[Federal Register Volume 59, Number 40 (Tuesday, March 1, 1994)]
[Unknown Section]
[Page 0]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 94-4555]
[[Page Unknown]]
[Federal Register: March 1, 1994]
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NUCLEAR REGULATORY COMMISSION
[Docket Nos. 50-295 and 50-304]
Commonwealth Edison Company (Zion Nuclear Power Station, Unit
Nos. 1 and 2)
Exemption
I
The Commonwealth Edison Company (the licensee), is the holder of
Facility Operating License Nos. DPR-39 and DPR-48 which authorize
operation of Zion Nuclear Power Station, Units 1 and 2, at a steady-
state power level not in excess of 3250 megawatts thermal. The facility
consists of two pressurized water reactors located at the licensee's
site in Lake County, Illinois. The licenses provide, among other
things, that they are subject to all rules, regulations and orders of
the Nuclear Regulatory Commission (the Commission) now and hereafter in
effect.
II
In a letter dated December 3, 1993, the licensee provided an
assessment of the reference temperature for pressurized thermal shock
(RTPTS) for the design life (32 effective full power years) for
the Zion Nuclear Power Station Units 1 and 2 (Zion 1 and 2) reactor
vessels and requested an exemption from determining the unirradiated
reference temperature (initial RTNDT) in accordance with NB-2331
of Section III of the ASME Boiler and Pressure Vessel Code (ASME Code),
as specified in 10 CFR 50.61(b)(2)(i). Prior correspondence commenced
with the licensee's letter dated December 13, 1991, that replied to the
amendment to 10 CFR 50.61 which was published in the Federal Register
on May 15, 1991, (56 FR 22300). In a letter dated March 13, 1992, the
licensee provided its flux reduction program to ensure the
intermediate-to-lower shell circumferential weld for Zion Unit 1 would
remain less than the screening criterion through 32 EFPY. In a letter
dated May 22, 1992, the licensee used data provided by the Babcock and
Wilcox Owners' Group (B&WOG) to address the initial RTNDT and
RTPTS for the Zion Unit 1 and 2 reactor pressure vessels (RPVs).
With this data, the licensee was able to show that the RPVs will
satisfy the pressurized thermal shock (PTS) screening criteria through
32 EFPY. After reviewing the licensee's submittals, the staff requested
additional information in a letter dated December 2, 1992. The licensee
responded in a letter dated January 28, 1993. On June 9, 1993, the
staff met with the licensee to discuss the performance of a modified
analysis utilizing improved analytical techniques. In a letter dated
September 1, 1993, the licensee provided a summary report demonstrating
that the Zion RPVs will not exceed the end of life PTS screening
criteria. In another letter dated October 5, 1993, the licensee
detailed the development of the methodology utilized in performing the
PTS evaluation for the Zion RPVs.
III
The Pressurized Thermal Shock (PTS) rule, 10 CFR 50.61, ``Fracture
toughness requirements for protection against pressurized thermal shock
events,'' adopted on July 23, 1985, establishes screening criteria that
define a limiting level of embrittlement beyond which operation cannot
continue without further plant-specific evaluation. The screening
criteria are given in terms of reference temperature, RTPTS. The
screening criteria are 270 deg.F for plates and axial welds and
300 deg.F for circumferential welds. The RTPTS is defined as the
sum of (a) the unirradiated reference temperature, (b) the margin to be
added to cover uncertainties in the initial properties, copper and
nickel contents, fluence, and calculation procedures, and (c) the
increase in RTPTS caused by irradiation. The amount of increase in
RTPTS is based on the amount of neutron irradiation and the amount
of copper and nickel in the material. The greater the amounts of
copper, nickel and neutron fluence, the greater the increase in
RTPTS for the material and the lower its fracture resistance. The
PTS rule requires that the unirradiated reference temperature be
determined from measurements as defined in the ASME Code, Section III,
Paragraph NB-2331. The amount of margin is dependent on whether: (a)
The material is a weld or a base metal, (b) the unirradiated reference
temperature is a generic value or a measured value, and (c) the
increase in RTPTS is from credible surveillance material or is
from the chemistry factor tables in the PTS rule.
The PTS rule was amended on May 15, 1991. The amended rule changed
the method of calculating embrittlement to the method recommended in
Regulatory Guide (RG) 1.99, Revision 2, ``Radiation Embrittlement of
Reactor Vessel Materials'', and requires licensees to consider the
effect of reactor vessel operating temperature and surveillance results
on the calculated RTpts value. The licensee provided this
assessment in a letter dated July 2, 1992, which contained the
licensee's response to Generic Letter (GL) 92-01, Revision 1, ``Reactor
Vessel Structural Integrity, 10 CFR 50.54(f)''. The purpose of GL 92-01
was to obtain information needed to assess compliance with requirements
set forth in 10 CFR Part 50, Appendices G and H and commitments made in
response to GL 88-11 regarding reactor vessel structural integrity. The
licensee's responses to GL 92-01 are being evaluated and will be
resolved as an issue separate from this exemption request.
Pressurized Thermal Shock (PTS) Evaluation
Licensee's Evaluation
The licensee reports that the beltline of each reactor vessel
consists of a forging, four plates, four longitudinal welds and three
circumferential welds. There are sufficient records to identify the
heat number and chemical composition (percentage copper and nickel) of
all beltline materials.
Unirradiated Reference Temperature
The unirradiated reference temperature for the beltline forgings
and plates was determined from test results from the materials. The
licensee used a generic value (-5 deg.F) for the unirradiated reference
temperature of all beltline weld metals, with the exception of the weld
metal identified as WF-70. The unirradiated reference temperature for
WF-70 weld metal was determined from drop weight tests and fracture
toughness tests from welds fabricated with WF-70 and WF-209-1 weld
metal. Since WF-70 and WF-209-1 welds were fabricated using the same
heat number of weld wire and the same type of flux, their material
properties are considered equivalent. The licensee's data will be
discussed in the Staff Evaluation of Unirradiated Reference Temperature
for WF-70.
The unirradiated reference temperature that is defined in Section
III of the ASME Code, Paragraph NB-2331 is determined from Charpy V-
notch (CVN) impact and drop weight tests. These tests have been
performed on WF-70 weld metal by the licensees for Zion and Oconee, the
B&WOG and Oak Ridge National Laboratory (ORNL). The test results
indicate that the unirradiated reference temperature varies from
-3 deg.F to +123 deg.F with a standard deviation of 43.1 deg.F and a
mean value of 49 deg.F. This wide variability was a surprise to the
staff because welds similar to WF-70 were reported to have a mean value
of -4.8 deg.F and a standard deviation of 19.7 deg.F. The staff
believes that the large uncertainty in unirradiated reference
temperature for WF-70 weld metal is due to the low upper-shelf behavior
of the material and that the definition of unirradiated reference
temperature in the ASME Code is not applicable for material with low
upper-shelf behavior like WF-70 weld metal. The licensee has proposed
to determine the unirradiated reference temperature from drop weight
and fracture toughness tests instead of the method defined in Section
III of the ASME Code. The licensee proposes to define the unirradiated
reference temperature as equal to the sum of: (a) the mean value for
the nilductility transition temperature, TNDT, from the drop
weight test data from WF-70 and WF-209-1 weld and (b) the two standard
deviation value determined from the drop weight test data. This method
results in a mean value for the TNDT of -56 deg.F and a standard
deviation of 14.8 deg.F for WF-70 weld metal. Using these values of
TNDT and standard deviation, the unirradiated reference
temperature is -26 deg.F for WF-70 weld metal. Since the licensee has
not followed the method in Section III of the ASME Code, the licensee's
method for determining the unirradiated reference temperature of WF-70
does not meet the requirements of 10 CFR 50.61. The licensee has,
therefore, requested an exemption from the requirement to determine the
unirradiated reference temperature (initial RTNDT) in accordance
with NB-2331 of Section III of the ASME Boiler and Pressure Vessel Code
(ASME Code), as specified in 10 CFR 50.61(b)(2)(i).
Increase in RTPTS and Margin
The increase in RTPTS for each beltline material, except WF-70
weld metal, was determined using the chemistry factor tables in the PTS
rule. The increase in RTPTS for WF-70 weld metal was determined
from Charpy impact tests on WF-70 weld metal irradiated in the Zion
Units 1 and 2 surveillance capsules. The increase in RTPTS for WF-
70 weld metal was determined using the methodology documented in
Section 2.1 of RG 1.99, Revision 2.
The amount of margin for each beltline plate and forging was the
amount identified in the PTS rule for base metal with measured
unirradiated reference temperature. The amount of margin for each
beltline weld, with the exception of WF-70, was the amount identified
in the PTS rule for weld metal with generic values of unirradiated
reference temperature. The amount of margin for WF-70 weld metal was
determined using the standard deviation for the increase in RTPTS
from irradiation in RG 1.99, Revision 2, when credible surveillance
data is available. This results in a margin value of 28 deg.F for WF-70
weld metal.
Paragraph 10 CFR 50.61(b)(3) requires that RTPTS values which
are modified by surveillance data be approved by the Director, Office
of Nuclear Reactor Regulation. The staff believes that using the
methodology in RG 1.99, Revision 2 for determining the increase in
RTPTS from surveillance material is an acceptable alternative to
the value determined from the chemistry factor tables in the PTS rule.
The staff believes that the amount of margin for WF-70 should be the
amount determined using the standard deviation for the increase in
RTPTS from irradiation in RG 1.99, Revision 2. This results in a
margin value of 28 deg.F and an unirradiated reference temperature of
-26 deg.F for WF-70. The reasons for not including the uncertainty of
the unirradiated reference temperature in the margin, but adding it to
the TNDT will be discussed in the Staff Evaluation of Unirradiated
Reference Temperature for WF-70.
RTPTS at Expiration of the Zion 1 and 2 licenses
The licensee has projected that at the expiration of their
licenses, WF-70 weld metal in Units 1 and 2 will have RTPTS values
of 230 deg.F and 172 deg.F, respectively. Both these values are
significantly below the PTS screening criteria in the PTS rule. As a
result of the licensee's evaluation of WF-70 weld metal, the limiting
material in Unit 1 is a circumferential weld fabricated using WF-154
weld metal and the limiting material in Unit 2 is a circumferential
weld fabricated using SA-1769 weld metal. The RTPTS values for
these welds at the expiration of the Units 1 and 2 licenses are
268 deg.F and 269 deg.F, respectively. Both of these values are
significantly below the PTS screening criterion, 300 deg.F, in the PTS
rule.
Staff Evaluation of Unirradiated Reference Temperature for WF-70
As discussed previously, the licensee and the B&WOG have concluded
that determination of unirradiated reference temperature via the CVN
procedure of NB-2331 of Section III of the ASME Code is not appropriate
for the Zion beltline welds fabricated with WF-70 weld metal. The staff
recognizes that the ASME Code procedure, when applied to lower upper
shelf materials such as WF-70, may not produce a reasonable
determination of unirradiated reference temperature. The staff has,
therefore, encouraged the licensee to pursue alternate approaches to
determine the unirradiated reference temperature for WF-70. The
approach selected by the licensee and the B&WOG involves analysis of
WF-70 fracture toughness data in accordance with the Draft ASTM
Standard on Fracture Toughness in the Transition Range (Draft 5, Rev.
3-3-93). The purpose of the licensee's analysis is to demonstrate that
the above methodology ``bounds'' the fracture toughness data and can be
indexed to the ASME fracture toughness reference curves. The indexing
to either the KIC or KIR curves is used to show that the
reference temperature determined from drop weight tests provides an
appropriate unirradiated reference temperature for WF-70.
At a meeting with the licensee on June 9, 1993, the staff
acknowledged the merit of the ASTM approach and encouraged the licensee
to pursue it to completion. At that time, the staff also indicated that
the licensee should consider constraint adjustments and strain rate
effects on the data. In particular, the staff questioned the basis for
directly indexing the Babcock and Wilcox (B&W) dynamic fracture
toughness data to the ASME KIR curve with respect to the differing
strain rates involved in generation of the data. The licensee
subsequently submitted a B&W report (BAW-2202, September, 1993) which
addresses its revised analysis for the determination of the
unirradiated reference temperature.
The staff has independently evaluated the data provided in BAW-2202
and the previous report (BAW-2100, January, 1993) in accordance with
the Draft ASTM Standard on Fracture Toughness in the Transition Range.
The staff analysis, presented in the attached Figure 1, considered both
constraint and rate effects on the data. Figure 1 presents the B&W
dynamic fracture toughness data as the open symbols. The solid symbols
represent the same data constraint corrected using the procedure
suggested by Anderson and Dodds, 1993. The ASTM curves (Kjc
median, 95% CL and lower bound) were derived from the constraint-
corrected data at 0 deg.F where it can be seen that the magnitude of
the correction was small. It is seen that the ASTM Kjc lower bound
curve effectively bounds all of the data with the possible exception of
the constraint-corrected point at +132 deg.F. However, the specimen at
+132 deg.F exhibited a significant amount of ductile tearing prior to
failure by cleavage. It is known that the Anderson-Dodds procedure will
``over-correct'' for constraint in such instances.
With respect to strain rate effects, the B&W dynamic data were
generated at a rate of approximately 7 x 104 ksi in/sec.
This rate is on the threshold of the rates achieved in the crack arrest
tests which constitute the ASME KIR curve. Figure 1 also shows a
direct comparison between the B&W dynamic fracture toughness data and
some recently available crack arrest data on WF-70 from the ORNL. While
the crack arrest data are generally conservative in comparison to the
B&W data, it is seen that the ASTM Kjc lower bound curve also
bounds the ORNL data. On the basis of this analysis, the staff finds
the methodology of indexing the B&W dynamic data to the KIR curve
acceptable.
In conclusion, the staff analysis which addresses constraint and
rate effects has shown the fracture toughness based procedure for
determination of unirradiated reference temperature to be acceptable
for WF-70. As shown in Figure 1, the ASME KIR curve, with a
reference temperature of -26 deg.F bounds all of the constraint-
adjusted data and the ASTM curves up to approximately 140 deg.F. This
analysis therefore supports an unirradiated reference temperature of
-26 deg.F for the WF-70 material.
Other procedures for determination of RTNDT may serve as
acceptable alternatives to NB-2331 contingent on staff review and
approval. However, it should be noted that the staff acceptance of the
alternative procedure in this evaluation was contingent on the analysis
of a significant amount of fracture toughness data for the WF-70 weld
metal. Acceptance of such a procedure in a case where little or no
fracture toughness data were available would be difficult in the
absence of an officially sanctioned consensus standard.
As part of the resolution of low-upper-shelf reference temperature
issues on a generic basis, the ASME Code has tasked the Failure Modes
of Components Committee of the Pressure Vessel Research Council (PVRC)
to consider alternate procedures for the determination of unirradiated
reference temperature. To this end, the PVRC recently held a \1/2\ day
workshop on ``KIR Curves and RTNDT'' on October 11, 1993,
where the ASTM fracture toughness based approach was highlighted. As a
result of the workshop, it is expected that the Committee will be able
to make recommendations to the ASME Code by December 31, 1994.
Irradiation Temperature and Surveillance Material Test Results
The methods of calculating the increase in RTPTS in the PTS
rule and in RG 1.99, Revision 2 were empirically derived from
surveillance data from U.S. commercially operated nuclear reactor
vessels. The methods are valid for a nominal irradiation temperature of
550 deg.F. Irradiation below 525 deg.F is considered to produce
embrittlement greater than the values predicted in the PTS rule and RG
1.99, Revision 2.
In its response to GL 92-01, the licensee reported that the cold
leg temperature during nuclear systems power operation varied linearly
between 547.0 deg.F at 0 percent power and 529.4 deg.F at 100 percent
power. Hence, irradiation occurred at temperatures exceeding 525 deg.F
and the methodologies in the PTS rule and RG 1.99, Revision 2 are
applicable to Zion Units 1 and 2.
Regulatory Guide and 1.99, Revision 2 indicates that about a best-
fit line to the surveillance data, scatter should be less than 28 deg.F
for welds and for fluence of two or more orders of magnitude, the
scatter should be less than 56 deg.F. Zion 1 has four irradiated
surveillance data points and Zion 2 has three irradiated surveillance
data points from WF-70 weld metal. The maximum difference between the
measured increase in reference temperature and the best fit line is
20 deg.F. Since this is less than 28 deg.F, the increase in RTPTS
and the associated standard deviation may be based on the methodology
in Section 2.1 of RG 1.99, Revision 2.
Conclusions
Based on the Zion 1 and 2 irradiation temperature and surveillance
data, the methodologies in the PTS rule and RG 1.99, Revision 2 are
applicable to Zion 1 and 2. As a result of its review, the staff
concludes that the licensee's method of determining the unirradiated
reference temperature is an acceptable alternative to the method
described in NB-2331 of Section III of the ASME Code because staff and
licensee analyses indicate that the fracture toughness data are bounded
by the ASME KIR curve with an unirradiated reference temperature
of -26 deg.F. However, since the unirradiated reference temperature was
not determined in accordance with the method in Section III of the ASME
Code, an exemption to the PTS rule is required. The RTPTS values
for all beltline materials will be below the PTS screening criteria
when the Zion 1 and 2 licenses expire. 10 CFR 50.12(a)(1) allows the
Commission to grant exemptions which are authorized by law, will not
present an undue risk to the public health and safety, and are
consistent with the common defense and security. Since the licensee's
method of determining the unirradiated reference temperature is an
acceptable alternative to the method in NB-2331 of Section III of the
ASME Code, RTPTS values for WF-70 weld metal that are calculated
using the licensee's method are authorized by law and will not present
an undue risk to the public health and safety and are consistent with
the common defense and security. For the same reason, the staff finds
that application of the regulation would not serve the underlying
purpose of the rule, which is to ensure that reactor pressure vessels
in service are not susceptible to fracture as a result of pressurized
thermal shock. On this basis, the staff finds that the licensee has
demonstrated that there are special circumstances present as required
by 10 CFR 50.12(a)(2).
References
(1) ``Properties of Weld Wire Heat Number 72105 (Weld Metals WF-
70 and WF-209-1), BAW-2100, January, 1993.
(2) ``Test Practice (Method) for Fracture Toughness in the
Transition Range,'' Draft 5, Rev. 3-3-93, presented at the ASTM E08
Committee Meetings, Atlanta, GA, May, 1993.
(3) ``Fracture Toughness Characterization of WF-70 Weld Metal,''
BAW-2202, September, 1993.
(4) ``Simple Constraint Corrections for Subsize Fracture
Toughness Specimens,'' T.L. Anderson and R.H. Dodds, Jr., ASTM STP
1024, 1993, pp. 93-105.
IV
Accordingly, the Commission has determined that, pursuant to 10 CFR
50.12, an exemption is authorized by law and will not endanger life or
property or the common defense and security and is otherwise in the
public interest and hereby grants the following exemption with respect
to a requirement of 10 CFR 50.61:
For Zion Nuclear Power Station, Units 1 and 2, the licensee's
method of determining the unirradiated reference temperature (initial
RTNDT) from drop weight and fracture toughness tests is an
acceptable alternative to the method in NB-2331 of Section III of the
ASME Code as specified in 10 CFR 50.61(b)(2)(i).
Pursuant to 10 CFR 51.32, the Commission has determined that the
granting of the subject exemption will not have a significant effect on
the quality of the human environment (59 FR 4727).
Dated at Rockville, Maryland this 22nd day of February 1994.
This exemption is effective upon issuance.
For the Nuclear Regulatory Commission.
Jack W. Roe,
Director, Director of Reactor Projects III/IV/V, Office of Nuclear
Reactor Regulation.
[FR Doc. 94-4555 Filed 2-28-94; 8:45 am]
BILLING CODE 7590-01-M
