estimation of initial rtndt for older reactor pressure … · © 2016 electric power research...

$: Estimation of Initial RTNDT for Older Reactor Pressure … · © 2016 Electric Power Research Institute, Inc. All ... the ASME Boiler & Pressure ... initial fracture toughness values$
© 2016 Electric Power Research Institute, Inc. All rights reserved.

W. Server1, T. Hardin2, N. Palm2,

and R. Gamble3

International Light Water Reactor Materials

Reliability Conference and Exhibition 2016

Chicago, IL, 3 August 2016

Estimation of Initial RTNDT

for Older Reactor Pressure

Vessels

1ATI Consulting 2EPRI 3Sartrex Corp.

2© 2016 Electric Power Research Institute, Inc. All rights reserved.

Background (1/2)

Until 1973, the ASME Boiler & Pressure Vessel Code, Section III (the Code) required that materials used in the pressure-retaining components of reactor pressure vessels (RPVs) be qualified by Charpy V-notch (CVN) impact tests on specimens oriented in the strong direction (L-T)– At 60°F below lowest service temperature, 3 CVN

specimens must average 30 ft-lb (41 J) with no single test less than 25 ft-lb (34 J)

In Summer 1972 Addenda, the Code revised approach and adopted Linear Elastic Fracture Mechanics with toughness characterized by RTNDT

RTNDT is defined by Pellini TNDT and Tcv (lower-bound Charpy T50 ft-lb (68 J) / T35 mils lateral expansion) for CVN specimens oriented in weak direction (T-L)

RTNDT = MAX [ TNDT, Tcv - 60°F ]


Background (2/2)

10 CFR 50, Appendix G, introduced in August 1973, required all U.S. operating power reactors to assess vessel integrity based on weak direction (T-L) properties– RTNDT; Upper Shelf Energy limits for beltline (weak direction)

For the vessels fabricated when only strong direction data was required, the U.S. Nuclear Regulatory Commission (NRC) published methods for estimating weak direction properties from strong direction (L-T) data: Branch Technical Position 5-3 (BTP 5-3, also known as MTEB 5-2)

In early 2014, a vendor reported to the NRC that BTP 5-3 for determining Initial RTNDT was potentially non-conservative and published paper PVP2014-28897


Evaluation of Conservatism of BTP 5-3

EPRI Project team assembled enhanced database: plate and

forging materials with measured CVN properties in both weak

(T-L) and strong (L-T) directions

– For each plate and forging material, weak direction properties were

estimated from the strong data using BTP 5-3 prediction methods

– Predicted weak properties were then compared to the measured

weak properties to determine conservatism or non-conservatism

– Comparison indicated that in some cases the BTP 5-3 methods were

less conservative than expected, but in line with prior observations

– The standard errors associated BTP 5-3 methods were quantified

– Regression analyses of the data also were performed for use in

probabilistic fracture mechanics (PFM) analyses using FAVOR


Example: Analysis of BTP 5-3 B1.1(3) – Forgings

B1.1(3) is a method to estimate weak-direction (T-L) T50 from strong-direction T50 (L-T)

Two different populations of data were clearly evident

Preliminary evaluation suggested populations may be differentiated by L-T USE [approx. 140 ft-lb (190 J)]

Further evaluation showed that all but one low USE forging were Rotterdam Drydock (RDM) forgings, and one non-RDM low USE forging was well-predicted by BTP 1.1(3)

Consequently, forgings were categorized as either RDM or non-RDM Note: T50 is the temperature at which

minimum Charpy energy is 50 ft-lb (68 J)


RPV Integrity Assessment

Objective was to assess the effect of uncertainty in Initial

RTNDT estimated from BTP 5-3 on Appendix G P/T limit

curves

– Is there a safety need for requiring plants to change P-T curves to

address the uncertainty in Initial RTNDT?

– How does uncertainty in initial properties affect overall structural

integrity?

Also check on deterministic calculation of RTPTS for most

limiting plate material in the PWR fleet


How Uncertainty in Initial RTNDT is Evaluated

Uncertainty in RTNDT is accounted for in the margin terms of

RTPTS and ART

P-T limit curves are based on the vessel material with

highest Adjusted Reference Temperature (ART), defined as

ART = Initial RTNDT + ΔRTNDT + Margin

Margin is defined as

– Margin = 2 x (σi2+ σΔ

2)0.5,

σi is the uncertainty in RTNDT(u) and

σΔ is the uncertainty in shift due to irradiation, ΔRTNDT


RPV Integrity Assessment

Currently, NRC and Industry assume σi = 0 whenever BTP 5-3 was used; it was assumed the BTP provided a conservative estimation

The issue: if BTP 5-3 is potentially non-conservative, then σi ≠ 0

– Plus, it is known that some uncertainty in RTNDT(u) does exist

– From a vessel safety perspective, how important is it to account for that uncertainty?

Metric used to assess the safety need to revise RPV integrity limits:

– Using probabilistic fracture mechanics (PFM) analyses, compare the conditional probability of failure (CPF) and the change in CPF (∆CPF) resulting from cooldown on the P-T curve for a vessel assumed to have a small surface-breaking flaw on the inside surface, for the 2 cases of Initial RTNDT:

– Current BTP method (e.g., σi = 0⁰F), and

– Mean Initial RTNDT and standard error for Initial RTNDT determined from regression analysis of experimental data


Materials Evaluated in RPV Integrity Assessment

Non-RDM forging with the highest 60-year Adjusted

Reference Temperature (ART) in the PWR fleet

– Limiting material can be in either the RPV beltline (highest fluence

region of the RPV shell) or the extended beltline (lowest fluence

region of the RPV shell)

RDM forging with the highest 60-year ART in the PWR fleet

– Limiting material is in the RPV extended beltline

RPV beltline plate with the highest 60-year ART in the PWR

fleet

– This case will be presented in following slides


Initial RTNDT for the Beltline Plate with the Highest 60-year ART

in the PWR Fleet

Mean RTNDT(u) = BTP estimate;σi = 0⁰F; EOLE ART = 237 ⁰F

Mean RTNDT(u) = BTP estimate; σi = 19.9⁰F; EOLE ART = 256 ⁰F

Mean RTNDT(u) = data regression

estimate; σi = 15.1⁰F; EOLE ART = 243 ⁰F

RPV wall thickness = 8.6-inch;

R/t = 10

Postulated circumferential inside

surface flaw with depth = 3% wall

thickness -100

-80

-60

-40

-20

0

20

40

60

80

100

-100 -80 -60 -40 -20 0 20 40 60 80 100

Init

ial R

TN

DT

= A

ctu

al T

ran

sver

se C

VN

50

ft-

lb T

emp

erat

ure

-6

0,

⁰F

Initial RTNDT = Estimated Transverse CVN 50 ft-lb Temperature - 60, ⁰F

Actual = Estimated; 1.1(3) (a) & (b); σi = 0⁰F or 19.9⁰F

Regression Mean RTNDT(u) for 1.1(3)(b); σi = 15.1⁰F

IS-P1; Regression mean = 24⁰F

IS-P2 & IS-P3; Regression Mean = 15.7⁰F

LS-P4; Regression mean = 15.1⁰F

LS-P5; Regression mean = -10.3⁰F

RPV beltline plate


0.0

0.5

1.0

1.5

2.0

2.5

0 100 200 300 400 500 600

Pre

ssu

re, k

si

Temperature at Vessel ID, ⁰F

RTNDT(u) = 21⁰F; σi = 0⁰F; Margin = 34⁰F; ART = 237⁰F

RTNDT(u) = 21⁰F; σi = 19.9⁰F; Margin = 52.3⁰F; ART = 256⁰F

RTNDT(u) = 15.7⁰F; σi = 15.1⁰F; Margin = 45.5⁰F; ART = 243⁰F

RPV beltline - Rolled and welded plateWall thickness = 8.62-inch R/t = 1050⁰F/hr cooldown from 523⁰F to 60⁰F

The P/T Limit Curves are input into the FAVOR software for the PFM CPF and ∆CPF analyses

Blue curve is based on the

current limiting material as

determined from the BTP

with σi = 0; black curve is

based on limiting ART using

regression analysis

Appendix G P/T Limit Curves for the Beltline Plate with the

Highest 60-year ART in the PWR Fleet

Curves are

generated per

ASME Section XI

Appendix G, and

ART is used as

the RTNDT:


Comparison of CPF and ∆CPF from BTP Current Practice with

Regression Analysis – Beltline Plate with Highest 60-year ART

PFM analyses using FAVOR

were performed for

cooldowns along each P-T

curve; the risks of failure

were compared

The cooldown transient for a

vessel with a postulated

small surface flaw has been

shown to be the highest

failure risk condition 1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

0 10 20 30 40 50 60 70 80 90 100

Co

nd

itio

nal

Pro

bab

ility

of

Ves

sel F

ailu

re, C

PF,

or

∆C

PF

Percent EOLE

CPF: Appendix G, mean = 21⁰F, σi = 0⁰F: RPV, mean = 21⁰F, σi = 19.9⁰F

CPF: Appendix G, mean = 21⁰F, σi = 19.9⁰F: RPV, mean = 21⁰F, σi = 19.9⁰F

∆CPF: Appendix G mean = 21⁰F, σi = 0⁰F or mean = 21⁰F, σi = 19.9⁰F

CPF: Appendix G, mean = 21⁰F, σi = 0⁰F: RPV, mean = 15.7⁰F, σi = 15.1⁰F

CPF: Appendix G, mean = 15.7⁰F, σi = 15.1⁰F: RPV, mean = 15.7⁰F, σi = 15.1⁰F

∆CPF: Appendix G, mean = 21⁰F, σi = 0⁰F or mean = 15.7⁰F, σi = 15.1⁰F

RPV beltline - rolled and welded plateWall thickness = 8.62-inch

R/t = 1050⁰F/hr cooldown from 523⁰F to 60⁰F along the

Appendix G P/T limit curves

→The insignificant difference in CPF shows that even rather large uncertainties in

initial fracture toughness values have negligible impact on vessel failure risk


RTPTS Evaluation for PWR Beltline Plate Closest to

Exceeding PTS Screening Criterion at 60 Years

→The RTPTS calculated from the mean RTNDT and associated σi obtained

from regression analysis of data specific to BTP B1.1(3)(b) shows this

beltline plate does not exceed PTS screening criteria of 10CFR50.61

through EOLE


RPV Integrity Assessment Conclusions

The ∆CPF associated with using either the BTP or regression

analysis to define Initial RTNDT is very small (less than 1E-7)

– Consequently, there is negligible safety benefit to be gained by changing the

BTP 5-3 procedure for estimating Initial RTNDT or its application for defining P-T

limit curves

– Uncertainty in initial fracture toughness values has negligible impact on vessel

failure risk

– These conclusions have been demonstrated for three material classifications:

plate, non-RDM forgings, and RDM forgings (in both the beltline and extended

beltline regions)

RTPTS assessment also shows that the most limiting PWR plate

material does not exceed the PTS screening limit at 60-year EOLE


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