[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