High-temperature oxidation and mutual interactions of materials during severe accidents in LWRs

Transcription

1 High-temperature oxidation and mutual interactions of materials during severe accidents in LWRs Seminar at the Institute of Nuclear Chemistry and Technology, 2 March 214, Warsaw, Poland Institute for Applied Materials IAM-AWP & Program NUSAFE 1 KIT University of the State of Baden-Wuerttemberg and National Research Center of the Helmholtz Association

2 Karlsruhe Institute of Technology Founded in 29 = FZK research center (1956) + University Karlsruhe (1825) = 9 employees = 24 students 2

3 M a t n S Nuclear Safety Research at KIT Safety of Nuclear Reactors Nuclear Waste Disposal Radiation Protection Nuclear Decommissioning Technologies ei n b ü c k 3

4 M a t n S Safety of Nuclear Reactors Severe Accident Research Expertise at KIT Reflood of an overheated core (CORA, QUENCH) Core melting phenomena and in-vessel corium behavior (LIVE) Direct containment heating (DISCO-H) Fuel coolant interaction (DISCO-FCI, DISCO-JET) Molten corium concrete interaction and ex-vessel corium coolability (BETA, COMET, MOCKA) Hydrogen behavior in reactor systems (HYKA) Development and application of SA code systems (MELCOR, ASTEC, WECHSL, GASFLOW, COM3D ) ei n b ü c k 4

5 QUENCH project at KIT (program NUSAFE) Investigation of hydrogen source term and materials interactions during LOCA and early phase of severe accidents including reflood CODES Application Validation Modelling Separate-effects tests Bundle experiments 5 PWR fuel element

6 QUENCH Facility Unique out-of-pile bundle facility to investigate reflood of an overheated reactor core electrically heated fuel rod simulators Extensive instrumentation for T, p, flow rates, level, etc. So far, 17 experiments on SA performed (1996-today) Influence of pre-oxidation, initial temperature, flooding rate B4C, Ag-In-Cd control rods Air ingress Advanced cladding alloys Debris formation and coolability 6 M. Steinbrück et al., Synopsis and outcome of the Quench experimental program, NED 24 (21),

7 QUENCH Separate-effects tests: Main setups INRRO TG QUENCH-SR BOX 7

8 Outline Phenomenology of severe accidents in light water reactors (LWR) High-temperature oxidation of zirconium alloys in various atmospheres Behavior of boron oxide control rods during severe accidents Silver-indium-cadmium control rod failure during severe accidents 8

9 LWR severe accident scenario Loss of coolant causes steady heatup of the core due to residual decay heat From ca. 1 C oxidation of zirconium alloy cladding becomes significant From ca. 125 C chemical interactions between the different core materials (stainless steel, Zr alloys, boron carbide ) lead to the local formation of melts significantly below the melting temperatures of the materials From ca. 18 C formation of melt pool in the core and relocation of melt/debris to the lower plenum (in-vessel, see TMI-2). Subsequently, failure of the RPV and release of corium melt into the containment (ex-vessel, see Fukushima) 9

10 Core materials in Light Water Reactors UO2(/PuO2) fuel: 1-2 t Zry cladding + grid spacers: 2-4 t Zry canister (BWR): 4 t Various steels, Inconel: >5 t (incl. RPV) B4C absorber (BWR, VVER, ):.3-2 t AgInCd absorber (PWR): 3-5 t Environment Water, steam Air Nitrogen After failure of RPV/primary circuit 1 PWR fuel assembly BWR control blade

11 High-temperature oxidation of zirconium alloys Most cladding alloys consist of wt% zirconium plus some alloying elements (Sn, Nb, Fe, Cr, ) Element Zircaloy-4 D4 M5 E11 ZIRLO Nb Sn Fe Cr <.1 In steam, oxygen, nitrogen, air, and various mixtures Temperature: 6-16 C 11

12 Oxidation of zirconium alloys chemical reactions Hf at 15 K Zr + 2 H 2O ZrO2 + 2 H 2 Zr + O2 ZrO2 Zr +.5 N 2 ZrN -585 kj/mol -183 kj/mol -361 kj/mol Release of hydrogen and heat Hydrogen either released to the environment or absorbed by Zr metal 12

13 Hydrogen detonation in Fukushima Dai-ichi NPPs main hydrogen source: zirconium - steam reaction 13

14 Oxidation in steam (oxygen) Most LOCA and SFD codes use parabolic oxidation correlations (determined by the diffusion of oxygen through growing oxide scale) 7 12 C Oxide thickness, µm 6 ZrO2 5 α-zr(o) Time, min Oxide thickness during oxidation of Zry at 12 C in steam 14 prior β-zr d ZrO2 = k m (T ) t µm 2 min at 12 C in steam

15 Breakaway oxidation Loss of protective properties of oxide scale due to its mechanical failure. Critical times and oxide thicknesses for breakaway strongly depend on type of alloy and boundary conditions (ca. 3 min at 1 C and 8 h at 6 C). During breakaway significant amounts of hydrogen can be absorbed (>4 at.%, 7 wppm) due to local enrichment of H2 in pores and cracks near the metal/oxide boundary ( hydrogen pump ). 15 Zry-4 Duplex Zirlo M5 E m/s, g/m Breakaway is caused by phase transformation from pseudo-stable tetragonal to monoclinic oxide and corresponding change in density up to ca. 15 C. 1 C in steam Time, s Zry-4 3 h, 1 C 8

16 In-situ investigation of hydrogen uptake during oxidation of Zry in steam by neutron radiography ch, wppm Breakaway oxidation time, s Zry-4, 1 C 3 g/h steam, 3 l/h argon 16 Rapid initial hydrogen uptake Further strong hydrogen absorption Grosse, 16th Intern. Symposium on after transition tom.zirconium breakaway in the Nuclear Industry (ASTM)

17 Oxidation in atmospheres containing nitrogen Late phase after RPV failure Air ingress into reactor core, spent fuel pond, or transportation cask Nitrogen in BWR containments (inertization) and ECCS pressurizers Prototypically following steam oxidation and mixed with steam Breach in the primary circuit Residual fuel elements RPV rupture Mid loop operation Open RPV lid Consequences: Significant heat release causing temperature runaway from lower temperatures than in steam Strong degradation of cladding causing early loss of barrier effect High oxygen activity influencing FP chemistry and transport 17 Spent fuel storage pool accident

18 Oxidation of Zr alloys in N2, O2 and air 25 ~ Linear oxidation kinetics 12 C m, % 2 r Ai 15 n Oxyge Parabolic oxidation kinetics 1 5 Nitrogen Parabolic reaction kinetics Time, s Oxidation rate in air is much higher than in oxygen or steam 18

19 Oxidation of Zr alloys in N2, O2 and air 1h C m, % 2 r Ai h n Oxyge Time, s 19 3h Nitrogen 8 1

20 Consequences of air ingress for cladding 1 hour at 12 C in steam 1 hour at 12 C in air Loss of barrier effect of cladding 2

21 Mechanism of air oxidation Diffusion of air through imperfections in the oxide scale to the metal/oxide interface Consumption of oxygen Remaining nitrogen reacts with zirconium and forms ZrN ZrN is re-oxidized by fresh air with proceeding reaction associated with a volume increase by 48% Formation of porous and nonprotective oxide scales initially formed dense oxide ZrO2 2 porous oxide after oxidation of ZrN 3 ZrO2 / ZrN mixture 4 α-zr(o)

22 Experiments on mechanism of nitrogen attack Temperature in C 25 Liquid L+α L+β 2 L+cub. ZrO2 L+α c=fcc t=fct m=mon. c α +cub. ZrO2-x β 15 β+α t α α +tetr. ZrO2-x 1 α +mon. ZrO2-x 5 Specimens: Pre-oxidized in t+m O2 at m 12 C ( ) ZrO Homogenized at.%o 3h at 14 C wt.%o in Ar Reaction in N2 1h at 12 C 2 Zr Zr 22 ZrO2 modif.:

23 Reaction of ZrOx with nitrogen C 12 m, % 1 8 Oxygen load: % (β -Zr).7% (β -Zr(O)) 5.7% (α -Zr(O)) 33.1% (substoich. ox.) 35% (stoichiometr. ox.) Time, s

24 Oxidation in mixed steam-air atmospheres Zry-4, 1 hour at 12 C H2O.7 H2O.3 air.3 H2O.7 air.1 H2O.9 air Increasing degradation with raising content of air in the mixture 24

25 Oxidation in mixed atmospheres 1 hour at 1 C in steam 1 hour at 1 C in 5/5 steam/n2 Strong effect of nitrogen on oxidation and degradation Nitrogen acts like a catalyst (NOT like an inert gas) Enhanced hydrogen source term by oxidation in mixtures containing nitrogen 25

26 QUENCH-16 bundle test with air ingress Hydrogen and nitrogen release during quench phase 2 H2 25 O Nitrogen consumption Temp Oxygen starvation Temperature, K Gas flow rate, mg/s N2 ZrN formation at the end of air ingress phase 5 Time, s Off-gas composition during the air ingress phase (after pre-oxidation in steam) 26 ZrN re-oxidation during quench phase

27 Absorber materials in LWRs Boron carbide AgInCd alloy Used in boiling water reactors (BWR), VVERs, some pressurized water reactors (PWR) Control rods (PWR) or crossshaped blades (BWR) Surrounded by stainless steel (cladding, blades) and Zry (guide tubes, canisters) Used in PWRs Surrounded by stainless steel cladding and Zry guide tubes Rods in Zry guide tubes combined in control rod assemblies Zry guide tube Zry guide tube Tmelt=19 C Tmelt=19 C SS cladding SS cladding Tmelt=145 C B4C Tmelt=245 C BWR control rod 27 PWR control rod assembly Tmelt=145 C BWR control blade AgInCd PWR control rod Tmelt=8 C

28 Degradation of B4C control rods (1-pellet) Post-test appearance and axial cross section of B4C/SS/Zry specimens after 1 hour isothermal tests at temperatures between 1 and 16 C ZrO 1 C 2 cap 12 C 13 C 14 C 12 C 13 C 14 C 15 C 16 C Zry-4 guide tube SS cladding B4C pellet ZrO2 cap 1 C PD 28 Melt formation Oxide shell 15 C 16 C Failure and B4C/melt consumption

29 Eutectic interaction of stainless steel with B4C 1 h at approx. 125 C Complete liquefaction of stainless steel 4 wt.% B4C 1 wt.% B4C 1/3 of SS liquefied.3 wt.% B4C 29

30 Eutectic interaction of stainless steel with B4C SS B4C 1 cm Rapid and complete melting of SS at 125 C starting at B4C/SS boundary 3

31 Oxidation of boron carbide; main chemical reactions B4C + 8H 2O( g ) 2 B2O3 (l ) + CO2 ( g ) + 8 H 2 ( g ) -76 kj/mol B4C + 6 H 2O( g ) 2 B2O3 (l ) + CH 4 ( g ) + 4 H 2 ( g ) -987 kj/mol B2O3 + H 2O( g ) 2 HBO 2 ( g ) +341 kj/mol Release of hydrogen, various carbon-containing gases and heat Formation of a superficial boron oxide layer and its vaporization 31

32 Oxidation kinetics of B4C in steam.12 H2 release rate, mole/(m²s) 12 C Framatome Framatome, low steam CODEX ESK pellet ESK pellet, low steam.1 porous pellets.8.6 dense pellets.4 3 g/h steam.2 3 g/h steam. 5 1 Time, s Strongly dependant on B4C structure and thermohydraulic boundary conditions like pressure and flow rate

33 Oxidation of B4C absorber melts Transient oxidation of B4C/SS/Zry-4 absorber melts in steam between 8 and 155 C 1 16 ZrO2 B4C SS Temperature 8 H2 release rate, l/h 5 B4C 95 SS Zry 9 B4C 81 SS 1 Zry 6 Zry Time, s before oxidation 33 after oxidation Oxidation rate during reaction of absorber melts and pure CR components in steam Temperature, C B4C 1 SS Zry

34 Degradation of B4C control blade (BWR bundle test) CORA-16 fuel 34 Complete loss of absorber blade Dissolution of cladding and fuel Massive melt relocation (B4C, SS, Zry, UO2)

35 Gas release due to oxidation of B4C (melts) Hydrogen Up to 29 g H2 per kg B4C Up to 5 kg additional H2 production for BWRs Carbon monoxide/dioxide Ratio depending on temperature and oxygen activity Non-condensable gases affecting THs and pressure CO combustible and poisonous Methane Would have strong effect on fission product chemistry (iodine!) Bundle experiments and SETs reveal only insignificant release of CH4 Boric acids Volatile and soluble in water Deposition at colder locations in the circuit 35

36 Energetic effects of B4C oxidation Oxidation of B4C in steam: 13 MJ/kgB4C Oxidation of B4C in oxygen: 5 MJ/kgB4C Significant contribution to energy release in the core For comparison: Oxidation Zr in steam: 6 MJ/kgZr Fuel value of mineral oil: 12 MJ/kgoil Fuel value of black coal: 3 MJ/kgcoal 36

37 Possible consequences for Fukushima accidents Boiling water reactors with cruciform-shaped blades 1 control blade = 7 kg B4C + 93 kg SS Complete liquefaction of the blade at T>12 C Fukushima Daiichi NPPs: Unit 1: 97 control blades Unit 2-4: 137 control blades Complete oxidation of B4C inventory by steam: 195/275 kg H2 27/38 kwh (1/14 GJ) BWR control rod 37

38 Failure of AgInCd absorber rod Ag-In-Cd control rods fail at temperatures above 12 C due to the eutectic interaction between SS and Zry-4 Failure is very stochastic (from local to explosive) with the tendency to higher temperatures for symmetric samples and specimens with inner oxidation No ballooning of the SS cladding tube was observed before rupture Burst release of cadmium vapour is followed by continuous release of indium and silver aerosols and absorber melt 38 Zry guide tube Tmelt=19 C SS cladding Tmelt=145 C AgInCd Tmelt=8 C

39 Different failure types of AgInCd absorber rod ormate durch Klicken bearbeiten Textmasterformate durch Klicken bearbeiten Zweite Ebene ene Dritte Ebene bene te Ebene Fünfte Ebene Vierte Ebene Fünfte Ebene SIC-2 (asym. rod) Local failure at 123 C 39 SIC-5 (symmetric rod) Global failure at 135 C

40 Explosive failure of SIC-11 w/o Zry guide tube No balloning before explosive failure! -3 s 4-2 s -1 s -.4 s. s

41 QUENCH-13 control rod appearance 5 mm 75 mm 54 C 17 mm 65 C 35 mm 85 C 55 mm 126 C 128 C 85 mm 1437 C 95 mm 1418 C 15 mm 1216 C No direct interaction between AIC and steel Increasing interactions between relocated AIC and Zry in gap with temp. Increasing interaction between melt and steel with increasing Zr content 41

42 QUENCH-13 bundle test: aerosol release First burst release of cadmium vapor, then aerosols mainly consisting of silver and indium 42

43 Summary Chemical interactions may strongly affect the early phase of a severe nuclear accident. The main hydrogen source term is produced by metal-steam reactions Exothermal chemical reactions can cause heat release larger than the decay heat and hence strongly contribute to the power generation in the core Nitrogen does not behave like an inert gas during the conditions of a severe accident Eutectic interactions between the various materials in the core (i.e. B4C-SS, SS-Zry) cause liquefaction of materials significantly below their melting temperatures Boron carbide may (at least locally) significantly contribute to release of heat, hydrogen and other gases 43

44 THANKS to The QUENCH team at KIT Bożena Sartowska and Wojtek Starosta for inviting me and hospitality YOU for your attention 44

45 High-temperature oxidation and mutual interactions of materials during severe accidents in LWRs Seminar at the Institute of Nuclear Chemistry and Technology, 2 March 214, Warsaw, Poland Institute for Applied Materials IAM-AWP & Program NUSAFE 45 KIT University of the State of Baden-Wuerttemberg and National Research Center of the Helmholtz Association

46 Phase diagram Zr - O Zirconium-Oxygen Phase Diagram* Temperature in C 25 Liquid L+ β 2 L+ α ZrO2 modif.: L+cub. ZrO2 L+ α c=fcc t=fct m=mon. c α +cub. ZrO2-x β 15 β+α t α α +tetr. ZrO2-x t+m 1 m α +mon. ZrO2-x 5 Zr Zr ZrO at.%o wt.%o

47 Phase diagram Zr - H 1 9 Temperature in C 8 Sieverts law: β Zr 7 6 β+δ α+β α Zr 5 δ ZrH2-x 4 3 α+δ δ + ε 1 Zr Zr Zr ph 2 with 2 47 ε ZrH2-x H = ks Zr at.%h wt.%h H/Zr ks = A e B RT

48 Phase diagram iron - boron B in wt.% C 21 L C 1538 C 15 C δ Fe C C ~97.5 Decrease of melting temperatures due to eutectic interactions 1174 C ~17 9 γ Fe FeB 11 Fe2 B Fe2B Temperature in C C α Fe 7 Fe B in at.% B

49 Basics kinetics m S = k m (T ) t n n=.5 protective, parabolic Mass gain n=.33 protective, cubic n=1 non-protective, linear n=.5 1 breakaway Time 49

50 Oxidation of Zr alloys in various atmospheres Air H2O C 5 5 O2 m, % 2 PO + Air α -Zr + N2 4 PO + N 2 15 N2 Air Time, s

51 Nitride formation under local and global oxygen starvation conditions Local oxygen starvation: Formation and re-oxidation of nitride phase at metal-oxide phase boundary Global oxygen starvation: Pre-oxidation in steam and subsequent reaction in pure nitrogen Nitride formation only in the absence of oxygen in the gas phase and in the presence of oxygen in the solid phase! 51

52 Reaction of α-zr(o) with nitrogen 12 C, 6.5 wt% O 52

53 QUENCH Program at KIT Investigation of hydrogen source term and materials interactions during LOCA and early phase of severe accidents including reflood Separate-effects tests CODES Application Validation Modelling Bundle experiments 53 PWR fuel element

54 QUENCH Separate-effects tests: Main setups Off gas to MS Specimen High-temperature furnace Gas inlet Ar/O2, N2, air Vacuum system Analytical balance m TC Thermobalance Ar + steam + H2 + CO + CO 2 + CH C 125 C (steam) 2 C ZrO2 tube Specimens: -2 cm MS coupling video system pyrometer Specimen 1-15 cm induction coil (water cooled) high frequency generator Controlled Liquid flow evaporator mixer controller 17 C Air lock CEM TC Furnace Specimen Gas flow controllers Mass spectrometer H2 Air or N2 quartz tube BOX Facility Water storage H2O Ar Gas supply system Ar H 2O Control center Oxidising, reducing atmosphere (incl. steam) two colour pyrometer or TC for calibration 15 C 54 MS coupling LAVA Furnace Inert, reducing atmosphere ZrO2 crucible Specimens: 1-2 cm W crucible (susceptor) MS coupling Specimens: 1-2 cm Transparent for neutrons Specimens: 15 cm Induction heating Zry-4 melt specimen INRRO Facility Oxidising, reducing atmosphere (incl. steam) 23 C MS coupling Computer system Induction heating ZrO2 crucible (debris catcher) Specimens: 1-2 cm Mixer QUENCH-SR Rig movable infraredpyrometer for crucible+susceptor

55 Core degradation - summary Melting temperatures [ oc] UO2 285oC B4C 245oC Zry 4 176oC SS 145oC AgInCd 8oC Melting of UO2, ZrO and (U, Zr)O2 Complete core degradation Melting of metallic Zry and α-zr(o) causes UO2liquefaction Severe core degradation Liquefaction due to eutectic interactions Late phase Early phase Local core degradation