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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 !!