01/09/2008-01/11/2008 Comparison of Leak Rates from Alloy … · 2012. 11. 30. · AREVA NP, Inc. >...

1> Jan. 10, 2008 NRC/EPRI LBB Workshop, Rockville, MDAREVA NP, Inc.

Comparison of Leak Ratesfrom Alloy 82/182 Butt Weld Cracksfor Leak-Before-Break Assessment

By Ashok D. Nana & Kenneth K. Yoon

Presented By:Ashok D. Nana

Advisory Engineer, New Plants EngineeringAREVA NP Inc.Lynchburg, VA

NRC/EPRI International Workshop on LBB in PWSCC SystemsJanuary 9-11, 2008 Hilton Washington DC/Rockville

Executive Meeting Center1750 Rockville Pike, Rockville MD 20852


Outline

> Objective> Background on LBB

PWSCC in Alloy 82/182 Pipe Butt Welds of PWRsFatigue Cracks versus PWSCC Crack Morphologies

> MRP LBB Task Group 2004/2005To address PWSCC LBB Issues for the Industry

> Leak Rate Analysis Using KRAKFLO (AREVA)Phase I - Benchmarking Code to IGSCC dataPhase II – Benchmarking Code to Field dataPhase III - Leak Rate Sensitivity Study on

5 LBB Piping Systems> Review 2005 EMC2 PVP Paper on Similar Issue> Summary & Conclusions> Recommendations for Future Study


Objective - MRP LBB Task Group

> At Alloy 82/182 (DMW) & Alloy 600 LBB Locations Re-assess 10 gpm leakage flaw sizes considering PWSCC/IGSCC type crack morphologies in lieu of conventional fatigue crack morphologies used in past LBB submittals. Compare against critical crack size(s)Demonstrate LBB margins are still met


Background on LBB

> PWSCC issues in Pipe Butt Welds Oct. 2000, Hot Leg Nozzle to Pipe Weld at V.C. Summer Plant, Similar issues occurred at 4 other plants (foreign & domestic)

> LBB evaluation procedures are provided in SRP 3.6.3 & NUREG 1061, Vol. 3. Per the Guidelines:

LBB cannot be applied if active degradation mechanisms (such as PWSCC) are present in the piping system

> At the time LBB analyses were performed and reviewed there were “no known active degradation mechanisms” that could potentially lead to through-wall cracking

> Most plants met Reg. Guide 1.45 that required leak detection capability of 1 gpm. Since a factor of 10 is required for LBB, a 10 gpm leakage crack size was typically evaluated.


Background

> PWSCC issue calls into question previous LBB Analysis Containing Alloy 82/182 materials and thatUsed fatigue crack morphology.

> Industry must justify present & past LBB submittals to the NRC relative to PWSCC

Demonstrate adequate LBB Margins are maintained with consideration of PWSCC Crack Morphology

> Fatigue cracks have large COD, relatively smooth surface roughness with relatively straight leakage path through the pipewall

> PWSCC cracks are tight cracks with a more tortuous flow path through the pipe wall than fatigue cracks

Source for Sketch: EMC2 2005 PVP Paper on Impact of PWSCC on LBB


Background on LBBV. C. Summer PWSCC Issue --- Hot Leg Piping Butt Weld

To-date PWSCC cracks in DM Butt Welds have been primarily axially oriented.

In October 2006, several Circ. flaw indications observed in Wolf Creek Pressurizer nozzles – addressed by MRP-216.


Background

> U.S. PWR plants with Alloy 82/182 PWSCC susceptible LBB locations:

Main Loop Piping -- 48 PlantsSurge Line Piping -- 17 PlantsShutdown Cooling Line -- 1 PlantSafety Injection Line -- 1 Plant

> AREVA study initiated under MRP LBB Task Group to address:

Initial benchmarking to SCC data & then perform sensitivity study

> MRP Task Group to assess impact of considering PWSCC on existing LBB approved PWR piping.


Phase I: -- Benchmarking KRAKFLO to IGSCC data

> AREVA Leak Rate Code KRAKFLOOriginally developed from NRC leak rate Code LKRateSimilar to LEAK-01 and PICEPPreviously benchmarked to extensive Number of IGSCC crack experiments in Battelle Phase II Programs

• Involving 82 experiments• Cracks ranging in size from 0.029”(0.74mm) to 1.1”(28mm)• Measured leak rates ranging from 0.0003 gpm (1.34 x 10-5

kg/s) to 3.84 gpm (2.00 x 10-1 kg/s)• Benchmarking to Battelle’s phase II data helped establish

recommended values for – Surface roughness 200μin (5μm) with associated – Number of 45-degree turns/inch = 24

• PICEP Code has similar recommended values


KRAKFLO Predictions against Battelle’sIGSCC Data

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

1.0E-05 1.0E-04 1.0E-03 1.0E-02 1.0E-01 1.0E+00

Measured Leak Rate (kg/s)

Pre

dict

ed L

eak

Rat

e (k

g/s)

Tests 1 - 6

Tests 19 - 82

Tests 7 - 18


Phase II – Benchmarking to Field Data

> Very limited field data exist involving SCC with measured leak rate.

> In 2004, KRAKFLO was benchmarked to two most notable events:

Duane Arnold (BWR) Event of June 1978• Complex crack at a leaking Inlet nozzle safe end• Continuous internal circ. crack with 80-deg.Thru-Wall crack

Palisades event of September 1993• Circ. crack ~ 3”(7.6 cm) long, observed in PORV safe end

> KRAKFLO recommended values for IGSCC used in benchmark analysis

Both cracks analyzed considering various modeling techniques for crack shapeUncertainty in loading considered for Duane Arnold case


Duane Arnold Event with IGSCC

Thermal Sleeve weld

Safe end

Crevice

Thermal Sleeve weld

Safe end

Crevice

Source for Sketch: Draft MRP -140 Report


Benchmarking of KRAKFLO to Field Data

Duane Arnold Inlet Nozzle Safe End Circ. Flaw

Case # Flaw Length (in)

Bending Moment Load (in-lbs)

Crack Shape1 Leak Rate(gpm)

1 7.2 225,000 Parallel 3.07 2 7.63 225,000 Divergent 3.43 3 7.2 negligible Parallel 1.5 4 7.63 negligible Divergent 1.7

1 Parallel – length of crack is constant through the wall thickness Divergent - crack at ID plane less than crack at OD or exit plane

Note: Cases 1 and 2 (with applied moment loads) compares favorably to measured leak rate

Comparison to measured data: Duane Arnold had an unidentified leakage rate of 3 gpm into the primary containment sump.


Benchmarking of KRAKFLO to Field DataPalisades PORV Safe End Circ. Flaw

Case #

Flaw Type and Length Crack Shape1

Leak Rate(gpm)

1 6 discontinuous cracks, each 0.5” long Parallel 6 x 0.0089= 0.05

2 6 discontinuous cracks, each 0.75” long Parallel 6 x 0.0265= 0.16

3 Single crack, 1.5” long Parallel 0.17 4 Single crack, 2.25” long Parallel 0.55 5 6 discontinuous cracks, each 0.5” long Convergent 6 x 0.019

= 0.11 6 6 discontinuous cracks, each 0.55” long Convergent 6 x 0.0245

= 0.15 7 Single crack, 2.25” @ ID and 3.0” @ OD Divergent 1.12

1 Parallel – length of crack is constant through the wall thickness Convergent – crack at ID plane greater than crack at OD or exit plane Divergent - crack at ID plane less than crack at OD or exit plane

Cases 5 and 6 represent realistic crack configuration.

Comparison to measured data: Palisades observed steam leak with leak rate of ~0.2 gpm.


Phase III: -- Leak Rate Sensitivity Study

> Following successful benchmarking of KRAKFLO to field & test SCC data, a sensitivity study was performed on:

5 different LBB piping systems involving Alloy 82/182 Butt Welds

> Objective: Determine percent increase in crack length when considering crack with a SCC morphology in lieu of a fatigue crack

> The results from this limited group of piping systems showed that:

On average, the percent increase in crack length is about 37%


KRAKFLO RESULTS OF SENSITIVITY STUDY

0

50

100

150

200

250

300

350

400

450

0 25 50 75 100 125 150 175 200 225 250Length of Leaking Fatigue Crack, mm

Leng

th o

f Lea

king

SC

C C

rack

, mm

IGSCC y = 1.37x

Length of leaking SCC Crack = 1.37*length of Fatigue Crack

5 LBB Piping Systems-- RV Outlet-- RV Inlet-- Pressurizer Surge-- Decay Heat -- Cold Leg to Safe End


2005 EMC2 PVP Paper on Similar Issue

> In 1995, improved model of crack morphology parameters proposed in NUREG/CR-6004

Parameters such as surface roughness, number of turns were determined to be based on ratio of COD and global roughness. Values calculated using interpolation procedure. Actual flow path length found to depend on number of turns. Equations established using Engineering Judgment

> This model incorporated in recent version (1.1) of SQUIRT leak rate code

> Sensitivity study performed on 6 different LBB piping systems

to study %increase in crack length when SCC morphology is considered


2005 EMC2 PVP Paper on Similar Issue

> PWSCC crack morphology parameters:Determined from limited in-service cracks using photomicrographsMean and standard deviation of crack morphology parameters were used for the sensitivity analysis

> Results of their sensitivity study showed the following average increase in SCC crack lengths over fatigue crack lengths:

69% increase in crack length for PWSCC morphology, and89% increase in crack length for IGSCC morphology


Summary & Conclusions

> A 3-Phase systematic approach was used to:Benchmark KRAKFLO to

• SCC tests and • to field service data

Perform sensitivity study on 5 LBB piping systems from two NSS Vendor designsAbove evaluations performed using consistent set of crack morphology variables

> From LBB perspective: SCC crack morphology over Fatigue Cracks:

KRAKFLO predicts a 37% increase for IGSCC morphologyEMC2 predicts a 89% increase for IGSCC morphologyEMC2 predicts a 69% increase for PWSCC morphology

> Disparity in IGSCC results between EMC2 and this study needs to be investigated further

> To-date, no V&V has been performed for new version of SQUIRT with improved model for SCC



Comparison of SQUIRT to Collier’s Battelle IGSCC data:

Only published work --based on earlier version of SQUIRT CodeSQUIRT predicted results showed significant scatter in data (+/- 50%) for cracks with a COD of 220 μm.Since these cracks produced leak rates in 2 to 3 gpm range, they are important for benchmarking for LBB



> KRAKFLO has been demonstrated to be a good pre-predictive leak rate code for SCC morphology against presently available SCC data

> EMC2 2005 paper – utilized a different approach:Based on morphology parameters - statistically determined for a probabilistic evaluationRecommends mean morphology parameters New SQUIRT Version 1.1, not benchmarked to Battelle Phase II data or field service data

> This study:Benchmarked to Battelle Phase II data & field service dataUses recommended morphology parameters from this benchmarking

> In both studies LBB methodology is followed:Deterministic evaluation Considers a safety factor of 10 to account for uncertainties in geometry, materials and loading


Knowledge/Data Gap

> Lab data for PWSCC cracks similar to Battelle’sIGSCC data

> A V&V Leak Rate Code that predicts reasonably accurately PWSCC leakages.


Recommendations for Future Study

> Develop a “Best Estimate” Deterministic L.R. (DLR) Model that:

Predicts with reasonable accuracy L.R. of IGSCC/PWSCC Lab data

• Are there any known PWSCC Lab data available for Benchmarking?

• IGSCC/PWSCC should include “photo micrographs” for accurate quantification of parameters i.e. surface roughness, type & number of 45 or 90degree turns.

• Should COD dependence model such as proposed in NUREG/CR-6004 be considered? Or modified with better enhancement.

• Is there a grain size effect between IGSCC and PWSCC in weld, butter, HAZ, Alloy 600 etc?

Compare against known Field dataNew L.R. Model - Help address disparity in SCC results between KRAKFLO and SQUIRT

> Following successful documented Verif.& Valid. (V&V) process, a Probabilistic Model can then be developed from this DLR Model

01/09/2008-01/11/2008 Comparison of Leak Rates from Alloy … · 2012. 11. 30. · AREVA NP, Inc. >...

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