CHARACTERIZATION OF OXYGEN DISTRIBUTION IN LOCA SITUATIONS

Transcription

1 CHARACTERIZATION OF OXYGEN DISTRIBUTION IN LOCA SITUATIONS Duriez C. 1, Guilbert S. 1, Stern A. 2, Grandjean C. 1, Bělovský L. 3, Desquines J. 1 1 IRSN ² IRSN post-doctorate, now at CEA 3 ALIAS Cz

2 Scope of the presentation DIFFOX: IRSN develops a simulation code aiming at modeling the oxygen profile within a cladding submitted to a LOCA transient. Validation of High Temperature oxidation Process: Oxidation kinetics and oxygen content profile compared to literature data Two main areas for DIFFOX improvement have been studied: Low Temperature oxide dissolution process: Influence from bare up to 70 microns thick LT oxide layer Influence of hydrogen on the Zy-O phase diagram: Effect of 600 ppm hydrogen content on the a/a+b and a+b/b transus 16th International Symposium on the Zirconium in the Nuclear Industry Chengdu, China 2/ 22

3 DIFFOX: main modeling options of the code latest DIFFOX version ZrO 2 a(o)-zr a(o)+b(o)-zr b(o)-zr Single and two-sided oxidation, Thermal transients modeled, Solving of diffusion equation with moving boundaries (1D, implicit finite differences), LT and HT oxide layers modeled, LT-ZrO2 HT-ZrO2 Partial dissolution of LT oxide layer during the HT transient, ZrO 2, a(o)-zr, b(o)-zr, a(o)+b(o)-zr layers modeled, Diffusion coefficients mainly derived from literature review, Influence of hydrogen on Zy-O phase diagram boundaries modeled: Temperatture ( C) b Zr a+b Zr Expected influence of H a Zr [H]=0 [H] O (Wt %) 16th International Symposium on the Zirconium in the Nuclear Industry Chengdu, China 3/ 22

4 DIFFOX: Oxide scale modeling high-temperature oxide low-temperature oxide Metal tetragonal sub-layer (<1200 C) monoclinic (<850 C) or mon + tet ( C) or tetragonal (>1200 C) monoclinic (<1100 C) or mon + tet (>1100 C) outer oxide surface t-zro 2 bulk oxide pre-oxide oxide (total ZrO 2 thickness) Validation on Leistikow C isothermal tests (1987) The development and validation of DIFFOX are major issue for IRSN 16th International Symposium on the Zirconium in the Nuclear Industry Chengdu, China 4/ 22

5 Tested materials Zirconium alloy: Stress Relieved Annealed (SRA) low-tin Zircaloy-4 tubes from CEZUS Low Temperature (LT) corrosion: Specimens were LT pre-oxidized at 500 C under pure oxygen to assess the influence of in-service corrosion 10 to 60 days exposure leading to 10 to 70 micrometers thick pre-oxide layers Example: 60 days exposure Bright light Polarized light 100 µm Hydriding: 600 wppm electrolytic hydriding of 50 cm long samples performed at EDF (160h) Hot extraction at 4 different locations confirmed values ranging between ppm. 24 h at 430 C heat treatment to achieve a quite uniform H distribution 16th International Symposium on the Zirconium in the Nuclear Industry Chengdu, China 5/ 22

6 High Temperature oxidation: protocol validation on bare Zy-4 Steam generator 1200 C 1100 C Ar 200 Billone 2008 (US-NRC) Hozer 2008 (KFKI) Ozawa 2000 (NDC) Kawasaki 1978 (JAERI) Brachet 2001 (CEA) present study 1000 C furnace Sample holder with alumina sample Weight gain (g/m²) Cathcart-Pawel correlation C quenching bath (water) Vertical furnace for HT oxidation Durations (s) Comparison to literature data The HT oxidation protocol gives oxidation kinetics consistent with literature data between 900 and 1200 C for a wide range of oxidation durations 16th International Symposium on the Zirconium in the Nuclear Industry Chengdu, China 6/ 22

7 HT dissolution of the LT oxide layer 16th International Symposium on the Zirconium in the Nuclear Industry Chengdu, China 7/ 22

8 HT dissolution of the LT oxide layer: main objectives Safety concern: The HT dissolution of the LT oxide layer potentially induces significant oxygen charging in the sample and can have a deleterious impact on the Post-Quench-Ductility. DIFFOX: The DIFFOX code has the potential of simulating this LT oxide layer dissolution but requires validation data. Experiments performed for validation of DIFFOX LT oxide layer dissolution modeling: HT vacuum annealing of pre-oxidized samples (maximizing the dissolution process), HT steam annealing of pre-oxidized samples (competition between HT oxidation and HT dissolution). HT conditions (900 C-6000s): At the present stage experimental conditions don t address the LOCA safety issue but the DIFFOX validation issue. The HT heat treatment is chosen to enhance dissolution of the LT oxide. 16th International Symposium on the Zirconium in the Nuclear Industry Chengdu, China 8/ 22

9 Oxidation: High Temperature oxidation of as-received samples temperature HT: 900 C 6000 s steam no LT ox time Post-quench optical metallography: As received Zy-4 + HT oxidation 100 µm 100 µm ZrO 2 and azr(o): layer thickness measurement at 8 angular locations (both: inner and outer layers): e ZrO2 = 18.5 ± 0.7 mircons e azr(o) = 34.5 ± 1.0 microns EPMA radial O concentration profiles and O distribution maps (O Ka line) Average H content measurements in the sample by hot extraction : negligible A similar procedure was systematically applied to all tested samples 16th International Symposium on the Zirconium in the Nuclear Industry Chengdu, China 9/ 22

10 HT Dissolution of the LT oxide layer: HT vacuum annealing Oxidation: temperature HT: 900 C 6000 s 500 C LT ox vacuum Oxygen Post-quench optical metallography: Pre-oxidized + HT vacuum ann. time 10 mm LT ZrO 2 20 mm LT ZrO 2 30 mm LT ZrO 2 40 mm LT ZrO 2 60 mm LT ZrO 2 70 mm LT ZrO 2 50 mm ZrO 2 and azr(o): layer thickness measurement at 8 angular locations (both: inner and outer layers): De ZrO2 = - 5 to -10 microns e azr(o) ~ 35.0 microns 16th International Symposium on the Zirconium in the Nuclear Industry Chengdu, China 10/ 22

11 HT Dissolution of the LT oxide layer: HT steam oxidation Oxidation: Post-quench optical metallography: LT oxide temperature 500 C LT ox Oxygen dark brown regions HT: 900 C 6000 s steam time Example: 20 mm LT ZrO 2 Pre-oxidized + HT oxidation 0.5 mm light brown regions LT oxide 25 mm 25 mm HT oxide ZrO 2 and azr(o): layer thickness measurement De ZrO2 ~ -5 microns De ZrO2 = +20 microns e azr(o) ~ 40.0 microns e azr(o) ~ 30.0 microns 16th International Symposium on the Zirconium in the Nuclear Industry Chengdu, China 11/ 22

12 HT Dissolution of the LT oxide layer: DIFFOX vs experiments 20 mm LT ZrO 2 followed by HT steam oxidation (900 C 6000 s) 30 Region with no HT oxide 30 Region with HT oxide DIFFOX simulation DIFFOX simulation Oxygen concentration (wt%) MeasuredO-profile: 20 mm LT ZrO 2 + HT steam oxidized Oxygenconcentration ( Oxygen concentration (wt%) Measured O-profile: 20 mm LT ZrO 2 + HT steam oxidized Distance to outer surface (µm) Distance to outer surface (µm) DIFFOX predicts a protective LT ZrO 2 layer No HT oxide expected, azr(o) layer thickness over-predicted in regions with HT oxide 16th International Symposium on the Zirconium in the Nuclear Industry Chengdu, China 12/ 22

13 HT Oxidation of as-received samples: DIFFOX vs experiments As-received cladding HT steam oxidation (900 C 6000 s) 30 Oxygen concentration (wt%) DIFFOX simulation Measured O- profile: as- received + HT steam oxidized Distance to outer surface (µm) Very good code-to-data agreement Polishing reveals only part of the azr(o) region 16th International Symposium on the Zirconium in the Nuclear Industry Chengdu, China 13/ 22

14 HT Dissolution of the LT oxide layer: DIFFOX vs experiments 20 mm LT ZrO 2 followed by HT vacuum anneal (900 C 6000 s) 30 Oxygen concentration (wt%) DIFFOX simulation Measured O- profile: 20 mm LT ZrO 2 + HT vacuum annealed Distance to outer surface (µm) Very good code-to-data agreement 16th International Symposium on the Zirconium in the Nuclear Industry Chengdu, China 14/ 22

15 Investigating the Influence of H on Zy-O phase diagram 16th International Symposium on the Zirconium in the Nuclear Industry Chengdu, China 15/ 22

16 Expected influence of Hydrogen on the oxygen distribution O content Temperatture ( C) b Zr a+b Zr a Zr(O) O (Wt %) 5 [H]=0 [H] 0 [H] [H] ZrO 2 azr(o) bzr Distance to the outer surface Hydrogen shifts the a/(a+b) and b/(a+b) boundaries to higher O concentrations: O solubility in the bzr phase is increased Few experiments addressing influence of hydrogen, It was thus decided to improve the description of this effect in DIFFOX code. 16th International Symposium on the Zirconium in the Nuclear Industry Chengdu, China 16/ 22

17 Strategy to evaluate the influence of 600 ppm H Temperature ( C) bzr [H]=0 a+b Zr azr(o) O (wt%) Expected influence of 600 wt ppm H a+b Zr expected a Zr expected (a+b) phase tests a phase tests 3 temperatures 21 samples tested 16th International Symposium on the Zirconium in the Nuclear Industry Chengdu, China 17/ 22

18 temperature 1050 C - 5 min Vacuum (quartz vial) Testing procedure steam 10 hr at 1000 C 3h at 1100 and 1200 C (CEA procedure) 3 temp. tested: 1000, 1100 and 1200 C Vacuum (quartz vial) Water quench (a+b Zr samples only) time Analysis performed Electrolytical H Charging at EDF H measurement (HE) H homogenization + Zr texture removal H,O measurement (HE) O measurement by weight gain O charging by sample oxidation duration adjusted (Cath.Paw.) H,O measurement (HE) O measurement by weight gain O dissolution and homogenization Melting point of ZrO 2 too hot H,O measurement (HE), Weight gain (not accurate), EPMA measurements (homogeneity + distribution) Perturbated by matr l loss not reliable [H] and [O] contents are used to check that the vials keep tight during HT steps 16th International Symposium on the Zirconium in the Nuclear Industry Chengdu, China 18/ 22

19 Determination of phase boundaries: After steam oxidation 25 Oxygen concentration (wt%) Measured O profile (#26 sample): steam oxidation 60 s at 1200 C DIFFOX simulation Oxide azr(o) Prior-bZr temperature 1050 C - 5 min Vacuum (quartz vial) steam 10 hr at 1000 C 3h at 1100 and 1200 C (CEA procedure) 3 temp. tested: 1000, 1100 and 1200 C Vacuum (quartz vial) Water quench (a+bzrsamples only) time a/(a+b) (a+b)/b Distance to metal / oxide interface (mm) Assuming slow diffusion of oxygen at RT, phase boundaries can be derived from EPMA profile after steam oxidation (before homogenization process), Low accuracy because EPMA measurement is not accurate at the interface Some improvements are still needed in the DIFFOX code. 16th International Symposium on the Zirconium in the Nuclear Industry Chengdu, China 19/ 22

20 200 µm Determination of phase boundaries: after homogenization Sample # 20 O concentration (wt %) Distance to the external surface (mm) O azr(o) prior-bzr temperature 1050 C -5 min Vacuum (quartz vial) steam 10 hr at 1000 C 3h at 1100 and 1200 C (CEA procedure) 3 temp. tested: 1000, 1100 and 1200 C Vacuum (quartz vial) Water quench (a+bzr samples only) time Fe Cr At equilibrium: direct measurement of O contents in both azr(o) and prior bzr Fe and Cr segregates to the b-phase during HT treatment 9/11 16th International Symposium on the Zirconium in the Nuclear Industry Chengdu, China 20/ 22

21 Determination of phase boundaries: Results Summary Chung & Kassner, 0 ppm H (1979) b Zr b/(a+b) a/(a+b) T ( C) a+b Zr This work, on oxidized This work, on homogenized Erickson ~600 ppmh (1964) Brachet 600 ppm (2001) 800 a Zr(O) O concentration (wt%) The results are consistent with expectations Some scatter in the data 16th International Symposium on the Zirconium in the Nuclear Industry Chengdu, China 21/ 22

22 Summary Overall agreement on the key role played by O-distribution during a LOCA transient DIFFOX is the IRSN calculation code dedicated to this assessment A set of experiments dedicated to the influence of LT oxide layer dissolution was performed showing that: - the LT oxide layer can be both protective or not, - DIFFOX calculations are consistent with a protective layer The influence of 600 ppm H on the Zy-O phase diagram was studied and new modeling data is provided. Further studies Further studies are required to provide validation data in the duration range of a LOCA (including shorter steam exposure and wider temperature range). Detailed influence of hydrogen on Zy-O phase diagram (300 and 1000 wppm) Transient oxidation-cooling experiments are planned to increase the validation field of DIFFOX 16th International Symposium on the Zirconium in the Nuclear Industry Chengdu, China 22/ 22