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A Review of Iodine Chemistry Under Severe Accident Conditions

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A Review of Iodine Chemistry Under Severe Accident Conditions •--'s AECL-9089 ATOMIC ENERGY £2"A LENERGIE ATOMIQUE OF CANADA LIMITED TjS9 DU CANADA, LIMITEE A REVIEW OF IODINE CHEMISTRY UNDER SEVERE ACCIDENT CONDITIONS EXAMEN DE LA CHIMIE DE L'lODE DANS DES SITUATIONS ACCIDENTELLES GRAVES D. J. Wren, J. Paquette, J. C. Wren, P. N. Clough*, H. C. Starkie" "Safety and Reliability Directorate UKAEA, Culcheth Whiteshell Nuclear Research Etablissement de recherches Establishment nucleaires de Whiteshell Pinawa, Manitoba ROE 1LO 1987 Copyright © Atomic Energy of Canada Limited, 1987 ATOMIC ENERGY OF CANADA LIMITED A REVIEV OF IODINE CHEMISTRY UNDER SEVERE ACCIDENT CONDITIONS by D.J. Wren, J. Paquette, J.C. Wren, P.N. Clough* and H.C. Starkie* * Safety and Reliability Directorate UKAEA, Culcheth Vhiteshell Nuclear Research Establishment Pinawa, Manitoba ROE 1LO 1987 AECL-9089 EXAMEN DE LA CHIMIE DE L'IODE DANS DES SITUATIONS ACCIDENTELLES GRAVES par D.J. Wren, J. Paquette, J.C. Wren, P.N. Clough* et H.C. Starkie* RESUME Dans ce rapport, on examine les progrès réalisés dans l'établissement d'une connaissance de base des facteurs qui détermineront le comportement de l'iode lors d'accidents hypothétiques de réacteurs refroidis à l'eau. Les sujets considérés sont les réactions thermiques, les réactions radiolytiques, les effets de l'impureté, la formation de l'iodure organique, les modèles et essais intégrais et le réglage de la volatilité. On a acquis de grandes connaissances dans un certain nombre de secteurs, plus particulièrement en bases de données cinétiques et thermodynamiques pour les réactions thermiques et radiolytiques de l'iode inorganique en solution. Toutefois, il subsite une compréhension limitée des mécanismes réglant la formation des iodures organiques ainsi qu'un besoin d'essais intégrais de comportement de l'iode dans des systèmes complexes 'sales' pour obtenir des données permettant la validation des modèles chimiques subissant une mise au point. *Safety and Reliability Directorate UKAEA, Culcheth Etablissement de recherches nucléaires de Whiteshell L'Énergie Atomique du Canada, Limitée Pinava, Manitoba ROE 1L0 1987 AECL-9089 A REVIEW OF IODINE CHEMISTRY UNDER SEVERE ACCIDENT CONDITIONS by D.J. Wren, J. Paquette, J.C. Wren, P.N. Clough* and B.C. Starkie* ABSTRACT This report reviews the progress that has been made in establishing a basic understanding of the factors which will determine the behaviour of iodine during postulated accidents in water-cooled reactors. The topics considered are thermal reactions, radiolytic reactions, impurity effects, organic iodide formation, integral models and tests and volatility control. There have been substantial gains in a number of areas, most notably in the kinetics and thermodynamics databases for thermal and radiolytic reactions of inorganic iodine in solution. However, there remains a limited understanding of the mechanisms controlling the formation of organic iodides and a need for integral tests of iodine behaviour in complex, 'dirty' systems to provide data for the validation of chemical models which are undergoing development. * Safety and Reliability Directorate UKAEA, Culcheth Atomic Energy of Canada Limited Whiteshell Nuclear Research Establishment Pinawa, Manitoba ROE 1L0 1987 AECL-9089 CONTENTS Page 1. INTRODUCTION 1 2. EQUILIBRIA AND KINETICS OF THE REACTIONS OF IODINE SPECIES IN AQUEOUS SOLUTION 2 2.1 THERMAL OXIDATION OF IODIDE 5 2.2 IODIDE HYDROLYSIS 6 2.3 DISPROPORTIONATION OF HYPOIODOUS ACID 6 2.4 TRI-IODINE FORMATION 9 2.5 IONIZATION OF HYPOIODOUS ACID 10 2.6 PROTONATION OF HYPOIODOUS ACID 10 2.7 IODINATION OF HYPOIODITE 10 2.8 PARTITIONING OF IODINE SPECIES 10 3. RADIOLYSIS 11 4. IMPURITY EFFECTS 17 5. ORGANIC IODIDES 18 5.1 SURFACE REACTIONS 19 5.2 GASEOUS-PHASE REACTIONS 21 5.3 AQUEOUS-PHASE REACTIONS 22 6. INTEGRAL MODELS AND TESTS 24 6.1 THE IMPAIR CODE 24 6.1.1 In-Vessel Release and Transport of Iodine 24 6.1.2 Iodine Behaviour in the Containment 24 6.1.3 Iodine Behaviour Following Containment Failure 25 6.2 EXPERIMENTAL PROGRAM AT KVU 26 6.3 THE VHITESHELL MODEL 28 7. VOLATILITY CONTROL 29 8. DISCUSSION AND CONCLUSIONS 33 8.1 THERMAL REACTIONS 34 8.2 RADIOLYTIC REACTIONS 34 8.3 IMPURITY EFFECTS 35 8.4 ORGANIC IODIDE FORMATION 35 8.5 MODELS AND INTEGRAL TESTS 36 8.6 VOLATILITY CONTROL 36 continued... CONTENTS (concluded) 9. SUMMARY AND CONCLUSIONS 37 REFERENCES 37 1. INTRODUCTION Radioisotopes of iodine have long been recognized as major contributors to the hazard if an accident at a water-cooled nuclear reactor results in an escape of activity from the core. The chemistry of iodine is complex and, although much research effort has been devoted over many years to defining the behaviour or radioiodine (notably 131I) in reactor accidents, many uncertainties are still outstanding. The most important question is the extent to which iodine will remain airborne in the reactor containment building following its release from the reactor coolant system, and thus be available for an immediate release to the atmosphere via containment leakage or failure. Iodine can exist in a variety of chemical forms, some of which are volatile and others essentially involatile. The volatile species include elemental iodine (I2), hydrogen iodide (HI), organic iodides (CH3I, etc.), and possibly hypoiodous acid (HIO). Involatile forms are anions, for example, the iodide (I") and iodate (I03~) ions, which will be completely dissolved in water in a post-accident situation. The volatile species are also soluble in water to varying degrees, and will be partitioned between the gaseous and aqueous phases. In assessing the potential for iodine release, it is necessary to know how this element is distributed amongst the following three physical states: (1) The gaseous phase. Certain fractions of the volatile forms will exist as true gases in the containment atmosphere, and so they will be available for immediate release. (2) Aerosols. In many post-accident conditions, significant amounts of aqueous aerosols will be airborne in the containment atmosphere for long periods. These aerosols may contain both involatile and volatile iodine species. The fractions of volatile species present in the aerosols will depend on their partitioning behaviour with respect to the gaseous. Aerosol-borne iodine will also be available for immediate release. (3) The bulk condensed phase. Some iodine will become adsorbed onto containment surfaces, but, by far, the most important sink will be the pools of water that will collect in containment sumps following any accident involving loss of coolant. Such pools will contain the involatile iodine species plus fractions of the volatile species, as determined by their partitioning behaviour. Iodine in these pools can be regarded as much less mobile than in the airborne fraction. However, resuspension during the depressurization that may accompany containment failure or, in the long term, following failure, is an important issue. This review is concerned with the distribution of iodine between the gaseous and aqueous phases following a reactor accident, an understand- ing of which is essential to assess its overall behaviour and release. Two main aspects are addressed, namely, what determines the speciation of iodine between involatile and volatile forms, and how do the volatile forms partition between the gaseous and aqueous phases. For a particular application, both aspects are incorporated in the overall distribution coefficient for iodine, defined as - 2 - Distribution coefficient H = Total iodine concentration in aqueous phase Z(volatile + involatiles) Total iodine concentration in gaseous phase (E volatiles) H will depend on many parameters, such as temperature, iodine concentra- tion, and water chemistry, and will be time dependent in an accident situation. Important in determining H are the equilibrium partition coefficients P(X) of individual species X, defined as Volume concentration of X in aqueous phase P(X) = Volume concentration of X in gaseous phase at equilibrium at temperature T. Some idea of the significance of H can be gained by considering the situation in a large pressurized-water reactor (PWR) containment following a loss-of-coolant accident (LOCA). If we assume that, after several hours, the aerosol fraction is small, the gas-borne fraction of iodine will be governed by the distribution between the atmosphere and the water pools. If both states are assumed to be well mixed, this fraction is given by F = Vg/(Vw-H + Vg) where Vg = volume of the gas space Vw = volume of the water pools By taking representative values for a modern PWR, Vg = 8-104 m3 Vw = 300 m3 (total RCS + accumulator water) a value H = 10 000 leads to a gaseous iodine fraction F = 2.6Z. This example also illustrates the accident context for which this review is most relevant. During the early stages in many accident situations, the airborne iodine in the containment will be mainly in aerosol forms, and the gaseous forms will be of minor importance if the containment fails at this stage. However, if the containment survives for an extended period, the aerosols will largely settle out, and their iodine contents will be redistributed between the gaseous phase and the water pools. In this case, the distribution of iodine between the pools and the containment atmosphere is a key question vith respect both to leakage from the containment and to immediate release if the containment fails. In the changed environment of a breached containment, the distribution coefficient will be important in determining the rate at which iodine will be released from residual pools. Recently, the state of knowledge of inorganic iodine behaviour relevant to severe accidents at PVRs and boiling-water reactors (BWRs) was extensively reviewed by two of the authors {1]. Likely conditions of water chemistry and radiation fields in the containments were defined. This review was based on the assumption that iodine would be initially released to the containment almost entirely in the form of involatile iodide, which would quickly become associated with water.
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