ML15222A833

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ENT000620 - PWROG Presentation, Material Orientation Toughness Assessment (Mota) for the Purpose of Mitigating Branch Technical Position (BTP) 5-3 Uncertainties (Feb. 19, 2015)
ML15222A833
Person / Time
Site: Indian Point  Entergy icon.png
Issue date: 02/19/2015
From:
Entergy Nuclear Operations
To:
Atomic Safety and Licensing Board Panel
SECY RAS
References
RAS 28133, ASLBP 07-858-03-LR-BD01, 50-247-LR, 50-286-LR
Download: ML15222A833 (45)
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ENT000620 Submitted: August 10, 2015 Westinghouse Non-Proprietary Class 3

© 2015 Westinghouse Electric Company LLC All Rights Reserved Global Expertise

  • One Voice Material Orientation Toughness Assessment (MOTA) for the Purpose of Mitigating Branch Technical Position (BTP) 5-3 Uncertainties Chris Koehler - Xcel Energy, Chairman PWROG MSC Elliot Long/Brian Hall - Westinghouse Date: February 19, 2015 P R E S S U R I Z E D WAT E R R E A C T O R O W N E R S G R O U P

Introduction

  • AREVA published a paper and sent an official letter to the U.S. NRC on January 30, 2014 identifying potential problems with Branch Technical Position (BTP) 5-3

- Position 1.1(4) of BTP 5-3 sometimes non-conservative in the determination of the initial Reference Nil-Ductility Transition Temperature (RTNDT) material property for reactor vessel materials made from SA-508, Class 2 forgings.

  • The BTP 5-3 methods to estimate initial RTNDT were invoked for reactor pressure vessels fabricated to an ASME Boiler & Pressure Vessel Code earlier than the Summer 1972 Addenda of the 1971 Edition because the RTNDT concept did not exist before that time.
  • BTP 5-3 provides estimation methods for conversion of measured Strong-Direction Charpy data, which was required pre-Summer 72, into Weak-Direction materials data, which was required afterwards
  • AREVAs finding calls into question the baseline RTNDT values of reactor vessels whose materials used this particular method 2

P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

NRC Response to AREVA Letter

  • The U.S. NRC actively began investigating this issue further in response to the AREVA letter.
  • The NRC technical evaluation included both forging and plate materials
  • This analysis confirmed non-conservatism of several BTP 5-3 Positions for estimation of initial RTNDT and initial Upper-Shelf Energy (USE) values.
  • The NRC presentation to various utility representatives on June 4, 2014 prompted the industry to request that EPRI continue addressing this issue on their behalf.

3 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

EPRI Quantification of BTP 5-3 Uncertainties

  • EPRI had already issued a survey on May 27, 2014 to utilities requesting additional information related to how RTNDT was determined for the plants in the U. S. Fleet.
  • EPRI investigated and quantified the BTP 5-3 uncertainties

- The results of this work are documented in, Assessment of Potential Non-Conservatisms of NUREG-0800 Branch Technical Position 5-3 Estimation Methods for Initial Fracture Toughness Properties of Reactor Pressure Vessel Steels.

  • Based on the EPRI Probabilistic Fracture Mechanics (PFM) evaluations, there is negligible safety benefit to changing BTP 5-3 B1.1(3) or its application

- The uncertainty in BTP 5-3, Position 1.3 (a) and (b) can be further addressed by the PWROG MOTA project 4

P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

PWROG Approach

  • The PWROG recognized that existing deterministic margin is potentially available in ASME Code Section XI, Appendix G and other NRC approved sources

- Regulatory Guide (RG) 1.161 is the NRC guidance on performing an Equivalent Margins Assessment (EMA) related to Upper Shelf Energy limited plants (< 50 ft-lbs at EOL)

- Current Pressure-Temperature limits, using the ASME Code, postulate axial flaws in plates/forgings and therefore, use Weak-Direction material properties

- ASME Code Case N-588 introduced methodology specifying that only circumferential flaws are required to be postulated in circumferential welds - Code provides stress intensity factor equation for circumferential (circ.) flaws

- As previously noted, BTP 5-3 provides estimation methods for conversion of measured Strong-Direction Charpy data, into Weak-Direction materials data

  • The following slides provide additional details on this approach By using the EMA RG and Code Case N-588 precedents, we can show significant inherent margin in Appendix G methodology sufficient to mitigate the uncertainties associated with use of BTP 5-3 methods used for vessel shell plates and forgings 5

P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

Regulatory Guide 1.161 Text

  • RG 1.161 states the following:

The CVN value should be for the proper orientation of the plate material (see Figure 2 [recreated in the next slide]). For example, for axial flaws the CVN value for the L-T (strong) orientation in the vessel wall should be used. Similarly, for circumferential flaws the CVN value for the T-L (weak) orientation should be used.

  • It is our contention that this approach, as defined in RG. 1.161, is technically valid for assessing BTP 5-3 uncertainty 6

P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

RG 1.161 Material Orientation Figure Match up Flaw to Material Orientation Properties in EMA is Allowed 7

P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

EPRI MRP Report Figure with Flaws Added Axial Flaw Circumferential Flaw 8 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

WRC-175 PVRC Recommendations on Toughness Requirements for Ferritic Materials

  • WRC-175 is basis for 1972 changes to ASME Section III, which brought in the requirements for flaw tolerance in Appendix G
  • With respect to shells:

Transverse properties for shells was not recommended by PVRC

  • ASME required transverse properties for all components, which is conservative 9

P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

ASME Code Case N-588

  • Implemented in 1997 time frame to add a more realistic methodology for the use of circumferential flaws when considering circumferential welds for P-T limit curves
  • Postulated that any flaws in a circumferential weld would be in the circumferential direction
  • ASME Code stress intensity factor (SIF) values for Axial Flaws are ~2 times the Circumferential Flaw results due to the higher pressure stresses (more details to come)
  • This Code case has been endorsed per RG 1.147 10 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

Pressure-Temperature Limit Curve Methodology Governing equation for P-T Limit Curve analysis:

C*Klm + Klt < Klc where, Klm = stress intensity factor caused by membrane (pressure) stress [ksi]

Klt = stress intensity factor caused by thermal stress [ksi]

Klc = fracture toughness, a function of the RTNDT of the material [ksi]

C = Safety Factor on membrane stress 11 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

Pressure-Temperature Limit Curve Margins

  • Appendix G allowable limits are established using the following three required margins
  • Postulate 1/4 thickness reference flaw with semi-elliptical (6:1) shape
  • Lower bound crack initiation (KIc curve) fracture toughness

- Material RTNDT and metal temperature

  • Safety factor, C, of 2 on membrane pressure stress 12 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

MOTA Margin Definition

  • The MOTA Margin is defined as the ART difference between an Axial Flaw based P-T limit curve and a Circumferential Flaw based P-T limit curve

- It is calculated by subtracting the Circumferential Flaw ART value (weak direction properties) from the original Axial Flaw ART value (strong direction properties) at the point of intersection

  • MOTA Margin is applicable to all base metal cylindrical shell sections away from discontinuities
  • The MOTA Margin compensates for the uncertainty that have been summarized in the EPRI BTP 5-3 assessment

- Applies to the full range of reactor vessel dimensions in the domestic PWR fleet This demonstration determines the margin that circumferential flaw (weak property) P-T curves have before they would become governing relative to the axial flaw (strong property) Appendix G curves 13 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

How to Determine MOTA Margin

- match up material property orientations to appropriate flaws

- provide a Circumferential Flaw stress intensity factor correlation

  • Extrapolation to P-T Limits:

- Use Strong Direction Axial Flaw ART with standard ASME Section XI Appendix G Pressure stress

- Use Weak Direction Circumferential Flaw ART with Code Case N-588

- Increase Circumferential Flaw ART value to force Circumferential Flaw curve to just intersect the Axial Flaw Curve

- The MOTA Margin can then be determined between the two flaw orientations 14 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

Parameters for Analysis

  • Plants selected to cover the geometries of the entire PWROG Fleet
  • Test Cases were run on four Westinghouse-Design, Two CE Design, and the B&W Design Plants
  • Utilized ASME Appendix G, KIc for Axial Flaws (Strong Properties) and Code Case N-588 for Circumferential Flaws (Weak Properties)
  • Bounding cases were performed for Steady-State Case and all heat-up and cooldown rates 15 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

General Plant Information and Geometries Utilized for MOTA Investigation Information Dimensions (in.)

Rated Vessel Plate / Vessel Vessel Plant Design Power Manufacturer Forging ID Wall Cladding (MW)

A W- 585 B&W Forging 132 6.5 0.156 2-Loop B W- 855 CE Plate 157 7.875 0.156 3-Loop C W- 1060 RDM Forging 173 8.465 0.156 4-Loop D CE 805 CE Plate 172.7 8.79 0.1875 E CE 1333 CE Plate 183.9 11.19 0.16 F W- 1048 CE Plate 173.375 8.625 0.21875 4 -Loop G B&W- 870 B&W Plate 171 8.44 0.1875 177 16 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

Input Values to P-T Curve Determination Information Dimensions (in.) Initial (Axial) ART Values (°F)

Plant Inside Outside 1/4T 3/4T Radius (in) Radius (in)

A 66.156 72.656 262 231 B 78.656 86.531 200 165 C 86.656 95.121 205.7 171.2 D 86.35 95.14 252.7 185.8 E 92.11 103.3 200 175 F 86.906 95.531 245 198.2 G 85.5 93.94 180 146 17 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

Steady-State Assessment Existing Axial KIc P-T Curve vs. Circ. Curve - Plant C Purple = Axial Flaw and Weak Initial RTNDT Red = Circ. Flaw and Weak Initial RTNDT (As Measured)

MOTA Margin 18 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

Steady-State Assessment Existing Axial KIc P-T Curve vs. Circ. ART to Intersect - Plant C Purple = Axial Flaw and Weak Initial RTNDT (As Measured)

Green = Circ. Flaw and ART MOVED to Match Existing Axial Curve (Raised Initial RTNDT) 19 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

1/4T MOTA Margin Calculation -

Plant C

  • Axial Flaw ART = 205.7°F (Purple Curves)
  • Circumferential Flaw ART Comparison =

205.7°F (Red Curve)

  • Increased Circumferential Flaw ART to Just Intersect Axial Curve = 265°F (Green Curve)
  • MOTA Margin = ART Circ. - ART Axial
  • MOTA Margin = 265°F - 205.7°F = 59°F 20 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

Cooldown - All Rates - Plant C Same ART Value Circ. Flaw Axial Flaw Solid Lines - Circ. Flaw Only Dashed Lines - Existing Axial only 21 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

Heat-up - All Rates - Plant C Same ART Value Circ. Flaw Axial Flaw Solid Lines - Circ. Flaw Only Dashed Lines - Existing Axial only 22 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

Final MOTA Margin (1/4T) Results Plate / MOTA Plant Design Plant Vessel Mfr.

Forging Margin (°F) 2-Loop A B&W Forging 66 3-Loop B CE Plate 61 Westinghouse C RDM Forging 59 4-Loop F CE Plate 58.5 2-Loop D CE Plate 60 CE Sys. 80 E CE Plate 61 B&W B&W-177 G B&W Plate 60 23 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

Effect of Heatup Curve (3/4T)

  • Flaw Orientation Limiting Locations

- Axial - Low Temperature/Pressure

- Circumferential - High Temperature/Pressure

  • 1/4T ART values dominate the steady-state curve

- High Temperature/Pressure, Steady-State is limiting as shown previously

  • 3/4T Value dominates the heatup curves

- Still applicable at high temperature and pressure regions

  • Further investigation was performed for MOTA margin on the heat-up transient, 3/4T ART value -

See slides below 24 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

100°F/hr Assessment (3/4T Limiting) Existing Axial KIc P-T Curve vs. Circ. Curve - Plant C Purple = Axial Flaw and Weak Initial RTNDT Red = Circ. Flaw and Weak Initial RTNDT (As Measured)

MOTA Margin 25 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

100°F/hr Assessment Existing Axial KIc P-T Curve vs. Circ. ART to Intersect - Plant C Purple = Axial Flaw and Weak Initial RTNDT (As Measured)

Green = Circ. Flaw and ART MOVED to Match Existing Axial Curve (Raised Initial RTNDT) 26 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

3/4T MOTA Margin Calculation -

Plant C

  • Axial Flaw ART = 171.2°F (Purple Curves)
  • Circumferential Flaw ART Comparison =

171.2°F (Red Curve)

  • Increased Circumferential Flaw ART to Just Intersect Axial Curve = 217.5°F (Green Curve)
  • MOTA Margin = ART Circ. - ART Axial
  • MOTA Margin = 217.5°F - 171.2°F = 46°F 27 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

Governing MOTA Margin (3/4T)

Results MOTA Margin (°F)

Vessel Plate /

Plant Design Plant Mfr. Forging 1/4T 3/4T 2-Loop A B&W Forging 66 61 3-Loop B CE Plate 61 50 Westinghouse C RDM Forging 59 46 4-Loop F CE Plate 58.5 48 2-Loop D CE Plate 60 62 CE Sys. 80 E CE Plate 61 40 B&W- 48 B&W G B&W Plate 60 177 28 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

Effect of Varying 1/4T ART Values

  • Selected Plant F for first sensitivity study as it had the lowest MOTA (1/4T) Margin
  • Investigated effect on MOTA Margin

- Low ART values (1/4T = 100°F)

- High ART values (1/4T = 390°F)

  • Analysis showed that 1/4T ART magnitude has no effect on the MOTA margin
  • Temperature point where Circumferential flaw curve intersected Axial flaw curve = the change in ART value, as shown on the following figures 29 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

Steady-State Assessment Nominal ART Axial KIc P-T Curve vs. Circ. ART to Intersect - Plant F 30 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

Steady-State Assessment Low ART Axial KIc P-T Curve vs. Circ. ART to Intersect - Plant F 31 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

Steady-State Assessment High ART Axial KIc P-T Curve vs. Circ. ART to Intersect - Plant F 32 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

Effect of Varying 3/4T ART Values

  • Selected Plant E for second sensitivity study as it had the lowest MOTA (3/4T) Margin and the thickest reactor vessel
  • Investigated effect on MOTA Margin

- Low ART values (3/4T = 75°F)

- High ART values (3/4T = 275°F)

  • Analysis shows that 3/4T ART magnitude has negligible effect on the MOTA margin
  • The largest variation is +/- 1.5°F across all RV thicknesses 33 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

100°F/hr Assessment Nominal ART Axial KIc P-T Curve vs. Circ. ART to Intersect - Plant E 34 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

100°F/hr Assessment Low ART Axial KIc P-T Curve vs. Circ. ART to Intersect -

Plant E 35 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

100°F/hr Assessment High ART Axial KIc P-T Curve vs. Circ. ART to Intersect -

Plant E 36 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

Final MOTA Margin Analysis Results

  • Minimum MOTA Margin Values Margin (°F)

Plant Plate/Forging 1/4T 3/4T C Forging 59 46 F Plate 58.5 48 E Plate 61 40

  • Maximum - Plant A (Forging)

- 1/4T of 66°F, 3/4T of 61°F

  • The analysis demonstrated that there is:

- no effect of the reactor vessel ART values on the MOTA Margin at the 1/4T location

- negligible effect of the reactor vessel ART values on the MOTA Margin at the 3/4T location.

  • MOTA Margin covers the entire US PWROG Fleet, with consistent results across all three plant designs 37 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

MOTA Conclusions

  • The axial flaw fracture behavior is governed by strong direction properties in both plates and forgings in the RPV cylindrical shell sections.
  • The issue of the conservatism of BTP 5-3 estimation methods pertains primarily with the uncertainty in the ability to estimate the weak Charpy impact properties from measured strong Charpy properties.
  • Since the forging and plate measured strong properties are coincident with the assessed 10 CFR 50, Appendix G axial flaw, the use of an RTNDT based on weak properties contains inherent margin.

38 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

MOTA Conclusions

  • The BTP 5-3 uncertainty in estimating RTNDT in the weak direction (circumferential flaw) identified by the industry should be compared to the margin identified herein for the circumferential flaw.
  • Plate and High USE Forging (>140 ft-lb) Plant Conclusion

- The minimum MOTA Margin (40°F) exceeds the maximum BTP 5-3 uncertainty effect on ART of 26°F

  • Low USE Forging (<140 ft-lb) Plant Conclusions

- The minimum MOTA Margin at 1/4T (59°F) exceeds the BTP 5-3 uncertainty effect on ART of 54°F

- The minimum MOTA Margin at 3/4T (46°F) is on par with the BTP 5-3 uncertainty effect on ART of 54°F

  • The current methods for developing P-T curves are acceptable in light of the identified BTP 5-3 estimation uncertainties.

39 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

Questions?

The Materials Committee is established to provide a forum for the The Materials Subcommittee is established to provide a forum for identification and resolution of materials issues including their the identification and resolution of materials issues including their development, modification and implementation to enhance the development, modification and implementation to enhance the safe, efficient operation of PWR plants.

safe, efficient operation of PWR plants.

40 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

DRAFT Back-Up Slide 41 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

EMA Visual Example: J-Applied vs.

J-Material for Axial and Circ. Flaws Solid Lines = Material Fracture Toughness Dashed Lines = Applied Stress Intensity Factor (SIF)

Strong direction properties are compared to axial flaw stress intensity factor Weak direction properties are compared to circ. flaw stress intensity factor Stress intensity is much lower for circ. flaws than for axial flaws Red = Axial Flaw and Strong Material Property Green = Circ. Flaw and Weak Material Property 42 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

Steady-State Assessment KIc vs. KIR Control - Plant C Equivalent ART values (205.7°F) show improved margin comparing KIR vs. KIc as a control step 43 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

Lower MOTA Margin for 3/4T?

  • Which ART Value Dominates at Axial-Circumferential Flaw Cross-Over Point, i.e.

top of the P-T curve?

- 1/4T, Steady-State Limited

- 3/4T, Heatup Transient Limited

  • Since 3/4T values are limiting at cross-over point for heat-up, the KIt thermal SIF component of P-T limit curves is non-zero
  • This higher stress state leads to a lower MOTA Margin 44 P R E S S U R I Z E D WAT E R REACTOR OWNERS GROUP

Global Expertise

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