Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555-0001
August 3, 1995
NRC GENERIC LETTER 95-05: VOLTAGE-BASED REPAIR CRITERIA FOR WESTINGHOUSE
STEAM GENERATOR TUBES AFFECTED BY OUTSIDE DIAMETER
STRESS CORROSION CRACKING
Addressees
All holders of operating licenses or construction permits for
pressurized-water reactors (PWRs).
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this generic letter to
give guidance to licensees who may wish to request a license amendment to the
plant technical specifications to implement alternate steam generator tube
repair criteria applicable specifically to outside diameter stress corrosion
cracking (ODSCC) at the tube-to-tube support plate intersections in
Westinghouse-designed steam generators having drilled-hole tube support plates
(TSPs) and alloy 600 steam generator tubing. In the past, the NRC has allowed
some licensees to implement alternate steam generator repair criteria for this
particular degradation mechanism on an operating cycle-specific basis. This
generic letter does not restrict the approval of such repair criteria to a
cycle specific basis. It is expected that recipients will review this
information for applicability to their facilities and consider actions, as
appropriate, to implement the alternate criteria. However, suggestions
contained in this generic letter are not NRC requirements; therefore, no
specific action or written response is required.
Background
The tubing of the steam generator constitutes more than half of the reactor
coolant pressure boundary (RCPB). Design of the RCPB for purposes of
structural and leakage integrity is a requirement under Title 10 of the Code
of Federal Regulations Part 50 (10 CFR Part 50), Appendix A. Specific
requirements governing the maintenance of steam generator tube integrity are
in plant technical specifications and in Section XI of the American Society of
Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (ASME Code).
These include requirements for periodic inservice inspection of the tubing,
flaw acceptance criteria (i.e., repair limits for plugging or sleeving), and
primary-to-secondary leakage limits. These requirements, coupled with the
broad scope of plant operational and maintenance programs, have formed the
basis for ensuring adequate steam generator tube integrity.
9507310085. GL 95-05
August 3, 1995
Page 2 of 7
Flaw acceptance criteria, termed "plugging" or "repair limits," are specified
in the plant technical specifications. Current plant technical specifications
require that flawed tubes be removed from service by plugging or be repaired
by sleeving, if the depths of the flaws exceed the repair limit, typically
40 percent through-wall. The technical specification repair limits ensure
that tubes accepted for continued service will retain adequate structural and
leakage integrity during normal operating, transient, and postulated accident
conditions, consistent with General Design Criteria (GDCs) 14, 15, 30, 31, and
32 of 10 CFR Part 50, Appendix A. Structural integrity refers to maintaining
adequate margins against gross failure, rupture, and collapse of the steam
generator tubing. Leakage integrity refers to limiting primary-to-secondary
leakage to within acceptable limits.
The traditional strategy for achieving the objectives of the GDCs related to
steam generator tube integrity has been to establish a minimum wall thickness
requirement in accordance with the structural criteria of Regulatory Guide
(RG) 1.121, "Bases for Plugging Degraded PWR Steam Generator Tubes."
Development of minimum wall thickness requirements to satisfy RG 1.121 was
governed by analyses for uniform thinning of the tube wall in the axial and
circumferential directions. The assumption of uniform thinning results in
development of a repair limit that is conservative for all flaw types
occurring in the field. The resultant 40-percent depth-based repair limit
typically incorporated into the technical specifications is conservative for
highly localized flaws such as pits, short cracks, and in particular ODSCC
that occurs at TSPs.
This generic letter offers guidance on the implementation of an alternate
repair criterion to be applied to predominantly axially oriented ODSCC at TSP
locations. This criterion does not set limits on the depth of ODSCC
indications to ensure tube integrity margins; instead, it relies on
correlating the eddy current voltage amplitude from a bobbin coil probe with
the more specific measurement of burst pressure and leak rate. The staff
recognizes that although total margin may be reduced following application of
the voltage-based repair guidance of this generic letter, this guidance does
ensure that structural and leakage integrity continues to be maintained with
an acceptable level of margin consistent with the applicable GDCs of 10 CFR
Part 50, Appendix A and the limits of 10 CFR Part 100. Since the
voltage-based repair criteria do not incorporate minimum wall thickness
requirements, there is a possibility that tubes with up to 100-percent
through-wall cracks can remain in service. Because of the increased
likelihood of through-wall cracks, the staff has included provisions for
augmented steam generator tube inspections and more restrictive operational
leakage limits in this generic letter guidance.
In taking the action described in this letter, the NRC staff is approving the
use of the specific voltage-based repair criteria described herein as an
acceptable measure for dealing with ODSCC tube degradation. This action .
GL 95-05
August 3, 1995
Page 3 of 7
should not be construed to discourage licensees from using better or further
refined data acquisition techniques, eddy current technology, and eddy current
data analysis techniques as they become available. The staff strongly
encourages the industry to continue its efforts to improve the nondestructive
examination (NDE) of steam generator tubes and continues to believe that
inspection methods and repair criteria based on physical dimensions (e.g.,
length and depth) of defects are the most desirable when they can be achieved.
This generic letter is intended to provide relief while maintaining an
acceptable level of safety for licensees having steam generators experiencing
this particular degradation mechanism while the staff pursues a longer term
resolution to the issue of steam generator degradation through the development
of a steam generator rule. The staff recognizes that licensees may wish to
pursue various alternatives to the guidance of this generic letter [e.g.,
alternative probability of detection (POD) calculations]. However, licensees
should recognize that pursuing such alternatives could have significant
schedular implications, since the NRC staff would be required to review and
approve the associated information and analyses.
Discussion
1. Overview of the Voltage-Based Approach
In order to use the voltage-based repair criteria, licensees should complete
the following actions:
. Perform an enhanced inspection of tubes, particularly at the TSP
intersections.
. Utilize NDE data acquisition and analysis procedures that are consistent
with the methodology used to develop the voltage-based repair criteria.
. Repair tubes that exceed the voltage limits.
. Determine the beginning-of-cycle (BOC) voltage distribution.
. Project the end-of-cycle (EOC) voltage distribution.
. For the projected EOC voltage distribution, calculate both the primary-to-
secondary leakage under postulated accident conditions and the conditional
burst probability. As an alternative, the actual measured EOC voltage
distribution can be used when it is impractical to complete the projected .
GL 95-05
August 3, 1995
Page 4 of 7
EOC calculation prior to returning the steam generators to service for the
purpose of determining whether the reporting criteria in GL Sections 6.a.1
and 6.a.3 apply.
2. Generic Letter Applicability
The criteria in this generic letter are only applicable to predominantly
axially oriented ODSCC indications located at the tube-to-TSP intersections in
Westinghouse-designed steam generators with alloy 600 steam generator tubing.
These criteria are not applicable to other forms of steam generator tube
degradation, nor are they applicable to ODSCC that occurs at other locations
within a steam generator. The voltage-based repair criteria can be applied
only under the following constraints:
. The repair criteria of this generic letter apply only to Westinghouse-
designed steam generators with 1.9-cm [3/4-inch] and 2.2-cm [7/8-inch]
diameter alloy 600 tubes and drilled-hole TSPs.
. The repair criteria of this generic letter apply only to predominantly
axially oriented ODSCC confined within the tube-to-TSP intersection (refer
to Section 1.a of Attachment 1 for further guidance).
. Certain intersections are excluded from the application of the
voltage-based repair criteria, as discussed in Section 1.b of
Attachment 1.
3. Voltage Repair Limits
The voltage repair limits are:
. Indications less than the lower voltage repair limit, as measured by
bobbin coil, may remain in service. For 2.2-cm [7/8-inch] diameter tubes,
the lower voltage repair limit is 2.0 volts. For 1.9-cm [3/4-inch]
diameter tubes, the lower voltage repair limit is 1.0 volt.
. Indications greater than the lower limit and less than or equal to the
upper voltage repair limit, as measured by bobbin coil, can remain in
service if rotating pancake coil (RPC) inspections do not confirm the
indications. The methodology for calculating the upper voltage repair
limit is specified in Section 2.a.2 and 2.a.3 of Attachment 1.
. Indications greater than the lower limit and less than or equal to the
upper voltage repair limit, as measured by bobbin coil, that are confirmed
by RPC, and indications greater than the upper voltage repair limit, as
measured by bobbin coil, must be repaired.
The voltage-based repair limits in this generic letter were determined
considering the entire range of design basis events that could challenge tube
integrity. The voltage-based repair criteria ensure structural and leakage .
GL 95-05
August 3, 1995
Page 5 of 7
integrity for all postulated design basis events. The structural criteria are
intended to ensure that indications subjected to the voltage repair limits
will be able to withstand pressure loadings consistent with the criteria of
RG 1.121. The leakage criteria ensure that for degradation subjected to the
voltage repair criteria, induced leakage under worst-case MSLB conditions
calculated using licensing basis assumptions, will not result in offsite and
control room dose releases that exceed the applicable limits of 10 CFR
Part 100 and GDC 19.
Requested Actions
Implementation of the guidance in this generic letter is voluntary. A
licensee that chooses to implement these criteria should include all of the
following in the proposed program:
. Implementation of the applicability requirements discussed in Section 1 of
Attachment 1. The applicability requirements ensure that the repair
criteria are applied only to those intersections for which the voltage-based
repair criteria were developed.
. Implementation of the inspection guidance discussed in Section 3 of
Attachment 1. The inspection guidance ensures that the techniques used to
inspect the steam generator tubes are consistent with the techniques used to
develop the voltage-based repair criteria.
. Calculation of leakage according to the guidance discussed in Section 2.b of
Attachment 1. This calculation, in conjunction with the use of licensing
basis assumptions for calculating offsite and control room doses, enables
licensees to demonstrate that the applicable limits of 10 CFR Part 100 and
GDC 19 continue to be met. This calculation is performed using the
projected EOC voltage distribution for the next cycle of operation. If it
is not practical to complete this calculation prior to returning the steam
generators to service, the measured EOC voltage distribution can be used
(from the previous cycle of operation) as an alternative (refer to
Section 2.c of Attachment 1) for the purposes of determining whether the
reporting criteria of Section 6.a.1 apply.
. Calculation of conditional burst probability according to the guidance
discussed in Section 2.a of Attachment 1. This is a calculation to assess
the voltage distribution for the next cycle of operation. The results are
compared against a threshold value. This calculation is performed using the
projected EOC voltage distribution for the next cycle of operation. If it
is not practical to complete this calculation prior to returning the steam
generators to service, the measured EOC voltage distribution can be used
(from the previous cycle of operation) as an alternative (refer to
Section 2.c) for the purposes of determining whether the reporting criteria
of Section 6.a.3 apply.
. GL 95-05
August 3, 1995
Page 6 of 7
. Implementation of the operational leakage monitoring program according to
the guidance discussed in Section 5 of Attachment 1. The operational leak
rate monitoring program is a defense-in-depth measure that provides a means
for identifying leaks during operation to enable repair before such leaks
result in tube failure.
. Acquisition of tube pull data according to the guidance discussed in
Section 4 of Attachment 1.
. Reporting of results according to the guidance discussed in Section 6 of
Attachment 1.
. Submittal of a technical specification (TS) amendment request that commits
to the preceding actions and provides TS pages according to the guidance
discussed in Attachment 2, including the associated "no significant hazards
consideration" (10 CFR 50.92) and supporting safety analysis.
Licensees planning to adopt this TS amendment are encouraged to follow the
guidance discussed in Attachments 1 and 2. Whenever practical, the staff
requests that licensees following the guidance of this generic letter submit
their TS amendment request at least 90 days prior to the beginning of the
outage during which the alternate repair criteria are to be implemented.
Backfit Discussion
Licensee action to propose TS changes under the guidance of this generic
letter is voluntary; therefore, such action is not a backfit under the
provisions of 10 CFR 50.109.
Paperwork Reduction Act Statement
The voluntary information collections contained in this request are covered by
the Office of Management and Budget clearance number 3150-0011, which expires
July 31, 1997. The public reporting burden for this voluntary collection of
information is estimated to average 120 hours per response, including the time
for reviewing instructions, searching existing data sources, gathering and
maintaining the data needed, and completing and reviewing the collection of
information. Send comments regarding this burden estimate or any other aspect
of this voluntary collection of information, including suggestions for
reducing this burden, to the Information and Records Management Branch
(T-6 F33), U.S. Nuclear Regulatory Commission, Washington, D.C. 20555-0001,
and to the Desk Officer, Office of Information and Regulatory Affairs,
NEOB-10202 (3150-0011), Office of Management and Budget, Washington, D.C.
20503.
. GL 95-05
August 3, 1995
Page 7 of 7
This generic letter requires no specific action or written response. If you
have any questions about this matter, please contact the technical contact
listed below or the appropriate Office of Nuclear Reactor Regulation (NRR)
project manager.
/s/'d by DMCrutchfield
Dennis M. Crutchfield, Director
Division of Reactor Program Management
Office of Nuclear Reactor Regulation
Technical contact: Emmett Murphy, NRR
(301) 415-2710
Lead project manager: Gordon E. Edison, NRR
(301) 415-1448
Attachments:
l. Guidance for a Proposed License Amendment to Implement an Alternate Steam
Generator Tube Repair Limit for Outside Diameter Stress Corrosion Cracking
at the Tube Support Plate Intersections
2. Model Technical Specifications
3. List of Recently Issued NRC Generic Letters. Guidance for a Proposed License Amendment to Implement
an Alternate Steam Generator Tube Repair Limit for Outside Diameter Stress
Corrosion Cracking at the Tube Support Plate Intersections
1. Introduction
This guidance is the NRC staff position on the implementation of the
voltage-based repair criteria in steam generators designed by Westinghouse for
outside diameter stress corrosion cracking (ODSCC) located at the tube-to-TSP
intersections. This guidance is not applicable to other forms of steam
generator (SG) tube degradation nor is it applicable to ODSCC that occurs at
other locations within the SG. The voltage-based repair criteria have been
developed for, and are currently applicable only to, Westinghouse designed SGs
with 2.2-cm [7/8-inch] or 1.9-cm [3/4-inch] diameter alloy 600 tubes with
drilled-hole TSPs. Application of the alternate repair criteria to other
vendor designed SGs would require both the development and NRC staff review
and approval of a comparable data base and the associated correlations for
each vendor's SG type.
The NRC staff emphasizes that although the NRC has approved the implementation
of the voltage-based repair criteria (described in this generic letter) as a
short-term measure, this guidance should not be construed as discouraging the
development and use of better acquisition techniques, eddy current technology,
and eddy current data analysis techniques. The staff strongly encourages the
industry to continue to improve the nondestructive examination (NDE) of SG
tubes.
1.a ODSCC
The voltage-based repair criteria are applicable only to indications at TSP
intersections where the degradation mechanism is dominantly axial ODSCC with
no NDE detectable cracks extending outside the thickness of the support plate.
For purposes of this guidance, ODSCC refers to degradation whose dominant
morphology consists of axial stress corrosion cracks which occur either
singularly or in networks of multiple cracks, sometimes with limited patches
of general intergranular attack (IGA). Circumferential cracks may sometimes
occur in the IGA affected regions producing a grid-like pattern of axial and
circumferential cracks, termed "cellular corrosion." Cellular corrosion is
assumed to be relatively shallow (based on data from tube specimens removed
from the field), transitioning to dominantly axial cracks as the cracking
progresses in depth. The circumferential cracks are assumed (based on
available data) to be of insufficient size to produce a discrete, crack-like
circumferential indication during field NDE inspections. Thus, the failure
mode of ODSCC is axial and the burst pressure is controlled by the geometry of
the most limiting axial crack or array of axial cracks.
It is also assumed for purposes of this guidance that the ODSCC is confined to
within the thickness of the TSP, based on data from tube specimens removed
from the field. Very shallow microcracks are sometimes observed on these
specimens to initiate at locations slightly outside the thickness of the TSP;
however, these microcracks are small compared to the cracks within the
thickness of the TSP and are too small to produce an eddy current response.
The degradation mechanism should be confirmed as dominantly axial ODSCC by
periodically removing tube specimens from the SGs and by examining and testing
them as specified in Section 4 of this guidance. The acceptance criteria
should consist of demonstrating that the dominant degradation mechanism
affecting the burst and leakage properties of the tube is axially oriented
ODSCC. In addition, results of inservice inspections with rotating pancake
coil (RPC) probes should be evaluated in accordance with Section 3.b of this
guidance to confirm the absence of detectable crack-like circumferential
indications and detectable ODSCC indications extending outside the TSP
thickness.
1.b Exclusion of Intersections
The voltage-based repair criteria of this guidance do not apply to
intersections meeting the criteria specified below.
1.b.1 The repair criteria do not apply to tube-to-TSP intersections where the
tubes with degradation may potentially collapse or deform as a result of the
combined postulated loss-of-coolant accident and safe shutdown earthquake
loadings (e.g., intersections near the wedge supports at the upper TSPs).
Licensees should perform or reference an analysis that identifies which
intersections are to be excluded.
1.b.2 The repair criteria do not apply to tube-to-TSP intersections having
dent signals greater than 5.0 volts as measured with the bobbin probe.
1.b.3 The repair criteria do not apply to intersections at which there are
mixed residuals of sufficient magnitude to cause a 1.0 volt ODSCC indication
(as measured with a bobbin probe) to be missed or misread.
1.b.4 The repair criteria do not apply to intersections with interfering
signals from copper deposits.
1.b.5 The repair criteria do not apply to the tube-to-flow distribution
baffle plate intersections except as discussed in Section 2.a.3.
2. Tube Integrity Evaluation
Licensees should perform an evaluation to confirm that the SG tubes will
retain adequate structural and leakage integrity until the next scheduled
inspection. The first portion of this evaluation, referred to as the
"conditional burst probability calculation," assesses the voltage distribution
left in service against a threshold value of 1 þ 10-2 probability of rupture
under postulated main steamline break (MSLB) conditions. The conditional
burst probability calculation is intended to provide a conservative assessment
of tube structural integrity during a postulated MSLB occurring at the end of
cycle (EOC). It is used to determine whether the NRC needs to focus
additional attention on the particular voltage repair limit application. If
the calculated conditional burst probability exceeds 1 þ 10-2, the licensee
should notify the NRC according to the guidance in Section 6.
The second portion of the tube integrity evaluation is intended to ensure that
the total leak rate from the affected SG during a postulated MSLB occurring at
EOC would be less than a rate that could lead to radiological releases in
excess of the licensing basis for the plant. If calculated leakage exceeds
the allowable limit determined by the licensing basis dose calculation,
licensees can either repair tubes, beginning with the largest voltage
indications until the leak limit is met, reduce reactor coolant system
specific iodine activity [refer to example technical specification (TS) pages
of Attachment 2], or reduce the length of the operating cycle. The analyses
discussed above may incorporate or reference previous analyses, or portions
thereof, to the extent that they continue to bound the conditions of the SG as
determined by inspection.
For plants in which the TSs do not require the pressurizer power-operated
relief valves (PORVs) to be operable during power operation, these tube
integrity analyses should be conducted for an assumed differential pressure
across the tube walls equal to the pressurizer safety valve setpoint plus
3 percent for the valve accumulation, less atmospheric pressure in faulted
SGs. For plants in which the TSs do require the PORVs to be operable, the
assumed differential pressure for the conditional burst probability
calculation may be based on the PORV setpoint in lieu of the safety valve
setpoint with similar adjustments. The TS requirements for operation with
PORV block valves closed due to leaking PORVs should be in accordance with
Enclosure A of Generic Letter 90-06, "Resolution of Generic Issue 70, `Power-
Operated Relief Valve and Block Valve Reliability,' and Generic Issue 94,
`Additional Low-Temperature Overpressure Protection for Light-Water Reactors,'
Pursuant to 10 CFR 50.54(f)." That is, electrical power to the block valves
must be maintained to allow continued operation with the block valves closed,
as required in the sample technical specification Section 3.4.4 of GL 90-06.
2.a Conditional Probability of Burst During an MSLB
For this generic letter, the conditional probability of burst refers to the
probability that the burst pressures associated with one or more indications
in the faulted SG will be less than the maximum pressure differential
associated with a postulated MSLB assumed to occur at EOC. A methodology
should be submitted for NRC approval for calculating this conditional burst
probability. After the NRC approves a method for calculating conditional
probability of burst, licensees may reference the approved method. This
methodology should involve (1) determining the distribution of indications as
a function of their voltage response at the beginning of cycle (BOC) as
discussed in Section 2.b.1, (2) projecting this BOC distribution to an EOC
voltage distribution based on consideration of voltage growth due to defect
progression between inspections as discussed in Section 2.b.2(2) and voltage
measurement uncertainty as discussed in Section 2.b.2(1), and (3) evaluating
the conditional probability of burst for the projected EOC voltage
distribution using the correlation between burst pressure and voltage
discussed in Section 2.a.1. The solution methodology should account for
uncertainties in voltage measurement [Section 2.b.2(1)], the distribution of
potential voltage growth rates applicable to each indication
[Section 2.b.2(2)], and the distribution of potential burst pressures as a
function of voltage (Section 2.a.1). Monte Carlo simulations are an
acceptable approach for accounting for these various sources of uncertainty.
2.a.1 Burst Pressure Versus Bobbin Voltage
An empirical model, for 7/8-inch or 3/4-inch diameter tubing, as applicable,
should be used to relate burst pressure to bobbin voltage response for
purposes of estimating the conditional probability of burst during a
postulated MSLB. The model should consider, at a minimum, the scale factors
for the coordinate system (e.g., linear or logarithmic), the detection and
treatment of outliers, the order of the regression equation, the potential
influence of measurement errors in the variables, and the evaluation of the
residuals following the development of a relationship. This model should
explicitly account for burst pressure uncertainty as indicated by scatter of
the supporting test data and should also account for the parametric (i.e.,
slope and intercept) uncertainty of the regression fit of the data.
Currently, an approved model consists of determining a linear first-order
equation between the burst pressure and the logarithm (base 10) of the bobbin
voltage amplitude with standard least-squares linear regression analysis. The
model may need to be changed as additional information is acquired; however,
such changes should be submitted to the NRC staff for approval. The
supporting data sets for 7/8-inch diameter and 3/4-inch diameter tubing should
contain all applicable data consistent with the latest revision of the
industry data base as approved by the NRC. The currently approved data base
for burst pressure as a function of voltage is given in Reference 7, as
supplemented by References 1 and 2.
2.a.2 Determination of the Upper Voltage Repair Limit for TSP Intersections
From the regression relationship (discussed above in Section 2.a.1), a lower
95-percent prediction bound should be determined for the burst pressure as a
function of bobbin voltage amplitude. The lower 95-percent prediction
interval is further reduced to account for the lower 95/95-percent tolerance
bound for tubing material properties at 650 oF. Using this reduced lower
prediction bound curve, the structural limit is determined for a free span
burst pressure of 1.4 times MSLB differential pressure (þPMSLB) consistent with
the structural limits in RG 1.121.
To determine the upper voltage repair limit, the structural limit is reduced
to account for flaw growth and voltage measurement uncertainty. The method
for determining the flaw growth allowance is discussed in Section 2.b.2(2) and
should be a plant-specific average growth rate or 30-percent per effective
full power year (EFPY), whichever is larger. The voltage measurement
uncertainty allowance should be the 95-percent cumulative probability value
for the voltage measurement uncertainty models. Eddy current voltage
measurement uncertainty is discussed in Section 2.b.2(1). Currently the
95-percent cumulative probability value is 20 percent of the BOC voltage
amplitude. The upper voltage repair limit should be determined prior to each
outage, using the most recently approved NRC data base.
2.a.3 Determination of the Upper Voltage Repair Limit for Flow Distribution
Baffle Plate Intersections
Because of the greater diametral clearances at the flow distribution baffle
plate and the potential for higher voltage growth rates at these
intersections, different tube repair limits may be necessary for indications
at the flow distribution baffle plate. To determine if the voltage-based
repair criteria can be applied to flow distribution baffle plate
intersections, the causal factors for the high voltage growth rates and the
applicability of these conditions at the plant should be assessed. This
assessment should be provided to the NRC for approval along with the original
amendment request.
If the assessment indicates that the voltage-based repair criteria can be
applied to indications at the flow distribution baffle plate, the methodology
used for calculating the upper voltage repair limit for the flow distribution
baffle plate intersections should be identical to the methodology discussed in
Section 2.a.2, except as noted below:
. The tube structural limit should be determined for a free span burst
pressure of 3 times normal operational differential pressure (þPNO) or
1.4þPMSLB, whichever is more limiting, because of the greater diametral
clearances at the flow distribution baffle plate.
. The growth rate of the indications at the flow distribution baffle plate
should be determined and should be compared to the growth rates of
indications at TSP elevations to determine if the growth rate at the flow
distribution baffle is considerably different from the growth rate at the
TSPs. If the growth rate of indications at the flow distribution baffle
plate is higher than the remainder of the indications, this growth rate, or
a higher one, should be use for determining the upper voltage repair limit
for flow distribution baffle plate indications. The growth rate should not
be less than 30-percent per EFPY, as discussed in Section 2.a.2. The upper
voltage repair limit for the flow distribution baffle should be determined
before each outage using the most recently approved NRC data base.
2.b Total Leak Rate During MSLB
A leak rate methodology is approved in Reference 8 and is described in
Reference 9. Licensees may reference the approved method. The leak rate
methodology involves (1) determining the distribution of indications as a
function of their voltage response at BOC as discussed in Section 2.b.1, (2)
projecting this BOC distribution to an EOC voltage distribution based on
consideration of voltage growth due to defect progression between inspections
as discussed in Section 2.b.2(2) and voltage measurement uncertainty as
discussed in Section 2.b.2(1), and (3) evaluating the total leak rate for the
projected EOC voltage distribution using a probability of leakage (POL) model,
as discussed in Section 2.b.3(1), and the conditional leak rate model, as
discussed in Section 2.b.3(2). The solution methodology should account for
uncertainties in voltage measurement [Section 2.b.2(1)], the distribution of
potential voltage growth rates applicable to each indication
[Section 2.b.2(2)], the uncertainties in the probability of leakage as a
function of voltage [Section 2.b.3(1)], and the distribution of potential
conditional leak rates as a function of voltage [Section 2.b.3(2)]. Monte
Carlo simulations are an acceptable method for accounting for these sources of
uncertainty, provided that the calculated total leak rate reflects an upper
95-percent quantile value at an upper 95-percent confidence bound.
2.b.1 Distribution of Bobbin Indications as a Function of Voltage at BOC
The frequency distribution by voltage of bobbin indications actually found
during inspection should be scaled upward by a factor of 1/POD to account for
non-detected cracks which can potentially leak or rupture under postulated
MSLB conditions during the next operating cycle. This adjusted frequency
distribution minus detected indications for tubes that have been plugged or
repaired should constitute, for purposes of the tube integrity analyses, the
assumed frequency distribution of bobbin indications at BOC as a function of
voltage. This can also be expressed as
Nl = (1/POD)(Nd) - Nr (1)
where:
Nl = assumed frequency distribution of bobbin indications
Nd = frequency distribution of indications actually detected
Nr = frequency distribution of repaired indications
POD = probability of detection of ODSCC flaws
POD should be assumed to have a value of 0.6, or as an alternative, an NRC
approved POD function can be used, if such a function becomes available.
Nd includes all flaw indications detected by the bobbin coil, regardless of
whether these indications are confirmed by rotating pancake coil (RPC)
inspection. Alternatively, a fraction of bobbin indications at locations
which have been inspected with an RPC probe, but where the RPC failed to
confirm the bobbin indication, may be excluded from Nd subject to NRC
approval.
If the steam generators have been chemically cleaned, the impact of the
chemical cleaning on the BOC voltage distribution needs to be evaluated.
2.b.2 Projected End-of-Cycle (EOC) Voltage Distribution
As discussed above, the calculation of both conditional burst probability and
leakage (during a postulated MSLB) requires the generation of the projected
EOC voltage distribution. To project an EOC voltage distribution from the BOC
voltage distribution determined above, requires consideration of (1) eddy
current voltage measurement uncertainty and (2) the addition of voltage growth
to account for defect progression. Monte Carlo techniques are an acceptable
means for sampling eddy current measurement uncertainty and the voltage growth
distribution to determine the projected EOC voltage distribution. Eddy
current measurement uncertainty and voltage growth are discussed below.
2.b.2(1) Eddy Current Voltage Measurement Uncertainty
Uncertainty in eddy current voltage measurements stems primarily from two
sources:
. voltage response variability (i.e., test repeatability error) which stems
primarily from probe wear
. voltage measurement variability among data analysts (i.e., measurement
repeatability error)
Each of these uncertainties should be quantified. An acceptable
characterization of these uncertainties is contained in EPRI TR-100407,
Revision 1, Draft Report August 1993, "PWR Steam Generator Tube Repair
Limits-Technical Support Document for Outside Diameter Stress Corrosion
Cracking at the Tube Support Plates" (Reference 3), Sections 2.4.1, 2.4.2, and
D.4.2.3, with the exception that no distribution cutoff should be applied to
the voltage measurement variability distribution. (However, the assumed
15 percent cutoff for the voltage response variability distribution in
Reference 3 is acceptable.)
2.b.2(2) Voltage Growth Due to Defect Progression
Potential voltage growth rates during the next inspection cycle (i.e.,
operating cycle between two scheduled SG inspections) should be based on
voltage growth rates observed during the last one or two inspection cycles.
For a given inspection, previous inspection results at tube-to-TSP
intersections currently exhibiting a bobbin indication should be evaluated
consistent with the data analysis guidelines in Section 3 below. In cases in
which data acquisition guidelines employed during previous inspections differ
from those discussed in Section 3, the evaluation of the previous data should
be adjusted to compensate for the difference. Voltage growth rates should
only be evaluated for those intersections at which bobbin indications can be
identified at two successive inspections, except if an indication changes from
non-detectable to a relatively high voltage (e.g., 2.0 volts).
The distribution of voltage growth rates (based on the change in voltage on an
intersection-to-intersection basis) should be determined for each of the last
one or two inspection cycles. When only the current or only the current and
previous inspections employed data acquisition guidelines similar to those
discussed in Section 3, only the growth rate distribution for the previous
cycle should be used to estimate the voltage growth rate distribution for the
next inspection cycle. If both of the two previous inspections employed such
similar guidelines, the most limiting of the two previous growth rate
distributions should be used to estimate the voltage growth rate distribution
for the next inspection cycle. However, the two distributions should be
combined if one or both of the distributions is based on a minimal number
(i.e., < 200) of indications. If the growth rate distribution, or combined
distribution from two cycles, consists of fewer than 200 indications, a
bounding probability distribution function of growth rates should be used
based on consideration of experience to date at similarly designed and
operated units.
It is acceptable to use a statistical model fit of the observed growth rate
distribution as part of the tube integrity analysis provided that the
statistical model conservatively accounts for the tail of the distribution.
It is also acceptable that the voltage growth distribution be in terms of
þ volts rather than percent þ volts, provided the conservatism of this
approach continues to be supported by operating experience. For the purposes
of assessing the conditional probability of burst and conditional leak rate,
negative growth rates should be included as zero growth rates in the assumed
growth rate distribution. However, for the purposes of determining the upper
voltage repair limit in accordance with Sections 2.a.2 and 2.a.3, it is
appropriate to consider negative growth rates as part of the estimate for
average growth rate.
If the steam generators have been chemically cleaned, the impact of the
chemical cleaning on voltage growth rates needs to be evaluated.
2.b.3 Calculation of Projected MSLB Leakage
Once the projected EOC voltage distribution is determined, the leakage for the
postulated MSLB is calculated utilizing the EOC voltage distribution and the
use of two models: (1) the probability of leakage model and (2) the
conditional leak rate model. As previously discussed in Section 2.b, Monte
Carlo techniques are an acceptable approach for accounting for the
uncertainties implicit in these models. These models are discussed below.
2.b.3(1) Probability of Leakage as a Function of Voltage
An empirical model, for 7/8-inch and 3/4-inch diameter tubing as applicable,
should be used to relate the probability of leakage (POL) to the bobbin
voltage response. This model should explicitly account for parameter
uncertainty of the POL functional fit of the data (i.e., "model fit"
uncertainty). Currently, the staff has approved a model which uses a log-
logistic function to fit the data. This model may need to be changed as
additional leakage data is acquired. Revisions to this model should be
submitted to the NRC for review and approval.
The supporting data sets for 2.2-cm (7/8-inch) diameter and 1.9-cm (3/4-inch)
diameter tubing should include all applicable data consistent with the latest
revision of the industry data base as approved by the NRC. The currently
approved data base for POL as a function of voltage is given in Reference 7,
as supplemented by References 1 and 2.
2.b.3(2) Conditional Leakage Rate under MSLB Conditions
An empirical model, for 7/8-inch or 3/4-inch diameter tubing as applicable,
should be used to relate the conditional leak rate to the bobbin voltage
response. This empirical model should account for both data scatter and
parameter uncertainty of the empirical fit. Currently, an approved model
consists of determining a linear first-order equation between the logarithm
(base 10) of the conditional leak rate and the logarithm (base 10) of the
bobbin voltage amplitude with standard least-squares linear regression
analysis. The model may need to be changed as additional information is
acquired; such changes should be submitted to the NRC staff for review and
approval.
Use of the linear regression fit of the logarithm of the conditional leak rate
to the logarithm of the bobbin voltage is subject to demonstrating that the
linear regression fit is valid at the 5-percent level with a "p-value" test.
If this condition is not satisfied, the linear regression fit should be
assumed to have zero slope (i.e., the linear regression fit should be assumed
to be constant with voltage).
The supporting data sets for 2.2-cm (7/8-inch) diameter and 1.9-cm (3/4-inch)
diameter tubing should include all applicable data consistent with the latest
revision of the industry data base as approved by the NRC. The currently
approved data base for conditional leak rate as a function of voltage is given
in Reference 7, as supplemented by References 1 and 2, with certain
exceptions. Specifically, data excluded under criteria 3a, 3b, and 3c in
References 1 and 2 should not be excluded pending NRC review and approval of
these criteria. In addition, an MSLB leak rate of 2496 liters per hour should
be utilized for the data point obtained from V.C. Summer tube R28C41, pending
staff review and approval of any proposed alternative estimate.
2.b.4 Calculation of Offsite and Control Room Doses
For the MSLB leak rate calculated above, offsite and control room doses should
be calculated utilizing currently accepted licensing basis assumptions.
Licensees should note that Attachment 2 to this generic letter provides
example TS pages for reducing reactor coolant system specific iodine activity
limits. Licensees who wish to take credit for reduced reactor coolant system
iodine activities (below 0.35 microcuries per gram dose equivalent I-131) in
the radiological dose calculation should provide a justification supporting
the request that evaluates the release rate data described in Reference 6.
Reduction of reactor coolant iodine activity is an acceptable means for
accepting higher projected leakage rates and still meeting the applicable
limits of Title 10 of the Code of Federal Regulations Part 100 and GDC 19
utilizing licensing basis assumptions.
2.c Alternative Tube Integrity Calculation
As discussed above, licensees should calculate (1) primary-to-secondary
leakage under postulated accident conditions and (2) conditional probability
of burst given an MSLB, to confirm that the SG will retain adequate structural
and leakage integrity until the next scheduled SG inspection. Section 6 of
this attachment contains reporting guidance that recommends that licensees
notify the staff prior to returning the SGs to service when conditional burst
probability exceeds 1 x 10-2 or when calculated accident leakage exceeds the
licensing limit. These calculations are to be performed using the projected
EOC voltage distribution; however, it may not always be practical to complete
these calculations prior to returning the SGs to service. Under these
circumstances, it is acceptable to use the actual measured bobbin voltage
distribution instead of the projected EOC voltage distribution to determine
whether the reporting criteria in Section 6 of this guidance are satisfied.
The actual measured bobbin voltage distribution should contain all bobbin
indications detected, regardless of whether the RPC probe confirmed the
degradation to be present and the NDE uncertainty distribution should be
sampled. The postulated accident leakage and the conditional probability of
burst should be calculated in accordance with Sections 2.a. and 2.b.3 of this
attachment. This calculation is intended to assess whether the SGs can be
returned to service and the plant can be operated until the full assessment
(submitted within 90 days of restart) from the projected EOC voltage
distribution is performed.
3. Inspection Criteria
The inspection scope, data acquisition, and data analysis should be performed
in a manner consistent with the methodology utilized to develop the voltage
limits (e.g., the methodology described in Reference 4, Appendix A, and
Reference 5, Appendix A) with the exceptions and clarifications noted below.
3.a Bobbin Coil Inspection Scope and Sampling
The bobbin coil inspection should include 100 percent of the hot-leg TSP
intersections and cold-leg intersections down to the lowest cold-leg TSP with
known ODSCC. The determination of TSPs having ODSCC should be based on the
performance of at least a 20-percent random sampling of tubes inspected over
their full length.
3.b Rotating Pancake Coil (RPC) Inspection
RPC inspections should be conducted as discussed below for purposes of
obtaining additional characterization of ODSCC flaws found with the bobbin
probe and to inspect intersections with significant bobbin interference
signals (due to copper, dents, large mix residuals) which may impair the
detectability of degradation with the bobbin probe or which may unduly
influence the bobbin voltage measurement. With respect to ODSCC flaw
characterization, a key purpose of the RPC inspections is to ensure the
absence of detectable crack-like circumferential indications and detectable
indications extending outside the thickness of the TSP. The voltage-based
repair criteria are not applicable to intersections exhibiting such
indications, and special reporting requirements pertaining to the finding of
such indications are described in Section 6.
3.b.1 RPC inspection should be performed for all indications exceeding
2.0 volts as measured by bobbin coil for 2.2-cm [7/8-inch] diameter tubes or
1.0 volt as measured by bobbin coil for 1.9-cm [3/4-inch] diameter tubes.
3.b.2 All intersections with interfering signals from copper deposits should
be inspected with RPC. Any indications found at such intersections with RPC
should cause the tube to be repaired.
3.b.3 All intersections with dent signals greater than 5 volts should be
inspected with RPC. Any indications found at such intersections with RPC
should cause the tube to be repaired. If circumferential cracking or primary
water stress corrosion cracking indications are detected, it may be necessary
to expand the RPC sampling plan to include dents less than 5.0 volts.
3.b.4 All intersections with large mixed residuals should be inspected with
RPC. For purposes of this guidance, large mixed residuals are those that
could cause a 1.0 volt bobbin signal to be missed or misread. Any indications
found at such intersections with RPC should cause the tube to be repaired.
3.c Data Acquisition and Analysis
3.c.1 The bobbin coil should be calibrated against the reference standard
used in the laboratory as part of the development of the voltage-based
approach by direct testing or through use of a transfer standard.
3.c.2 Once the probe has been calibrated on the 20-percent through-wall
holes, the voltage response of new bobbin coil probes for the 40-percent to
100-percent American Society of Mechanical Engineers (ASME) through-wall holes
should not differ from the nominal voltage by more than ñ 10 percent.
3.c.3 Probe wear should be controlled by either an inline measurement device
or through the use of a periodic wear measurement. When utilizing the
periodic wear measurement approach, if a probe is found to be out of
specification, all tubes inspected since the last successful calibration
should be reinspected with the new calibrated probe. Alternatives to this
approach, which provide equivalent detection and sizing and are consistent
with the tube integrity analyses discussed in Section 2, may be permitted
subject to NRC approval.
3.c.4 Data analysts should be trained and qualified in the use of the
analyst's guidelines and procedures. Data analyst performance should be
consistent with the assumptions for analyst measurement variability
[Section 2.b.2(1)] utilized in the tube integrity evaluation (Section 2).
3.c.5 Quantitative noise criteria (resulting from electrical noise, tube
noise, calibration standard noise) should be included in the data analysis
procedures. Data failing to meet these criteria should be rejected, and the
tube should be reinspected.
3.c.6 Data analysts should review the mixed residuals on the standard itself
and take action as necessary to minimize these residuals.
3.c.7 Smaller and larger diameter probes can be used to inspect tubes where
it is impractical to utilize a nominal-size probe (i.e., 0.610 inch diameter
for 3/4-inch tubing and 0.720 inch diameter for 7/8-inch tubing) provided that
the probes and procedures have been demonstrated on a statistically
significant basis to give an equivalent voltage response and detection
capability when compared to the nominal-size probe. This can be demonstrated
on a plant-specific or generic basis. Data supporting the use of alternate
probe sizes should be submitted for NRC approval.
3.c.8 Data analysts should be trained on the potential for primary water
stress corrosion cracking to occur at TSP intersections. The analysts should
be sensitized to identifying indications attributable to primary water stress
corrosion cracking.
4. Tube Removal and Examination/Testing
Implementation of voltage-based repair criteria should include a program of
tube removals for testing and examination as described below. The purpose of
this program is to (1) confirm axial ODSCC as the dominant degradation
mechanism as discussed in Section 1.a; (2) monitor the degradation mechanism
over time; (3) provide additional data to enhance the burst pressure,
probability of leakage, and conditional leak rate correlations described in
Sections 2.a.1, 2.b.3(1), and 2.b.3(2), respectively; and (4) assess
inspection capability.
4.a Number and Frequency of Tube Pulls
Two pulled tube specimens with an objective of retrieving as many
intersections as is practical (a minimum of four intersections) should be
obtained for each plant either during the plant SG inspection outage that
implements the voltage-based repair criteria or during an inspection outage
preceding initial application of these criteria. On an ongoing basis, an
additional (follow-up) pulled tube specimen with an objective of retrieving as
many intersections as is practical (minimum of two intersections) should be
obtained at the refueling outage following accumulation of 34 effective full
power months of operation or at a maximum interval of three refueling outages,
whichever is shorter, following the previous tube pull.
Alternatively, the request to acquire pulled tube specimens may be met by
participating in an industry sponsored tube pull program endorsed by the NRC
that meets the objectives of this guidance. Such a program would have to
satisfy the following objectives: (1) to confirm the degradation mechanism for
plants utilizing the generic letter for the first time, (2) to continue
monitoring the ODSCC mechanism over time, (3) to enhance the burst pressure,
probability of leakage, and conditional leak rate correlations, and (4) to
assess inspection capability.
4.b Selection Criteria
Selection of the tubes to be removed should consider the following criteria:
4.b.1 There should be an emphasis on removing tube intersections with large
voltage indications.
4.b.2 Where possible, the removed tube intersections should cover a range of
voltages, including intersections with no detectable degradation.
4.b.3 As a minimum, selected intersections should ensure that the total data
set includes a representative number of intersections with RPC signatures
indicative of a single dominant crack as compared to intersections with RPC
signatures indicative of two or more dominant cracks about the circumference.
4.c Examination and Testing
Removed tube intersections should be subjected to leak and burst tests under
simulated MSLB conditions to confirm that the failure mode is axial and to
permit enhancement of the supporting data sets for the burst pressure and
leakage correlations. The systems for future tests should accommodate, and
permit the measurement of, as high a leak rate as is practical, including leak
rates that may be in the upper tail of the leak rate distribution for a given
voltage. Leak rate data should be collected at temperature for the
differential pressure loadings associated with the maximum postulated MSLB.
When it is not practical to perform hot temperature leak tests, room
temperature leak rate testing may be performed as an alternative. Burst
testing may be performed at room temperature. The burst and leak rate
correlations and/or data should be normalized to reflect the appropriate
pressure and temperature assumptions for a postulated MSLB.
Subsequent to burst testing, the intersections should be destructively
examined to confirm that the degradation morphology is consistent with the
assumed morphology for ODSCC at the tube-to-TSP intersections. The
destructive examinations should include techniques such as metallography and
scanning electron microscope (SEM) fractography as necessary to characterize
the degradation morphology (e.g., axial ODSCC, circumferential ODSCC, IGA
involvement, cellular IGA, and combinations thereof) and to characterize the
largest crack networks with regard to their orientation, length, depth, and
ligaments. The purpose of these examinations is to verify that the
degradation morphology is consistent with the assumptions made in Section 1.a
of this attachment. This includes demonstrating that the dominant degradation
mechanism affecting the tube burst and leakage properties is axially oriented,
ODSCC.
5. Operational Leakage
5.a The operational leakage limit should be reduced in the TSs to 150 gallons
per day (gpd) through each SG.
5.b Licensees should review their leakage monitoring measures to ensure that
should a significant leak occur in service, it will be detected and the plant
will be shut down in a timely manner to reduce the likelihood of a potential
tube rupture. Specifically, the effectiveness of these procedures for
ensuring the timely detection, trending, and response to rapidly increasing
leaks should be assessed. The licensee should consider the appropriateness of
alarm setpoints on the primary-to-secondary leakage detection instrumentation
and the various criteria for operator actions in response to detected leakage.
5.c SG tubes with known leaks should be repaired prior to returning the SGs
to service following an SG inspection outage.
6. Reporting Requirements
6.a Threshold Criteria for Requiring Prior Staff Notification To Continue
With Voltage-Based Criteria
This guidance allows licensees to implement the voltage-based repair criteria
on a continuing basis after the NRC staff has approved the initial TS
amendment. However, in several situations, the NRC staff must receive
notification to enable the staff to assess whether a licensee can continue
with the implementation of the voltage-based repair criteria:
6.a.1 If the projected EOC voltage distribution results in an estimated
leakage greater than the leakage limit (determined from the licensing basis
calculation), then the licensee should notify the NRC of this occurrence and
provide an assessment of its significance prior to returning the SGs to
service. If it is not practical to complete this calculation prior to
returning the SGs to service, the measured EOC voltage distribution can be
used (from the previous cycle of operation) as an alternative (refer to
Section 2.c). If it is determined that the projected calculated leakage will
exceed the leakage limit (during the operating cycle) after the SGs are
returned to service, then licensees should provide an assessment of the safety
significance of the occurrence, describe the compensatory measures being taken
to resolve the issue, and follow any other applicable reportability
regulations.
6.a.2 If indications are identified that (1) extend beyond the confines of
the TSP, or (2) appear to be circumferential in nature, or (3) are
attributable to primary water stress corrosion cracking, the NRC staff should
be notified prior to returning the SGs to service.
6.a.3 If the calculated conditional probability of rupture under postulated
MSLB conditions based on the projected EOC voltage distribution exceeds
1 þ 10-2, licensees should notify the NRC and provide an assessment of the
significance of this occurrence prior to returning the SGs to service. This
assessment should address the safety significance of the calculated
conditional probability and can account for operator actions to prevent
primary pressure from reaching the PORV or safety valve setpoint provided that
the assessment includes a probabilistic assessment of the operator actions.
If it is not practical to complete this calculation prior to returning the SGs
to service, the measured EOC voltage distribution can be used (from the
previous cycle of operation) as an alternative (refer to Section 2.c).
6.b Information To Be Provided Following Each Restart
The following information should be submitted to the NRC staff within 90 days
of each restart following an SG inspection:
(a) The results of metallurgical examinations performed for tube
intersections removed from the SG. If it is not practical to provide
all the results within 90 days, as a minimum, the burst test, leakage
test and morphology conclusions should be provided within 90 days. The
remaining information should be submitted when it becomes available.
(b) The following distributions should be provided in both tabular and
graphical form. This information will enable the staff to assess the
effectiveness of the methodology, determine whether the degradation is
changing significantly, determine whether the data supports different
voltage repair limits, and perform confirmatory calculations. The
voltages reported should be adjusted to account for differences between
the laboratory standard and the standard used in the field (i.e.,
transfer standard corrections should be made).
(i) EOC voltage distribution - all indications found during the
inspection regardless of RPC confirmation
(ii) cycle voltage growth rate distribution (i.e., from BOC to EOC) -
the data should indicate whether the distribution has been
adjusted for the length of the operating interval, and the length
of the operating interval should be provided (i.e., in EFPYs).
The planned length of the next operating interval should also be
provided (in EFPYs).
(iii) voltage distribution for EOC repaired indications - distribution
of indications presented in (i) above that were repaired (i.e.,
plugged or sleeved)
(iv) voltage distribution for indications left in service at the
beginning of the next operating cycle regardless of RPC
confirmation - obtained from (i) and (iii) above
(v) voltage distribution for indications left in service at the
beginning of the next operating cycle that were confirmed by RPC
to be crack-like or not RPC inspected
(vi) non-destructive examination uncertainty distribution used in
predicting the EOC (for the next cycle of operation) voltage
distribution
(c) The results of the tube integrity evaluation (calculated accident
leakage and conditional burst probability) described in Section 2,
including the repair limits that were implemented (i.e., the upper
voltage repair limit, the average growth rate at the tube support plates
and flow distribution baffle, if applicable, the measurement variability
allowance, and the correlation used). Note that if the leakage and
conditional burst probability were calculated using the measured EOC
voltage distribution for the purposes of addressing the 6.a.1 and 6.a.3
reporting criteria, then the results of the projected EOC voltage
distribution should be given in this report.
.7. References
1. Letter dated April 22, 1994, to Jack Strosnider, NRC, from David A.
Steininger, EPRI, "Exclusion of Data from Alternate Repair Criteria (ARC) Data
Bases Associated with 7/8 inch Tubing Exhibiting ODSCC."
2. Letter dated June 9, 1994, to Brian Sheron, NRC, from David J. Modeen,
Nuclear Energy Institute.
3. EPRI TR-100407, Revision 1, Draft Report August 1993, "PWR Steam Generator
Tube Repair Limits-Technical Support Document for Outside Diameter Stress
Corrosion Cracking at the Tube Support Plates."
4. WCAP-12985, Revision 1, "Kewaunee Steam Generator Tube Plugging Criteria
for ODSCC at Tube Support Plates," Westinghouse Electric Corporation, January
1993, Westinghouse Proprietary Class 2.
5. WCAP-13522, "V.C. Summer Steam Generator Tube Plugging Criteria for
Indications at Tube Support Plates," Westinghouse Electric Corporation,
Westinghouse Proprietary Class 2.
6. J. P. Adams and C. L. Atwood, "The Iodine Spike Release Rate During a
Steam Generator Tube Rupture," Nuclear Technology, Vol. 94, p. 361 (1991).
7. EPRI Draft Report, NP-7480-L, "Steam Generator Tubing Outside Diameter
Stress Corrosion Cracking at Tube Support Plates - Data Base for Alternate
Repair Limits," Volume 1, Revision 1, September 1993, "7/8 Inch Diameter
Tubing," and Volume 2, October 1993, "3/4 Inch Diameter Tubing."
8. Safety Evaluation by the Office of Nuclear Reactor Regulation Related to
Amendment No. 54 to Facility Operating License NPF-72, Commonwealth Edison
Company, Braidwood Station, Unit 1, Docket No. STN 50-456, as documented in an
amendment package titled, "Issuance of Amendment (TAC NO. M89697)" dated
August 18, 1994.
9. WCAP-14046, "Braidwood Unit 1 Technical Support for Cycle 5 Steam
Generator Interim Plugging Criteria," Westinghouse Electric Corporation,
Westinghouse Proprietary Class 2.. ABBREVIATIONS
ARC alternate repair criteria
ASME American Society of Mechanical Engineers
BOC beginning of cycle
EFPY effective full power year
EOC end of cycle
EPRI Electric Power Research Institute
GDCs General Design Criteria
IGA intergranular attack
MSLB main steamline break
NDE nondestructive examination
NEI Nuclear Energy Institute
NRC Nuclear Regulatory Commission
ODSCC outside diameter stress corrosion cracking
POD probability of detection
POL probability of leakage
PORV power-operated relief valve
PWR pressurized-water reactor
RCPB reactor coolant pressure boundary
RG regulatory guide
RPC rotating pancake coil
SEM scanning electron microscope
SG steam generator
STS standard technical specifications
TS technical specification
TSP tube support plate. Model Technical Specifications
The model technical specifications (TSs) are based on the "Standard Technical
Specifications (STS) for Westinghouse Pressurized Water Reactors," NUREG-0452,
Revision 4a. The changes are indicated in italics. Note that the model TS
changes described below also contain an example change to reduce reactor
coolant system specific activity. The model TSs that appear below should be
adopted consistent with the licensing basis. It should be noted that in the
improved STS, some of these surveillance requirements have been relocated to
the "Administrative Controls" section.
3/4.4.5 REACTOR COOLANT SYSTEM
4.4.5.2 Steam Generator Tube Selection and Inspection
[Add the following paragraphs:]
b. 4. Indications left in service as a result of application of
the tube support plate voltage-based repair criteria shall
be inspected by bobbin coil probe during all future
refueling outages.
d. Implementation of the steam generator tube/tube support plate repair
criteria requires a 100-percent bobbin coil inspection for hot-leg
and cold-leg tube support plate intersections down to the lowest
cold-leg tube support plate with known outside diameter stress
corrosion cracking (ODSCC) indications. The determination of the
lowest cold-leg tube support plate intersections having ODSCC
indications shall be based on the performance of at least a
20-percent random sampling of tubes inspected over their full
length.
4.4.5.4 Acceptance Criteria
a. As used in this specification:
6. Plugging Limit means the imperfection depth at or beyond
which the tube shall be removed from service and is equal to
40 percent of the nominal wall thickness. This definition
does not apply to tube support plate intersections for which
the voltage-based repair criteria are being applied. Refer
to 4.4.5.4.a.10 for the repair limit applicable to these
intersections.
10. Tube Support Plate Plugging Limit is used for the
disposition of an alloy 600 steam generator tube for
continued service that is experiencing predominantly axially
oriented outside diameter stress corrosion cracking confined
within the thickness of the tube support plates. At tube
support plate intersections, the plugging (repair) limit is
based on maintaining steam generator tube serviceability as
described below:
a. Steam generator tubes, whose degradation is
attributed to outside diameter stress corrosion
cracking within the bounds of the tube support
plate with bobbin voltages less than or equal to
the lower voltage repair limit [Note 1], will be
allowed to remain in service.
b. Steam generator tubes, whose degradation is
attributed to outside diameter stress corrosion
cracking within the bounds of the tube support
plate with a bobbin voltage greater than the
lower voltage repair limit [Note 1], will be
repaired or plugged, except as noted in
4.4.5.4.a.10.c below.
c. Steam generator tubes, with indications of
potential degradation attributed to outside
diameter stress corrosion cracking within the
bounds of the tube support plate with a bobbin
voltage greater than the lower voltage repair
limit [Note 1] but less than or equal to the
upper voltage repair limit [Note 2], may remain
in service if a rotating pancake coil inspection
does not detect degradation. Steam generator
tubes, with indications of outside diameter
stress corrosion cracking degradation with a
bobbin voltage greater than the upper voltage
repair limit [Note 2] will be plugged or
repaired.
d. [If applicable] Certain intersections as
identified in [reference report] will be
excluded from application of the voltage-based
repair criteria as it is determined that these
intersections may collapse or deform following a
postulated LOCA + SSE event.
e. If an unscheduled mid-cycle inspection is
performed, the following mid-cycle repair limits
apply instead of the limits identified in
4.4.5.4.10.a, 4.4.5.4.10.b, and 4.4.5.4.10.c.
The mid-cycle repair limits are determined from
the following equations:
where:
VURL = upper voltage repair
limit
VLRL = lower voltage repair
limit
VMURL = mid-cycle upper voltage
repair limit based on
time into cycle
VMLRL = mid-cycle lower voltage
repair limit based on
VMURL and time into cycle
þt = length of time since
last scheduled
inspection during which
VURL and VLRL were
implemented
CL = cycle length (the time
between two scheduled
steam generator
inspections)
VSL = structural limit voltage
Gr = average growth rate per
cycle length
NDE = 95-percent cumulative
probability allowance
for nondestructive
examination uncertainty
(i.e., a value of 20-
percent has been
approved by NRC)
Implementation of these mid-cycle repair limits should
follow the same approach as in TS 4.4.5.4.10.a,
4.4.5.4.10.b, and 4.4.5.4.10.c.
Note 1: The lower voltage repair limit is 1.0 volt for 3/4-inch diameter
tubing or 2.0 volts for 7/8-inch diameter tubing
Note 2: The upper voltage repair limit is calculated according to the
methodology in Generic Letter 95-xx as supplemented. VURL may
differ at the TSPs and flow distribution baffle.
4.4.5.5 Reports
d. For implementation of the voltage-based repair criteria to tube
support plate intersections, notify the staff prior to returning the
steam generators to service should any of the following conditions
arise:
1. If estimated leakage based on the projected end-of-cycle (or if not
practical, using the actual measured end-of-cycle) voltage
distribution exceeds the leak limit (determined from the licensing
basis dose calculation for the postulated main steamline break) for
the next operating cycle.
2. If circumferential crack-like indications are detected at the tube
support plate intersections.
3. If indications are identified that extend beyond the confines of the
tube support plate.
4. If indications are identified at the tube support plate elevations
that are attributable to primary water stress corrosion cracking.
5. If the calculated conditional burst probability based on the
projected end-of-cycle (or if not practical, using the actual
measured end-of-cycle) voltage distribution exceeds 1 X 10-2, notify
the NRC and provide an assessment of the safety significance of the
occurrence.
REACTOR COOLANT SYSTEM
3/4.4.6 REACTOR COOLANT SYSTEM LEAKAGE
3.4.6.2 Reactor Coolant System leakage shall be limited to:
a. No PRESSURE BOUNDARY LEAKAGE,
b. 1 GPM UNIDENTIFIED LEAKAGE,
c. 150 gallons per day of primary-to-secondary leakage through any one
steam generator,
d. 10 GPM IDENTIFIED LEAKAGE from the Reactor Coolant System, and
e. GPM CONTROLLED LEAKAGE at a Reactor Coolant System pressure of
2235 ñ 20 psig.
f. 1 GPM leakage at a Reactor Coolant System pressure of 2235 ñ 20 psig
from any Reactor Coolant System Pressure Isolation Valve specified
in Table 3.4-1.
For licensees who want to reduce RCS specific iodine activity, the following
TS pages apply:
REACTOR COOLANT SYSTEM
3/4.4.8 SPECIFIC ACTIVITY
3.4.8 The specific activity of the primary coolant shall be limited to:
a. Less than or equal to [reduced value] microcurie per gram DOSE
EQUIVALENT I-131, and
b. Less than or equal to 100/þ microcuries per gram.
APPLICABILITY: MODES 1, 2, 3, 4, and 5.
ACTION:
MODES 1, 2, and 3*:
a. With the specific activity of the primary coolant greater than
[reduced value] microcurie per gram DOSE EQUIVALENT I-131 for more
than 48 hours ...
MODES 1, 2, 3, 4, and 5:
a. With the specific activity of the primary coolant greater than
[reduced value] microcurie per gram DOSE EQUIVALENT I-131 or greater
...
[Revise Figure 3.4-1 to lower the line by a factor corresponding to the
reduction in specific activity. The lowered line should parallel the
original.]
.REACTOR COOLANT SYSTEM
BASES
3/4.4.5 STEAM GENERATORS
The voltage-based repair limits of SR 4.4.5 implement the guidance in GL 95-XX
and are applicable only to Westinghouse-designed steam generators (SGs) with
outside diameter stress corrosion cracking (ODSCC) located at the tube-to-tube
support plate intersections. The voltage-based repair limits are not
applicable to other forms of SG tube degradation nor are they applicable to
ODSCC that occurs at other locations within the SG. Additionally, the repair
criteria apply only to indications where the degradation mechanism is
dominantly axial ODSCC with no significant cracks extending outside the
thickness of the support plate. Refer to GL 95-XX for additional description
of the degradation morphology.
Implementation of SR 4.4.5 requires a derivation of the voltage structural
limit from the burst versus voltage empirical correlation and then the
subsequent derivation of the voltage repair limit from the structural limit
(which is then implemented by this surveillance).
The voltage structural limit is the voltage from the burst pressure/bobbin
voltage correlation, at the 95-percent prediction interval curve reduced to
account for the lower 95/95-percent tolerance bound for tubing material
properties at 650 oF (i.e., the 95-percent LTL curve). The voltage structural
limit must be adjusted downward to account for potential flaw growth during an
operating interval and to account for NDE uncertainty. The upper voltage
repair limit; VURL, is determined from the structural voltage limit by
applying the following equation:
VURL = VSL - VGr - VNDE
where VGr represents the allowance for flaw growth between inspections and VNDE
represents the allowance for potential sources of error in the measurement of
the bobbin coil voltage. Further discussion of the assumptions necessary to
determine the voltage repair limit are discussed in GL 95-XX.
The mid-cycle equation in SR 4.4.5.4.10.e should only be used during unplanned
inspections in which eddy current data is acquired for indications at the tube
support plates.
SR 4.4.5.5 implements several reporting requirements recommended by GL 95-XX
for situations which the NRC wants to be notified prior to returning the SGs
to service. For the purposes of this reporting requirement, leakage and
conditional burst probability can be calculated based on the as-found voltage
distribution rather than the projected end-of-cycle voltage distribution
(refer to GL 95-XX for more information) when it is not practical to complete
these calculations using the projected EOC voltage distributions prior to
returning the SGs to service. Note that if leakage and conditional burst
probability were calculated using the measured EOC voltage distribution for
the purposes of addressing the GL section 6.a.1 and 6.a.3 reporting criteria,
then the results of the projected EOC voltage distribution should be provided
per the GL section 6.b (c) criteria.
3/4.4.6 LEAKAGE LIMITS
The leakage limits incorporated into SR 4.4.6 are more restrictive than the
standard operating leakage limits and are intended to provide an additional
margin to accommodate a crack which might grow at a greater than expected rate
or unexpectedly extend outside the thickness of the tube support plate.
Hence, the reduced leakage limit, when combined with an effective leak rate
monitoring program, provides additional assurance that should a significant
leak be experienced in service, it will be detected, and the plant shut down
in a timely manner.
3/4.4.8 SPECIFIC ACTIVITY
Reduction of the RCS specific activity to levels less than 0.35 microcurie per
gram DOSE EQUIVALENT I-131 requires an evaluation (provided with the TS
amendment request) of the release rate data described in Reference 6 of
GL 95-XX.
