Unit 2 CRDM Vessel Head Penetration

Justification for Safe Operation

November 20, 2001

ENCLOSURE 2

American Electric Power Meeting with

Nuclear Regulatory Commission

Donald C. Cook Nuclear Plant Unit 2 CRDM Vessel Head Penetration Justification for Safe Operation Until

January 19, 2002

Slide 2

Agenda

"* Introduction, Background, and Overview

"* Deterministic Model

"- Probabilistic Risk Assessment

"* Inspection Plans

"* Closing Summary

Scot Greenlee

Warren Bamford

James Hawley

Daniel Garner

Scot Greenlee

Slide 3MR -

introduction, Background, and Overview

Scot Greenlee Director

Nuclear Technical Services Organization American Electric Power

Slide 4

Background

", October 1994 - Inspected 71 of 78 Unit 2 Penetrations - Proactive response to industry issue

- Used eddy current examination on ID - Three axial indications found on one penetration - NRC approved operation with indications until next outage

"* April 1996 - Inspected Most Susceptible Penetrations (5) - No significant growth in previous indications

- Repaired previous indications

- No new indications

"* September 1997 to June 2000 - Extended Shutdown ide 5 = 4,SlI

Background

"* May 2001 - Planning begins for Unit 2 visual and NDE inspections for

November 2001 outage - includes purchase of robotic repair arm

"- August 3, 2001 - NRC Issues Bulletin 2001-01 - Cook Unit 2 in Moderate Susceptibility Range

- Previous indications require additional justification for operation past December 31, 2001

", August I September 2001 - Forced outage - Unit 2 in cold shutdown >30 days

- Refueling outage moved to January 19, 2002

- Started head purchase activities Slide 6

Presentation Overview

"- Cold Shutdown Period Following Bulletin Issue - No crack growth in cold shutdown

- Operation to January 19, 2002, with cold shutdown period, provides less crack growth than operation to December 31, 2001, without cold shutdown

"* Extensive Plant Specific Analyses Completed - Deterministic model conservatively shows more than 40 years

to critical circumfrential crack

- Probabilistic analysis shows more risk in early shutdown than operation past December 31, 2001

Slide 7

Deterministic Model of Penetration Integrity

Warren Bamford Q)Westinghouse

Slide 8 OT

Presentation Outline

m Background

m Stress Analysis

m Crack Growth Model

m Comparison to Babcock & Wilcox Plants

m Circumferential Crack Growth Calculations

m Results: Safety Case

Slide ~i- ISlide 9

Background

m Circumferential Cracking Has Been Found in Three Babcock and Wilcox Plants

m Cook Unit 2 Manufactured With Tubes From Huntington Alloys Stock

Slide 10 Ell'

Stress Analysis Overview

"m Elastic Plastic (E-P) Analysis Approach

"m Model for E-P Finite Element Analysis

"= Loadings

"* Stress Strain Curve

"* Results: Axial Stress Dominant

"* Independent Confirmations - Framatome

- EdF

- CE

- B&W

Slide 11

Elastic Plastic Analysis Approach

"* 3-D Isoparametric Bricks and Wedges

"- Model - Wedge of the head

- Half a penetration

- Takes advantage of symmetry

"* Materials - Tube and weld: Alloy 600

- Vessel: carbon steel

"* Welding Process Model Produced Ovality - Closely Matched Field Profiles

Slide 12

Three-Dimensional Model of the Outermost Penetration

Slide 13r� �T�V

Vessel Head Penetration Ovality Regression Analysis (All Heads)

Vessel Head Penetration Ovality Regression Analysis (All Heads)

E E

0 E

E

3.100

2.50

2.00

1.5(1

1 .(1(}

0.50

0.00

-0.500.0

Linear Regression

f(x)=2.(l426E-2"x+ -1.1922E- 1

R^2=8.7738E-ISlide 14

125.0

P".

Stress Analysis Results

"* Highest Stresses at Outermost Penetrations

"* During Operation - Vessel displaces radially

- Tubes displace toward position perpendicular to head

* Gap Tends to Open Between Head and Tube

m Hoop Stress Exceeds Axial Stress Significantly on All Hill Side Locations

Slide 15

Stress Analysis Results

"* Highest Stresses in Outermost Penetrations Are Hoop - On the tube ID - Below and near the weld

- Corresponds to crack locations in all but the B&W Tubular Products (BWTP) penetrations

"* Highest Axial Stresses Just Above and Along Plane of Weld

m Stress Analysis Cracking Found

Consistent With No Circumferential in Huntington Materials

Slide 16

Loadings Evaluated

- Steady State - Governing Loading

m Hydrotest

* Operating

"* Boltup

"* Seismic

"* Welding

"* Interference

Slide 17

transients

Weld Model Load Sequence

m First Weld Pass

m Second Weld Pass

m Cold Hydrotest (Shop @ 3107

m Cold Hydrotest (Field @ 3107

psi)

psi)

m First Cycle of Steady State Loading I Unloading

m Second Cycle of Steady State Loading Slide 18

Stress Strain Curves

"* Based on Testing Material From Outermost Penetration of Non - Irradiated Plant

"* Monotonic and Cyclic Curves Obtained

"* Cyclic Stress - Strain Curve Used - Multiple loading cycles during fabrication / service

- Different yield strength materials converge to a single cyclic curve

"* Used Cyclic Curve for One Half the Life of Penetration

Slide 19

Cyclic Stress - Strain Curve and Test Data

ALLOY 600 HEAD PENETRATION@ 600 -F(316 -C)

CYCLIC STRESS-STRAIN CURVE AND TEST DATA

Monotonic CURVE

600

500 G

LLJ 400 0

n __J

300 :•

200. Lj Ctn (Y

Est'd. Stress-Strain Curve From Data

I Block Test

10 Block Test

0

52 Blocks of 84 Block Test

100

024

% TOTAL STRAIN AMPLITUDE

100

MATERIAL

80

60

40

V)

LU C) D

ELl

Ur) LU

I-c'n

Slide 20

00

Axial Stresses in CRDM Above Weld Region

Load step = 12 fterat~on = 5 Tim,- 12

Neutral file - NRFVv78D5 RES

Variable = S33 Max, 163667.

Min. -230727

50000,

*-25000,

-50000. S-7,5000,

Slide 21'/. -WECAN/PLUS Ve 97 Sep 62m/1 7:06:33 WOG78

-100000,W in

Crack Growth Model Overview

m Development of Scott Model

", Independent Confirmation

"* Available Test Data

"* DC Cook ISI Confirmation

"* MRP Recommendations

"* Comparison With NRC Model of November Report

"* Temperature Correction

ide 22SI

Crack Propagation

"* Mechanism Is Primary Water SCC "* Key Variables

- Temperature

- Applied Stress Intensity Factor

"* Temperature Effect Modeled on Observed Cracking in Alloy 600

"* Base Crack Growth Rate of Scott Modified to Account for Temperature Effects

"* Results to Be Discussed - Propagation through wall

- Propagation along circumference of penetration

Slide 23

Crack Growth Data 325C, Huntington Heats

1.E-09

I.E-10

E

"0

0 1.E-11

0 10 20 30 40 50 60 70 80

Stress Intensity Factor, K (MPa*sqrt(m))

A

SCOT MODEL da/dt=2.23e-12(K-9)11.16

Huntington (315-331C)

+ Test Temp.(302.6, 340.2, 353.3, 343.0 corrected to 325C) COOCK 2

1.E-12

1+

- +-

4-

Slide 24

.1,

Comparison to Babcock and Wilcox Plants

"* B&W Plants Use BWTP Tubes

"* BWTP Tubes Are Different From Huntington Alloy

Tubes - Lower grain boundary carbide coverage leads to shorter

initiation times for BWTP heats

- PWSCC crack growth rates are 2-3 times faster for BWTP heats

- Majority of BWTP tubes were roto-straightened > High hardness on the OD

» Higher PWSCC susceptibility on the OD

", Cook Unit 2 Uses Huntington Alloy Tubes

Slide 25

Crack Growth Data 325C, B&W Tubular Products Heats

SCOTT MODEL da/dt=2.23e-12(K-9)1.16

B&W(315-331 C)

x Test temp.(290, 291.1, 309.2, 337.9, 339.6, 340.8, 341.6, 353.5, 360, 360.6,363.3 corrected to 325 C)

0 10 20 30 40 50 60 70 80

Stress Intensity Factor, K (MPa*sqrt(m))

1.E-09

1.E-10

E

U i.E-i1

1.E-12

xx x

x

I r-- - -I

Slide 26

Summary of Crack Growth Temperature Effects

TEMPERATURE, DEG. C M333157 0-352

I H z 0

cj� -J

w H .4:

I H

0

0

0

F-i

0.1 F '.1155

RECIPROCAL TEMPERATURE, 1/DEG. K

Key:

Slide 27 0 *RDM.ab ees Data

All others are S/G Tube Gab & F I Data

;� ii

Crack Growth Calculations For Circumferential Flaws

s Development of a Leak Path

m OD Initiation and Growth Toward the ID

m Circumferential Growth to Failure

Slide 28

Development of a Leak Path

"- Three Possible Paths - ID above weld

- OD up past weld

- Through the weld

", ID Above Weld Is Preferred Path For All Except B&W Plants

"* Cook Unit 2 ID Verified Flaw Free in 1994, Except One Penetration - Repaired in 1996

Slide 29

OD Initiation and Growth Toward ID

"* Assume Leakage Occurs by Any of Three Paths

"* Nine Circumferential Locations Studied

"* Only Uppermost Hillside Location Had Stresses

Leading to Through - Wall Penetration

"* Compressive Stresses Lead to Relatively Slow

Growth

"* Raju and Newman Stress Intensity Factor Used

"* Through - Wall Growth Would Take About 24 Years

lide 300SI

Circumferential Growth to Failure

"* Propagation Follows the Top of the Weld

"* Stress Decreases as the Flaw Propagates - Nominal axial residual stresses

- Displacement - induced stresses

"* Stress Intensity Factor Determined from Finite Element Modeling Accounts for These Factors

"- Results Obtained for Two Crack Growth Models - Scott Model - NRC Model

Slide 31 W.•%•I> !•

Scott Crack Growth Model

* Assume Through-wall Circumferential Flaw - Occurred just after the Cook inspection in fall 1994 - Length of 10 percent of circumference

m Over 40 Years Unacceptable

Required to Grow Flaw to Length

m Cook Unit 2 Will Have Less Than 4 EFPY of Operation From 1994 Inspection to January 19, 2002

- Any Circumferential Flaw Occurring in 1994 Would Still Be at Acceptable Level

Slide 32 -

Scott Crack Growth Model

0 15 30 45 60

PWSCC PERIOD (YRS)

180

160

140

120

100

80

60

CD Li.

40

20

0

Slide 33

75

NRC Crack Growth Model

"* Assume Through-wall Circumferential Flaw - Occurred just after the Cook inspection in fall 1994 - Length of 10 percent of circumference

"* Over 5 Years Required to Grow Flaw to Unacceptable Length

"* Cook Unit 2 Will Have Less Than 4 EFPY of Operation From 1994 Inspection to January 19, 2002

m Any Circumferential Flaw Occurring in 1994 Would Still Be at Acceptable Level

Slide 34

NRC Crack Growth Model

1E2 3 4 5 6 7 8

PWSCC PERIOD (YRS)

CD

LL

180

160

140

120

100

80

60

40

20

0

0

Slide 35

9 10

lO

Probabilistic Risk Assessment

James Hawley Supervisor

Probabilistic Risk Assessment American Electric Power

Slide 36

RISKASSESSMENT

"m NRC Requested Justification for Operation Past December 31, 2001

"* Risk Assessment Complements Deterministic Assessment

"m Looks at Operation Past December 31, 2001 From Incremental Perspective (Incremental Core Damage Probability - ICDP)

"m Comparison Made to Unplanned Outage Risk

lide 37SI

RISK INPUTS

"* Leak Frequency: Based on Statistical Analysis of Industry Data

"* Four Types of Leaks / LOCAs: Seepage, Leak, Small LOCA, Medium LOCA - Fractions conservatively chosen as 0, 0.9, 0.08 & 0.02 based on

Engineering Judgement

- Industry data for fractions: 0.965 (28/29), 0.035 (1/29), 0, 0

"* Conditional Core Damage Probability (CCDP) - From recently updated /IWOG certified PRA model

Slide 38 =r

CDF Risk Assessment Model Event Tree

1_.OK- seepage 0

2 OK -leak ~20

p3CD -leak fl 3.310-05

1.07E-02 4 OK - small LOCA

S5 CD - small LOCA 3.310E-03

6 OK- Medium LOCA

0.02 7CD - Medium LOCA

4.520E-03 Total CDF =

ICDP for 19 DAYS = 4.14E-06 X 19/365 = 2.16E-07

Slide 39

3.20E-07

2.83E-06

9.67E-07

4.14E-06

9

Leak I Type of Conditional Sequence End State Frequency Freq. Leak core damage

I or LOCA Probability

CDF Risk Assessment Model Event Tree Using Leak/Inspection Data Results

1

2

3

4

5

6

OK- seepage

OK - leak

CD - leak

OK - small LOCA

CD - small LOCA

OK - Medium LOCA

0/29-1 7 CD - Medium LOCA 4.520E-03

Total CDF =

ICDP for 19 DAYS = 1.22E-08 X 19/365 = 6.35E-10

1.22E-08

0.00E+00

0.0OE+00

1.22E-08

[Leak 1 Type of Conditional Sequence End State Frequency

~Freq. Leak core damage _ _ __1

Slide 40.�..

RISK RESULTS

"* Risk of Operating 19 Days Past 12/31101 = 2.16 E-07

"* Risk of One Unplanned Day in Mode 5 is 6.1E-08 - Based on Shutdown Safety Monitor Model

"* Four Extra Days In Mode 5 Causes Increase In Overall Plant Risk

Slide 414 f

Cook Unit 2 Inspection Plans

Daniel Garner Project Manager

American Electric Power

Slide 42

Inspection Plans

"* Visual Examination - 100% effective visual examination using robotic crawler

- Evaluating qualified visual examination

"* Eddy Current I Ultrasonic Exam - All J-groove welds and outside diameter metal of interest for all

CRDMs and thermocouples

- Inside diameter of most susceptible penetrations - (19)

- Inside diameter for seven part length rod locations

lide 43SI

Susceptibility Summary

* WCAP 14907 Cook Units

(Not Docketed) Prepared in 1997 for

. Each Penetration Analyzed for Probability ofAchieving 75% Through Wall Cracks

m Data Provided at 10 EFPY Intervals

m Cook 2 Is at -15 EFPY - Linear interpolation Between 10 and 20 EFPY

Used

Slide 44 2

Probabilities of Reaching Through Wall Cracks

75%

53 12.9 42,44 11.5

43,45-48 11.1

Penetration 15 Years 38-41 9.7

61 8.0 60 6.9

50, 51 5.3 54, 55, 57, 59 4.7

49 3.9

52 3.0 32 3.0

31,33 1.630. 34-37

Penetration 15 Years

17 0.7 10, 11,12 0.7

13 0.6 18-21 0.5 14, 16 0.5

8 0.5 7 0.5

15 0.4 9 0.1 6 0.1

2,3,4,5 0.1 1 0.0

Most Susceptible Locations

Part Length Rod Locations

Slide 45

I I I

UNIT 2 REACTOR VESSEL HEAD PENETRATION MAP

R

P

N

M

L

K

J

H

(3

F

F

D

C

B

A

Slide 46

o Most Susceptible Locations

o Unsleeved Dummy

o Part Length Rod

o Thermocouple

o Head Vent

15 14 13 12 11 10 9 8 , 6 5 4 3 21

Closing Summary

Scot Greenlee Director

Nuclear Technical Services Organization American Electric Power

Slide 47

Closing Summary

Based on Conservative Analysis

Cook Unit 2 Is Safe to OperateUntil January 19, 2002

Slide 48