'Crack Propagation Analysis for Circumferential Cracks in ...

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CRACK PROPAGATION ANALYSIS -

FOR CIRCUMFERENTIAL CRACKS ,

IN ALLOY 600 N0ZZLE SAFE-ENDS:

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TR-MCC-307

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OCTOBER 1993

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ABB COMBUSTION ENGINEERING NUCLEAR SERVICES ,

COMBUSTION ENGINEERING, INC.~

1000 PROSPECT HILL ROAD

WINDSOR, CONNECTICUT '

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9310220100 931015 EL *

PDR ADOCK 05000255 hP PDR k;-

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-TABLE OF CONTENTS ,|;

TITLE PAGE

INTRODUCTION 1 ,

CRACK GROWTH RATE MODEL 1

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CRACK GE0 METRIC AND PROGRESSION ASSUMPTIONS 2-

LOADING AND INITIAL CONDITION ASSUMPTIONS .3

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INPUT TO COMPUTATIONS 3

RESULTS 3

REFERENCES 4

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INTRODUCTION4

This report documents crack propagation studies performed for selected Alloy600 components in the pressurizer of the Palisades NSSS. The componentsinclude:

e Safety Valve Flangee PORV Safe-End

e Surge Line Safe-Ende Spray Line Safe-End

The assumed mechanism (based on recent failure analysis results) for crackpropagation in all analyses is primary water stress corrosion cracking(PWSCC). The methodology used in these studies was developed for specificapplication to the problem of crack propagation in CEDM nozzles (Reference 1).

CRACK GROWTH RATE MODEL

The basic crack propagation model used in the analyses is based on a subset ofthe Alloy 600 data published by Smialowska et.al. (Reference 2). The onlydata specifically excluded were high (>7 ppm) lithium data not representativeof Palisades primary chemistry. All experiments utilized in the finalcorrelation analysis were performed at a temperature of 330*C.

The crack growth rate correlation is given by:

in di - A + B in in (K-C)dt

where: dl = crack growth rate (m/s)dt

A = -25.942B = 3.595K = Applied Stress Intensity (MPa s/ m)

C-Kisce - 0 (for this case)

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The crack growth rate versus stress intensity is shown in Figure 1. It shouldbe noted that no explicit threshold stress intensity is required for crackpropagation in this correlation form. This feature was incorporated to permitinvestigation of crack growth for CEDM nozzles in which observed cracks wereextremely shallow (<.4 mm).

Two corrections are necessary to the growth rate correlation for applicationto the Palisades study. The first involves component temperature. The effect t

of temperatures unequal to 330*C is accommodated using an Arrhenius rate lawwith an activation energy of 33 Kcal/ mole. This is consistent with Reference1. The second correction reflects the effect of the prior cold work presentin the Smialowska data. Scott (Reference 3) has suggested reduction in growthrate by a factor of between 5 and 10. A factor of 5 is utilized in the

present work.

CRACK GE0 METRIC AND PROGRESSION ASSUMPTIONS

One-dimensional Linear Elastic Fracture Mechanics (LEFM) models were used inconjunction with the crack rate correlation to compute crack progression. Theassumed configuration for the initial crack is an embedded semi-ellipticalsurface flaw with Mode I opening. The applied stress is computed using theequation given in Figure 2.3.3 of Reference 4.

K, - 1.95g v' a/$

where K, - applied stress intensity

rr - section tensile stress

a - flaw depth

Q - complete elliptic integral of the second kind

Evaluation of crack growth is performed by numerical integration using thecrack growth rate correlation and the stress intensity equation. The requiredinput includes operating temperature, initial flaw depth and initial stressintensity, which in conjunction with the two governing equations completely

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define the mathematical system. |

LOADING AND INITIAL CONDITION ASSUMPTIONS ]|

In the absence of detailed finite element computations, an assumption of netsection yield was made for all components. No credit was taken for reductionof yield strength with temperature. This assumption is consistent, however,with finite element analysis results documented in Reference 1 for CEDMnozzles in the partial penetration weld region.

Initial flaw depth was assumed to be approximately 3 mils, consistent with )results of detailed experiments performed by Boursier et.al. (Reference 5) in 1

which various regimes of PWSCC crack growth were studied for Alloy 600. This-assumed depth is deemed appropriate for separating the initiation andpropagation parts of the PWSCC phenomenon and probably represents thebeginning of the true fracture mechanics regime.

lThe final major assumption concerned the initial (and final) aspect ratio of _

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the flaw. For the purposes of the Palisades components a range of flawaspects was used ranging from a semi-circular (2/1) to a long semi-elliptical(6/1) aspect ratio flaw. ,

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INPUT TO COMPUTATIONS j|

The input for each Palisades component is given in Table 1. In the case of j

the PORY Fafe-End computations were performed for two levels of appliedstress. The first corresponded to the 77 KSI yield strength cited in theCERTS for the material. The second corressponded to a 40 KSI assumed value ,

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obtained from material near or in a softened weld heat affected zone. For the~

other components CERT yield strength values were used in the analysis.

RESULTS

The results of the crack growth assessment for the Palisades PORV safe-end areshown in Figure 1 for the 77 KSI applied stress and in Figure 2 for the 40 KSI

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appl.ied stress. The safety valve safe-end crack growth assessment is shown inFigure 3 for a 77 KSI applied stress. The surge line and spray line safe-endcrack growth assessment, are shown in Figures 4 and 5 respectively.

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Table 2 is a derivative of the information contained in Figures 1-5. The,

crack depths shown in the table represent the largest cracks which wouldtheoretically permit operation for 15 effective full power months prior to

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throughwall propagation. The Palisades PORV Safe-End case at 77 KSI appliedstress is the most limiting with a required crack detection threshold of 40mils.

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'REFERENCES

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1. ABB Combustion Engineering, " Safety Evaluation of the Potential for and;'

Consequences of Reactor Vessel Head Penetration Alloy 600 ID InitiatedNozzle Cracking," CEN-607, May 1993.

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2. Smialowska, Z. S. et a1. , " Effects of pH and Stress Intensity on CrackGrowth Rate in Alloy 600 in Lithiated and Borated Water at High I

Tempertures," Proceedings, Fifth International Symposium on,

Environmental Degradation of Materials in Nuclear Power Systems - WaterReactors, Monterey, Aguust, 1991.

3. Scott, P. M., "An Analysis of Primary Water Stress Corrosion Cracking in '

PWR Steam Generators," Proceedings, Specialists Meeting on OperatingExperience with Steam Generators, Brussels Belgium, September 1991.

4. R. L. Cloud et al., Brittle Fracture Desian Guide, ASME,1971. |

5. J. Boursier et al., " Stress Corrosion Cracking of Alloy 600 in Water: !

Influence of Strain Rate on the Different Stages of Cracking," EUROCORR'92, June 1992.

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|Table 1

INPUT FOR PALISADES CRACK STUDIES.

Iinitial Stress intensity

W all Applied Crack TemperatureComponent Thickness Stress Depth . (*F)' 2/1 4/1 6/1

(in) (KSI) (in) (KSI V in) -;

PORV .438 77 0.0032 640 5.36 7.08 7.58Safe-End

PORV .438 40 0.0032 640 2.78 3.68 3.94Safe-End

Safety 1.3 77 0.0032 640 5.36 7.08 7.58ValveFlange

Surge Line .75 51 0.0032 640 3.54 4.69 5.02Safe-End

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Spray Line .625 77 0.0032 540 5.36 7.08 7.58Safe-End

Table 2

CRACK DETECTION REQUIREMENTS i

Time to Throughwall. . Crack Depth 15 Months iComponent Crack Prior to T/W Crack

(Months) .' ' (inches):: 1

PORV Safe-End (77 KSI) 20 .039

PORV Safe-End (40 KSI) 42 .175f

Safety Valve Flange 38 .550

Surge Line Safe End 41 .310|

Spray Line Safe-End > 350 .600 1

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