Grosse, Mirco

Maeng, J.

Steinbrueck, Martin

Sung, J.

English

KITopen

KIT – The research university in the Helmholtz association  KIT / Institute for Applied Materials / Program NUSAFE     www.kit.edu Experimental Investigations and Modelling of the Reaction of Zirconium Alloys in Nitrogen/Oxygen or Nitrogen/Steam Atmospheres M. Grosse, M. Steinbrueck, Y. Maeng, J. Sung Institute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 Outline Introduction Experiments with contradictory results Atomistic description of the process Proposed model of the kinetics First experimental validations Hydrogen uptake during oxidation in steam/nitrogen mixture Conclusions and further plans Institute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 Introduction High temperature oxidation of the zircaloy claddings in nitrogen/steam atmosphere can occur for instance:  During severe accidents in BWR where the containment is inerted by nitrogen Spent fuel pool (SFP) accidents (The hydrogen released consumes the oxygen from the air and steam/nitrogen atmosphere remains at the fuel rods).  The reaction   2 Zr + N2 = 2 ZrN  (very simplified)       occurs only under oxygen and steam starvation conditions.  Modelling in severe accident simulation codes is unsatisfying      Institute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 Experiments with contradictory results 0 5000 10000 15000 200000200400600800 N2 conc., %     0     0.1     0.2     0.5     0.7     1     2     5   10   50   70   80   90 100m, g/m²time, sH2O+N2H2ON2Mass gain of Zircaloy-4 oxidized at 800°C in steam-nitrogen mixtures (annealing in TG ) Institute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 Experiments with contradictory results 0 100 200 300 400 5000,000,010,020,030,040,05 0% N2 1% N2 80% N2mass increase, gtime, min800°CWhat make the difference?  Main difference between the two furnaces is the gas flow! Mass gain of Zircaloy-4 oxidized at 800°C in steam-nitrogen mixtures (annealing in BOX furnace ) Institute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 Atomistic description oxide     metal Diffusion of oxygen vacancies into the oxide Diffusion of oxygen into the metal Oxygen or steam transport to the surface   eOVZrOZrO O 222)(OZrOZr 222 2)2(2 ZrOOeVZrO O HZrOOHeVZrO O 222 222  oxygen or steam saturation conditions: Wagner (1968) Institute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 Atomistic description Oxygen and steam starvation conditions:     ZrNZrONeVZrO O 22)2(4 222  How many oxygen vacancies are available at the surface reaction zone?  How many oxygen vacancies reach this zone per time unit? Do we know it?  YES, we know!  The molar amount of  oxygen vacancies is equal the molar amount of oxygen reacted under oxygen saturation conditions. *22tKnn oxOnoxV 2* oxideDDKt oxideInstitute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 Atomistic description The nitrogen concentration  in the oxide is given by:       If a critical nitrogen concentration is reached then zirconium nitride precipitations are formed.    )(22*2 222 reactingnnntKn NOHOnremainVoxO  022221 Nt nOHON ndttKnnnox Oxygen and steam starvation conditions: Institute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 Atomistic description Why exists a critical nitrogen concentration?  - formation enthalpy has to be greater than the interface energy     between nitride and oxide  - solubility of nitrogen in the oxide (possibly)   If new oxygen becomes available then ZrN is re-oxidized. Zirconium nitride act as an “oxygen vacancy storage”.    ZrN precipitation is connected with crack formation.  Institute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 Atomistic description Cracks are barriers for oxygen vacancy diffusion  Oxygen vacancies will be more or less completely consumed in the outer cells.  → reoxidation of ZrN  High vacancy concentration in the cells between metal/oxide interface and first crack parallel (= reaction zone) to this interface.  → enhanced ZrN formation Nitrides in the oxide layer at Zry-4 (1100°C, 40min, 20% steam + 80% nitrogen Placement work of Laetitia Ott Institute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 Experiments with contradictory results Metallographic image of the nitride containing layer at the metal/oxide interface (oxidation in air at 850°C, air and argon flow rates are 12 l/h and 4 l/h, respectively). Placement  work of  Yunhwan Maeng  Institute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 Atomistic description 2 cases for the spatial extension of the starvation:  global (the whole sample surface) or  local (e.g. in cracks)   2 cases for the general degree of the starvation:  partial (only oxygen and steam starvation + nitrogen saturation) or total    (oxygen and steam starvation + nitrogen starvation) Institute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 Atomistic description Air oxidation: 2 cases of starvation conditions in cracks: Case I:  Complete starvation (both, O2 and N2 are consumed completely)             -  Consumption of O2 and N2 in the reaction zone results in a total pressure   drop in the crack. To compensate it, the same amount of air as it was   consumed has to be sucked into the crack.   → The reaction rate is completely independent of the oxide layer         thickness as well as of shape, length and cross section of the         crack.    → It depends on the oxygen + nitrogen flux   Institute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 Atomistic description Case II:  O2 starvation but N2 saturation (similar valid for steam/nitrogen oxidation)             -  Enrichment of N2 in the crack, Oxygen has to diffuse through the nitrogen  to the reaction zone. The same happens if other gases are in the mixture  (H2, Ar). However, the gas diffusion rate is high and the deceleration of the   process weak.     → The reaction rate slightly depends on the oxide layer thickness,         shape, length and cross section of the crack.   → It depend s on the oxygen flux but not on the nitrogen flux    Air oxidation: 2 cases of starvation conditions in cracks: Institute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 Proposed simplified model of the kinetics From the atomistic description of the process the following can be concluded: The deviation from the parabolic reaction kinetics occurs if a critical nitrogen concentration in the oxide is reached.       Consequences: In the case of N saturation, the smaller the O2 flow rate, the earlier is the nitride formation and with it the transition of the kinetics. Otherwise it increases with the N2 flow rate. On the basis that the size of the nitrides does not depend on the nitrogen concentration, their number NZrN is linearly proportional to the amount of absorbed nitrogen.           criticalNttNttOHOnN nstarvationNfordtnsaturationNfordtnntKnox)(2)(22*22020121 22Institute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 - The number of cracks Ncracks in the oxide formed due to the ZrN precipitation is linear proportional to the number of nitrides and with       it its interface fraction.   - The reaction rate in the cracks is exactly or nearly constant depending on the starvation conditions.            Proposed simplified model of the kinetics .constncrack Ncracks nAIF Institute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 - The effective reaction rate is the sum of the reactions via the cracks and via a more or less undamaged oxide layer weighted by their interface fractions IFcracks  and (1 - IFcracks), respectively:    with     Proposed simplified model of the kinetics  cracksparaboliccrackscrackseffective IFnIFnn  1)(2)(222222saturationNforndtnnnAstarvationNforndtnAIFcriticalNtOHOeffectivecriticalNtNcracksConsequences: in the case of only partial starvation, the smaller the oxygen flux, the higher the reaction rate because        .  In the case of total starvation  only the O2 + N2 gas flows determine the reaction rate. Therefore, the reaction rate is proportional to the O2 + N2 gas flow. paraboliccracks nn  Institute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 First experimental validations 0 500 1000 1500 2000 2500 3000050100150 Time, sm, g/m2 SF1 SF2 SF4 SF6 SF8 SF10850°C  in steam /nitrogen    in air .::22constnnn ArNOH 0 1000 2000 3000 4000 5000050100150  AF1 AF2 AF4 AF6 AF8 AF10m, g/m2Time, sInstitute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 Experimental validation 0 60 120 180 240 300 360 420 480 540 600050100150200250300350400450500 AF1 AF2 AF4 AF6 AF8 AF10m, g/m2Time, s.::22constnnn ArNO 0 60 120 180 240 300 360 420 480 540 600050100150200250300350400450500 SF1 SF2 SF4 SF6 SF8 SF10Time, sm, g/m21100°C  in steam /nitrogen    in air Institute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 Experimental validation changednconstnn ArNO  .,, 22 850°C, low gas flux  in steam /nitrogen    in air 0 1000 2000 3000 4000 5000050100150 A3PP1 A3PP2 A3PP4 A3PP6 A3PP8 A3PP10m, g/m2Time, s0 500 1000 1500 2000 2500 3000050100150m, g/m2Time, s S1PP1 S1PP2 S1PP4 S1PP6 S1PP8 S1PP10Institute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 Experimental validation changednconstnn ArNO  .,, 22 850°C, high gas flux  in steam /nitrogen    in air 0 1000 2000 3000 4000 5000050100150  A18PP1 A18PP2 A18PP4 A18PP6 A18PP8 A18PP10m, g/m2Time, s0 500 1000 1500 2000 2500 3000050100150m, g/m2Time, s S6PP1 S6PP2 S6PP4 S6PP6 S6PP8 S6PP10Institute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 Experimental validation changednconstnn ArNO  .,, 22 1100°C, low gas flux  in steam /nitrogen    in air 0 60 120 180 240 300 360 420 480 540 600050100150200250300350400450500 A3PP1 A3PP2 A3PP4 A3PP6 A3PP8 A3PP10m, g/m2Time, s0 60 120 180 240 300 360 420 480 540 600050100150200250300350400450500m, g/m2Time, s S1PP1 S1PP2 S1PP4 S1PP6 S1PP8 S1PP10Institute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 Experimental validation changednconstnn ArNO  .,, 22 1100°C, high gas flux  in steam /nitrogen    in air 0 60 120 180 240 300 360 420 480 540 600050100150200250300350400450500 A18PP1 A18PP2 A18PP4 A18PP6 A18PP8 A18PP10m, g/m2Time, s0 60 120 180 240 300 360 420 480 540 600050100150200250300350400450500m, g/mTime, s S6PP1 S6PP2 S6PP4 S6PP6 S6PP8 S6PP10Institute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 Hydrogen uptake during oxidation in steam/nitrogen mixture Mirco Grosse et al. facility ICON neutron source SINQ institution PSI Villigen resolution ~ 25 µm time per image 300 s Institute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 Hydrogen uptake during oxidation in steam/nitrogen mixture Mirco Grosse et al. Correction of the effect of oxygen:  (former work: M. Grosse et al.;  Nucl. Instr.& Meth. In Phys. Res.  A 651, (2011), 253)   Correction of the effect of nitrogen:  Using the samples annealed in synthetic air:          mean thickness of the nitride      containing layer   mgmSamO 132.0ZrNN mm 160.0ZrNInstitute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 Hydrogen uptake during oxidation in steam/nitrogen mixture Mirco Grosse et al. 0 100 200 300 400 5000,00,51,01,52,02,53,0Mass increase (%)Time [min] 0 % N2 1 % N2 80 % N2Mass increase at 800°C0 100 200 300 400 5000500Concentration (ppm)Time [min] 0 % N2 1 % N2 80 % N2Hydrogen concentrations at 800°C8 h, 0 % N2   8 h, 1 % N2          2 h, 80 % N2 Institute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 Hydrogen uptake during oxidation in steam/nitrogen mixture Mirco Grosse et al. 0 5 10 15 20 25 30 35 40 45024681012141618Mass increase at 1000°CTime (min) 0 % N2 1 % N2 80 % N2Mass increase at 1000°C0 5 10 15 20 25 30 35 40 4502004006008001000120014001600Concentration (ppm)Time [min] 0 % N2 1 % N2 80 % N2Hydrogen concentrations at 1000°C80 % N2, 5 min        20 min             40 min Institute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 Hydrogen uptake during oxidation in steam/nitrogen mixture Mirco Grosse et al. 0 5 10 15 20 25 30 35 40 45051015202530Mass increase (%)Time (min) 0 % N2 1 % N2 80 % N2Mass increase at 1100°C0 5 10 15 20 25 30 35 40 45-1000010002000300040005000Concentration (ppm)Time [min] 0 % N2 1 % N2 80 % N2Hydrogen concentrations at 1100°C40 min, 0 % N2    1 % N2           80 % N2 Institute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 Hydrogen uptake during oxidation in steam/nitrogen mixture Mirco Grosse et al. “Hydrogen pump” effect known for breakaway oxides:  - Steam penetrates into the cracks and reaches the oxide/metal interface   - Steam reacts at the interface, hydrogen remains in the cracks resulting in a high hydrogen partial pressure    - Higher hydrogen partial pressure results in a stronger hydrogen uptake by the metallic Zry (Sieverts law). The total pressure in the cracks decreases. New steam is sucked into the cracks and the process continues at the beginning. Institute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 Summary and Conclusions • The reaction of zirconium in air and/or steam/nitrogen mixtures depends strongly on the quantitative amount of oxygen and steam starvation.  • 2 times 2 cases can be divided: • by spatial extension: global or local starvation • by general degree of starvation: partial or total  • Partial starvation:  The lower the oxygen flow rate, the earlier is the parabolic to linear kinetics transition and the higher is the reaction rate.  • Total starvation: The higher the oxygen + nitrogen flow rates, the higher is the reaction rate.  Institute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 Summary and Conclusions • In the model proposed three fitting parameters have to be determined: • The critical nitrogen concentration             at which ZrN precipitation starts • The constant reaction rate in the reaction zone at the crack tip  • The coefficient A connecting nitrogen concentration and interface fraction of the cracks  • Influence of oxygen partial pressure: Investigation by varying of the argon flow while the oxygen or steam flow was kept constant.  Result: No clear influence of the reaction kinetics on the oxygen partial pressure.  • The hydrogen uptake is enhanced by the crack formation. It shows a similar behaviour like the mass gain.  criticalNncracksnInstitute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 Thanks  I thank all involved in this investigations in particular our students Fabio da Silva and Laetitia Ott. The results of their placement works improve our understanding of the parameters influencing the reaction kinetics.  Thanks to all colleagues in particular Sanggil Park for the fruitful discussions of  the mechanisms of the reactions of zirconium with air and steam/nitrogen mixture.    KIT – The research university in the Helmholtz association  KIT / Institute for Applied Materials / Program NUSAFE     www.kit.edu Experimental Investigations and Modelling of the Reaction of Zirconium Alloys in Nitrogen/Oxygen or Nitrogen/Steam Atmospheres M. Grosse, M. Steinbrueck, Y. Maeng, J. Sung Institute for Applied Materials  Program NUSAFE Mirco Grosse et al  Pohang (Rep. Korea), July 22. 2016 Proposed simplified model of the kinetics  tcriticalNOHOeffectivecracks ndtnnnAIF222    is recursive because    depends on IFracks . It can be calculated numerically by: effectiven  criticalNi ncrackscrackcracksOHOcracks ntitKiIFniIFininAiIFox)(*2))1(1()1()(2)(1)(22

Experimental investigations and modelling of the reaction of zirconium alloys in nitrogen/oxygen or nitrogen/steam atmospheres

https://publikationen.bibliothek.kit.edu/1000057797/3875546

Experimental investigations and modelling of the reaction of zirconium alloys in nitrogen/oxygen or nitrogen/steam atmospheres

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