Radioiodine Chemistry: The Unfinished Story€“ Effect of iodine on the oxidation of metal...
Transcript of Radioiodine Chemistry: The Unfinished Story€“ Effect of iodine on the oxidation of metal...
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J.C. WrenDepartment of Chemistry
The University of Western OntarioLondon, Ontario, CANADA
The First European Review Meeting on Severe Accident ResearchAix-en-Provence, France
15 Nov 2005
Radioiodine Chemistry: The Unfinished Story
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Background
• Radioiodine has been a long-standing concern in safety analysis – The single most important nuclide for public dose – Iodine volatility in containment is a critical parameter in accident analysis– Inappropriate assumptions lead to inappropriate safety design decisions, and
emergency management plans and provisions.
• Complex reaction & transport kinetics under accident conditions– Considerable worldwide effort over the last 15 years
• OECD status report on iodine chemistry • Krausemann, E., 2001. EUR 19752 EN• Wren et al., 2000. Nucl. Tech., 129, 297.
• This presentation– Limited to a discussion on iodine during the ‘chemistry’ phase of an accident
scenario
3Iodine Behaviour in Containment
‘Chemistry Phase’
Water Radiolysis
H2O •OH, eaq–, H•, HO2•, H2, H2O2, H+
Iodine dissolved in water+
Non-Volatile Iodine ↔ Volatile Iodine
Volatile Iodinein the gas phase
Public DosePublic Dose
I−, I3 −, HOI, IOx
− ↔ I2 ↔ RII−, I3 −, HOI, IOx
− ↔ I2 ↔ RI
I2 ↔ RII2 ↔ RIIxOyIodine Released as CsI
From Fuel / Fuel ChannelInto Containment
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Evolution of Licensing Basis Assumptions
• Prior to TMI-2 accident – US NRC TID 18444– Conservative assumptions, based on limited knowledge of accident progression
• 50% of the core inventory released to containment • 50% deposited onto surface• Initial iodine speciation: 91% I2, 5% aerosols, 4% volatile organic iodides
• Actual situation at TMI-2– Severe damage to the fuel in the core – But an extremely small fraction of iodine was found airborne (and less released)– Triggered a large effort to understand the progression of beyond design basis
accidents, a significant component of the effort on iodine behaviour• A Newer treatment – US NUREG-1465
– Still contains substantial conservatism, based on data available prior to 1995 • Iodine speciation in containment: 95% CsI, 5% I2, 0.15% volatile organic iodides
– Considerable opportunity to further improve source term predictions with a technically sound basis
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Current Status
(1) Significant progress over the last 15 years in our understanding of complex radiolysis-driven iodine chemistry and transport
• Integrated-effects tests in intermediate-scale radiation facilities– RTF at AECL, Canada, CAIMAN at IRSN Cadarache, France– Some control of test conditions and on-line measurements– Systematic and parametric studies in multi-component environments – Valuable in establishing the relative importance of various processes
DODO
ORP
pHpH
AAIS
Gas Recirculation Loop
H2 Sensor
AqueousRecirculation
Loop
Gas Ventilation LoopCharcoal Filter
AqueousSampling
Loop
GasSampling
Loop
pH control
pHpH
OnlineGamma pHpH
OnlineGamma Lead Canister
Main Vessel
131I tracer
• Supporting bench-scale tests– The integrated effects tests are difficult to interpret and not directly applicable to
real containment conditions • Likewise, the PHEBUS results cannot be used directly.
– The international iodine community has performed valuable experiments at more fundamental levels
– Essential for the development of iodine models with better predictivecapabilities.
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(2) Iodine models, with predictive capabilities, now available in different safety analysis codes
• Models of varying degrees of sophistication and validation – Comprehensive codes, LIRIC, INSPECT and MELCOR-I– Simplified codes, IODE, IMPAIR, AIM and IMOD, for incorporation in larger
system-level codes. • Used to establish/prioritize key processes • Collaboration of the international community to improve the models
through ISP-41 and 46 – Considerable spread in the results of exercises on iodine modeling – Demonstrated the contribution of many user-defined model parameters, or
input parameters that were not well defined in the tests.– Not a reflection of adequacy of the fundamental reaction chemistry data, rather
the uncertainty in modeling the influence of the ‘real’ environment
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(3) The focus of research can now shift to address the more challenging problem of dealing with ‘real’ accident environments
• A common focus required to develop consistent and convergent predictive capabilities– Range of potential reaction partners influencing iodine chemistry could be
overwhelming – Multiple, parallel attempts to develop simplifying approximations could lead a
situation where disagreement between predictive tools becomes difficult to disentangle
• Room for individual exploration of separate effects– Sustaining a critical approach to safety analysis tool development.
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I−, I3 −, HOI, IOx
− ↔ I2 ↔ RII−, I3 −, HOI, IOx
− ↔ I2 ↔ RI
I2 ↔RII2 ↔RIIxOy
I−, I3 −, HOI, IOx
− ↔ I2 ↔ RII−, I3 −, HOI, IOx
− ↔ I2 ↔ RII−, I3 −, HOI, IOx
− ↔ I2 ↔ RII−, I3 −, HOI, IOx
− ↔ I2 ↔ RI
I2 ↔RII2 ↔RII2 ↔RII2 ↔RIIxOy
I−, I3 −, HOI, IOx
− ↔ I2 ↔ RII−, I3 −, HOI, IOx
− ↔ I2 ↔ RII−, I3 −, HOI, IOx
− ↔ I2 ↔ RII−, I3 −, HOI, IOx
− ↔ I2 ↔ RI
I2 ↔RII2 ↔RII2 ↔RII2 ↔RIIxOy
I−, I3 −, HOI, IOx
− ↔ I2 ↔ RII−, I3 −, HOI, IOx
− ↔ I2 ↔ RII−, I3 −, HOI, IOx
− ↔ I2 ↔ RII−, I3 −, HOI, IOx
− ↔ I2 ↔ RI
I2 ↔RII2 ↔RII2 ↔RII2 ↔RIIxOyIxOy
Multiple Processes
• Strong coupling between scale-dependent and scale-independent processes
Mass Transport
Homogeneous Reactions in the aqueous phase
Homogeneous Reactions in the gas phase
Heterogeneous Reactions on surfaces
Independent of the geometry and the makeup of materials
Strongly dependent on the containment design
– Difficult to separate the effects of individual parameters– Difficult to establish a simple scaling factor
• Complex chemistry in the presence of ionizing radiation
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Water Radiolysis in the presence of impurities
I2
I−, HOI, I3−, IO3 −, etc
Surfa
ce /
collo
ids
I(ad )
Gas Phase
Aqueous Phase
RI I−
Water Radiolysis in the presence of impurities
Water Radiolysis in the presence of impurities
I2
I−, HOI, I3−, IO3 −, etc
I2
I−, HOI, I3−, IO3 −, etc
Surfa
ce /
collo
ids
I(ad )
I(ad )
Gas Phase
Aqueous Phase
RI I−RI I−
Complex Aqueous Chemistry
Iodine in various oxidation states
Large # of iodine reactions to model
Wide range of chemical and transport behaviour ⇒
Ionizing radiation
Kinetics of each reaction path to model
Continuous production of reactive species
Water radiolysis ⇒
Iodine conversion water radiolysis driven
Reactions of water radiolysis products need to be modeled
Small [I]
Water radiolysisaffected by impurities
⇒
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Homogeneous Reactions in the aqueous phase
Mass TransportHomogeneous Reactions
in the gas phase
Heterogeneous Reactions on surfaces
Water Radiolysis in the presence of impurities
Water Radiolysis in the presence of impurities
I2
I−, HOI, I3−, IO3 −, etc
Surfa
ce /
collo
ids
I(ad)
Gas Phase
Aqueous Phase
RI I−RI I−
Fe 2+⁄Fe 3+
Fe 2+⁄Fe 3+
O2, H2
Humid Air Radiolysis
NO32−
RH
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Basic Modeling Capabilities
I−, HOI, I3−, IO3 −, etc I2
Reactions with water radiolysis products•OH, •H, eaq
−, •O2−, H2O2, HO2•, etc
Thermal iodine reactionsI2 + H2O = HOI + I− + H+; I− + I2 = I3−; etc
Reactions with water radiolysis products•OH, •H, eaq
−, •O2−, H2O2, HO2•, etc
Thermal iodine reactionsI2 + H2O = HOI + I− + H+; I− + I2 = I3−; etc
Key Oxidations
I– + •OH •I + OH–
•I + •I I2
Key Oxidations
I– + •OH •I + OH–
•I + •I I2
Key Reductions
I2 + H2O HOI + I– + H+
I2 + •O2– •I2– + O2
I2 + H2O2 2I– + 2 H+ + O2
Key Reductions
I2 + H2O HOI + I– + H+
I2 + •O2– •I2– + O2
I2 + H2O2 2I– + 2 H+ + O2
(a) Thermal and Radiolytic Iodine Reactions
Water RadiolysisH2O •OH, •H, eaq
−, H+, H2 , H2O2
HO2 , O2, •O2−
Water RadiolysisH2O •OH, •H, eaq
−, H+, H2 , H2O2
HO2 , O2, •O2−
(b) Radiolysis of Clean Water
•H + RH + •R, RH
•H + RH + •R, RH
RO2• →→ RCOOH →→ RH + CO2
+ O2RO2• →→ RCOOH →→ RH + CO2
+ O2
+ •I+
RII2
+ •I+
RII2
: Rxns with radiolysis products
: Rxns with radiolysis products
RH RH •R•R•R•R
(c) Radiolytic Organic Iodide Formation
The focus of research can now shift to address the more challenging roles of other chemical species and reactive surfaces on iodine chemistry.
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Trace metal ions RH H2
NO2−, NO3
−
Air radiolysis
surface
O2
Water Radiolysis in the Presence of Impurities
Water RadiolysisH2O •OH, •H, eaq
−, H+, H2 , H2O2
HO2 , O2, •O2−
Water RadiolysisH2O •OH, •H, eaq
−, H+, H2 , H2O2
HO2 , O2, •O2−
(1) Role of Metal Ions• Catalytically react with
water radiolysis products• Different metals have
different impacts on radiolysis chemistry
Reactions of Fe2+ / Fe3+
Fe2+ + •OH, H2O2, O2
Fe3+ + •H, eaq–, •O2
–
Reactions of Fe2+ / Fe3+
Fe2+ + •OH, H2O2, O2
Fe3+ + •H, eaq–, •O2
–
Advanced Modeling Capabilities
• A UWO program to investigate the effects of different metal species– To develop a basis for understanding– To provide insight into which metal ion species has the greatest impact. – To establish a methodology for inclusion of metal ion chemistry into advanced
radiolysis models
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(2) Submerged Metal/Metal Oxide Surfaces
Water Radiolysis
H2O •OH, eaq–, H•, HO2•, H2, H2O2, H+
Oxid
es (F
eI I /Fe
I IIph
ases
)
Fe
Fe Steel oxidation releases Fe2+
Radiolysis products influence corrosion
Surface decomposition of H2O2 affects radiolysis chemistry
Reactions of Fe2+ / Fe3+ with Radiolysis Products
Fe2+ + •OH, H2O2, O2
↔ Fe3+ + •H, eaq–, •O2
–
• These surfaces interact synergistically with water radiolysis products • React readily with iodine to form metal iodides that may or may be not
soluble– If soluble (FeI2), the formation of metal iodide catalytically increases dissolution of
metal ions, or slows down the iodine conversion – If insoluble (e.g., AgI), it could reduce iodine volatility
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(2) Submerged Metal/Metal Oxide Surfaces
• Nature of the oxide layer on structural metals is an important factor influencing the reaction rates of aqueous iodine species– The nature and the thickness may change with iodine adsorption
• UWO program on the interaction of the metal/metal oxides– Carbon steel, stainless steel, Zn and Ag – Interaction with H2O2 with and without γ-irradiation– Effect of iodine on the oxidation of metal surfaces and the dissolution of the
corrosion products • Chemistry of steel surfaces with iodine present is important
– Steel is used in containment buildings – Used in the engineering-scale iodine behaviour studies
15(3) Organic Surfaces
• RH dissolved in water affects iodine volatility– In large-scale tests, organic surfaces are the source of dissolved organic impurities – Difficult to unequivocally separate the homogeneous and heterogeneous reactions
• Prediction of iodine volatility depends on understanding the dominant mechanism active in these tests
– Some data available, however, a systematic study is warranted to better understand the competing processes
• AECL program in Canada, and EPICUR program at IRSN in France
•H + RH + •R, RH
•H + RH + •R, RH
RO2• →→ RCOOH →→ RH + CO2
+ O2RO2• →→ RCOOH →→ RH + CO2
+ O2
+ •I+
RII2
+ •I+
RII2
: Rxns with radiolysis products
: Rxns with radiolysis products
surfa
ce
RH RH •R•R
•R•RRH
16(4) Effects of Humid Air Radiolysis
NOx, Nitric acid, Nitrate Formation
N2, O2, H2O in air •OH, HO2•, •O, O3, •N, eaq-, etc
Radiolysis of Humid Airγ, β
I2
IxOy, IodateFormation
Organic Iodide decomposition
RI
• Air radiolysis responsible for IxOy formation, RI decomposition and nitric acid ⁄ nitrate formation
– IxOy and RI reactions lower iodine volatillity– Nitric acid / nitrate semi-catalytically react with water radiolysis products
• The challenge is to determine the concentrations of the air radiolysisproducts in the presence of reactive containment walls and aerosols
– Nitric acid and ozone formation rates decrease in the present of reactive surfaces• The PARIS project
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(5) Gaseous Iodine Sorption on Surfaces
I2 loading in N2
More adsorptionLess corrosion
I2 loading in airLess adsorptionMore corrosion
• Metal surfaces a major iodine “sink”• Considerable variation in T and RH dependence of the adsorption rates• The uncertainty in the adsorption rates, combined with the difficulty in
accurately establishing the mass transport conditions, significantly contributed to the spread in ISP 41 results
• UWO program on gaseous iodine adsorption on metal/metal oxides– Effects of reaction conditions and the type of metal/metal oxide on the iodine
incorporation in the oxide layer and corrosion rates.
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(6) Integrated Effects Tests
• To maintain the integrated-effects test capability to develop and validate whole system models of iodine behaviour
– Small-scale tests involve probing the reaction processes at fundamental levels– As our understanding improves, our ranking of the relative importance of
various chemical and transport processes may change• Test capability that can examine combinations of sub-sets of multi-
component environments, but with good control of conditions and on-line measurements, is most useful
– e.g., CAIMAN facility in France; ThAI facility in Germany
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Summary
• We have made great strides through the concerted research and development efforts of the international community.
• As a result, we have good basic models for use in predicting iodine chemistry under accident conditions.
• The challenge for the future is to add to the basic models the effects of additional chemical species and chemically reactive surfaces on iodine chemistry.
• The development of more sophisticated models will allow us to improve our capabilities to address more complex environments.
• Several key additions have been identified, and experimental programs to address them and other areas for model improvement have been initiated.
• Through these programs and related studies, and the valuable results of future large-scale tests we can anticipate success in further enhancing our understanding of radioiodine chemistry.
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Thank you !!