Risk-Informed Assessment of PWSCC Issue in CANDU Feeder Pipingenvdeg2015.org/final-proceedings/ENVDEG/presentations/... · 2015. 10. 19. · PWSCC observed in PWR/BWR primary heat

›- Copyright -

›Risk-Informed Assessment of PWSCC Issue in CANDU Feeder Piping

›Xinjian Duan, Min Wang, Candu Energy Inc. ›Ming Li, Ontario Power Generation

17th International Conference on Environmental Degradation of Materials in Nuclear Power Systems-Water

Reactor, August 9-13, 2015, Ottawa, Ontario, Canada.

Background

- Copyright - © [2015] SNC-Lavalin Inc. and its member companies. All rights reserved. Unauthorized use or reproduction is prohibited. 2

PWSCC observed in PWR/BWR primary heat transport piping

Dissimilar Metal Weld (DMW)

CANDU feeder DMW: SA-106 Grade B to Alloy 600 with Alloy

82/182 filler material

Based on the EDY method, DMWs in some CANDU outlet

feeders are approaching high risk category to be susceptible to

PWSCC

Cracking inspection on DMWs is challenging due to high dose

and access limitations. As an alternative, demonstration of LBB

mode of failure is required.

oFor susceptible PWSCC, inspection is not required if LBB is demonstrated

oFor active PWSCC, inspection (scope and frequency) is required

Objective


Demonstrate LBB through a risk-informed methodology

(composite of the three methods)

oDeterministic LBB

oAFEA/XFEM flaw evaluation

oProbabilistic assessment

Methodology


›Deterministic LBB FFSG

SRP 3.6.3

›FLAW Evaluation AFEA/XFEM

›Probabilistic Assessment PRAISE-CANDU 1.0

Probabilistic

Assessment AFEA/XFEM

Flaw Evaluation

Deterministic

LBB

• Factor on Crack Length • Factor on Load • Factor on Leak Detection • Factor on time from leakage to rupture • Consequential leakage

• Realistic margin

on time from

leakage to

rupture

• Likelihood of

failure

• Effect of inspection

Deterministic LBB


›Key Inputs Geometry and material properties

Loads

oLoad Case 1 operational leakage calculation

oLoad Cases 2/3 for the stability evaluation

Leak detection limit: 25 kg/h based on operating procedure and OPEX

Leak rate model (SQUIRT), leak rate factor of 5 as per the FFSG requirement to take into account the uncertainties in leak rate calculation

Crack growth model (PWSCC and fatigue)

›Results LBB margins generally decreases with decreasing piping size

40/44 feeders satisfy the LBB requirements

4 feeder (two 2.5" + two 3") with reduced margins (3.7 – 4.8) on leak rate

Advanced FEA (AFEA) or Extended FEM (XFEM)


›Purpose Deterministic calculation

To grow the crack of a postulated inner surface crack

oConsidering the effect of Weld Residual Stresses (WRS)

oCalculating the natural or non-constrained crack shape

›Methodology AFEA: same as MRP-216

XFEM: new development

›Preliminary AFEA Results Sufficient time to credit operator action to the leakage

oTime from incipient leakage to orderly shutdown leakage (5×25 kg/h): 160 days

oTime from orderly shutdown leakage to immediate shutdown leakage (1800 kg/h): 58 days

Weld Residual Stresses-Overview


›Most uncertainty mechanical parameter

›Large experimental program to quantify this parameter

Sample Fabrication

oLiburdi Automation

Measurements

oNeutron Diffraction: Canadian Neutron Beam Centre and Rutherford

Appleton Lab

oX-ray Diffraction: Proto Manufacturing and Open University

oContour Method: Open University

FE Modeling

oCandu Energy Inc (Abaqus) and Carleton University (VrWeld)

Probabilistic Assessment


›Purpose

Provide additional technical basis for exempting inspection

Provide supplemental information for deterministic calculation (deterministic

LBB+AFEA/XFEM)

oQuantify low probability of rupture

›PFM Code – PRAISE-CANDU 1.0

Full compliance with CSA N286.7

Joint effort between Structural Integrity Associates and Candu Energy Inc.

State-of-the-art deterministic models and uncertainty treatment

›Advantages

Uncertainties: aleatory, epistemic, and combined aleatory and epistemic

Quantify the effect of crack initiation, inspection, and leak detection

PRAISE-CANDU 1.0


›Two-loop Architecture

Same as xLPR 1.0/2.0

Separation of uncertainties

o Aleatory uncertainty

o Epistemic uncertainty

o Aleatory + Epistemic

Important parameter/uncertainty study

›PC based, Monte Carlo only

o Computationally efficient

o Use of large number of CPUs

›Three Levels of Validations

o Benchmarking with other generally accepted and documented PFM codes

o Comparing PFM calculations with the rupture events of non-nuclear piping

o Phenomenon based validation

START

END

Epistemic

Loop Done?

Yes

No

Sample Epistemic

Random Variables

Aleatory Loop

Done?

Yes

No

Sample Aleatory

Random Variables

Deterministic

Models

Time Loop

PRAISE-CANDU Benchmarking on PWSCC


1E-08

1E-07

1E-06

1E-05

1E-04

1E-03

1E-02

1E-01

1E+00

0 5 10 15 20 25

P(L

OC

A)

Time (years)

PRAISE-CANDU

WinPRAISE

0

0.2

0.4

0.6

0.8

1

0 10 20 30 40 50 60P

rob

ab

ilit

y o

f R

up

ture

Time (years)

95 percentile: PRAISE-CANDU 1.0

95 percentile: xLPR 1.0

Mean: PRAISE-CANDU 1.0

Mean: xLPR 1.0

›Excellent Agreement with

WinPRAISE

xLPR 1.0

PRAISE-CANDU vs WinPRAISE PRAISE-CANDU vs xLPR 1.0

PWSCC Initiation Model 1


›Amzallag et al (1999)

i: temperature index, exp(-Qi/RT)

i: stress index, pn

im: material index

A: model coefficient, including heat-to-heat and within-heat uncertainties

QI: activation energy, calibrated to be 193 kJ/mol

pn: applied stress

𝑡𝐼1

𝑖𝑖𝑖𝑚

𝑡𝐼 =1

𝐴𝑖𝑚𝑝𝑛e𝑄𝐼 R𝑇 for 𝑝 > 𝑡ℎ

Results


The rupture probability is low, < 1.410-8

HAZ (Alloy 600) has higher rupture probability than weld center

(Alloy 82)

1E-09

1E-08

1E-07

1E-06

1E-05

1E-04

1E-03

1E-02

1E-01

1E+00

0 5 10 15 20 25

Pro

ba

bil

ity

Time (a)

P(Init) P(TW) P(LOCA)

1E-09

1E-08

1E-07

1E-06

1E-05

1E-04

1E-03

1E-02

1E-01

1E+00

0 5 10 15 20 25

Pro

ba

bil

ity

Time (a)

P(Init) P(TW) P(LOCA)

Weld Center (Alloy 82) HAZ (Alloy 600)

Effect of Activation Energy


Activation Energy, QI = 0, i.e., multipel pre-existing initial cracks

The rupture probability is low, < 1.010-4

1E-09

1E-08

1E-07

1E-06

1E-05

1E-04

1E-03

1E-02

1E-01

1E+00

0 5 10 15 20 25

P(L

OC

A)

Time (a)

QI = 193 kJ/mol QI = 0

Effect of Leak Detection Limit


25 kg/h vs. 50 kg/h, 125 kg/h, no leak detection

Rupture probability increases

1E-09

1E-08

1E-07

1E-06

1E-05

1E-04

1E-03

1E-02

1E-01

1E+00

0 5 10 15 20 25

P(L

OC

A)

Time (years)

No leak detection

OLRL = 125 kg/h

OLRL = 50 kg/h

OLRL = 25kg/h

Power Law Model

Effect of In-Service Inspection


No inspection vs. inspection every 3 years with good POD

Rupture probability slightly decreases

1E-09

1E-08

1E-07

1E-06

1E-05

1E-04

1E-03

1E-02

0 5 10 15 20 25

P(L

OC

A)

Time (years)

No Inspection Good POD

Parameter Ranking


Activation energy (activation energy 193 vs 0 KJ/mol ), leak

detection capability (25 vs 125 kg/h) and welding residual stress

are top three factors .

0

4

8

12

16

20Im

po

rta

nce

Factors

Uncertainty Separation


Forty billion (40×109) Monte Carlo simulations

Uncertainty of mean residual stress being epistemic uncertainty

Rupture probability: 5.9510-8 at 95th percentile vs. 1.4010-8 at

mean value

Conclusions


A tiered composite RI-informed LBB assessment has been

developed and applied to PWSCC issue on feeder DMW. The

results have been conditional accepted by the regulator.

Deterministic LBB assessment demonstrates 90% DMWs satisfy

the LBB requirements.

AFEA demonstrates sufficient margins on time from leak to

rupture for the remaining 10% DMWs.

Probabilistic assessment demonstrates the probability of rupture

is low.

The sensitivity study shows the PWSCC initiation model, leak

detection capability, and WRS are important, while the in-service

inspection has little impact on the rupture probability.

The uncertainty can be separated through two-loop PFM code,

PRAISE-CANDU 1.0 for prioritizing the resources.

Acknowledgement


CANDU Owners Group and Ontario Power Generation for the

funding the work.

Documents

Risk-Informed Assessment of PWSCC Issue in CANDU Feeder Pipingenvdeg2015.org/final-proceedings/ENVDEG/presentations/... · 2015. 10. 19. · PWSCC observed in PWR/BWR primary heat