Radiolysis in CANDU Coolant and its Effect on Chemistry and Materials International Association for the Properties of Water and Steam Workshop May 11 & 12, 2009 Jungsook Clara Wren Department of Chemistry University of Western Ontario

Presentation Outline Overview of NSERC/AECL Industrial Research Chair Program Preliminary Results from -Radiolysis Induced Carbon Steel Corrosion at 150oC Importance of Steady-State Radiolysis on Steel Corrosion 2

NSERC/AECL IRC Program 2006 - 2010 Water Radiolysis H2O •OH, eaq–, H•, HO2•, H2, H2O2, O2, H+ Corrosion Metal Oxidation/Dissolution Catalytic Reactions Corrosion products + Radiolysis products Steady-State Water Radiolysis - Effects of Dissolved Impurities H2/O2/H2O2, Transition Metals, Nitrate/Nitrite, Organic Compounds Corrosion Kinetics In non-irradiated but chemically-simulated radiation environments Corrosion Kinetics under -Irradiation Temperature Ranges: < 90oC  150oC  320oC (2nd Term ?) 3

Experimental Techniques Surface Analyses (coupons/particulates) Raman, SEM/EDX, pXRD, XRD, XPS, Auger, Reflectance FTIR, Confocal Microscopy Electrochemistry (Ex-situ/In-Situ Spectroscopy) Current-potential profile, Corrosion potential, Electrochemical Impedance spectroscopy, Linear polarization Chemical Analyses (gas & aqueous speciation) GC/MS+TCD+ECD (H2, NOx, organics, etc) UV-Vis + chemical titration (H2O2, NO3/NO2, Fe2+/Fe3+) pH Chemical Kinetics & Transport Modelling Interfacial Transport COMSOL MultiPhysics (Finite Element Method) Water Radiolysis Kinetics FACSIMILIE (Differential Rate Eq Solver) Solution Thermodynamics OLI (Thermo. database + solver)

-Radiolysis on Carbon Steel Corrosion Coupon Studies at T  150oC Dose Rate 6.4 kGy/ h These studies are performed as a function of pH, T, irradiation time, dissolved H2 and O2 (cover gas), buffer

Chemical Production in the Irradiated System Aqueous and Gas Phases Steam Radiolysis H2, H2O2, O2, •OH, •H, •eaq, •O2 H2, O2 H2, O2 Corrosion Products H2, Fe2+, Fe3+ Liquid Water Radiolysis H2, H2O2, O2, •OH, •H, •eaq, •O2 6

Effect of pH & -Irradiation at 150oC 1 m 5x No Rad 21 h @150oC Ar 3 m pH25=10.6 pH25= ~7 5x 5x Rad 3 m 1 m 7

Effect of pH & -Irradiation at 150oC 1 m 5x No Rad H2 (0.6%) H2 (0.8%) 21 h @150oC Ar 3 m pH25=10.6 pH25= ~7 H2 (2.3%) H2 (2.2%) 5x 5x Rad 3 m 1 m 8

66 h Irradiation at 150oC, Ar pH25 = 10.6 pH25 = 7 Cross Section Aq. Phase Outer oxide layer 2.2 m 5x Inner layer 3 m 9 Base metal 1 m pH25 = 7 Cross Section 3 m Aq. Phase Outer oxide layer 760 nm 5x Inner layer 3 m 9 Base metal 200 nm

Fe3O4 / -Fe2O3 / FeII/FeIII(OH)x Raman Spectroscopy Cross Section 66 h Irradiation, pH 10.6, Ar -Fe2O3 -FeOOH -Fe2O3 Fe3O4 Raman Intensity Raman Shift Irradiated Sample A Mixed Phase ?? Fe3O4 / -Fe2O3 / FeII/FeIII(OH)x No significant -Fe2O3 Confocal Raman Microscopy, small angle XRD for depth profile are underway

Steam vs Liquid 1 m 1 m 150oC pH25=7 Ar, 21 h 1 m 3 m No Rad 5x 5x 1 m 1 m No Rad No Rad Liquid Phase Steam Phase Rad Rad 150oC pH25=7 Ar, 21 h 5x 5x 1 m 3 m 11

Preliminary Conclusions Coupon Studies at 150oC Kinetics is still important at 150oC Steady-state irradiation enhances surface oxide formation The type of oxide depends on the rate of oxidation and pH Aqueous corrosion on CS under -irradiation is uniform, and does not show localized corrosion At pH25 10.6, -irradiation appears to promote more compact oxide Corrosion in the steam phase is more inhomogeneous Steady-State Radiolysis affects carbon steel corrosion behaviour

Water Radiolysis Solvent Oriented Process Physical (chemical) Stage Primary Radiolysis Yields (G-values) Bulk Phase Chemistry Stage 13

Importance of Steady-State Radiolysis Continuous production Aqueous chemistry control by pH, chemical additives Radical & reactive species quickly decay away Surface Oxidation Metal Dissolution Interfacial Transfer Steady-State Concentrations Steady-state concentrations determine corrosion behaviour Different steady states in different aqueous environments 14

Steady-State Radiolysis Model Aqueous Reaction Kinetics Database ~ 40 reactions Pulse Radiolysis has been a very useful tool for obtaining G-values and rate constants of fast reactions of radicals and ions •OH + H2 325oC 150oC 25oC 80oC G-value (T) x water(T)

Steady-State Radiolysis Model Steady-State Model Validation Individual reaction components in the database are sound The model as a whole has not been validated except at room T Missing key reactions? What are the rate controlling reactions? Do they change with T, pH, impurity? Validation of the model as a function of temperature ( 150oC) under different chemical and interfacial conditions are on-going 16

Effect of pH on Radiolysis Steady-State Concentrations G-values 25oC 25oC Without steady-state analysis such pH dependence was not envisioned Model sensitivity analyses to understand why such behaviour is observed 17

Effect of Temperature on Radiolysis Steady-State Concentrations G-values pH25 = 10.6 G-value (T) x water(T) Without steady-state analysis such T dependence was not envisioned 18

Summary at T  150oC Radiolysis affects carbon steel corrosion Thermodynamic considerations are not sufficient Steady-state concentrations of radiolytic products determine surface oxide formation/transformation Steady-state radiolysis behaviour strongly depends on pH, T, chemical additives, dose rate, etc, These dependences are not well established Pulse radiolysis studies are not sufficient Molecular, not radical, products are more important for aqueous corrosion in basic solutions The relative importance of radical species may increase with T Significant implication in chemistry control

Acknowledgement Dr. Jamie Noel Dr. X John Zhang Dr. Peter Keech Dr. Jiju Joseph Dr. Sriya Peiris Dr. Sergey Mitlin Dong Fu Sarah Pretty Kevin Daub Katy Yazdanfar Pam Yakabuskie Susan Howett

Thermodynamics predicts -Fe2O3 to be the most stable oxide at 150oC Pourbaix Diagram OLI 150oC pH25 = 10.6 pH25 = 6 Ref: B. Beverskog, I. Puigdomenech, Corrosion 38, 2121-2135, 1996 Magnetite stable in a small part of water stability region For aqueous phase, Fe(OH)4+ stable over a significant area Copy Thermodynamics predicts -Fe2O3 to be the most stable oxide at 150oC 21

Catalytic Interaction NSERC/AECL IRC Program Synergistic Interaction of Radiolysis & Corrosion Water Radiolysis Corrosion Metal oxidation-reduction Dissolution of metal oxides •OH, eaq–, H•, HO2•, H2, H2O2, H+, O2, O2•–  H2O Creates unusual solution conditions A wide range in chemical reactivity, redox and transport property Metal Oxides, MOx Bulk Metal, M Mn+(aq) Catalytic Interaction •OH, H2O2, O2 Fe2+(aq) Fe3+(aq) Depends on surface and solution-redox conditions •H, eaq–, •O2– Catalytic interaction of dissolved metal and radiolysis redox species

Effect of Radiation 150oC 3 m pH25=10.6 Ar 3 m 3 m No Rad 21 h 66 h 5* 3 m 3 m 23

Effect of Radiation 150oC 3 m pH25=6 1 m Ar 1 m 3 m 3 m 3 m 5* 150oC pH25=6 Ar 3 m 1 m No Rad 21 h 66 h Rad 5* 5* 5* 1 m 3 m 3 m 3 m 24

Mostly Fe3O4 + -FeOOH + possibly FeII/FeIII(OH)x 66 h Irradiation, pH 7, Ar 3 m SEM of Cross Section Outer oxide layer 760 nm 5x Inner layer 3 m 25 Base metal -Fe2O3 -FeOOH -Fe2O3 Fe3O4 Raman Intensity Raman Shift Irradiated Sample Need to compare pH 10.6 vs pH 6 cases Mostly Fe3O4 + -FeOOH + possibly FeII/FeIII(OH)x

Water Radiolysis Kinetic Model (constant radiation field) •OH + H2 ~ 40 Elementary Reactions & Rate constants (T) Continuous production G-values (T) Steady State Concentrations water(T), Kwater(T), etc Coupled reaction rate equations are solved using numerical integration software FACSIMILE Input Dose Rate T, pH Output Concentrations as a function of time 26

Steady-State Radiolysis Database Rate constants Equilibrium constants •OH + H2 325oC 150oC 25oC 80oC Individual rxn components are sound The model as a whole has not been validated except at room T What are the rate controlling reactions? Do they change with T, pH, impurity? Missing key reactions? T dependence well established For most reactions, it follows Arrhenius T dependence At high T, all reaction rates approach diffusion limited Diffusion rate  T/ 27

pH on Steady-State Radiolysis Behaviour Model not validated at > 80oC 25oC 300oC ~ 3 orders of magnitude changes at pHs around pKa of eaq + H+ = •H pH dependence diminished due to increase in the reaction of eaq with H+ Without steady-state analysis such pH dependence not easily predicted 28

Chemical Additives/Dissolved Species Effect of [Fe2+]o on [H2O2]SS Ar, pH = 10.6 Effect of Radiolysis on Iron Solubility [Fe2+]o = 0, 510-5, 110-4 M •OH, H2O2, O2 Synergistic interaction between corrosion products and radiolysis products Fe2+(aq) Fe3+(aq) •H, eaq–, •O2– 29

Steady-State Radiolysis Model Aqueous Reaction Kinetics Database ~ 40 reactions Pulse Radiolysis has been a very useful tool for obtaining G-values and rate constants of fast reactions of radicals and ions (G-value x water density) does not vary significantly with T Solvation is important Water vapour (dry steam) has different G-values G-value (T) x water(T)

Aqueous Reaction Kinetics Database Pulse Radiolysis has been a very useful tool for obtaining G-values and rate constants of fast reactions of radicals and ions Rate constants •OH + H2 325oC 150oC 25oC 80oC T dependence well established For most reactions, it follows Arrhenius T dependence At high T, all reaction rates approach diffusion limits Diffusion rate  T/