Birchley, J.

Fernandez-Moguel, L.

Park, S.

Prasser, H.M.

Seifert, H.J.

Steinbrück, M.

English

Prasser, H. M.

Seifert, H. J.

KITopen

Wir schaffen Wissen – heute für morgen A mechanism of nitridation process in the Zr-O-N system during air oxidation  Sanggil Parka, Leticia Fernandez-Moguela, Martin Steinbrückb, Jonathan Birchleya, Horst-Michael Prassera,c, Hans Jürgen Seifertb aPaul Scherrer Institute, Switzerland bKarlsruhe Institute of Technology, Germany cETH Zürich, Switzerland  NuMat 2014: The Nuclear materials Conference 27-30 October 2014, Hilton Clearwater, Florida  2 Outline  Motivation and Introduction of the air oxidation modeling  Part I. Phases in the Zr-O-N system  Part II. Conceptual nitridation process  Part III. Mechanism of nitridation process  Summary and Outlook  NuMat 2014 conference, Florida, 27 October 2014  3 Motivation – Air Ingress scenarios Air Ingress scenarios Late phase after RPV failure Refueling: RPV head removal Taken from: Wikipedia Taken from: www.cleanenergyinsight.org Spent fuel pool draining Taken from: www.josephmiller.typepad.com Dry storage cask drop during transport Taken from: www.power-eng.com • Reactor sequences – Late phase after RPV failure – Mid loop operation: Refueling • Spent fuel sequences – Spent fuel pool draining – Dry storage cask drop NuMat 2014 conference, Florida, 27 October 2014  4 Motivation – Nitrogen reservoirs in reactors Nitrogen reservoirs in reactors Taken from: www.lke.mavt.ethz.com • BWR – Inerted containment by nitrogen – to prevent hydrogen combustion • PWR – Hydroaccumulator filling by nitrogen – Passive high pressure injection system  Nitrogen Taken from: www.lke.mavt.ethz.com NuMat 2014 conference, Florida, 27 October 2014 Introduction – Role of nitrogen during air oxidation 1st role of nitrogen: cladding degradation by volume mismatches between ZrO2 and ZrN  micro porous and macro cracked oxide forms due to ZrN formation and reoxidation 2nd role of nitrogen: exothermic heat release during ZrN formation and reoxidation  accelerate the oxidation kinetics and enhance the heat release   5  Is nitrogen an inert gas?  Dissociation of double bond = +497 kJ/mol  Gibbs energy of formation ∆GZrO2= -896 kJ/mol (1073.15K)  Dissociation of triple bond = +945 kJ/mol  Gibbs energy of formation ∆GZrN= -264 kJ/mol (1073.15K)  After oxygen is sufficiently consumed, nitrogen plays an active role! NuMat 2014 conference, Florida, 27 October 2014 Introduction – Purpose of the present study  Limitations in current state of knowledge on the nitridation process Post-test investigation  The results of post-test investigations reveal no metastable state and phase transformations during the process  The understanding of nitridation process is phenomenological  Recently, Zr-O-N ternary compounds were detected by Raman investigation Binary system analysis: Zr-O and Zr-N  Only binary compounds, ZrO2 and ZrN, are involved during the process  6 A mechanism of nitridation process in the Zr-O-N system during air oxidation Theory Experiment Simulation Zr-O-N system analysis Thermodynamics Thermo-Calc calculation using Zr-O-N database KIT SETs data Literature findings NuMat 2014 conference, Florida, 27 October 2014 7  Zr-O-N ternary system:       ZrO2-Zr3N4 pseudo binary system Taken from: http://www.chem.ufl.edu/~itl/4411/lectures/lec_16.html Zr-O-N system Electronegativity Zr 1.33 O 3.44 N 3.04 ΔEn(Zr-O) = 2.11  ΔEn(Zr-N) = 1.71  • ZrO2: Ionic bond • Zr3N4: Ionic-covalent bond (Polar bond) • ZrN: Metallic bond  Nitrogen rich Zr3N4 phase is a mestastable state and thus is decomposed to ZrN at high temperatures Part I – Phases in the Zr-O-N system NuMat 2014 conference, Florida, 27 October 2014 8  Zr-O binary system  Zr-N binary system m-ZrO2 t-ZrO2 c-ZrO2 o-Zr3N4 c-Zr3N4 c-ZrN Part I – Phases in the Zr-O-N system NuMat 2014 conference, Florida, 27 October 2014 9  Zirconium oxynitrides phases: 𝐙𝐙𝐙𝐙𝐙𝐙𝟐𝟐−𝟐𝟐𝟐𝟐𝐍𝐍𝟒𝟒𝟐𝟐𝟑𝟑 Formula Zr3N4 mol fraction (x) β’ 𝑥𝑥 = 314 Zr7O11N2 0.214 β’’ 𝑥𝑥 ~ 4.814 ~ Zr7O9.5N3.0 ~ 0.343 β 𝑥𝑥 = 614 Zr7O8N4 0.418 γ 𝑥𝑥 = 34 Zr2ON2 0.75 Taken from: (Lerch; 1998) Part I – Phases in the Zr-O-N system Zr (+4) O (-2) N (-3) 𝐙𝐙𝐙𝐙𝐙𝐙𝟐𝟐−𝟐𝟐𝟐𝟐𝐍𝐍𝟒𝟒𝟐𝟐𝟑𝟑 +4*1 -2*(2-2x) -3*(4x/3) = 0  Zriconium oxynitride phases retain the cubic fluorite structure of c-ZrO2   - oxygen vacancies formed by the nitrogen incorporation, the original cubic lattice has been gradually distorted.     Zr7O11N2 Zr7O8N4 Zr2ON2 NuMat 2014 conference, Florida, 27 October 2014 10 Part II – Conceptual nitridation process  O2/N2 dissolution to  α-Zr and oxide formation after saturation of α-Zr(O)         Stabilization of t-ZrO2 near the interface - High compressive stress, low grain size and sub-stoichiometry near the interface.       O2 consumption by forming α-Zr(O) and ZrO2-X    N2 incorporation by stabilizing c-ZrO2      Oxygen and nitrogen dissolution Oxide formation Undulation of the metal/oxide interface t-ZrO2 thin layer formation NuMat 2014 conference, Florida, 27 October 2014 11  Oxide/Nitride metastable system near the interface (c-ZrO2 /o-Zr3N4) − After solubility limit of the concentration of oxygen vacancies VO°°       c-ZrO2 + ⅔ N2 → ⅓ o-Zr3N4 + 2O (-14.34% molar volume shrinkage from c-ZrO2) − After solubility limit of α-Zr(O,N)      Zr + ⅔ N2 → ⅓ o-Zr3N4  (from α-Zr(O,N))   Nitrogen rich part of Zr3N4/γ-Zr2ON2 system near the interface      ½ ZrO2 + ½ Zr3N4 → Zr2ON2  − Nitrogen rich Zr3N4 phase is a mestastable state and thus is decomposed to ZrN (and/or) γ-Zr2ON2 at high temperatures (above 800ºC)   Zr3N4 → 3ZrN + ½ N2 4 Zr3N4 +3O2−2N2 6 γ−Zr2ON2 +9O2−6N2 12 m−ZrO2  ZrN and γ-Zr2ON2 are optically golden-yellow color.   Part II – Conceptual nitridation process NuMat 2014 conference, Florida, 27 October 2014 12  Decomposition of Zr3N4/γ-Zr2ON2 system to ZrN and m-ZrO2 and β-type oxynitride phases from 800ºC 𝐙𝐙𝐙𝐙𝐙𝐙𝟐𝟐−𝟐𝟐𝟐𝟐𝐍𝐍𝟒𝟒𝟐𝟐𝟑𝟑 Formula Zr3N4 mol fraction (x) β’ 𝑥𝑥 = 314 Zr7O11N2 0.214 β’’ 𝑥𝑥 ~ 4.814 ~ Zr7O9.5N3.0 ~ 0.343 β 𝑥𝑥 = 614 Zr7O8N4 0.418 γ 𝑥𝑥 = 34 Zr2ON2 0.75 Taken from: (Lerch; 1998)  Accelerated self-sustaining nitridation process (solid solution reaction and reoxidation) 2N(ZrN) + 3OOX ⟶ 2NO′ + VO°° + 3O(ZrN) - Accelerated by low activation energy 2NO′ + VO°° + 3O ⟶ N2 + 3OOX  - Self-sustained by newly generated nitrogen Part II – Conceptual nitridation process NuMat 2014 conference, Florida, 27 October 2014 13 Part III – Mechanism of nitridation process  Reactions I. Nitridation (solid-gas reaction): N2 + 3OOX ⟶ 2NO′ + VO°° + 3O  (reducing condition; nitrogen activity and partial pressure are higher than oxygen’s) II. Nitridation (solid solution reaction): 2N ZrN + 3OOX ⟶ 2NO′ + VO°° + 3O ZrN  (reducing condition; nitrogen activity and partial pressure are higher than oxygen’s) III. Reoxidation: 2NO′ + VO°° + 3O ⟶ N2 + 3OOX  (oxidizing condition; nitrogen activity and partial pressure are lower than oxygen’s) NuMat 2014 conference, Florida, 27 October 2014 14 Part III – Mechanism of nitridation process NuMat 2014 conference, Florida, 27 October 2014 15  𝐩𝐩𝐙𝐙𝟐𝟐 /𝐩𝐩𝐍𝐍𝟐𝟐stability diagram (from Thermo-Calc. calculation)   Cladding degradation by self-sustained nitridation-reoxidation Nitridation: ∆Vc−ZrO2→o−Zr3N4/ γ−Zr2ON2 = −11.49%  - micro porous media  Reoxidation: ∆VZrN→m−ZrO2 = +42.45%  - micro cracked media (mainly circumferential cracks) Zry-4 air oxidation at 1000ºC for 1 hour Part III – Mechanism of nitridation process NuMat 2014 conference, Florida, 27 October 2014 16 Nitridation process Volume change and heat generation In the beginning: t-ZrO2 and α-Zr(O,N) ∆Vt−ZrO2→c−ZrO2 = +0.15 %   ∆HO−N = −500 kJ/mol N ∆HVO°° = −190 kJ/mol VO°° Step I. Nitrogen incorporation (gas-solid reaction) near the oxide-metal interface: Step II. Oxide/Nitride metastable system near the oxide-metal interface ∆Vc−ZrO2→o−Zr3N4/ γ−Zr2ON2 = −11.49% Step III. Nitrogen rich part of Zr3N4/ γ-Zr2ON2  system near the interface Step IV. Decomposition of Zr3N4/ γ-Zr2ON2  system to ZrN and m-ZrO2 and β-type oxynitride phases from 800°C ∆VZrN→m−ZrO2 = +42.45%   O2 g + ZrN → m − ZrO2 + N2 g  ∆HReoxidation = −732 kJ/mol ZrN   Step V. Accelerated self-sustaining nitridation process (solid solution reaction and reoxidation) In the end: ZrO2 Part III – Mechanism of nitridation process NuMat 2014 conference, Florida, 27 October 2014 Summary and Outlook 17 Limitations in current state of knowledge on nitridation process  Post-test investigations and binary system analysis Different approach to develop a detailed conceptual model  Theory: Zr-O-N ternary system analysis from different fields and thermodynamics  Simulation: Thermo-Calc calculation (using TTZR1 and ALCHYMY databases)  Experiment data: KIT SETs data and literature findings A mechanism of nitridation process in the Zr-O-N system I. Solid-gas nitridation II. Solid solution nitridation III. Reoxidation   NuMat 2014 conference, Florida, 27 October 2014 Thank you for your attention. Questions ? 18 NuMat 2014 conference, Florida, 27 October 2014 

A mechanism of nitridation process in the Zr-O-N system during air oxidation

https://publikationen.bibliothek.kit.edu/230098040/3816695

A mechanism of nitridation process in the Zr-O-N system during air oxidation

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