Hydriding Induced Corrosion Failures in BWR Fuel

Transcription

1 ASTM 17th International Symposium on Zirconium in the Nuclear Industry, Hyderabad, India Hydriding Induced Corrosion Failures in BWR Fuel Dan Lutz 1, Yang-Pi Lin 2, Randy Dunavant 2, Rob Schneider 2, Hartney Yeager 2, Aylin Kucuk 3, Bo Cheng 3, Jim Lemons 4 1 Global Nuclear Fuel Americas, Sunol, CA. 2 Global Nuclear Fuel Americas, Wilmington, NC 3 Electric Power Research Institute, Palo Alto, CA 4 Tennessee Valley Authority, Chattanooga, TN

2 Outline Key features of corrosion failures Summary of failure mechanism Unresolved issues Environmental factor contribution Hydride localization under BWR conditions Results Implications of environmental factor to the BWR fleet 2

3 Corrosion Failures Browns Ferry 2, Cycle 12 ( ) 63 assemblies (9x9), starting at ~30 GWd/MTU, 7 month into 24 month cycle Only second cycle fuel failed Cladding type: Zircaloy-2 with barrier, over 3 million fuel rods experience base Failed rods from multiple ingots; initial failures involved low alloy content, common lot material operated successfully in 12 other reactors to end-of-life Two rounds of hot cell investigations for root cause and failure mechanism Browns Ferry 3, Cycle 11 ( ) 3 assemblies, near end of 3 cycle life 1 assembly discharged after 1 cycle, not corrosion related 3

4 Corrosion Features Elevated, coalesced nodular corrosion and oxide spallation Oxide + Crud thickness Mostly oxide Excessive corrosion, spallation, hydriding and the primary failure perforations near 2.4 m (95 in) and above Low corrosion at assembly spacer (Zircaloy-2 ferrule with Inconel springs) locations Large azimuthal variation in corrosion Spacer Location 4

5 Failure Mechanism Schardt et al, Fuel Corrosion Failures in the Browns Ferry Nuclear Plant, 2004 International Meeting on LWRFP, Orlando, FL, September Lutz et al, Investigation of BWR Fuel Failures, TopFuel 2012, Manchester, UK, September Elevated corrosion on cladding with inadequate corrosion resistance for the exposed environment resulting in high hydrogen pickup Spallation of oxide locally Hydrogen localization leading to hydride lens near cladding OD Cladding stressed from cladding creep down and pellet expansion Failure from cracks initiated in brittle hydride Sound Rod Failed Rod Shared similarity in corrosion characteristics with previous CILC from s: failure involved crud intrusion into oxide, thermal insulation and autocatalytic corrosion 5

6 Unresolved Issues Environmental factor for elevated corrosion Material factor alone cannot explain elevated corrosion; expect coolant water chemistry effect Coolant s role in the failures not readily identifiable from review of water chemistry data Hydrogen localization under BWR conditions Are local thermal gradients at discontinuities in oxide thickness sufficient to cause localization? 6

7 Hydride Localization Oxide plateau Oxide spalling Oxide Crud Cladding 7

8 Objectives and Scope Environmental SIMS analyses of oxide (Failed vs. sound, plants, elevation and common material) Assess hideout return investigation data (water chemistry during reactor shutdown) Hydrogen localization under BWR conditions Finite element simulation 8

9 Samples Analyzed Using SIMS Failed Rod Earlier Reload Another BWR Early discharge Light corrosion Sound Rod, Heavy corrosion Sample Assembly Rod Condition Rod Average Exposure (GWd/MTU) Reactor Insertion Date Residence Time (Days) Sample Elevation (mm/inch) Average Oxide Thickness (microns) A YJS734 H2 Failed 47.3 BF2 May ~2286 / 90 n/a* B YJK363 B3 Sound 35.1 BF2 Oct ~3023 / C YJ1380 D1 Sound 68.9 L Jul ~2413 / D YJS614 G9 Sound 34.5 BF2 May ~3023 / E 724 / YJS616 B8 Sound 41.1 BF2 May F 2311 / 91 23* 1 Cycle Rod Common Lot (to E and F) G 762 / YJN587 E9 Sound 17.0 BF3 Oct H 2362 / I 699 / YJP354 E8 Sound 44.0 H Nov J 2286 / * Severely Spalled 9

10 Li in Oxide: SIMS Line Scans Failed BF2 H2 D, Plant BF2 METAL METAL C, Plant L High Li level in failed BF2 rod Lower Li level in sound, low corrosion BF2 rod Low Li in sound rod from plant L 6 Li implies natural source METAL 10

11 Line Scans High [Li] with 6 Li in sound BF2 rod at upper elevation confirmed. Lower elevation E: BF2 B8 ~2 ppm Upper elevation F: BF2 B8 ~300 ppm 1 cycle BF3 rod shows lower [Li], higher and with 6 Li at upper elevation G: BF3 E9 <1 ppm H: BF3 E9 ~3 ppm 3 cycle Plant H rod shows low [Li]; trend with elevation reversed compared with BF2 and BF3 rods I: H E8 ~3 ppm J: H E8 <1 ppm 11

12 BF2 B8 Upper elevation SIMS Imaging OXIDE CRUD BF3 E9 Upper elevation METAL OXIDE Association of Li and B implies similar trapping site and entry path from coolant 12

13 After: Shutdown Hideout Return Chemistry at Browns Ferry-2 and Hatch-2. EPRI, Palo Alto, CA: Hideout Return, BF2 and H, Feb 2003 Plant H had cladding common to failed BF2 rods No corrosion failures at Plant H Reactor Cold Zero power Reactor Hot Zero power Reactor Hot Full power Li below detection limit at Plant H in hideout return study (detected in SIMS) Sulfate, chloride, Ca return also higher at BF2 relative to Plant H Steady state sulfate and chloride reflect hideout return results Concentration factor predicted for BWR; possible ppm level of Li in BF2 Together with SIMS results indicative of environmental factor effect 13

14 Hydrogen Localization Model Fuel Cladding Thickness, TH 0.7 mm Fuel Cladding Section Axial Height Modeled, HT 25 mm ID O/M Interface Thick Oxide Thickness, OX THICK Spall Depth or Oxide Plateau Height, OX THIN /15 15 microns microns Axial Height of Spall or Thin Oxide Plateau, OX HT 2.5 mm, Spall 0.25 mm, Plateau mm LHGR 250 (7.6) W/cm (kw/ft) (0) DD αα cccc ss Ref. [3] cccc2 (0) 3 DD δδ ss QQ αα Ref. [3] RR 4170KK Ref. [3] QQ δδ RR 5730KK Ref. [3] QQ QQ αα RR 3015KK Ref. [3] QQ δδ RR 653KK Ref. [3] TTTTTTTT TTTTTTTT ee RRRR pppppp Ref. [4] ee RRRR pppppp TT 533KK ee RRRR pppppp TT > 533KK Ref. [4] 14

15 Thermal Gradient Oxide Spalling: T increases with oxide discontinuity (OX THICK OX THIN ) Oxide thickness (OX THICK ) affects temp at ID and O/M interface T not strongly sensitive to oxide thickness (OX THICK ) Axial Height, HT/2 (mm) C 317 C 349 C 317 C 250 W/cm 60 µm oxide layer 15 µm discontinuity OD T = 5 ID T = Clad Thickness, TH (mm) ID Axial Height, HT/2 (mm) W/cm 60 µm oxide layer 45 µm discontinuity OD T = 16 ID T = Clad Thickness, TH (mm) O/M Interface Oxide Plateau: Similar trend as Oxide Spalling cases T less than corresponding Oxide Spalling cases Axial Height, HT/2 (mm) C 317 C 349 C 317 C 250 W/cm 60 µm oxide layer 15 µm discontinuity OD T = 3 ID T = 1 Axial Height, HT/2 (mm) W/cm 60 µm oxide layer 45 µm discontinuity OD T = 10 ID T = Clad Thickness, TH (mm) Clad Thickness, TH (mm) 15

16 Hydrogen Diffusion Agreement with data from Sawatzky and Vogt (Mathematics of Thermal Diffusion of Hydrogen in Zircaloy-2, AECL-1411, 1971.) 16

17 Oxide Discontinuity and Hydride Localization Oxide Spalling: Localization increases with H content and T 5 o C T can cause localization with 200 ppm H No significant localization at 100 ppm H, even at 16 o C T Localization not sensitive to oxide thickness (OX THICK ), similar T Oxide Plateau: Similar trend as Oxide Spalling cases Less localization due to lower T 3 o C T insufficient to cause localization with 200 ppm H 60/15 micron oxide/spall 60/15 micron oxide/plateau 1000 hrs 1000 hrs 60/45 micron oxide/spall 60/45 micron oxide/plateau 17

18 Hydride Localization Hydride Localization Hydride localization possible if H is high (above 100 ppm) and with sufficient T from elevated corrosion and spalling (and high heat flux) all needed for failure mechanism to operate 18

19 Implications / Discussion Corrosion failures result from inadequate cladding corrosion resistance for the environment Li in oxide associated with corrosion failures Control blade absorber tube leakage (B 4 C) can be a source of 7 Li, consistent with Li and B association but B 4 C leakage is common; presence of 6 Li implies natural source Evidence of underperforming water chemistry cleanup system, e.g. sulfate spikes indicative of degraded cation resin Since the corrosion failures, cladding corrosion resistance and water chemistry control improved (Fe & Zn control, filter efficiency, monitoring) Confirmation of environmental factor provides added assurance for preventive measures No recurrence in ~10 years 19

20 Summary Elevated corrosion associated with Li in oxide, indicating environmental factor; Li as initiator not proven Hideout return study supported higher concentration of multiple species, including Li, than in reference plant Finite element modeling confirm significant thermal gradient associated with oxide spall or plateau can cause hydride localization if sufficient hydrogen is present Failure mechanism: cracking of brittle hydride under stress; hydride localization from thermal gradient at discontinuities of thick oxide; elevated corrosion from inadequate cladding corrosion resistance under demanding environment 20