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© 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 ]
3© 2016 Electric Power Research Institute, Inc. All rights reserved.
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
4© 2016 Electric Power Research Institute, Inc. All rights reserved.
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
5© 2016 Electric Power Research Institute, Inc. All rights reserved.
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)
6© 2016 Electric Power Research Institute, Inc. All rights reserved.
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
7© 2016 Electric Power Research Institute, Inc. All rights reserved.
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
8© 2016 Electric Power Research Institute, Inc. All rights reserved.
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
9© 2016 Electric Power Research Institute, Inc. All rights reserved.
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
10© 2016 Electric Power Research Institute, Inc. All rights reserved.
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
11© 2016 Electric Power Research Institute, Inc. All rights reserved.
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:
12© 2016 Electric Power Research Institute, Inc. All rights reserved.
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
13© 2016 Electric Power Research Institute, Inc. All rights reserved.
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
14© 2016 Electric Power Research Institute, Inc. All rights reserved.
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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