned previously, the data for I are much more reliable than those for HIO. The ionic species of iodine are quite non-volatile at temperatures below 150°C. As an (20) -32 example, Eggleton has calculated a value of 1.6 x 10 atm* for the partial pressure of I in equilibrium with a solution containing 1 mol-dm of I~. The partial pressure of H 10 was similarly calcu- -72 lated to be of the order of 10 atm. Since these values represent less than one molecule for the whole containment building, the volatility of *"hese ionic species can be ignored. No data are availabne for I0~ and I0~ but, on the basis of charge and size, their equilibrium partial pressures would be similar to those for I and 1^10 . The neutral molecule, HIO , would be expected to be volatile to a certain extent, but there are no literature values for AG°(HlO.Cg)). Thus, the concentration of iodine above an aqueous solution will depend on the concentration of I,(aq), HlO(aq) and, possibly, HIO,(aq) in the solution. Speciation is affected by factors like temperature, pH, oxidation potential, the overall concentration of dissolved iodine and the presence of other ions that might form complexes or precipitates with iodine.

1 atm = 101.3 kPa - 12 -

An examination of Figure 2 shows that the potentially volatile species, I (aq), HlO(aq) and HIO_(aq), become predominant only under acidic, oxidizing conditions. Figure 2(d) shows that I (aq) becomes a less important species at high temperatures and low iodine concentrations. From considerations of these thermodynamic diagrams, one expects that the concentration of iodine in a gas phase in equilibrium with an aqueous medium will be ninimized by basic pH, reducing conditions, low temperature and low concentration of dissolved iodine.

4.4 DISTRIBUTION DIAGRAMS

The exact distribution of iodine among the various aqueous species can be calculated by considering together the equilibrium constants for all the possible reactions in the iodine/water system. Such calculations have been performed for various pH, temperature and potential conditions and for a wide range of total dissolved iodine concentrations by using the Gibbs energy values from Table 1 and a computational iterative procedure. Figure 3 shows some of the results plotted as distribution diagrams. Figures 3(a) and (b) are for a total dissolved —9 —3 iodine concentration of 10 mol-dm and a strongly oxidizing medium (oxygen-saturated water which corresponds closely to water in equilibrium with air). As expected from the potential/pH diagrams, under these conditions 10 is the predominant ionic species. Among the potentially volatile species, HI0.(aq) predominates, followed by HlO(aq). Elemental

iodine, I?(aq), is predicted to be present in very low concentrations and is not expected to contribute very much to the overall volatility of iodine under these conditions.

As can be seen in Figure 3(b), raising the temperature from 25°C to 100°C increases the equilibrium concentration of both HIO,(aq) and HlO(aq); however, the increase is much more marked for HIO. The concentration of iodine in the gas phase is, therefore, predicted to increase with temperature for two reasons: the aqueous equilibrium - 13 -

concentrations of volatile species increase with the temperature and, as mentioned previously, the volatility of these species increases with temperature (cf. Table 2).

Figures 3(c) and (d) illustrate the results of similar calculations performed for a less oxidizing medium, corresponding to a partial pressure of oxygen of about 10 atm. It is obvious that a change in the oxidation potential will markedly affect the distribution of iodine in aqueous media and thus its transport to the gas phase. Calcu- lations were also done for a total iodine concentration of 10 mol-dm , as shown in Figure 4. In this case, there is a significant increase in the IT and HIO concentrations and the gas phata concentration of iodini' will increase accordingly.

At the low total iodine concentrations characteristic of accident conditions, the concentration of KI0(aq) in solution is predicted to be always greater than the concentration of I (aq), except under very specific conditions (i.e., in an acidic, moderately oxidizing medium). However, at equilibrium, iodine would be converted predomi- nantly to non-volatile 10 and I .

4.5 GAS-LIQUID PARTITION COEFFICIENTS

From the previous distribution calculations, the partition of iodine between the aqueous and the gas phase can be obtained. The partition coefficient of iodine is usually expressed as:

P = [I I(aq)]/[Z I(g)] ri3)

-3 where the concentrations are in mol-dm . This partition coefficient is calculated easily from the results of the distribution calculations and the aqueous/gas equilibrium constants reported in Table 2. Since no Gibbs energy data are available for HIO (g), we have considered only HIO and I. for the calculations. - 14 -

Figure 5 is a plot of log P against pH, for very oxidizing -9 -3 conditions and a total dissolved iodine concentration of 10 mol-dm The partition coefficient increases exponentially with pH under these conditions, due to the steady decrease in the equilibrium concentrations of HlO(aq) and I (aq) as the pH increases. As expected, an increase in temperature lowers the partition coefficient. Thus, low temperature and basic pH minimize the transfer of iodine to the gas phase.

In Figure 6, log P is plotted against the partial piessure of oxygen above the solution, for various temperatures and L. total iodine concentration in solution of 10 mol-dm . The effective oxygen partial pressure is an equivalent thermodynamic expression of the oxidation potential in the solution. It is apparent from this figure that the oxygen partial pressure (or oxidation potential) is very important in controlling the distribution of iodine between the aqueous and gas phases. Under very oxidizing conditions (oxygen-saturated water), the partition coefficient is relatively high. As the oxygen partial pressure is lowered, the partition coefficient decreases, passes through a minimum, and then increases rapidly as the oxygen partial pressure decreases further. The position of the minimum depends on the pH and, for solutions which are neutral to slightly basic, it is located around -4 -8 oxygen partial pressures of 10 to 10 atm. The strong dependence of the partition coefficient on the potential can be rationalized with the aid of the thermodynamic diagrams shown in Figures 2 and 3. On these diagrams one can see that, among the volatile iodine species, HIO is dominant in aqueous media under very oxidizing conditions. When the conditions become less oxidizing, the proportion of I (aq) increases markedly. Since I,(aq) is more volatile than HlO(aq), the partition coefficient decreases. As the potential decreases further, I becomes more and more stable relative to I-Caq) and the partition coefficient increases. At constant potential and pH the partition coefficient Is independent of the total iodine concentration. - 15 -

Again it is emphasized that the accuracy of these results depends on the uncertainties in the data base. The equilibrium constants for reaction (12) (Table 2) are probably accurate to within 10%. How- ever, the constant for the equilibrium between HI0(g) and HlO(aq) at 100°C can be estimated at best to be between 0.2 and 20. Moreover, preliminary measurements in our laboratory suggest that HIO volatility may be less than the calculated values at 25°C. Therefore, for conditions where HIO is calculated to be a major component of the iodine in the gas phase, actual partition coefficients may be in error by one or t«o orders of magnitude.

Based on the currently available thermodynamic data, the predominant chemical form of iodine in the gas phase is predicted to be HIO, except under moderately oxidizing conditions where I and HIO would be present in roughly equal concentrations. Note that this does not take organic iodide formation into account.

4.6 KINETIC FACTORS INFLUENCING IODINE SPECIATION IN AQUEOUS SOLUTION

Consideration of the thermodynamics of the iodine system provides a relatively detailed picture of the situation at equilibrium. However, if reaction rates are slow, the chemical forms of the iodine for the time period of interest may be markedly different from those indicated for equilibrium. For example, if at equilibrium the vapour phase contains even 1% by volume oxygen, then 99.95! of iodine (in any initial form) will have been converted to iodate (for 5 < pH < 10) and_ thus the amount of iodine available to the vapour phase will be limited. However, this behaviour is relevant to a LOCA only if the iodine is in a form which will rapidly be converted to iodate.

The thermodynamic parameters for most iodine species in aqueous solution are fairly well known for 25°C and can be estimated for somewhat higher temperatures. However, the kinetic factors governing the approach - 16 -

to equilibrium, especially for very dilute iodine solutions near neutral pH, are less well understood. Data are usually available for very specific pH conditions, and often only for 25°C. The most relevant reactions controlling the iodine speciation are:

I,(aq) + HO - HlO(aq) + H+ + i" (1)

I2(aq) + I~ __ - lj (2)

HlO(aq) _ H+ + I0~ (4>

2HI0(aq) ^ HlO^aq) + i" + H+ (14)

+ HIO2(aq) + HlO(aq) _ T0~ + i" + 2H (15)

+ il" + 4H + 02 3> 2I2(aq) + 2H2O (16)

If the iodine in solution is originally present only as iodide, from Csl or from other sources (HI, Zrl,, etc.), then under oxidizing conditions this may be converted to I and 10 . In acidic solutions the uncatalyzed oxidation of iodide to iodine by oxygen (reaction (16)) is slow, unless the concentration of acid is quite high (pH ^ 0) The rate at 25°C, for pH < 1, was found to be first order in acid concentration by Sigalla and Herbo *• ' who reported the rate of forma- + tion of iodine to be 8 x 10~ [02][l~][H ] mol-dm~ -min" . This suggests that the reaction would be even slower in the pH range of concern here (5 < pH < 10). Reaction (16) is catalyzed when the solution is 2+ irradiated with light (A < 500 nm), and by trace metals such as Cu and Fe . This last effect, in particular, may accelerate formation of I- (and/or HIO) from iodide if conditions in the containment building are oxidizing and acidic.

This species has been postulated as an intermediate by Liebhafsky and Roe(21) ancl by Noyes et al.^2,23)> j,ut has not yet been unambiguously identified. - 17 -

If the initial form of the released iodine in solution is

I2(aq), or if I^Caq) is formed by oxygen oxidation of I , the iodine would form an equilibrium concentration of iodate. The overall equilibrium

+ 3I2(aq) + 3H2O - 5I~ + I0~ + 6H (17)

is a combination of the hydrolysis equilibrium and the disproportionation equilibria (14) and (15). At 25°C the equilibrium (17) is to the right, for total iodine concentrations <^ 10 mol-dm and pH > 5. The iodide so produced can then be slowly oxidized to form more I (aq) (reaction (16)). The overall oxidation process can then be written

+ 2I2(aq) + 5O2 + 2^0 > 4T0~ + 4H (18)

The reverse reaction of equilibrium (17) has been studied extensively but the forward reaction less so. The hydrolysis of iodine t'97 9 ft ^ (the forward reaction of equilibrium (1)) is rapid ' , and is important above pH=6 for £ I >_ 10 mol-dm , and at even lower pH for lower iodine concentrations. Therefore the forward reactions of equilibria (27) (14) and (15) may be important. The work of Thomas et al. indicates that the rate of the forward reaction (17) is dependent on the concentration of iodine atoms nominally in the +1 oxidation state (X I(+l)) to the second power, at pH=9 (where I K+l) = [I2J + [I~] + [HIOj + [I(T]). The important reactive species was assumed to be hypoiodous acid (7 < pH < 10), and the rate in this range was reported as approximately d E I(+l)/dt = 2.5 x 10+2 [HI0]2 dm3-s"1-mol"1. Hypoiodite ion, formed by dissociation of HI0 (the forward reaction of equilibrium (4)), may also be reactive^ ' but is relatively unimportant below pH=10. For 7 < pH < 10, the rate equation predicts that,at 25°C,a solution with E K+l) = 10~6 mol-dm"3 initially will still contain 5 x 10~8 mol-dnf3 9 3 (I2 + HI0) after one day. Further, if initially £ I(+l) = 10" mol-dm" , the solution will contain 8 x 10 mol-dm of these species after ten days. Juznic^ *' has also studied the forward rate for equilibrium - 18 -

(17), but at higner iodine concentrations. His results and proposed (27) mechanism arc inconsistent with those of Thomas et al. and he suggests that the reaction rate is even slower. Thus, if the rate of the forward reaction of equilibrium (14) controls the conversion, in solution, of iodine from an inorganic volatile form to a non-volatile form, concen- -8 -9 -3 trations of volatile species of the order of 10 - 10 mol-dm will persist for very long periods of time. This will occur regardless of whether the iodine is present as I, or HIO. At higher temperatures, destruction of the dissolved I. or HIO should be more rapid.

The overall rate for the uncatalyzed oxygen oxidation of iodide to iodate will depend on the rate of reaction (16) and, for the conditions of interest, this rate is slow. However, as mentioned pre- (8 9} viously, strong radiation fields ' are known to oxidize low concentrations of I~ rapidly to non-volatile I0~, the thermodynamically stable form in oxidizing, neutral solutions. It should be noted that following a LOCA, radioactive decay of iodine isotopes will not markedly affect the rates of the iodine reactions, or the position of the thermodynamic equilibria, since it will not lower the total iodine concentration in solution by more than 15% even after several weeks. However, the actual radioactivity of the iodine would decrease substantially over such a period of time.

4.7 HYDRAZIHE REACTIONS

Hydrazine is a fairly strong reducing agent and, if added to the dousing sprays and the emergency core coolant, could be expected to maintain reducing conditions in the water pool after a LOCA. However, hydrazine is also thermodynaroically unstable in aqueous solutions and the rate of hydrazine decomposition is enhanced at high temperatures. Therefore, if the hydrazine is to have any effect it must survive the injection process where it may, for short periods of time, be in contact with hot fuel. Thus, lowered hydrazine concentrations in the final solution would be expected. The amount of hydrazine decomposed would be specific to the system and the accident conditions, and will not be discussed. 19 -

The main effect of hydrazine on aqueous !„ solutions is to produce I ions, as follows.

+ 2I2 + N2H4 ^ N2 + 41" + 4H (19)

This reaction is much slower in acid solutions than in neutral or basic solutions. This has led some worker? to suggest that H10 is the main oxidizing agent^ '. The reaction Is quite rapid near pH=7.

Hydrazine will a1so react with dissolved oxygen:

20 + N H , 2H 0 + N (20)

The reaction rate is highest in weakly alkaline solutions '. Tho reaction rate wi;h oxygen apparently is greatly accelerated at higher temperature. Therefore, if a considerable quantity of I is slowly released into the water, the oxygen-hydrazine reaction may well consume the hydrazine before all the iodine has reacted. Conversely, if only trace amounts of oxygen are present, all the iodine will rapidly be converted to iodide, and residual oxygen, which might slowly re-form volatile forms of iodine, will instead be scavenged, thus keeping the iodine in solution.

Any iodate which is produced by radlolysis will probably not be readily reconverted to iodine Dy hydrazine. The reaction

+ 3N2H^ + 2IO3 > 3H + 3N2 + 2I~ + 6H2O (22) is reasonably rapid at pH=l and 25°C but, at least for low 10 concen- + + 2 trations, the rate varies as [N H ][H ] and thus is much slower at (32) higher pH . There is also evidence that the mechanism of the iodate (33) reduction may be different at high pH - 20 -

Ic should be mentioned that although hydrazine-iodine reactions primarily produce gaseous nitrogen as a by-product, many other reactions involving the oxidation of hydrazine produce ammonia and/or (33) hydrazoic acid .

Temperature-dependent rate constant data are available for the reaction of methyl iodide with hydrazine ^ in aqueous solution. Data are also available for the hydrolysis reaction and its dependence on hydroxyl ion concentration . Below pH=10, the fraction of CH I reacting with hydroxyl ion is small compared to that which reacts directly with water. The hydrazine reaction and hydrolysis are both assumed to proceed by Kucleophilic attack on the methyl iodide, to form methylhydrazine and methanol respectively, as well as iodide ions. The kinetic data are summarized in Table 3.

At low temperatures (near 25°C) and in th° absence of hydrazine, CH I can persist for long pa-iods of time. Even a moderate increase in temperature to 50°C will destroy only half the soluble CH.,1 -3 -3 each day by hydrolysis. The addition of hydrazine at 10 mol'dm will markedly accelerate decomposition of the methyl iodide. At 80°C aqueous methyl iodide should rapidly disappear (within a day) even in the absence of hvdrazine.

4.8 SUMMARY

Under equilibrium conditions, which do not depend on initial iodine chemical forms, the liquid/gas partition coefficients are large and for each pH value exhibit a minimum at a specific oxidation potential. For example, if a 10 mol-dm solution of iodine had a total activity of 10 Ci (3.7 x 10 Bq), then only "- 10 Ci (3.7 x 10 Bq) would be found in the gas phase under the worst oxidation potential conditions at pH=7 and 25°C. The conditions required to minimize equilibrium transport of iodine from aqueous solution to the gas phase are: low temperature, high pH, and a strongly reducing medium. If these - 21 -

19 1 f\ conditions are maintained, partition coefficients > 10 - 10 may be possible. However, it must again be stated that oxidation potential is the important variable and that the minimum partition functions occur at a potential between air-saturated water and reducing conditions.

Chemical kinetic considerations can significantly affect this analysis and therefore the initial chemical form of the iodine becomes important. If iodine is released as an iodide (e.g., Csl or HI dissolved in water), then oxidation may be slow. If the initial iodine is released as 1^, iodine entering solution will rapidly hydrolyze to 1!IO, which could persist in solution for some time. The calculated distribution coefficients for HIO (Table 2) are in the range 10 to 200 under LOCA conditions. Since there is evidence that HIO reacts with hydrazine (and possibly other reducing agents), control of HIO may not be difficult. However, the behaviour of HTO in solution is not well characterized and further work is needed to establish the accuracy of HIO thermodynamic values. Until this work is done, the "HIO question", along with CH,I production, is a major unknown in assessing iodine behaviour in the containment.

5. GAS PHASE BEHAVIOUR

5.1 INTRODUCTION

The preceding analysis suggests that HIO, T and, possibly, HIO could be airborne inorganic iodine species in a LOCA. The quantities of these species will depend on the conditions described previously and on the initial chemical forms of the iodine, where reaction rates are important. If conditions are chosen so that the volatility of inorganic iodine species is small, then methyl iodide becomes the key iodine species. - 22 -

Much important information on the behaviour of iodine in the gas phase comes from the large-scale tests summarized in Appendix A. Because of the difficulty in controlling the variable conditions, it is inappropriate to compare the results of these tests quantitatively. Nevertheless, there is agreement on several important processes, as summarized in Appendix A. Spray-removal and deposition rates appear to be reasonably well understood, providing the chemical forms of the iodine are known. However, current knowledge is not sufficient to explain the persistent concentrations of airborne iodine species, which appear to involve CH,I, and perhaps HIO. We have previously discussed HIO volatility and production, but a similar analysis of CH I is not possible since its origins have not been well characterized.

5.2 GAS PHASE REACTIONS

5.2.1 Inorganic Iodine

The chemical forms of iodine released to containment will depend on the chemistry of the primary system. Reducing steam conditions, for example, likely result in the release of Csl, which will immediately dissociate, on contact with water, to produce I . However, a hot Csl/steam mixture contacting air in the containment building may be oxidized, to yield I . At low concentrations, I will rapidly and quantitatively hydrolyze to HIO and I when the spray water is con- tacted. The equilibrium constant for the gas phase hydrolysis

I2(g) + H2O(g) ' _ • • HIO(g) + HI(g) (23)

-22 is calculated to be ^ 10 at 25°C- We have used a mass spectroscopic method to show that I- is stable in air/water vapour mixtures to at least 110°C at a total I concentration of ^ 100 ug/g. Under these conditions, HIO will be formed only when liquid water (or possibly wet steam) is present and the concentration of HIO in the gas phase will be - 23 -

due to its volatility. However, this does not take into account unknown radiolysis, or surface effects.

Unfortunately, there is no unambiguous direct evidence for or against the existence of gas phase HIO and there is only limited characterization of HIO in aqueous solution. However, several workers have reported radioactive iodine transport, which they attribute to

HIQ(9,10,36-38)_

5.2.2 Methyl Iodide

The chemistry of methyl iodide and its formation in contain- (39) merit has been extensively reviewed by Parsly and by Postma and Zavadoski . Up to 2.55! of the iodine released under accident conditions is converted to organic iodides (^ 85% CH I), but the formation mechanisms are not well known. Thermodynaroic calculations fail to predict the observed concentrations.

Radiation effects are generally considered to be much more (39-41) important than thermal reactions . The main product in the radiolysis of CH and I is CH I(1°'42'43' and the rate of formation is increased by the presence of excess water vapour, but decreased by the presence of oxygen* .

The main species formed by the radiolysis of water vapour are 44 45 OH, H, H2 and 02* ' *. The OH radical can abstract a proton from CK^ to form methyl radicals which react rapidly with I, to form CH,I. Oxygen reduces the radical yield* ' and, consequently, the amount of f.H.I formed. However, CH,I will itself be decomposed by radiation and by reactions with H and OH

Additional work is required to evaluate the kinetics of radiolysis at radiation levels ;:,wer than have been used so far, and also to understand adequately the kinetics and mechanisms of CH I formation - 24 -

on containment surfaces and coatings. Evidence for the presence of CH I in large-scale tests in the absence of strong radiation fields makes these studies important.

Although methyl iodide is reasonably soluble in water (0.1 mol-dm at 25 °C and one atm) , sprays are not effective in removing airborne CH»I, even when doped with Ha SO (see Appendix A). This 5.s explained by the reaction rates indicated in Table 3. The results shown in Table 3 were obtained using low initial concentrations

of CH3I (<_ 10 mol-dm ). It is apparent that CH^I can persist in water for hours or days depending on temperature, pH and whether hydrazine (N.H,) is added to the spray.

5.3 IODINE SURFACE ADSORPTION AND DESORPTION

The deposition of iodine on surfaces has been studied by many workers, both in large-scale test facilities and laboratory-scale experiments. All studies agree that the deposition rate is a first order process which can be expressed as:

(24)

where C(s) is the mass of iodine deposited on the surface, C(g) is the iodine concentration in the gas phase, A is the surface area and v, is the deposition velocity. The kinetics of the desorption process are not as well established. The three parameters of primary interest in accident safety are the deposition velocity, the desorption velocity and the ultimate surface loading capacity for iodine. These three parameters are a function of the surface material, roughness, oxidation, temperature and humidity. Discussion of these parameters is found in Appen- dix C.

Inorganic iodine reacts with a wide variety of surfaces. However, methyl iodide does not have significant deposition velocities - 25 -

onto metals or metal oxides. This probably is associated with the stability of methyl iodide and its inability to participate in corrosion mechanisms. Methyl iodide can react with many paints and deposition velocities have been measured by Genco et al. . In general, deposition velocities for CH I onto paints are lower than those for iodine although there are exceptions (e.g., deposition on polyaminopropyl- (49) methylsilane). Rosenberg et al. have studied the effects of CH.I concentration and temperature on the deposition velocity onto paints. The data are quite scattered and there are only weak general trends to higher deposition velocities at low CH,I concentration and higher temperatures (temperature range 30 to 170°C). Deposition velocites are diffusion limited in cases where the surface reaction rates are large.

Iodine deposited on containment surfaces may not be irreversibly bound, particularly if the surface temperature is raised. The amount of irreversibly adsorbed I on paints varies between 35% and 100% of the initial loading, while the amount of irreversibly adsorbed CH I (49) varies between 75% and 100% of the initial loading . The reasons for this are not known since chemical changes in the paints have not been characterized.

Desorption of iodine from surfaces other than paints is a function of the particular surface and the iodine on the surface form. For metals on which the iodine is bound as an iodide (e.g., copper), there is essentially no desorption at temperatures < 150°c' . For surfaces where iodine is physically bound as iodine, desorption is much more extensive . It is clear that, depending on the nature of containment surfaces, desorption could represent an important source term for long-term iodine emissions into the gas phase. For example, Cline (52) et al. have suggested that surface desorption is responsible for the long-term airborne radioiodine measured at Three Mile Island subsequent to the accident. However, there is considerable scope for improved understanding of the nature of permanently adsorbed iodine and to spe- cify coatings that make use of this knowledge. - 26 -

5.4 SUMMARY

Spray systems are expected to remove iodine rapidly from the gas phase, and gas phase concentrations will depend on volatility. Large-scale tests have shown that there are low, but persistent, airborne iodine concentrations that are not greatly affected by sprays. Methyl iodide and possibly HIO are the likely chemical forms.

Important areas for further study include Csl oxidation, HIO gas phase behaviour, production of CH.I by radiolysis and surface chemistry. Iodide surface characterization studies could lead to improved containment coatings that have larger fractions of irreversibly bound iodine. New abatement technology for CH I control may be another option for limiting iodine gas phase concentrations

6. CONCLUSIONS

An evaluation of iodine chemistry shows that airborne iodine emissions during a LOCA should be small. There are a number of options for controlling chemical conditions in the containment to ensure that iodine release to the environment is minimal. The small amount of iodine that remains airborne can be controlled by the appropriate choice of containment liners, the addition of other containment systems (e.g., vault coolers), and by developing new abatement technology. It is clear that there is enormous scope for iodine control in CANDU power plants, which is predicated on our understanding of basic iodine chemistry.

Specific areas for continuing research to refine our analysis are as follows:

1. Some of the values used in the analysis require verification. For example, thermodynamic and kinetic data for HIO are not - 27 -

well characterized. Further work is required to establish HIO stability limits in dilute iodine solutions and to assess possible reactions that can convert HIO to less volatile species. The volatility of HIO needs to be verified using direct detection methods and gas phase behaviour of HIO needs to be assessed.

2. Hydrazine is an effective agent for buffering pH and the oxidation potential at desirable values. The chemistry of hydrazine at high temperatures and in high radiation fields must be assessed if containment water is to be recirculated through the reactor core.

3. The production of CH,I under postulated accident conditions requires further characterization.

4. The physical and chemical behaviour of Csl under a variety of primary system conditions needs to be assessed. - 28 -

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5. R.A. Lorenz, M.F. Osborne, J.L. Collins, S.R. Manning and A.P. Malinauskas, "Behaviour of Iodine, Methyl Iodide, Cesium Oxide and Cesium Iodide in Steam and Argon", Oak Ridge Nation- al Laboratory Report, ORNL/NUREG/TM-25 (1976).

6. R.A. Lorenz, J.L. Collins, A.P. Malinauskas, O.L. Kirkland and R.L. Towns, "Fission Product Release from Highly Irradiated LWR Fu.l", Oak Ridge National Laboratory Report, ORNL/NUREG/ TH-287/R2 (1980).

7. R.A. Lorenz, J.L. Collins and A.P. Malinauskas, "Fission Product Source Terms for the Light Water Reactor Loss-of- Coolant Accident", Nucl. Tech. 4_6, 404 (1979).

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10. C.C. Lin, "Chemical Effects of Gamma Radiation on Iodine in Aqueous Solutions", J. Inorg. Nucl. Chem. ^2, 1101 (1980).

11. R.F. Rolsten, Iodide Metals and Metal Iodides, Wiley, New York, 1961. - 29 -

12. M. Nakashima and E. Tachikawa, "Relative Reactivities of Radioiodine Released from U3O8 in Oxygen and Helium Atmo- spheres Toward Propane", J. Nucl. Sci. Tech. 15, 849 (1978).

13. R.K. Ghai, unpublished information.

14. C.C. Lin, "Behaviour of Radioiodine Studies: Iodine Partition between Aqueous Solution and Gas Phase", General Electric Company Report, NEDO-12583 (1975).

15. CM. Criss and J.W. Cobble, "The Thennodynamic Properties of High Temperature Aqueous Solutions. V. The Calculation of Ionic Heat Capacities up to 200°C. Entropies and Keat Capaci- ties above 200°C", J. Amer. Chem. Soc. j}6, 5390 (1964).

16. CM. Criss and J.W. Cobble, "The Thermodynamic Properties of High Temperature Aqueous Solutions. IV. Entropies of the Ions up to 200°C and the Correspondence Principle", J. Amer. Chem. Soc. jte, 5385 (1964).

17. R.J. Lemire and P.R. Tremaine, "Uranium and Plutonium Equi- libria in Aqueous Solutions to 200°C", J. Chem. Eng. Data 25_, 361 (1980).

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21. H.A. Liebhafsky and G.M. Roe, "The Detailed Mechanism of the Dushman Reaction Explored by Computer", Int. J. Chem. Kinetics .11, 693 (1979).

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23. K.R. Sharma and R.M. Noyes, "Oscillations in Chemical Systems. 13. A Detailed Molecular Mechanism for the Bray-Liebhafsky Reaction of lodate and Hydrogen Peroxide", J. Amer, Chem. Soc. 98, 4345 (1976).

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28. M. Eigen and K. Kustin, "The Kinetics of Halogen Hydrolysis", J. Amer. Chem. Soc. 84., 1355 (1962).

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33. L.F. Audrieth and B.A. Ogg, The Chemistry of Hydrazine, Wiley, New York, 1951.

34. R.A. Hasty, "The Rate of Reaction of Methyl Iodide and Hydra- zine in Aqueous Solution", J. Phys. Chem. 73. 317 (1969).

35. W. Eguchi, K. Adachi and T. Haoka, "Reactions of Methyl Iodide in Aqueous Sodium Hydroxide Solutions. II", Kyoto Diagaku Genshi Enerugi Kenkyusho Iho ^4, 52 (1973). (Chem. Abst. 80- 119802e).

36. M.J. Kabat, "Selective Sampling of Hypoiodous Acid", Pro- ceedings of the 14th ERDA Air Cleaning Conference, Sun Valley, Idaho, 2-4 August, 1976. M.W. First, ed., p. 490 (1977). CONF-760822.

37. J.H. Keller, F.A. Duce, D.T. Pence and W.J. Maeck, "Hypoiodous Acid: An Airborne Inorganic Iodine Species in Steam-Air Mixtures", Proceedings of the 11th AEC Air Cleaning Conference, Richland, Washington, 31 Aug. - 3 Sept., 1970. M.W. First and J.M. Morgan, Jr., eds., p. 467 (1970). CONF-700816. - 31 -

38. C.A. Pelletier and R.T. Hemphill, "Nuclear Power Plant Related Iodine Partition Coefficients", Electric Power Research Insti- tute Report, EPRI-NP-1271 (1979).

39. L.F. Parsly, "Chemical and Physical Properties of Methyl Iodide and its Occurrence under Reactor Accident Conditions", Oak Ridge National Laboratory Report, ORNL-NSIC-82 (1971).

40. A.K. Postma and R.W. Zavadoski, "Review of Organic Iodide Formation under Accident Conditions in Water-Cooled Reactors", Battelle Pacific Northwest Laboratories Report, WASH-1233 (1972).

41. R.H. Barnes, J.F. Kircher and C.W. Townley, "Chemical-Equili- brium Studies of Organic-Iodide Formation under Nuclear- Reactor-Accident Conditions", Battelle Memorial Institute Report, BKI-1781 (1966).

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45. R.S. Dixon, "The Dissociation of Water Vapour by Photolytic, Radiolytic and Electron Impact Methods", Radiat. Res. Rev. 2_, 237 (1970). Also AECL-3580.

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49. H.S. Rosenberg, G.E. Cremeans, J.M. Genco, D.A. Berry and D.L. Morrison, "Fission-Product Deposition and its Enhancement under Reactor Accident Conditions: Development of Reactive Coatings", Battelle Memorial Institute Report, BMI-1874 U969). - 32 -

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TABLE 1

GIBBS ENERGIES OF FORMATION (J-mol"1) FOR AQUEOUS, GASEOUS AND SOLID SPECIES USED IN THE EVALUATION OF IODINE BEHAVIOUR (25°C - 150°C)

Species 25°C 60°C 100°C 150°C

I2(c) 0 -4 174 -9 200 -15 860

I2(D 3 322 -2 100 -8 659 -17 340 19 360 10 170 -505 -14 070

I2(aq) 16 400 10 951 3 318 -8 126 KIO(g) -85 800 -94 800 -105 300 -118 600 HlO(aq) -93 700 -102 100 -107 100 -115 100 + H2IO -106 500 -109 900 -113 800 -118 800 10" -38 410 -37 420 -34 510 -28 500 j~ -51 920 -55 420 -58 870 -62 450 -51 920 -60 250 I3 -69 210 -79 650 10" -126 300 -130 400 -134 700 -139 600

HIO3(aq) -130 900 -136 000 -141 800 -149 000 -409 000 -407 800 -406 000 -403 400 H2IO63 -492 000 -509 000 -536 300 -581 100 H3IO-2 -478 700 -484 600 --493 100 -506 300 -517 000 -523 200 -531 500 -543 400 H4IO6 -51 800 -59 020 -67 440 -78 140 I04 H5IO6(aq) -535 600 -542 600 -551 600 -564 000

UI3(s) -482 000 -490 600 -500 800 -514 200

UI4(s) -523 000 -533 500 -545 600 -561 500

ZrI2(s) -258 000 -263 400 -270 000 -278 900

ZrI3(s) -395 000 -402 300 -411 200 -422 900

ZrI4(s) -482 000 -491 400 -502 300 -516 600

U02(s) -1 031 800 -1 035 000 -1 038 000 -1 043 000

ZrO2(s) -1 042 800 -1 045 000 -1 047 000 -1 050 000 TABLE 2

VOLATILITY OF IODINE SPECIES EQUILIBRIUM CONSTANTS IN THE 25°C - 150°C TEMPERATURE RANGE

Temperature Reaction CO 25 60 100 150

s 3.30 0.292 0.185

HIO(g) V HlO(aq) 187 14 1.8 0.36 > - 35 -

TABLE 3

THE DISAPPEARANCE OF METHYL IODIDE FROM AQUEOUS SOLUTION AS A FUNCTION OF TIME

6 % CH I remaining <[CH3I]o £ 10 mol•dm )

1 min 1 h 1 d 1 month

25°C Water, pH=10 100 100 98 60

Water, pH=10 100 99 79 0.1 10~3 mol-dm"3 N,H,

50°C Water, pH=10 100 98 56 <.1O"6

Water, pH=10 100 85 3 - 10"3 mol-dm"3 N H

80°C Water, pH=10 99 43 -x. 10"7 -

Water, pH=10 95 6 -o ID"27 - 3 3 10~ mol-dm" N2H4 - 36 -

oxidizing steam: I?(g)

• reduc ing gas phase liquid phase condit:.oils: I

oxidation oxidizing complexation aerosols 1 to In conditions: I2/HIC precipitation

washout emission of volatiles

plateout

FIGURE 1: Possible Behaviour of Iodine Released Into a Reducing Steam environment. Boxes marked • in the upper right corner represent "sinks" for iodine in the containment building. g 0.4 I III X CO gO.Ol u

"0.4 [

2[l]=10"9mol dm'3 T= 25°C "0.81 2. 8. 10. 12. N

FIGURE 2a: Potential-pH Diagram for the Iodine/Water System E[I]aq = 10"9 mol-dm"3, T = 25°C ui

2[l]=10"mol dm"3 T=100°C

-0.8

FIGURE 2b: Potential-pH Diagram for the Iodine/Water System E|I]aq = 10 y mol-dm"-3, T = 100°C 0.4 2 1 w ui 35°-° 5 HI -0.4

Z[l]=10~6mol dm"3 T=2S°C -0.8 2. 8. 10. 12. 14. N PHU

FIGURE 2c: Potential-pH Diagram for the Iodine/Water System T[I]aq = 10"6 mol-dnT3, T = 25°C \.c ^£H-HIO3 C.q) ' 1

1 3 «i

0.8 ^-^ 1O3

\ I 0.4 §

SHE ! o So.o l~ I ui "^

-0.4 _

2[l]=tO"6 mol dm"3

T=KK.-C| , -0.8 I | "7^^— 1 0. 2. 4. a 10. 12. PHT

FIGURE 2d: Potential-pH Diagram for the Iodine/Water System E[I]aq = 10~6 mol-dnT3, T = 100°C 12

FIGURE 3a: Iodine Species Distribution Diagram for ?0 = 1 atm (0.1013 MPa) £[I]aq = 10"9 mol-dm"3, T = 25°C

B ' 1 • ioo°c' - - 8 103 10 - " 12 O ;^ 3,4 -2 16 \

18 L \

i i 1 1 ~6T 8 10 12 N PHT

FIGURE 3b: Iodine Species Distribution Diagram for P02 = 1 atm (0.1013 MPa) Efljaq = 10"9 mol-dm"3, T = 100°C - 42 -

1 1 ' 1 4 c 25 °C - IO3 6 = 8 - w do -

14 », 16 ' • i LJ

PHT

FIGURE 3c: Iodine Species Distribution Diagram for Po2 = 1 atm (0.1013 MPa) E[I]aq = 10~° mol'dm"3, T = 25°C

FIGURE 3d: Iodine Species Distribution Diagram for Pm = 1 atm (0.1013 MPa) P02 £II]aq = 10~n-66 mol-dm"3, T = 100°C - 43 -

A ' i i < 1 25°C' 8 - - IO3 -

O 12 \ I- 2,4 -

18 \\ i i i 8 i 1 1 1 10 12

FIGURE 4a: Iodine Species Distribution Diagram for 6 P02 = 10~ atm (0.1013 Pa) E[I]aq = 10"9 mol-dm"3, T = 25°C

B ' 1 ' ' ioo°c' 8 - I"

10 IO3 g 12 \ N 16 18

i i • ••I 8 10 12 N

FIGURE 4b: Iodine Species Distribution Diagram for P02 = 10"6 atm (0.1013 Pa) Zfl]aq = 10~9 mol-dm-3, T » 100°C - 44 -

FIGURE 4c: Iodine Species Distribution Diagram for 6 P02 = 10~ atm (0.1013 Pa) Z[I]aq = 10~6 mol-dm"3, T = 25°C

1 r • 1 1 D1 100 "c 6 IO3 8 - - = l0 OI2 *\ 10" ' 14 16 ^\ N^\ ^- 1 1 1 1 6f 8 10 12 N pH,

FIGURE 4d: Iodine Species Distribution Diagram for P02 = 10-6 atm (0.1013 Pa) Efljaq = 10"6 mol-din'3, T = 100°C - 45 -

10 12 PH.

FIGURE 5: Aqueous/Gas Partition Coefficient for Iodine as a Function of pH. Efljaq = 10"9 mol-dnT3. -24 -20 -16 -12 -8 -4 0 log P<>2

FIGURE 6: Aqueous/Gas Partition Coefficients for Iodine as a Function of the Partial Pressure of Oxygen at Various Temperatures. The pH is the neutral pH at the temperature of interest and E[I]aq = 10~9 - 47 -

APPENDIX A

SUMMARY OF LARGE-SCALE TESTS - IODINE IN CONTAINMENT

A.I. TNTRODUCTION

The concentration of radioiodine in containment as a function of time is a key concern of reactor safety in the event of an accident. A number of test facilities have been used to study the behaviour of simulated fission product releases in various environments. The primary objectives of the tests performed in these facilities were characterization of the airborne iodine species as a function of time, measurement of the rate of iodine deposition on surfaces and the rate of airborne iodine removal by dousing sprays. In Table A-l, a brief list of some of the tests is compiled. This list is by no means all inclusive, but contains a representative sample of the different experiments. Evalua- tion of the different tests in terms of the behaviour of airbot.-.e iodine is very difficult. Each test is unique with respect to such important parameters as geometry, surfaces, spray flow rate and dura- tion, and iodine release mechanism. However, there is qualitative agreement on several important processes. In the following sections, brief statements of the most important qualitative results from the tests of Table A-l are presented and a general summary of the test results is given. - 48 -

A.2. RESULTS

A.2.1 CONTAMINATION-DECONTAMINATION EXPERIMENT - CDE^ ;

1. Particulates from melted fuel are small (0.001 - 0.7 |jm) and contain tin from Zircaloy cladding.

2. Deposition of I and particulates on surfaces depends on surface orientation due to gravity settling.

3. Deposition of I_ on surfaces increases in the following order: glass < plastics < metal < paints. The rate of deposition on paints increases if the temperature is above the softening point.

4. Deposition rates of I. are higher on surfaces where steam condenses than on surfaces where steam does not condense.

A.2.2 CONTAINMENT RESEARCH INSTALLATION - CRI^A'2^

1. Iodine is initially removed by a fast chemisorption process.

2. Deposition is fastest on painted surfaces.

3. Iodine (I.) is rapidly removed by boric acid-Na.S-O,, sj-rays but CH I is not.

4. Airborne concentration of CH I increases with time to a maximum 20 h after iodine release at 0.3% iodine inventory.

A.2.3 NUCLEAR SAFETY PILOT PLANT - KSPP

1. Initially 97% of airborne radioiodine is present as !• after 500 min, 73% of airborne radioiodine is CH I. - 49 -

2. Observed deposition rates are qualitatively consistent with a diffusion-controlled process from a non-uniform initial distribution.

3. Natural gravity deposition gives DRF (dose reduction factors) of 2 - 200 after two hours.

4. Retention of I is greatest on carbon steel, lower by a factor of two on brass, copper, aluminum, zinc and paint and lower by a factor of 10 on stainless steel.

5. Iodine (I9) is effectively removed by sprays - decontamination factors > 1000.

6. Rates of removal of I? by neutral and acidic solutions have been underestimated by theory.

7. Methyl iodide (CH I) is much less effectively removed by sprays - the removal rate can be increased by decreasing drop size and increasing flow rate.

8. Particles are removed at a rate between that for I~ and that for

A. 2.4 DIDO/PLUTO(A'6)

1. The concentration of airborne I,, undergoes initial rapid reduction, then is more slowly reduced.

2. The phenomenon described in (1) is independent of the operation of scrubbing systems.

3. Behaviour of airborne iodine as a function of time is independent of the initial iodine concentration over the range 0.1 - 10 ug-m

127 4. Introduction of I causes previously adsorbed radioiodine to be released to the air. - 50 -

5. Deposition velocities of I_ onto copper, charcoal paper, mild steel, paint and alumialuminun m are all ^0.2 cm-s , ^ 10 times faster than onto polyethylene.

6. Deposition of I? onto metals and metal oxides is probably via chemisorption and is diffusion controlled.

A.2.5 WINFRITH (ZENITH)(A "? 'A" 8)

At Winfrith, tests of iodine deposition were conducted at very 5 -3 high concentrations of airborne iodine, i 1 x 10 lJg*m , compared to similar tests at DIDO/PLUTO, ^ 0.35 ug-m . These tests show:

1. Rate of iodine deposition on mild steel is independent of initial !„ concentration (result of comparison with DIDO/PLUTO tests).

2. Deposition rate is diffusion controlled.

3. Iodine loadings are i> 600 monolayers on mild steel and concrete, i* 21 monolayers on paint and ^ 6 monolayers on Teflon.

4. Half-lives for iodine desorption are: mild steel - 17 days, concrete - 10 days, paint - 7 days and Teflon - 2 days. r 5. Mild steel loses !„ rapidly at 250°C and concrete loses I, at

300°C.

A.2.6 PSICO 10(A'9)

1. Rate of removal of iodine by spray methods is independent of initial iodine concentration.

2. Spray removal rate is faster by a factor of 10 than natural deposi- ! tion. - 51 -

3. High iodine removal rates have been observed for water (pH=7.1 - 7.6) not specially treated and containing iron at 0.01 g-dm and _3 calcium at 0.2 g-dm

4. Removal rate is faster by a factor of two with 1% Na S 0 added to above water.

5. Removal is 50% faster if spray water is colder than containment vessel.

6. No iodine release was observed from recirculated spray water after 11 h at 80°C.

A. 2. 7 CONTAINMENT SYSTEMS EXPERIMENT - CSE(A'10"A'"" 2)

1. Iodine leakage through natural pathways was too low to measure due to condensation of sceam in the pathways.

2. Measured leakage of iodine through a natural path (existing pipe with a butterfly valve) showed a decontamination factor of 34 compared to one for an artificial leak (inserted tubing).

3. Iodine leaked through a valve was 65% CH I.

4. Mixing of airborne iodine was uniform within a chamber, but non- uniform between chambers.

5. Inorganic iodides were rapidly removed by sprays of boric acid (pH=5), boric acid + NaOH (pH=9.5) and boric acid + NaOH + Na^CXj (pH=9.5). An iodine dose reduction factor of 50 was observed for basic borate sprays.

6. Fast initial washout was extended to 1% of initial airborne iodine concentration. Further reduction to 0.1% was obtained over 24 h with spray recirculation. - 52 -

7. Spray removal rate was successfully modelled by stagnant drop and well mixed drop models.

8. Methyl iodide spray removal was slow, but increased by a factor of 10 with addition of Na.S.O,; 2 h dose reduction factor = 1.5.

9. Particles were effectively removed by sprays: large particles (y 10 ym) by impaction, and small particles (< 1 pm) by diffusio- phoresis, interception and Brownian collection.

A. ?.. 8 MARVIKENTA'13^

The results of tests on radioiodine behaviour were very poor owing to an insufficient number of testing points and a poor iodine injection system. The results given below must be considered with caution:

1. Methyl iodide injected before blowdown was 89 - 90%. hydrolyzed.

2. Methyl iodide injected after blowdown remained airborne.

3. There was no significant effect of short-term spray cooling on CH.I airborne concentration.

4. Heavy deposits of I« were formed on metal surfaces near the injection point.

A.3. SUMMARY

The results of the large-scale tests agree on several important points. These are listed below. - 53 -

Spray Removal

1. Iodine as I or particulates is rapidly removed from the containment atmosphere to ^ 1% of the initial concentration by spray dousing.

2. The initial removal rate depends on characteristics such as drop size, but is adequately predictable by theoretical n.udels.

3. Removal rate of I. is essentially independent of pH over the range 5 < pH < 9.5, and independent of additives.

A. Methyl iodide is poorly removed by sprays, even with Na S 0 added.

Deposition

1. Iodine is very rapidly deposited on all metal and painted surfaces within the reactor.

2. The deposition rate is independent of the concentration of airborny iodine and is diffusion controlled.

3. Deposition is fastest on paints and mild steel at loadings greater than a monolayer.

4. Deposition is fastest on surfaces where steam is condensing.

5. With particulates present, deposition is greatest on surfaces where gravity settling can occur. - 54 -

REFERENCES

A.I W.A. Freeby, L.T. Lakey and D.E. Black, "Fission Product Behaviour under Simulated Loss-of-Coolant Accident Conditions in the Contamination-Decontamination Experiment", Idaho Nuclear Corporation Report, IN-li.72 (1969).

A.2 G.W. Parker, G.E. Creek and W.J. Martin, "Fission Product Transport and Behaviour in the Stainless Steel Lined Con- tainment Research Installation (CRI)", Oak Ridge National Laboratory Report, ORNL-4502 (1971).

A.3 L.F. Parsly, L.F. Franzen, P.P. Holz, T.H. Row and J.L. Wantland, "Behaviour of Iodine in the Nuclear Safety Pilot Plant Model Containment Vessel", Oak Ridge National Laboratory Report, ORNL-P-1265 (1964).

A.4 J.L. Wantland and T.H. Row, "Behaviour of Iodine in Air - A Resume of the First Seven Runs Conducted at the Nuclear Safety Pilot Plant", Oak Ridge National Laboratory Report, ORNL-4050 (1967).

A.5 L.F. Parsly, "Spray Program at the Nuclear Safety Pilot Plant", Nucl. Tech. K), 472 (1971).

A.6 A.C. Chamberlain, A.E.J. Eggleton, M.J. Megaw and J.B. Morris, "Physical Chemistry of Iodine and Removal of Iodine from Gas Streams", Reactor Sci. Tech. ll_, 519 (1963).

A.7 J.F. Croft, R.E. Davis and R.S. lies, "Experiments on the Surface Deposition of Airborne Iodine of High Concentration", Atomic Energy Authority, Winfrith Report, AEEW-R-265 (1963).

A.8 J.F. Croft and R.S. lies, "Experimental Releases of Iodine in the Zenith Reactor Containment", Atomic Energy Authority, Winfrith Report, AEEW-R-172 (1963).

A. 9 S. Barsali, R. Bovalini, F. Fineschi, B. Guerrini, S. Lanza, M. Mazzini and R. Mirandola, "Removal of Iodine by Sprays in the PSICO 10 Model Containment Vessel", Nucl. Tech. T±, 146 (1974).

A.10 R.K. Hilliard, A.K. Postma, J.D. McCormack and L.F. Coleman, "Removal of Iodine and Particles by Sprays in the Containment Systems Experiment", Nucl. Tech. _10, 499 (1971). - 55 -

A. 11 M.E. Witherspoon and A.K. Posttna, "Leakage of Fission Products from Artificial Leaks in the Containment Systems Experiment", Battelle Pacific Northwest Laboratories Report, BNWL-1582 (1971).

A.12 A.K. Postma and B.M. Johnson, "Containment Systems Experiment. Final Program Summary", Battelle Pacific Northwest Laboratories Report, BNWL-1592 (1971).

A.13 "The Marviken Full Scale Containment Experiments. Behaviour of Iodine in the Containment during the Blowdown Runs", Results MXA-3-201, Discussion of Results MXA-3-301 (1974). - 56 -

TABLE A-l

LARGE-SCALE TESTS OF IODINE BEHAVIOUR IN CONTAINMENTS

Deposition Test Volume Release Sprays Surfaces

Contamination - carbon steel, Decontamination 2.4 m3 melted fuel - stainless steel, Experiment (CDE) paint

Containment Research boric acid, steel, paint 3 Installation (CRI) 4.5 m I2, melted fuel boric acid + NaOH +

Nuclear Safety borax, borax- steel, brass, 3 Pilot Plant 38.3 m Nal + UO2 thiosulfate, copper, zinc, (NSPP) boric acid aluminum, paint

Winfrith 27 m3 - steel, concrete, (ZENITH) paint, Teflon

3 PSICO 10 95 m h H20 + Na2S203 - 3 Containment Systems 750 m I2, CH3I, H3BO3 + NaOH - Experiment (CSE) U0 aerosol + Na2S2°3

3 T fll T Marviken 2518 m H2O - I2, CH3I DIDO/PLUTO 7000 m3 steel, copper, h aluminum, paint, charcoal paper, polyethylene - 57 -

APPENDIX B

THERMODYNAMIC DATA

Table B-l contains a summary of the thermodynamic data criti- cally evaluated for use in this report. The origin of these data is discussed below.

B.I. GIBBS ENERGY AND ENTROPY VALUES AT 25°C

Values of AG° and S° for I2(c), I,(g) and I (aq) were taken to be the CODATA recommended valuesk ' '. The standard enthalpy of formation of the aqueous iodide ion is very important in the calculation of the Gibbs energies and entropies for many of the other species discussed here. The CODATA value is -56.90 ± 0.84 kJ-mol~ . Recently, Johnson ' and Ryabukhin^B"3^ have independently obtained AH°(I~) = -56.79 ± 0.07 -1 -1 kJ'inol and -56.83 kJ'iuol , respectively. These do not vary greatly from the CODATA value but suggest smaller error limits. To maintain consistency with the CODATA assessment ' , we have used AH° _i r = -56.90 ± Q.bO kj-mol throughout. The Gibbs energy and entropy of 1,(1) were taken from the JANAF tables^8' .

For I2(aq), an equation for the temperature dependence of the solubility of iodine in water has been calculated by Lindenberg^B"6^. 1 AH aq)) This leads to AG°(I (aq)) = 16. A0 ± 0.08 kj-mol" , °oln U2 < = f 1 23.0 ± 0.8 kJ-mol" and, hence, AS°oln (I2 (aq)) = 22.0V2.7 J-K'^mol" at 25DC. Using this with the CODATA value for S°(I (c)) gives S°(I (aq)) = 138.2 +2.7 J-K"1-mol"1. - 58. -

The enthalpy of formation of the triiodide ion at 25°C is given by CODATA. Values for the equilibrium constant of the reaction:

I2(aq) + I~ ^====f; I~ (B-l)

at 25°C have been obtained by many workers. The average value (equal weightings) from three of the most recent studiesv ' ' ' is K = 746 + 23 dm -mol" , leading to AG = -16.40 + 0.08 kJ-mol"1 and thus -1 AG° (I ) = -51.92 ± 0.81 kJ-mol . The entropy of reaction, 0.13 -1 -1 - 1-7 J-K -mol , was used to obtain S°(I ) = 245.0 + 2.9 J-K -raol .

Howard and Skinner ' determined the enthalpy of reaction of potassium iodate with aqueous hydrazine dihydrochloride. The enthalpy of formation of the aqueous hydrazine dihydrochloride (AH°(N_H.•2HC1, -1 tit 3000H 0) = -342 ± 4 kJ-mol ) was calculated from the enthalpy of solu- 1 11 tion (25.31 1 ±^ R0. 12 kJ-mol" )^" -* and, thus, AH° (N9H.-2HCl(c)) = -367 -1fB 12} ± 4 kJ-mol . The original source of the value for the solid hydrazine dihydrochloride could not be found and the error limit iis our estimate based on comparison with earlier literature values (B.14) CODATA values were used with heats of dilution from Parker's monograph to calculate the enthalpies of formation of KI and HC1 at the experimental concentrations. A value of AH°(KK>3(c)) = -499.0 ± 4.0 kJ-mol~ was obtained. The heat of solution of KIO in water used was that of Vanderzee et al/B'15) (-27.68 ± 0.03 kJ-mol"1). With AH|(K+(aq)) from C0DATA(B' 1\ this gives AH°(IO~) = -219.1 ± 4.0 kJ-mol" . The Gibbs energy of solution of KIO (c) (7.07 ± 0.41 kJ-mol"1) was taken from Johnson et al. "16^. This leads to AS° , (KI0_(c)) = 69.2 + 1.4 J-K"1.!™)!"1, and using soln 3 , , ,„ ,,v S°(KIO (c)) = 151.5 + 0.4 J-K -mol v ', S°(I0~) = 119.6 ± 1.5 -1-1 J-K -mol . From the enthalpy of formation and the entropy, the Gibbs energy of the aqueous iodate ion was calculated to be -126.3 ± 4.0 kJ-mol"1.

Woolley et al. have determined that the enthalpy of ionization of HIO is -2.76 ± 0.52 kJ-mol"1. Pethybridge and Prue^8'1 ' - 59 -

found a dissociation constant of 0.157. Therefore, the Gibbs energy and entropy of ionization for HIO,(aq) are 4.59 + 0.08 kJ-mol and -24.7 -1 -1 i 1.7 J-K -mol , respectively. Using our selected values for the aqueous iodate ion we have calculated AG°(HIO.(aq)) = -130.9 ± 4.0 -t -1 -T kJ-mol and S°(HIO (aq))= 144 ± 2 J-K -mol . Polymeric iodic acid species occur only iii concentrated solutions and are not considered here. (B 9} Burger and Liebhafsky ' have reported values for the equilibrium constant of the reaction:

+ I2(aq) + H20 -_ HI0(aq) + H + i" (B-2)

for the temperature range 0 - 56°C. Their data give values of AG° -1 -1 rx = -70.0 ± 0.8 kJ-mol and AH° = -63 ± 11 kJ-mol . From these, -1 -1 rX AS° =23+40 J-K -mol can be calculated. rx

Using auxiliary data from CODATA^ ' ' (and values for I.(aq) as discussed previously) we have calculated AG£(HIO(aq)) = -98.7 i 0.9 kJ-mol"1 and S°(HIO(aq)) = 79 ± 40 J-K^-mol"1 for 25°C.

Chia^B"20^ reports K = C.6 + 0.6) x 10~3 for the reaction:

I0~ + 21" + H20 ^ I~ + 20H" (B-3)

Thus, AG° = 12.8 ± 0.3 kJ-mol"1 and AG°(I0~) = -38.4 + 3.4 kj-mol"1. A value of AH°(IO~) = -107.5 kJ-mol'1 has been tabulated ^. with no original source or error limits given. We have assumed error limits of ± 4 kJ-mol"1 and calculated S°(I0~) = -6 ± 18 J^K^-raol"1. The existence of H 10 is not well established. Bell and Gelles -11 report K = (1.2 + 0.3) x 10 at 25°C for the reaction:

v + I2(aq) + H20 - H2I0 + l" (B-4) - 60 -

The Gibbs energy of reaction is then 62.3 ± 0.7 kJ-mol and the Gibbs

energy of formation of H2I0 is -106.5 +0.7 kJ-mol" at 25°C. Other /TJ Q T3 O 1 "i workers have obtained markedly different values ' . The use of Burger and Liebhafsky's equilibrium constant at 50°C (K = 3 x 10 ) with that of Bell and Gelles at 25°C results in an unreasonable enthalpy of deprotonation of HjIO of ^ -100 kJ-mol . The results of Christensen et al. suggest that a reasonable entropy of deprotonation for reactions such as:

+ + H2A + H20 ^ HA + H30 (B-5)

is ^ 17 + 20 J-K~1-mol~1. Use of this gives S°(H I0+) =95+46 ^1

/•o 03 R 2i 1 It is not clear from the work of Lin ' ' exactly what error limit should be assigned to his free energy of vapourization of HI0 from aqueous solution. Both papers lack experimental details.

We have estimated AG°a = 13.0 + 3.0 kJ-mol . Using our calculated value of AG|(HI0(aq)), this gives AG°(HIO(g)> = 85.8 ± 5.1 V.J-mol""1. The difference in the entropy of HCl(g) and HI(g) and of Cl,,(g) and I-(g) is 19.2 ± 0.4 J-K~ -mol" per halogen atom^B" '. The entropy of —1 —1^B 12 ^ HClO(g) is 236.6 ± 0.4 J-K -mol v ' ' (the error limit is our esti.nate); therefore, we have used S°(HIO(g)) = 256 ± 4 J-K~ -mol" (the error limits being chosen to reflect the uncertainty in the method used for the calculation). Evidence for HIO and I0_ is even more tenuous than that for HIO and 10 and thus such I (III) species are not considered here.

Mercer and Farrarv * ' have reported the heat of reaction of 3 1 NaI04(c) with 0.536 mol-dm" HI(aq) at 25°C to be -546.97 + 1.05 kJ-mol" .

We have used this value, the heats of solution of Nal(c) and I (c) in _o /-o 2 S ^ 0.536 mol-dm HI(aq) determined by the same workers * , the relative partial molal enthalpy of HI(aq) in 0.536 mo.l-dm HI(aq) from Parker's monograph ' , and values consistent with the C0DATA assessment - 61 -

for AH°(NaI(c)), AH°(K+) and AK°(H 0), to obtain AH°(NalO,(c)) = -427.4 ± 1.3 kj-mol . Mercer and Farrar also give -36.5 kJ-mol" for the 3 enthalpy of solution of NaI04(c) in 0.1 mol-dm" NaC104 at 25°C. By assuming that this value is the same as the zero ionic strength value , within +1.0 kj-mol , that the enthalpy for the reaction ' :

H IO 4 6 "* I0~ + 2H2O (K = 41) (B-6)

is 45.6 ±2.0 kj-mol (our error estimate) and taking the CODATA value for AH°(K+), we obtain AH°(I0~) = -149.5 ±2.6 kj'mol"1. The Gibbs energy of solution of KIO,(c) calculated by Johnson et al. , in combination with the enthalpy of solution of this solid from Shidlovskii and Voskresenskii(B'27), gives AS , (KIO,(c)) = 147.4 ± 3.1 J-K~1-mol~1. ,° .,.. soln 4 . From this, an estimated^ ; value of S°(KI0 (c)) (159 ± 8 J-K -mol ), and the CODATA value for S°(K+)(B'1), we obtain S°(io7) = 206 ± 9 -1-1 - J*K -mol . The entropy and enthalpy of formation of 10, were used to calculate AG°(I0~) = -51.8 ± 3.4 kJ-mol"1.

The data of Crouthamel et al. ' ' give AG°x and AH° for reaction (B-6). By using these and the corresponding values for 10,, we have calculated AG°(H,I0~) = -517.0 ± 3.4 kJ-mol"1 and S°(H,I07) 1 -I (B 251 = 162 ± 13 J-K -mol . Calorimetric measurements by Mercer and Farrar^ ' ' give 10.7 ± 2.9 kj-mol for the enthalpy of ionizatioa of HsI0,, while Crouthamel et al. ' found 0.0 kj-mol from the variation of the ionization constant with temperature. We have chosen to use the calorimetry result to obtain AH°(H 10 ) = -777.4 ± 4.0 kj-mol"1. The averaged results of Crouthamel et al. ' and of Haladjian and coworkers give log -K = -3.26 ± 0.05 for the ionization of H 10, and hence AG^H.IO,) = -535.6 ± 3.4 kj-mol"1 and S°(H,.IO,) = 189 ± 18 J-K^-mol"1. (Vt 26 B °8") Ionic strength corrections for the dilute solutions ' ' have been neglected.

_,., , (B.26,B.28,B.29) . There are a number of values availablne for the equilibrium constant of the reaction: - 62 -

+ 2 H + H3IO6 (B-7)

We have selected log K = -8.31 ± 0.03 (I -»• 0) determined by Buist et (R ?Q) — 1 al. '. The Gibbs energy of reaction (47.45 kJ-mol ) leads to AG°(H,I0g2) = -478.7 ± 3.4 kJ-mol"1. The enthalpy of reaction (B-7) is -12.6 ±1.7 kJ-mol from the weighted average of the caiorimetric and spectrophotometric results. Thus AS = 201 ± 6 J-K~ -mrl~ and S°(H,I0~2) = 145 + 11 J-K~1-mol~1. j 6

_2 The dissolution constant of H_I0, is given by Haladjian et al.k as log K = -12.1 ± 0.1 (I •v 0.005) and by Buist and v(R 291 coworkers ' ' as log1QK = -12.2 + 0.3 (corrected to I = 0). We have used log JK = -12.2 ± 0.2 giving AG° = 69.6 ± 1.5 kJ-mol"1 and -V -i rx thus AG°(H 10, ) =-409 ± 4 kJ-mol . The enthalpy of ionization of -2 f -1 H,iO, was roughly estimated as 17 ± 17 kJ-mol from some of the data in reference B.29. This enabled us to calculate AH°(H«IO7 ) = -717 ± 17 -1 -3 -1-1 o kJ-mol and S (H,I06 ) = -33 ± 54 J-K -mol . For the dimerization reaction: 2HI02 H : ^ ' ° 2H2° (B~8) values of K = 123 (I ^ 0.005)(B-28) and K = 141 (I = 0.1)(B-29) have been reported. We have attempted to correct these to zero ionic strength but, because of the high charges on the ions, this procedure is very crude. We have taken log K = 1.6 ± 0.5,giving AG° = -9.1 l -4 -1 rx fR 79"! L + 2.9 kJ-mol and AG°(H2I 0 £) = 492 ± 5 kJ-mol . Buist et al. *• ^' give AH° = 61 ± 4 kJ-mol for reaction (B-8) and hence we calculate 1 1 1 1 ASrx =234+6 J-K" -mol" and S°(H212O~Q) = 383 ± 17 J.K" -mol" .

Fuger and Brown ' have measured the heats of reaction of -3 -3 UI^(s) with 1 mol-dm and 6 mol•dm hydrochloric acid and found these to be -289.9 kJ-mol and -240.2 kj-mol~ respectively. Calculation of AH£(UI,(S)) has been redone using more recent values for - 63 -

AH°(UC1 (s)) ' and for the enthalpies of formation and dilution of HI^B" . We obtain an average value of AH°(UI,(s)) = -524.4 ± 7.0 —1 kJ-raol . For S°(UI,(s)) and for AH° and S°(UI (s)), we have accepted ^R 39} (R 1 ^ the values in Rand and Kubaschewski's compilation ' . CODATA vaiaes for S°(U) and S°(I (c)) were, used to obtain AG°(UI,(s)) = -523 ± 8 kJ-mol"1 and AG°(UI (s)) = -482 ± 9 kJ-mol"1.

Turnbull's^ " ' heats of reaccion of Zrl^(s) and ZrCl^s) with water and with hydroxide, when combined with recent data for the enthalpy of formation of ZrCl.(s) " , and CODATA values for iodide and chloride,give AH°(ZrI,(s)) = -488.7 ± 3.3 kJ-mol . Cubitxiotti 35 et al. * ^ have recently determined S°(ZrI4(s)) = 250.2 ± 0.8 J-K -mol and hence we have calculated ' AS°(Zri, (s)), and 1 f 4 AG°(ZrI (s)) = -482 ± 3 kJ-mol . Values for AG° and S° of Zri (s) fB 57 and Zri (s) were taken from the J. JAF tablesk ' ; the error limits f R 1 \ are our estimates. Values for U02(s) and ZrO2(s) are from CODATA and the NBS tables J respectively.

B.2. HEAT CAPACITIES

Heat caDacities for I.(c) and I?(l) were taken from i:he JANAF tables^B-5^ whil that for I (g) was from Kelley^B"36'. For ^ the heat capac ty was taken as the sum of C (^(c)) plus AC for the dissolution reaction" ' . The heat capacity for KI0(g) was estimated from that of HClO(g) ' by comparing the heat capacities of various gaseous chlorine and iodine compounds. For H 10 (aq) and HI0(aq) the empirical formula noted by Naumov et al. ' gives —1 —1 209 and 280 J-K •mol respectively. We have used these values, although the error limits are probably very large. The same formula gives 238 J-K"1-mol~1 for HI0 (aq). Woolley and Hepler^8"38^ obtained C°(HI0,(aq)) = -40 J-K -mol from flow calorimetry results using AH . (HIO (aq)) = -10 kJ-mol . Our recalculation of Woolley and - 64 -

Hepler's data,using AH. (HIO (aq)) = -2.8 kJ-mol"1^*18^, gives 3.OHXZ- 3 ..

C°(HIO3(aq)) = 0.8 ± 5 J-K~ -mol~ . We have used this value and, by comparison, suggest that the error limits on our estimated heat capacities of H 10 and HlO(aq) should be at least + 200 J-K^-mol"1.

We have used experimental 25°C heat capacities for C°(I07)(B-39) and c°(r)(B-'4O'B-41) (based on C°(H+) 5 0.0). Values for P 3 p . p other ionic species were calculated using a modified Criss-Cobble extrapolation' ' * ^ and represent average values for the 25 to 150°C temperature range.

The heat capacity of UI (s) was taken from Rand and Kubaschewski^B"32\ while that for Ulfs) was estimated as C°(UI,(s)) - -1 -1 3 P hC°(l2(c)) = 1C3 ± 5 J-K -mol . Values of C°(ZrI3) and C°(ZrI2) were obtained from the JANAF tables^B'3') and that for C°(ZrI,(s)) from Cubicciotti et al. ' . For U0 , C° is from Lemire and Tremaine and C°(ZrO.) was taken from NBS-270-5^B'34). P 2 - 65 -

REFERENCES

B.I "CODATA Recommended Key Values for Thermodynamics, 1977. Report of the CODATA Task Group on Key Values for Thermody- namics, 1977", J. Chem. Thermodyn. ]J3, 903 (1978).

B.2 G.K. Johnson, "The Standard Enthalpy of Formation of the Aqueous Iodide Ion", J. Chem. Thermodyn. % 835 (1977).

B.3 A.G. Ryabukhin, "Standard Thermal Constants of Iodide Ion in an Aqueous Solution", (U.S.S.R.) Deposited Document, VINITI 962-77 (1977). (Chem. Abst. 90-77327c (1979)).

B.4 V.B. Parker, D.D. Wagman and D. Garvin, "Selected Thermochemi- cal Data Compatible with the CODATA Recommendations", U.S. National Bureau of Standards, Washington, NBSIR 75-968 (1976).

B.5 D.R. Stull, H. Prophet, J. Chao, A.T. Hu, E.W. Phillips, G.C. Karris, S.K. Wollert, S. Levine, J.L. Curnutt, ot al., "JANAF Thermochemical Tables, 2nd Edition", T.S. National Bureau of Standards, Washington, NSRDS-NBS-37 (1971).

B.6 A.B. Lindenberg, "Equations thermodynamiques pour la solubility de l'iode dans l'eau, do 0 a 112.3°C. Enthnlpie et entropie d'hydratation de la vapeur d'iode", C.R. Acad. Sci. Ser. C 272, 1129 (1971).

B.7 M. Davies and E. Gwynne, "The Iodine-Iodide Interaction", J. Amer. Chem. Soc. _74, 2748 (1952).

B.8 L.I. Katzin and E. Gebert, "The Iodide-Iodine-Triiodide Equilibrium and Ion Activity Coefficient Ratios", J. Amer. Chem. Soc. 77., 5814 (1955).

B.9 J.D. Burger and H.A. Liebhafsky, "Thermodynamic Data for Aqueous Iodine Solutions at Various Temperatures", Anal. Chem. 45, 600 (1973).

B.10 P.B. Howard and H.A. Skinner, "Thermochemistry of Some Reac- tions of Aqueous Hydrazine. Part II. KIO3 and HIO3", J. Chem. Soc. (A) 269 (1967).

B.ll E.C. Gilbert and A.W. Cobb, "Studies of Hydrazine; Heats of Solution of Hydrazonium Salts at 25°C", J. Amer. Chem. Soc. 57, 39 (1935). - 66 -

B.12 D.D. Wagman, W.H. Evans, V.B. Parker, I. Halow, S.M. Bailey and R.H. Schumm, "Selected Values of Chemical Thermodynamic Properties. Tables for the First Thirty-Four Elements in the Standard Order of Arrangement", U.S. National Bureau of Stan- dards Technical Note, Washington, NBS-270-3 (1968).

B.13 F.D. Rossini, D.D. Wagman, W.H. Evans, S. Levme and I. Jaffe, "Selected Values of Chemical Thermodynamic Properties", U.S. National Bureau of Standards Circular 500, Washington (1952).

B.14 V.B. Parker, "Thermal Properties of Aqueous Uni-Univalent Electrolytes", U.S. National Bureau of Standards, Washington, NSF j-NBS-2 (1965).

B.15 C.E. Vanderzee, D.H. Waugh, N.C. Haas and D.D. Clyde, "Rela- tive Apparent Molar Enthalpies and Standard Enthalpies of Solution of Potassium Bromate and Potassium Iodate in Water at 298.15 K", J. Chem. Thermodyn. U_, 111 (1980).

B.16 G.K. Johnson, P.N. Smith, E.H. Appelman and W.N. Hubbard, "The Thermodynamic Properties of the Perbromate and Bromate Ions", Inorg. Chem. 9, 119 (1970).

B.17 J.E. Ahlberg and W.M. Latimer, "The Heat Capacities and Entropies of Potassium Bromate and Iodate from 15° to 300° Absolute. The Entropies of Bromate and Iodate Ions", J. Amer. Chem. Soc. 56_, 856 (1934).

B.18 E.M. Woolley, J.O. Hill, W.K. Hannan and L.G. Hepler, "Ther- modynamics of lonization of Aqueous Iodic Acid, an "Almost- Strong" Electrolyte", J. Solution Chem. y_, 385 (1978).

B.19 A.U. Pethybridge and .I.E. Prue, "Equilibria in Aqueous Solu- tions of Iodic Acid", Trans. Faraday Soc. 63, 2019 (1967).

B.20 Y.T. Chia, "Chemistry of +1 Iodine in Alkaline Solution", Tiiosis, Uiiverslty of California (1958). U.S. Atomic Energy Commission Report, UCRL-831].

B.21 R.P. Bell and E. Gelles, "The Halogen Cations in Aqueous Solution", J. Chem. Soc. 2734 (1951).

B.22 J.J. Christiansen, J.H. Rytting and R.M. Izatt, "Thermodynamics of Proton Dissociation in Dilute Aqueous Solutions. VIII. pK, AH" and AS° Values for Proton Ionization from Several Pyrimidines and their Nucleosides at 25°C", J. Phys. Chem. 7J., 2700 (1967). - 67 -

B.23 C-C. Lin, "Chemical Behaviour of Radioiodine in BWR Systems", J. Inorg. Nucl. Chem. 4^, 1093 (1980).

B.24 C-C. Lin, "Behaviour of Radioiodine Studies: Iodine Partition Between Aqueous Solution and Gas Phase", General Electric Company Report, NEDO-12583 (1975).

B.25 E.E. Mercer and D.T. Farrar, "A Calorimetric Study of Periodate Species in Aqueous Solution", Can. J. Chem. 4j5, 2679 (1968).

B.26 C.E. Crouthamel, A.M. Hayes and D.S. Martin, "Ionization and Hydration Equilibria of Periodic Acid", J. Amer. Chem. Soc. T3, 82 (1951).

B.27 A.A. Shidlovskii and A.A. Voskresenskii, "Heat of Formation of Lithium, Strontium, Lead and Silver Iodates and Potassium Metaperiodate", Russ. J. Phys. Chem. (Eng. Trans.) 39^, 810 (1965).

B.28 J. Haladjian, R. Sabbah and P. Bianco, "Recherches sur le point isohydrique et les ph^nom&nes de condensation ou d'asso- ciations", J. Chim. Physicochim. Biol. ^5_, 175X (1968).

B.29 G.J. Buist, W.C.P. Hipperson and J.D. Lewis, "Equilibria in Alkaline Solutions of Periodates", J. Chem. Soc. 307 (1969).

B.30 J. Fuger and D. Brown, "Thermodynamics of the Actinide Ele- ments, Part IV. Heats and Free Energies of the Tetrachlo- rides, Tetrabromides, and Tetraiodides of Thorium, Uranium and Neptunium", J.C.S. Dalton Trans. 428 (1973).

B.31 J. Fuger and F.L. Oetting, "The Chemical Thermodynamics of Actinide Elements and Compounds, Part 2, The Actinide Aqueous Ions", International Atomic Energy Agency, Vienna (1976).

B.32 M. Rand and 0. :.,Daschewski, The Thennochemical Properties of Uranium Compounds, Oliver and Boyd, Edinburgh, 1963.

B.33 A.G, Turnbull, "Thermochemistry of Zirconium Halides", J. Phys. Chem. 65_, 1652 (1961).

B.34 D.D. Wagman, W.H. Evans, V.B. Parker, I. Halow, S.M. Bailey, R.H. Schumm and K.L. Churney, "Selected Values of Chemical Thermodynauic Properties. Tables for Elements 54 through 61 in the Standard Order of Arrangement", U.S. National Bureau of Standards Technical Note, Washington, NBS-270-5 (1971).

B.35 D. Cubicciotti, K.H. Lau and M.J. Ferrantr, "Thermodynamics of Vaporization and High Temperature Enthalpy of Zirconium Tetra- iodide", J. Electrochem. Soc. 125_, 972 (1978). - 68 -

B.36 K.K.. Kelley, "Contributions to the Data on Theoretical Metal- lurgy. XIII. High-Temperature Heat-Content, Heat-Capacity, and Entropy Data for the Elements and Inorganic Compounds", U.S. Bureau of Mines, Washington, Bulletin 584 (1960).

B.37 G.B. Naumov, B..N. Ryzhenko and I.L. Khodakovsky, "Handbook of Thermodynamic Data", translated by G.J. Soleimani, U.S. National Technical Information Service Report, PB 226 722 (1974). p. 41.

B.38 E.M. Woolley and L.G. Hepler, "Heat Capacities of Weak Elec- trolytes and Ion Association Reactions: Method and Applica- tion to Aqueous MgSCty and HI03 at 298 K", Can. J. Chem. 5J>, 158 (1977).

B.39 J.J. Spitzer, I.V. Olofsson, P.P. Singh and L.G. Hepler, "Apparent Molar Heat Capacities and Volumes of Aqueous Elec- trolytes at 25°C: NaI03, KMn04 and MnCl2", Thermochim. Acta 28, 155 (1979).

B.40 J.-L. Fortier, P.-A. Leduc and J.E. Desnoyers, "Thermodynamic Properties of Alkali Halides. II. Enthalpies of Dilutions and Heat Capacities in Water at 25°C", J. Solution Chem. _3, 323 (1974).

B.41 J.E. Desnoyers, C. deVisser, G. Perron and P. Picker, "Re- examination of the Heat Capacities Obtained by Flow Micro- calorimetry. Recommendation for the Use of a Chemical Stan- dard", J. Solution Chem. 5_, 605 (1976).

B.42 CM. Criss and J.W. Cobble, "The Thermodynamic Properties of High Temperature Aqueous Solutions. IV. Entropies of the Ions up to 200°C and the Correspondence Principle", J. Amer. Chem. Soc. 86_, 5385 (1964).

B.43 R.J. Lemire and P.R. Tremaine, "Uranium and Plutonium Equi- libria in Aqueous Solutions to 200°C", J. Chem. Eng. Data lb_, 361 (1980). - 69 -

TABLE B-l

CODATA CONSISTENT PARAMETERS FOR IODINE THERMODYNAMIC CALCULATIONS

-1 • mol = Species AG°/kJ-mol S°/J-K X mol A + 10~3BT + 105C/T2

A E C

I2(c) 0.0 116.139 i 0.080 -50.646 246.91 27.974

I2(D 3.32 ± 0.04 150.36 i 0.04 80.67

I2(g) 19.36 ± 0.08 260.567 i 0.063 37.405 0.5858 -0.7113

I2(aq) 16.40 ± 0.08 138.2 + 2.7 205.04 246.91 27.974 HIO(g) -85.8 ± 5.1 256 ± 4 37.66 + a H2IO -106 .5 ± 0.7 95 ± 46 14.93 HlO(aq) -98.7 + 0.9 79 ± 40 280 10" -38 4 ± 3 4 -6 i 18 -396.2a y "" -51 92 + 0 84 106.70 ± 0.20 -121.3 -51 92 245.0 + 2 9 a l3 ± 0 85 -124.9 HIO3(aq) -130 9 ± 4 0 144 ± 2 3.3 -126 0 119.6 ± 1 5 I03 3 ± 4. -73 H5IO6(aq) -535.6 ± 3.4 189 ± 18 209 a H4IOg(aq) -517.0 + 3.4 162 ± 13 266.9 •51. 8 ± 3.4 206 ± 9 28.2a I0^-2 -478.7 ± 3.4 145 ± 11 413.2a 3 -3 -409 ± 4 -33 ± 54 -67.4a -492 ± 5 383 ± 17 1779a H2I2°10

UI3(s) -482 ± 9 238 ± 13 102.5

UI4(s) -523 ± 8 280 ± 13 129.7

ZrI2(s) -258 ± 15 150 ± 15 94.3

ZrI3(s) -395 + 15 205 ± 15 103.8

ZrI4(s) -482 ± 3 250.2 ± 0.8 125.1 uo 2(s) 1031.8 + 1.0 77.03 ± 0.21 68.91 22.62 10.40

ZrO2(s) 10*2.8 + 0.2 50.38 ± 0.21 56.19

from a modified Criss-Cobble treatment^B-42>B-43). - 70 -

APPENDIX C

FACTORS AFFECTING SURFACE ADSORPTION AND DESORPTION OF IODINE

C.I. SURFACE LOADING CAPACITY

A single monolayer of !„ deposited on a surface corresponds to -2 (CD 0.3 ug-cm of iodine " . Due to thr high polarizability of I« and its relatively high boiling point (184°C), iodine has a strong affinity for all surfaces and is deposited at loadings up to 10 monolayers thick, even on inert surfaces. On surfaces which are capable of reaction with iodine, surface loadings greater than 10 monolayers have been measured, and for metals, such as copper, there does not appear to be any limit to the surface loadings^ ' ~ . In the case of copper, this may be due (C 5) to the solubility of I, in Cul . In general, surface loadings increase when steam is present (as opposed to loadings in dry air) and decrease with Increasing temperature.

The decrease in iodine loading with temperature is simply explained by the higher volatility of iodine at increased temperature. This will reduce the concentration of physically adsorbed iodine and, by decreasing the residence time of neutral iodine molecules on the surface, decrease the rate for chemical absorption. The effect of humidity is not well understood. It is well known that moist iodine vapour promotes corrosion of many metals including stainless steels ' . The process is generally considered to occur through formation of small metal iodide centres which subsequently dissolve and corrosion is assumed to occur electrochemically in the aqueous phase. Support for the existence of corrosion centres Is found In the data of Rosenberg et (C 8^ al. ' . Very little is known about the kinetics of the process or the - 71 -

amount of iodine consumed. Experiments in our laboratory, for example, have found iodine deposited on copper to be bound as iodide.

(C 3) Geuco et al. " have speculated on the mechanism for deposition of iodine on paints and propose loadings of iodine molecules per unit area.M ,to be given by:

L L

nt = / C(x)dx + / v| (N - N) dx (C-l) o o

where L = paint thickness, x = axial distance within paint, C(x) = local iodine concentration limited by diffusion, N and N are the unreacted and reacted active sites and v is the number of iodine molecules con- o sumed per active site. In the limit of infinite time for iodine diffusion, the loading is limited by the number of active sites. They have also suggested that the active sites in the most absorbent paints are amines and phosphines,which react with iodine to form charge-transfer complexes, as follows:

1 + R-N or R.P + I2 > R.Nl" " or R.,PI + I~ (C-2)

While this is a reasonable basis for the theoretical efficiency of paints, the mechanism has not been rigorously confirmed.

Methyl iodide is highly volatile at temperatures likely to be encountered in a containment vault (b.p. CH.I = 42.4°C) and does not (C 3) deposit on metal or concrete surfaces. However, Genco et al. report significant loadings of CH.I on some paints which they interpret as being due to a mechanism whereby the methyl iodide reacts to form quaternary ammonium salts:

(C-3)

+ or CH3I + R2NH * [R2NHCH3] I~ (C-4) - 72 -

This mechanism has not been confirmed. Also, deposition loadings for CH I in an accident where I is present will be complicated by CH.I J (C.9) J formation on the surface. Bennett et al. have observed on the order of 10 % of the iodine in contact with a variety of paints to be converted to CH^I, and the percentage may increase for smaller airborne iodine concentrations which are closer to those that might occur under accident conditions than those used in the experiment.

Maximum surface loadings are not a factor in the overall iodine retention during an accident. Release of 25% of the core inventory of iodine for a single unit CANDU reactor corresponds to release of i- 1 kg of I . The surface area available for deposition in a single unit reactor is estimated to be 2.2 x 10 cm . Hence, the average iodine loading, assuming all surfaces have the same iodine affinity, _2 is 4.5 ug*cm . All of the commonly used paint, metal and concrete surfaces have loading capacities which exceed this figure. Hydrolysis of iodine by the dousing water, as discussed previously, results in a further increase of the excess surface loading capacity.

Surface deposition of iodine in the primary coolant system may be minimal. Kress ' has studied iodine adsorption on stainless _2 and mild steel at 260°C and found saturation levels of 0.04 ug-cm for _2 type 316 stainless steel and 0.76 ug-cm for "10-15 carbon steel". Osborne et al. ' found equilibrium surface loadings of 0.3 Mg-cm on iron powder at 400°C and four times lower loadings on Fe.,0, powder.

C.2. DEPOSITION VELOCITIES

(C.4 C.7 C.12—C 17) Deposition velocities onto many materials ' ' ' under varying conditions of temperature and relative humidity have been reported. With the exception of some experiments carried out by - 73 -

(C 8 ^ Rosenberg et al. ', all reported deposition velocities have been determined as a long-time average. Sample coupons were exposed to airborne iodine over a prescribed time interval and then removed for counting of radioactive tracer iodine. Experience in our laboratory indicates that the deposition of iodine over a long time does not occur with a single deposition velocity when the ratio of iodine concentration (C 7) to surface area is large. This result is supported by other workers ' .

The mechanism controlling the rate of iodine adsorption onto various reactor containment surfaces is not well known. Large-scale tests have generally determined that the nominal deposition velocities are rate limited by the diffusion of the airborne iodine to the reactive surface, for those surfaces which exhibit the highest deposition velocities. In laboratory tests where this is not the controlling factor, the surface reaction mechanism is important. Tt is generally conceded that iodine adsorption on surfaces which exhibit high loading capacities is via chemisorption. Parameters affecting deposition velocity are discussed below.

C.2.1 SURFACE ROUGHNESS

(C 3) Genco et al. ' have studied the effect of surface roughness on iodine deposition. They have found a regular decrease in the mass of iodine deposited as the surface is increasingly polished smooth,causing a decrease in the nominal deposition velocity. This is intuitively correct for surfaces where the adsorption is physical, due to an increased surface area with increased roughness. In contrast, Croft et (C 2) al. ' reported no variation with surface roughness for the deposition of iodine onto mild steel. In cases where the absorption is chemical, the reaction mechanism may dominate surface roughness effects. - 74 -

C.2.2 SURFACE OXIDATION

The role of surface oxidation is primarily to reduce the rate of iodine deposition. Shelton et al. ' have observed a regular

decrease in the deposition velocity on copper as the copper oxide layer (C J 8 } increases in thickness. Davis et al. ' observed a factor of 50 decrease in the iodine absorption in dry air for stainless steel with a 20 - 30 nm oxide layer compared to stainless steel with a thin oxide (C 2) layer. In contrast, Croft et al. ' reported that the deposition velocity of iodine on mild steel in a humid atmosphere is independent of the degree of surface oxidation. It appears that the role of surface oxidation is strongly connected to the as yet undetermined chemisorption

mechanism.

C.2.3 SURFACE TEMPERATURE

In general, deposition velocities increase as the temperature of the surface and air above it increases ' ' . For all materials, there is a maximum temperature above which desorption com- petes to reduce the deposition velocity. In cases where the surface is cold enough to permit condensation, deposition velocities depend not on the temperature but on the presence of a film of liquid on the surface. (C.20) Natalizio and Fluke ' have recently developed a plateout model that takes into account iodine sorption and desorption from the water film.

C.2.4 AIRBORNE IODINE CONCENTRATION

Experiments at various concentrations of airborne iodine have verified that the iodine adsorption is first order and the deposition velocity is independent of the airborne iodine concentrations. Experi- ments at the Winfrith and Pluto reactors, with five orders of magnitude variation in the airborne iodine concentration, found essentially the same deposition velocity on mild steel - 75 -

C.2.5 HUMIDITY

With the exception of steel and particularly stainless steel, steam has little effect on deposition velocities. Our studies have shown that, at relative humidities greater than 10%, the iodine deposition velocity onto stainless steel is dramatically increased. Further increases in humidity do not affect the deposition velocity. Apparently there is a critical moisture level at which surface iodides deliquesce to permit an electrochemical reaction with atmospheric oxygen. The (C 19) precise requirements are not known. The work of Freeby et al. suggests that condensation to form a water film on the surface acceler- ates deposition onto stainless steel and carbon steel but not onto paint. The decrease for paint may be due to an added diffusion hold-up for iodine migration toward active paint sites. However, surface plateout experiments involving condensing steam are complicated by thcrmo- phoresis and the quantitative effect of thin condensate films. - 76 -

REFERENCES

C.I J.S. Campbell, R.L. Moss and C. KembalJL, "The Adsorption of Iodine on Evaporated Films of Tungsten", Trans. Faraday Soc. 56, 1481 (1960).

C.2 J.F. Croft, R.E. Davis and R.S. lies, "Experiments on the Surface Deposition of Airborne Iodine of High Concentration", Health Physics 11,, 1 (1965).

C.3 J.M. Genco, H.S. Rosenberg and D.L. Morrison, "Iodine Depo- sition and its Enhancement under Reactor Accident Conditions", Nuclear Safety^, 226 (1968).

C.4 J.B. Morris and B. Nicholls, "The Deposition of Iodine Vapour on Surfaces", Atomic Energy Research Establishment Report, AERE-R 4502 (1965).

C.5 R.F. Rolsten, Iodide Metal and Meta] Iodides, Wiley, New York, 1961.

C.6 J.W. Mellor, A Comprehensive Treatise on Inorganic and Theo- retical Chemistry, Supplement II, Part I, Longmans, Creen and Co,, New York, 1356.

C.I R.M. Watkins, "Corrosion of Stainless Steel by Iodine Vapour in the HTGR Fission Product Purge System", USAEC General Atomic Report, GAMD-1330 (1960).

C.8 H.S. Rosenberg, J.M. Genco and D.L. Morrison, "Fission-Product Deposition and its Enhancement under Reactor Accident Condi- tions: Deposition on Containment-System Surfaces", Battelle Memorial Institute Report, BMI-1865 (1969).

C.9 R.L. Bennett, R. Slusher and R.E. Adams, "Reactions of Iodine Vapour with Organic Paint Coatings", Oak Ridge National Labora- tory Report, ORNL-TM-2760 (1970).

CIO T.S. Kress, "Parameters of Isothermal Fission-Product Depo- sition", Oak Ridge National Laboratory Rep rt, ORNL-TM-1330 Revised (1966).

C.ll M.F. Osborne, E.L. Compere and H.J. de Nordwall, "Studies of Iodine Adsorption and Desorption on HTGR Coolant Circuit Materials", Oak Ridge National Laboratory Report, DRNL-TM- 5094 (1976). - 77 -

C.12 R.T. Hemphill and C.A. Pelletier, "Surface Effects in the Transport of Airborne Radioiodine at Light Water Nuclear Power Plants", Electric Power Research Institute Report, ERPI-NP-876 (1978).

C.13 H.S. Rosenberg, G.E. Cremeans, J.M. Genco, D.A. Berry and D.L. Morrison, "Fission-Product Deposition and its Enhancement under Reactor Accident Conditions: Development of Reactive Coatings", Battelle Memorial Institute Report, BMI-1874 (1969).

C.14 A.C. Chamberlain, A.E.J. Eggleton, W.J. Megaw and J.B. Morris, "Physical Chemistry of Iodine and Removal of Iodine from Cas Streams", Reactor Science and Tech. 1]_, 519 (1963).

C.15 G.W. Keilhcltz and C.J. Barton, "Behaviour of Iodine in Reactor Containment Systems", Oak Ridge National Laboratory Report, ORNL-NSIC-4 (1965).

C.1C G.W. Parker, G.3. Creek and W.J. Mar .n, "Fission Product Transport and Behaviour in the Stainless Steel Lined Contain- ment Research Installation (CRI)", Oak Ridge National Labora- tory Report, ORNL-4502 (1971).

C.17 R.A.J. Shelton, S. Blairs and D. Margrave, "Effect of a Thin Oxide Film on the Reaction of Iodine Vapour with Copper", Nature 19TJ, 1183 (1961).

C.18 M.E. Davis, W.E. Browning, Jr., G.E. Creek, G.W. Parlor and L.F. Parsly, "The Deposition of Fission-Product Iodine on Structural Surfaces", presented at the International Symposium on Fission Product Release and Transport under Accident Condi- tions, Oak Ridge, Tennessee, April, 1965. CONF-650407. p. 730.

C 19 W.A. Freeby, L.T. Lakey and D.E. Black, "Fission Product Behaviour under Simulated Loss-of-Coolant Accident Conditions in the Contamination-Decontamination Experiment", Idaho Nuclear Corporation Report, IN-1172 (1969).

C.20 A. Natalizio and R.J. Fluke, "Fission Product Behaviour in Containment", presented at Simulation Symposium, Ecole Poly- technique, Montreal (1980). ISSN 0067.0367 ISSN 0067-0367

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