Hydriding Induced Corrosion Failures in BWR Fuel · Corrosion Features • Elevated, coalesced...

Hydriding Induced Corrosion Failures in BWR Fuel Dan Lutz1, Yang-Pi Lin2, Randy Dunavant2, Rob Schneider2, Hartney Yeager2, Aylin Kucuk3, Bo Cheng3, Jim Lemons4

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

ASTM 17th International Symposium on Zirconium in the Nuclear Industry, Hyderabad, India

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 ASTM 17th International Symposium on Zirconium in the Nuclear Industry, Hyderabad, India

Corrosion Failures • Browns Ferry 2, Cycle 12 (2001 – 2003)

– 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 (2002 – 2003) – 3 assemblies, near end of 3 cycle life – 1 assembly discharged after 1 cycle, not corrosion related


Corrosion Features • Elevated, coalesced nodular corrosion and oxide spallation

• 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


Oxide + Crud thickness Mostly oxide

Failure Mechanism • Schardt et al, “Fuel Corrosion Failures in the Browns Ferry Nuclear Plant,” 2004 International Meeting on LWRFP, Orlando, FL, September

2004. • Lutz et al, “Investigation of BWR Fuel Failures,” TopFuel 2012, Manchester, UK, September 2012.

• 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

• Shared similarity in corrosion characteristics with previous CILC from 1970 - 1980s: failure involved crud intrusion into oxide, thermal insulation and autocatalytic corrosion


Sound Rod

Failed Rod

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?


Hydride Localization


Oxide

Cladding

Crud

Oxide spalling

Oxide plateau

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


Samples Analyzed Using SIMS 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 1999 1211 ~2286 / 90 n/a*

B YJK363 B3 Sound 35.1 BF2 Oct 1997 1751 ~3023 / 119 21.1

C YJ1380 D1 Sound 68.9 L Jul 1992 2708 ~2413 / 95 21.5

D YJS614 G9 Sound 34.5 BF2 May 1999 1040 ~3023 / 119 21.9

E YJS616 B8 Sound 41.1 BF2 May 1999 1040

724 / 28.5 28

F 2311 / 91 23*

G YJN587 E9 Sound 17.0 BF3 Oct 1998 548

762 / 30 18

H 2362 / 93 17

I YJP354 E8 Sound 44.0 H Nov 1998 1498

699 / 27.5 18

J 2286 / 90 10


* Severely Spalled

Sound Rod, Heavy corrosion

Earlier Reload

Another BWR Early discharge Light corrosion

Failed Rod

1 Cycle Rod

Common Lot (to E and F)

Li in Oxide: SIMS Line Scans D, Plant BF2

C, Plant L

METAL

METAL

METAL

• High Li level in failed BF2 rod • Lower Li level in sound, low corrosion BF2 rod • Low Li in sound rod from plant L • 6Li implies natural source


Failed BF2 H2

Line Scans • High [Li] with 6Li in sound

BF2 rod at upper elevation confirmed.

• 1 cycle BF3 rod shows lower [Li], higher and with 6Li at upper elevation

• 3 cycle Plant H rod shows low [Li]; trend with elevation reversed compared with BF2 and BF3 rods


E: BF2 B8 ~2 ppm

F: BF2 B8 ~300 ppm

G: BF3 E9 <1 ppm

H: BF3 E9 ~3 ppm

I: H E8 ~3 ppm

J: H E8 <1 ppm

Lower elevation Upper elevation

SIMS Imaging

Association of Li and B implies similar trapping site and entry path from coolant

BF2 B8 Upper elevation

BF3 E9 Upper elevation


OXIDE CRUD

METAL OXIDE

Hideout Return, BF2 and H, Feb 2003

After: Shutdown Hideout Return Chemistry at Browns Ferry-2 and Hatch-2. EPRI, Palo Alto, CA: 2004. 1009448

Reactor Hot Full power

Reactor Hot Zero power

Reactor Cold Zero power

• Plant H had cladding common to failed BF2 rods

• No corrosion failures at Plant H

• 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 103 - 104 predicted for BWR; possible ppm level of Li in BF2

• Together with SIMS results indicative of environmental factor effect


Hydrogen Localization Model


Fuel Cladding Thickness, TH 0.7 mm

Fuel Cladding Section Axial Height Modeled, HT 25 mm

Thick Oxide Thickness, OXTHICK 60

30 microns

Spall Depth or Oxide Plateau Height, OXTHIN 45/15

15 microns

Axial Height of Spall or Thin Oxide Plateau,

OXHT � 2.5 mm, Spall0.25 mm, Plateau mm

LHGR 250 (7.6) W/cm (kW/ft)

𝑄𝑄𝛼𝛼∗

𝑅𝑅 3015𝐾𝐾 Ref. [3]

𝑄𝑄𝛿𝛿∗

𝑅𝑅 653𝐾𝐾 Ref. [3]

𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇 1.28 × 105𝑒𝑒−36540𝑅𝑅𝑇𝑇� 𝑝𝑝𝑝𝑝𝑝𝑝 Ref. [4]

𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇 � 1.07 × 104𝑒𝑒−21028

𝑅𝑅𝑇𝑇� 𝑝𝑝𝑝𝑝𝑝𝑝 𝑇𝑇 ≤ 533𝐾𝐾 5.26 × 104𝑒𝑒−28068

𝑅𝑅𝑇𝑇� 𝑝𝑝𝑝𝑝𝑝𝑝 𝑇𝑇 > 533𝐾𝐾

Ref. [4]

ID O/M Interface

𝑇𝑇𝛼𝛼(0) 2.17 × 10−3 𝑐𝑐𝑝𝑝

2

𝑠𝑠 Ref. [3]

𝑇𝑇𝛿𝛿(0) 1.09 × 10−3 𝑐𝑐𝑝𝑝

2

𝑠𝑠 Ref. [3]

𝑄𝑄𝛼𝛼𝑅𝑅

4170𝐾𝐾 Ref. [3] 𝑄𝑄𝛿𝛿𝑅𝑅

5730𝐾𝐾 Ref. [3] 𝑄𝑄∗

Thermal Gradient

ASTM 17th International Symposium on Zirconium in the Nuclear Industry, Hyderabad, India 15

Oxide Spalling: • ∆T increases with oxide

discontinuity (OXTHICK – OXTHIN) • Oxide thickness (OXTHICK) affects

temp at ID and O/M interface • ∆T not strongly sensitive to oxide

thickness (OXTHICK)

Clad Thickness, TH (mm)

Axi

al H

eigh

t, H

T/2

(mm

)

OD ∆T = 5ID ∆T = 5

250 W/cm60 µm oxide layer15 µm discontinuity

0 0.2 0.4 0.60

5

10

320330340


Axi

al H

eigh

t, H

T/2

(mm

)

OD ∆T = 16ID ∆T = 15


0 0.2 0.4 0.60

5

10

310320330340

317°C 349°C 317°C 349°C


Axi

al H

eigh

t, H

T/2

(mm

)

OD ∆T = 3ID ∆T = 1


0 0.2 0.4 0.60

5

10

320330340


Axi

al H

eigh

t, H

T/2

(mm

)

OD ∆T = 10ID ∆T = 5


0 0.2 0.4 0.60

5

10

310320330340

317°C 349°C 317°C 349°C

Oxide Plateau: • Similar trend as Oxide Spalling

cases • ∆T less than corresponding Oxide

Spalling cases

ID O/M Interface

Hydrogen Diffusion


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

Oxide Discontinuity and Hydride Localization


Oxide Spalling: • Localization increases with H

content and ∆T • 5 oC ∆T can cause localization

with 200 ppm H • No significant localization at 100

ppm H, even at 16 oC ∆T • Localization not sensitive to oxide

thickness (OXTHICK), similar ∆T

Oxide Plateau: • Similar trend as Oxide Spalling

cases • Less localization due to lower ∆T • 3 oC ∆T insufficient to cause

localization with 200 ppm H

60/15 micron oxide/plateau

60/45 micron oxide/plateau

60/15 micron oxide/spall

60/45 micron oxide/spall

1000 hrs

1000 hrs



• 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


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 (B4C) can be a source of 7Li,

consistent with Li and B association but B4C leakage is common; presence of 6Li 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

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


Hydriding Induced Corrosion Failures in BWR Fuel · Corrosion Features • Elevated, coalesced...

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