issues regarding the technical basis for reactor pressure

Issues Regarding the Technical Basis

for Reactor Pressure Vessel Closure Flange Rulemaking

Westinghouse Owners Group

Presentation to NRC

August 28, 2001

Warren Bamford

Westinghouse Electric Company, LLC

p o:\smt\Closure Head\Vessel Flange Rqmt. A1Atachment Cpp,

Background

Summary of Revisions to WCAP-14040

Reactor Pressure Vessel (RPV) Closure Flange Requirement

Basis of the RPV Closure Flange Requirement

Plant Geometries Considered

Stress Analyses

Fracture Analysis Methods

RPV Closure Flange Integrity Evaluation

Proposed Elimination of RPV Closure Flange Requirement

Safety Impact of Eliminating RPV Closure Flange Requirement for PWRs

Summary and Conclusions

Future ActionsoAsmt\Closure Head\Vessel Flange Rqmt.ppt

Background

"* WCAP-14040 Submitted to NRC to Obtain Review and

Approval of Methodology used to Develop RCS Heatup

(H/U) and Cooldown (CID) Limit Curves and Cold Overpressure Mitigating System (COMS) Setpoints

"* Approved Methodology Allows Relocating RCS H/U and CID

Limit Curves and COMS Setpoints from Tech Specs to a

Pressure and Temperature Limits Report (PTLR)

"* NRC approved WCAP-14040 in October 1995

o:'smt\Closure Head\Vessel Flange Rqmtppt

Background (cont.)

"* Several changes have been made in H/U and C/D Limit Curve Development Methods, and Incorporated into Appendix G of Section XI of the ASME Code since 1995

"* WCAP-14040 is being revised to incorporate these changes into an updated Topical Report that contains the current Methodology used to Develop HIU and C/D Limit Curves

"* These changes are incorporated as options, to allow plants the flexibility of implementing the changes, if desired

o:\smt\Ciosure Head\Vessel Flange Rqmtppt

Summary of Revisions to WCAP-14040

* Code Case N514: Low Temperature Overpressure Protection (February 12, 1992)

• Code Case N640: Alternate Reference Fracture Toughness for Development of P-T Limit Curves (February 26, 1996)

"* Code Case N588: Alternative to Reference Flaw Orientation of Appendix G for Circumferential Welds in the Reactor Vessel (December 12, 1997)

"* Code Case N641: Alternative Pressure-Temperature Relationship and Low Temperature Overpressure Protection System Requirements (January 17, 2000)

"* Proposed Elimination of Flange Requirement

o:\smt\Closure Head\Vessel Flange Rqmtppt

RPV Closure Flange Requirement

"* Required to be Included by 10CFR50 Appendix G

"* High stresses in the closure head flange region during boltup

"• OD surface stresses don't increase much between boltup and normal operating pressure, but the distribution changes from bending to membrane


RPV Closure Flange Requirement (cont.)

"° Since boltup is performed at low temperatures, fracture margin is important there

"* The original flange requirements were developed

because of the relatively low toughness used at

the time: Kia

", The recent approval of the use of Kic eliminates

the need to include the flange requirement


2500

2250

2000

1750

1500

S1250

1000

75O

25

- Kic- based

- - - - --- - -

K11 -1based

- RTNDT+120F 0

12, F 25O30E5

0 50 100 150 200

Temperature (F)

Figure 1-1 Illustration of the Impact of the Flange Requirement for a Typical PWR Plant

Introduction o: \4951.doc:lb-102599

1-2

50(

350250 300

UctOber 1",9

TOP HEAD DOME TORUS TO FLANGE WELD

T NOTE: ALL DIMENSIONS ARE IN INCHES 27.25

18.0 •, 27.625

NOTE: DIMENSIONS DO NOT INCLUDE CLADDING

UPPER HEAD REGION

Typical Geometry - Closure Head/Flange Regionppt o:\smnt~smtlette rs\Clo sue Head/Vessel Flange R~qmn.

Basis of the RPV Closure Flange Requirement (From Neil Randal's discussion in the FR, 11/14/80)

"* Consider closure head/flange region "* Stresses are higher at OD; use outside surface flaw " A/T = 0.25 "* Safety factor = 2 "* For this combination, K* = 92.7 ksi in. " Neil Randall's calculation was more conservative; K* = 98.3

ksi in. (AIT = 0.1, stress = 40-50 ksi) "* Using the K, curve, boltup should be at RTNDT + 120 "* Since this is unrealistic, the requirement was changed to

allow pressure up to 20% of design hydro before imposing the temperature requirement

o:\smt\CIosure Head\Vessel Flange Rqmtppt

Plant Geometries Considered

(2 loop)

(3 loop)

Westinghouse (4 loop)

CE B&W

GE (design 1)GE (design 2)

GE (design 3)

o:\smt\Closure Head\Vessel Flange Rqmt

Design

Westinghouse

Westinghouse

Thickness

5.7 5.8

7.0

7.4

6.8

3.6

4.0

4.8

ppt

Stress Analyses

"° All cases were finite element results "* ASME code minimum properties are used "* Axisymmetic models are used "* Steady state stress is very similar for all designs

- Mostly membrane stress

- Bending stresses higher for BWRs "* Boltup stress is mostly bending "° Comparisons were not available for the

Westinghouse 2 loop plants

- Conservatively covered by the 4 loop results ppt o:\smt\Closure Head\Vessel Flange Rqrnt.

Axial Stress Comparison: Steady State Operation @ 2250 psi

Membrane Bending

Plant Stress Stress Stress

W 4 Loop

W 3 Loop

CE

B&W

22.8

20.9

46.4

55.7

o:\smt\Closure Head\Vessel Flange Rqmt

OD

12.810.0

11.6

12.8

19.0

9.3

33.6

36.7

ppt

Stress Comparison:

Boltup E

Boltup vs Steady State

3oltup SS SS

Plant Membrane Bending Membrane Bending

1.1

2.1

0.8

4.3

14.2

14.5

22.8

27.6

oAsmt\Closure Head\Vessel Flange Rqmt

W 4 Loop

W 3 Loop

CE

B&W

10.0

11.6

12.8

19.0

12.8

9.3

33.6

36.7

ppt

Fracture Analysis Methods

0 Stress Intensity Factor: Raju and Newman

° Fracture Toughness: Kia and KIC

* Irradiation Effects Negligible

ppt o:\smt\Closure Head\Vessel Flange Rqmt

RPV Closure Flange Integrity Evaluation

* Semi-elliptic surface flaw postulated on head OD

° Orientation parallel to the weld

• Boltup cases analyzed to determine maximum value of K for any flaw depth

"* PWR and BWR cases considered "* Typical boltup temperatures are:

- 60 F for PWRs -80 F for BWRs

"* Using the KIc toughness, significant margin exists in all cases

- Not true for KIa, the reason for the original concern


Proposed Elimination of RPV Closure Flange Requirement

"* Consider developing a set of boltup requirements, using the

following assumptions:

- Postulated flaw depth - T/10

- Safety factor = 2.0

- Kia or K1 c lower bound curves

"* Using Kia, the governing case is RTNDT + 118F, closely matching

the original requirement of RTNDT + 120F

" Using K1 , the requirement for PWRs is RTNDTto RTNDT + 41F

Since RTNDT is typically 1OF, boltup would be at 10-51F Typically boltup is at 60F -: no requirement needed

"* Using KIC, the requirement for BWRs is RTNDT to RTNDT + 56F Since RTNDT is typically IOF, boltup would be at 10-66F Typically boltup is at 80F -> no requirement needed

oAsmt\Closure Head\Vessel Flange Rqmtppt

Boltup Requirements: K1cVS Kia

Comparison of Stress Intensity Factors

Plant K (a/t=0.1)* K (SF--2)w T-RTNDT (KJ T-RTNDT (KOa) W4 Loop 19.7 39.4 0.0 F 1.0 F W 3 Loop 19.4 38.8 0.0 F 0.0 F (315 30.0 - 60.0f 13.0 F 6.0 F

B&W 39.4- 79.B 41.0 F 10 F

Note: All units in ksi f/in.

7 (1.387 .4 38.0 97.0Y B (2) 48.0 96.0 56.0 11- 8 .0i

bVR(3) 25.1 50.2 0 43.o

ptoAsmt\Closure Head\Vessel Flange Rqmtppt

B&W REACTOR VESSEL CLOSURE HEAD/FLANGE WELD BOLTUP OUTSIDE SURFACE STRESS INTENSITY FACTOR vs a/t

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

a/t (crack depth/weld thickness)0.9

Stress Intensity Factor vs Flaw Size:

o:\smt\Closure Head\Vessel Flange Rqmt.

60

50

£ I-"

a 40

0 Z W I

cn

0 , 30

I-

z w z 20

I-

10

0

B&W Plant (t = 6.82 inches)

ppt

RPV Closure Flange Integrity Summary

Design (Depth, alt

CE

B&W

W 4 Loop

W 3 Loop

BWR Design 1

BWR Design 2

BWR Design 3

41

56 31

32

56

69

37

(0.42)

(0.60) (0.44)

(0.44)

(0.42)

(0.40)

(0.42)

1 1 1

o:\smt\Closure Head\Vessel Flange Rqrnt

Kia

89.6 89.6

89.6

89.6

17.3

17.3

17.3

52.7 52.7 52.7

52.7

61.4

61.4

61.4

ppt

Safety Impact of Eliminating RPV Closure Flange Requirement for PWRs

"* Current RPV closure flange requirements can cause severe operational limitations, after accounting for instrument uncertainty

"• The lower limit of pressure is 20% of hydrotest, or 621 psig until the flange limit of RTNDT + 120F is exceeded

"• Minimum pressure to cool the RCP seals is 325 psi


Safety Impact of Eliminating RPV Closure Flange

Requirement for PWRs (cont.)

"° The operating window can become very small

"* Example: For one plant, the operating window would increase from 121 psig to 262 psig

"* This change would significantly reduce the potential of an RCP seal failure (small LOCA)

oAsmt\Closure Head\Vessel Flange Rqmtppt

6-2

I

0. I.

L"

S

621

Figure 6-1

/

/ /

/ TOP etpoin{

/ "-Heatup Curve

Instrument Uncertainty

__LPump Seal Limit 325 psi

TemperatureRTndt+120

Illustration of the Flange Requirement and its Effect on the Operating Window for a Typical Heatup Curve

Are Flange Requirements Necessary? o:\4951.doc:1b-110399

October 1999

6-3

LIMITING MATERIAL: INTERMEDIATE SHELL FORGING 5P-5933 (using surv. capsule data)

LIMITING ART VALUES AT 12 EFPY: 114T, 70OF 314T, 60°F

] 27563556009 213.1 I l II .,. . - ; II i , ,I.

]LIAX TEST LI1XIT

UNACCEPTABLE OPERATI ON

r I I

1250 l -_oATUP RATE 1250 ;PTO 100 F/Hr. ACCEPTABLE

OPERATION

l I I I I I I.. I I ., I I I i

1000

750

500

~A+HrA~h+ +{I++~~-PORY LTOP SETPOINTS. I I-I I I I , , , , ' 1 1,1 1 1*LTOP OPERATING WINDOW

UNUIiMUrM DILlv ur TEMP. AT 60'F !!! "KC SEAL LIMIr: 3Z5 PSI

I ---I I I I I I I I I I II I

CXITICALITT LINll RAZED ON - 1-1INIREV1CI 1Ti.3OT IlC tIYT

TI P • T • S S F o l I I I I I1 1 s isiici FExI oD UP 0o 12.0 3??

S I I I I IIIII

0 50 100 150 200 250 300 350 460 450 500

Indicated Temperature (Deg.F)

Figure 6-2 Illustration of the Actual Operating Window for Heatup of Byron Unit 1, a Low Copper Plant at 12 EFPY

October 1999Safety Implications of the Flange Requirement o:\4951.doc:lb-110399

I I

2500

2250

2000

1750

1500

C1

II

i iJiI

11111:250 -

n

S. . . . . . . . • '? ' , , ,

l l '; l'• l l r lJ -L -1l l ' i i ' i i 1 I I I I i i I { I I /R

T I I ' I l l t i l t l l [ t l l l/ l 'I= l

A L I I I I 1I -1- I I 1 1 1 1 1I

I . i i ;~ .; i i.i i i I . I . . - . I I . I . . I I . . I . I . . I .I I

I] I

6-4

LIMITING MATERIAL: INTERMEDIATE SHELL FORGING 5P-5933 (using surv. capsule data)

LIMITING ART VALUES AT 12 EFPY: 1/4T, 70°F 314T, 60OF

2500 ' I I l

2250

txD

"-' 2000 - - - - - - - - -

I /I U

1500

1250

1000

750

500

250

0

Figure 6-3

5'0 100 150 260 250 300 350 400 450 500

Indicated Temperature (Deg.F)

Illustration of the Actual Operating Window for Cooldown of Byron Unit 1, a

Low Copper Plant at 12 EFPY

Are Flange Requirements Necessary? October 1999 o:\4951.doc:b- 110399

C.)

C-)

Summary and Conclusions

• The RPV closure flange requirement originated over 20 years ago, when the standard practice was to use the Kia reference toughness curve

The development and approval of Code Case N640, allowing the use of K1c has significantly improved the H/U and C/D curves

* Use of Code Case N640 significantly improves operational safety, by increasing the operating window between the P-T curve and the RCP Seal cooling pressure


Summary and Conclusions (cont.)

The benefits of Code Case N640 are severely

limited by the RPV closure flange requirement

* Use of flange

K1c has demonstrated that the RPV closure requirement is not required


Future Actions

0 Schedule for Rulemaking

• Treatment of Exemption Requests

0 Schedule for submittal of WCAP 14040 Rev. 3

o:\smt\Closure Head\Vessel Flange Rqmt.ppt

issues regarding the technical basis for reactor pressure

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