corrosion and hydriding model for zircaloy-2 pressure ...€¦ · corrosion and hydriding model for...

Corrosion and Hydriding Model for Zircaloy-2 Pressure tubes of Indian PHWRs

S K Sinha & R K SinhaS.K. Sinha & R.K. SinhaBhabha Atomic Research Centre, Mumbai

India

ASTM-2013, Hyderabad1Feb.5, 2013

Scope of presentation

• Introduction of the component• Background • Role of analytical models in the life management

programme of pressure tubes• Modelling work• Modelling work• Results• Conclusion

Feb.5, 2013 ASTM-2013, Hyderabad 2

The pressure which is a part of the Coolant Channelassembly is structurally simple in design but operatesunder severe in-reactor environmentunder severe in-reactor environment

•(Zr-2/Zr-Nb) •(Zr-2)

(Zr+2 5Nb

PRESSURE TUBE

•(Zr+2.5Nb+0.5Cu)

•[573 K]•[350 K]

•(SS 403)

1 2

220 MWe PHWR: 306 Channels, typically 5.2 m PT Length & 83 mm PT ID

Temp = 250 – 300 C, Pressure = 10 MPa, Neutron flux = 2.5 x 1017 n/m2/sec

Feb.5, 2013 ASTM-2013, Hyderabad

3

540 MWe PHWR: 392 Channels, typically 6.2 m PT Length & 104 mm PT ID

The components have limited service life due to operatingcomplex in-reactor material degradation mechanisms.

Mechanisms PT (Zr-2/ Zr-2.5Nb) CT (Zr-2/ Zr-4)

GS (Zr-2.5Nb-0.5Cu)

EF (SS-403)

Fast neutron Irradiation Enhanced Creep & G th

Dimensional changesSagging

Sag, Axial force on End Shield (?)

Relaxation of tight-fit (?)

& Growth (?)In-service Corrosion & H d idi

• Local & Gross Hydride Embrittlement

H d id bli t D l d

(?) Hydride Reorientation

Hydride

(Hydrogen migrationHydriding • Hydride blisters, Delayed

hydride cracking (DHC)• Reduction in fracture

toughness

, Hydride blisters,DHC (?)

migration to PT ends)

toughness Fast neutron Irradiation Enhanced

Lowering of fracture toughness and ductility

Yes Yes Yes

4

Embrittlement

ASTM-2013, HyderabadFeb.5, 2013

In early 90s, Five units having Zircaloy-2 pressure tubes and loosefit design of garter springs were in the early years of operations ( 3

4 FPYs)– 4 FPYs).

S f ti f th it• Safe operation of the units• Life limiting issues

– PT-CT contact as result of shift of garter spring spacersPT CT contact as result of shift of garter spring spacers– Accelerated corrosion and hydriding– Possibility of hydride blistering at the contact locations

• Constriants– Life management programme was under initial stage of– Life management programme was under initial stage of

development.– Un-inspected reactor core


Efforts were put on to develop methodology for assessment of fitnessfor service of the pressure tubes of these units by developing andintegratingintegrating

Analytical Models- SCAPCADiagnostic - SCAPCA- BLIST- HYCON-DELHYC

and Inspection Tools

- NIVDT C- NIVDT- BARCIS-WEST

Post Irradiation ExaminationAssessment Examination

- Microstructure- Mechanical Testing- H analysis

Methodology

H analysis-DHC velocity-Fracture toughness- Non-Destructive Life Extension

Feb.5, 2013ASTM-2013, Hyderabad

6

ExaminationTools– INGRES

Life management approach for an un-inspected reactor coreinvolved greater role of analytical models to reduce the inspectionload.

Conservatively analysed withUn-inspected Core

Conservatively analysed withworst possible inputs toidentify the pressure tubesmost likely to have hydridey yblisters at contact locationsfor the targeted time

Diagnostic andmonitoring toolsconfirmed theLikely hood of possibility ofhydride blisters atcontact locationsISI confirmed PT-CT

contacts

nucleation & growth of hydride blisters established anal ticall

Feb.5, 2013 ASTM-2013, Hyderabadwith 7

contactsanalytically

Analytical models developed for in-reactor degradationmechanisms for pressure tubes as a part of life managementprogramme had the key roles inprogramme had the key roles in

• reducing inspection burden• helping in taking safety related decisions

Built-in margin of conservatism initially provided to compensate for the lack of complete knowledge was progressively reduced with in-flow of knowledgein flow of knowledge.

8

Facts related to detrimental effects of hydrogen onZirconium alloy pressure tube materialy p

• Absorption of hydrogen in Zirconium alloys and its deleterious effect onstructural properties was identified as early as 1953.

• Most of Failures in pressure tubes in the history of CANDU type PHWRsoccurred in 70s and 80s have hydrogen as the root cause. Someexamples are– DHC at rolled joints

• 1974, Pickering-3, 17 Nos, g ,• 1975, Pickering-4, 61 Nos• 1982, Bruce-2, 2 Nos

– Blister formation at cold spots, cracking of series of blisters andBlister formation at cold spots, cracking of series of blisters andsudden rupture of PT

• 1983, Pickering-2 G16


Thus, hydriding was identified as an important lifelimiting parameter of Zircaloy 2 pressure tubes andlimiting parameter of Zircaloy-2 pressure tubes andtherefore required continuous monitoring.


10

Hydrogen ingress in pressure tube under operating conditionscan happen through various sources

• As an impurity in the as manufacturedpressure tube

• Corrosion reaction of zirconium alloyt b ith hi h t t h

This accounts foralmost all the in-reactor hydrogen

pressure tube with high temperature heavywater coolant

• Radiolysis of heavy water coolant

pick-up andnumerical modelsimulates theki ti f thi• Radiolysis of heavy water coolant

• Diffusion of hydrogen from the outer surfaceof pressure tube

kinetics of thisreaction.

p

• Diffusion of hydrogen from the SS Endfitting near the rolled joint region

ASTM-2013, Hyderabad 11Feb.5, 2013

Characteristics of corrosion & hydrogen pick-up inZircaloy-2 pressure tubeZircaloy-2 pressure tube

• Flat profile along the pressure tube length till certain oxide thickness (critical thickness or transition thickness)A l t d i• Accelerated corrosion

• Hydrogen pick-up profile follows oxide thickness profile

• In a typical high flux channel, the transition occurs approximately around 7 years of operation.


Oxide and hydrogen pick-up variation in one of pressuretubes subjected to post-irradiation examination

Peak value

Peak value

Pre transition Pre transition

Peak value

Plateau region Pre transition Plateau region

Salient features• Plateau region• Transition point


13

• region of acceleration• Peak point.

Oxidation Kinetics and Hydrogen pick-upOxidation Kinetics and Hydrogen pick-up

ASTM-2013, Hyderabad14Feb.5,

2013

Oxidation Kinetics & Hydrogen pick-up

• Zircaloy-2 (two stage kinetics)– Cubic pre-transition {dw3 =K *t}

K = A*exp(-Q/RT)Where, A t tCubic pre-transition {dw =Kpre t}

– Linear post-transition {dw = Kpost*t}

Wh

A = constantQ = Activation energyT = Temperature

Where, dw = oxide weight gainKpre = pre-transition rate constant

Hydrogen pick-up = fpup * Hcor(dw)

Wherep

Kpost = Post-transition rate constnatKp = parabolic rate constantt = time

Where,Hcor(dw) = Hydrogen evolved to

form ‘dw’ oxide f = fraction of Hydrogent time fpup fraction of Hydrogen

evolved picked-up by metal


Indian observations on Irradiation effect on Oxidation Kinetics

• Zircaloy-2– Out of pile pre-transition rate enhanced but no noticeable effect of

neutron fluence until the transition– Dominant effect of neutron flux during post transition

• Effect of flux during post transition corrosion is taken care of by increasing the rate constant by Kpost which is defined aspost

Kpost = C1**Exp(-Q/RT)

Where,Kpost = Increase in rate constantC1 = Constant(n/cm2-sec) = Neutron fast flux (>1 MeV)( ) ( )


PIE Data on oxide and hydrogen pick-up in Zircaloy-2Pressure tubes removed from Indian PHWRs

• Nos of PTs: 8• FPYs Range: 4.8 – 8.5

Units FPYs Nos of TubesMAPS-1 6 24 – 7 0 2MAPS-1 6.24 – 7.0 2MAPS-2 4.8 1RAPS –2 8.25 – 8.5 5


Steps involved in the development of Model

• Two stage oxidation kinetics have been implemented in the model The formTwo stage oxidation kinetics have been implemented in the model. The formof the correlations used are

– dwpre3= Apre*Exp (-Qpre/RT) * t (1)

dw = (A +C *)*Exp( Q /RT) * t (2)

[1] Billot Ph. And Giordano A., “ Comparison of Zircaloy Corrosion Models from the Evaluation of In-

t d O t f Pil L– dwpost = (Apost+C1 ) Exp(-Qpost/RT) t (2)

• Assumption: Qpre and Qpost are same[1]reactor and Out-of-Pile Loop Performance”, ASTM STP 1132,

Apre, Qpre

Oxide thickness < critical thickness

Available PIE data on Oxide thickness

Hydrogen pick-up fraction during pre and post transition

Critical oxide thickness for rate transition (?)

Measured Hydrogen pick-up

Apost, C1

post transition

Oxide thickness > critical thickness

ASTM-2013, Hyderabad 18

Qpost

Feb.5, 2013

Determination of Critical Oxide Thickness-1/2


2013

Determination of Critical Oxide Thickness-2/2

Observations:

T i i i ki l d 10 Transition is taking place around 10 microns.

ASTM-2013, Hyderabad20

Feb.5, 2013

Determination of constants and activation energy forpre-transition stage

Pre-transitionAverage Flux (n/cm2-s)

Apre Qpre (J/mole-K)

Oxide thickness <= 10 micronswere subdivided in differentgroups depending upon the fast

( )

7.87E+12 2.86E+09 87772.7 groups depending upon the fastflux at the axial location towhich the data belonged.

7.87E+12 2.86E+09 87772.7

1.02E+13 2.35E+11 109170.6

1 25E+13 2 20E+06 53515 1 Constants and Activation energy

evaluated for different groupshave wide range and do not

1.25E+13 2.20E+06 53515.1

1.72E+13 2.86E+09 87430.7

show any correspondence withfast flux.

1.49E+13 7.81E+02 19441.6

2.12E+13 3.11E+02 16336.7


Comparison of pre-transition rate constant of Indian pressuretubes with the Hilner out of pile rate and the pre-transitionrate observed in the N-reactor PTrate observed in the N reactor PT

Corrosion rateconstant is

• 1000 times theout of pile rate (N–reactor PT)

• 25 times the outof pile rate (IndianPHWR)


Determination of constants and activation energy forpost-transition stage

Post transition corrosion• Oxide thickness > 10 micronsOxide thickness > 10 microns• Activation energy during pre-transition and post-

transition are assumed to be same. []• Flux effect is taken into account by assuming the• Flux effect is taken into account by assuming the

rate constant to be a linear function of the flux.

Values of Constants

Constants ValuesApre 2.0E12Qpre /Qpost 113369Apost 1.5E10


C1 0.0243Feb.5, 2013

Methodology adopted in the evaluation of pick-upfractions

Pick-up fraction =Hydrogen absorbed in the metal

Hydrogen evolved during y g gcorrosion reaction

Pick-up fraction for the pre-transition stage wereevaluated from the data of MAPS and RAPS pressureevaluated from the data of MAPS and RAPS pressuretubes.

Post transition pick-up fraction was evaluated fromp pRAPS pressure tubes only.


Evaluation of Pick-up fractions for the pre-transitionstage


Feb.5, 2013

Pick-up fractions for the pre-transition stage: someobservations

Large scatter but is conspicuously showing adecreasing trend with flux

95% confidence line is considered in the modelfor evaluation of pick-up fraction.


Evaluation of Pick-up fractions for the post-transitionstage

0.9

1.0 Pick-up fractions estimated

from measured data Best fit line

0 6

0.7

0.8

ract

ion Observations

0.4

0.5

0.6

n pi

ck-u

p fr

Large scatter

Mean line also does notshow much variation.

0.2

0.3

Hydr

ogen

Max. Pick-up fraction valueis 0.85 and mean value is0.6.

0.5 1.0 1.5 2.0 2.5 3.00.0

0.1


Neutron flux(>1 MeV)*1.0E13 (n/cm2-sec)Feb.5, 2013

Application of the model for evaluating oxide thicknessgrowth and the resulting hydrogen pick-up in theoperating pressure tubes requires its incorporation in aoperating pressure tubes requires its incorporation in aComputer programme which

Coolant temperatureTime, oxide thi kComputer programmePressure tube fast

Neutron fluxthickness, hydrogen pick-up

Reactor operating history (time, capacity factorcapacity factor, availability factor)


Comparison of Model predictions with the hydrogenComparison of Model predictions with the hydrogenpick-up measured in the sliver samples removed frompressure tubes of RAPS unit 1, NAPS unit 1.p ,


2013

RAPS-1: 10.6 HOYs

40

50 RAPS1 (10.6 HOYs)

Estimation based on pre-transition pick-up fraction derived from RAPS pressure tubes

Hydrogen pick up measured by DSC

30

40

p (p

pm)

Hydrogen pick-up measured by DSC Hydrogen pick-up measured by HVEQMS

20

gen

pick

-up

10Hyd

rog

100 150 200 250 300 350 4000

Distance(cm) as measured from pressure tube inlet end


Distance(cm) as measured from pressure tube inlet end

Feb.5, 2013

NAPS – 1: 9.8 HOYs


2013

NAPS – 1: 10.8 HOYs


2013

Conclusion

The model developed gives an upper bound envelop withThe model developed gives an upper bound envelop withreasonable conservatism.

It h l d i ifi tl i f ti f th it ithIt helped significantly in safe operation of the units withzircaloy-2 pressure tubes for ~12 years.

The last of the units was taken for retubing ~3 years back.


2013

ACKNOWLEDGEMENTSACKNOWLEDGEMENTS

• Shri J.N. Kayal, REDy ,• Shri B.B. Rupani• Shri K. Madhusoodnan, RED• All the Colleagues in PIED• All the Colleagues in PIED


2013

Thanks for kind attentionThanks for kind attention


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