Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
WASHINGTON, D.C. 20055
January 25, 1988
TO ALL LICENSEES OF OPERATING BOILING WATER REACTORS (BWRS), AND HOLDERS
OF CONSTRUCTION PERMITS FOR BWRS
Gentlemen:
SUBJECT: NRC POSITION ON IGSCC IN BWR AUSTENITIC STAINLESS STEEL PIPING
(Generic Letter 88-01)
Intergranular stress corrosion cracking (IGSCC) near weldments in BWR
piping has been occurring for almost 20 years. Early cases were in
relatively small diameter piping. In early 1982, cracking was identified
in large-diameter piping in a recirculation system of an operating BWR
plant in this country. Since then, extensive inspection programs have
been conducted on BWR piping systems. These inspections have resulted in
the detection of significant numbers of cracked weldments in almost all
operating BWRs.
A number of domestic and foreign BWR owners have replaced or plan to
replace piping systems that have experienced IGSCC with more resistant
material. Other owners are implementing countermeasures such as Stress
Improvement (SI) or Hydrogen Water Chemistry (HWC) to reduce the
susceptibility of the piping to IGSCC. In many cases, cracked weldments
have been repaired by reinforcing them with weld overlay.
Substantial efforts in research and development have been sponsored by the
BWR Owners Group for IGSCC Research. The results of this program, along
with other related work by vendors, consulting firms, and confirmatory
research sponsored by the NRC, have permitted the development of revised
Staff Positions regarding the IGSCC problems.
The technical bases for these positions are detailed in NUREG-0313, Rev. 2
"Technical Report on Material Selection and Process Guidelines for BWR
Coolant Pressure Boundary Piping." This revision to NUREG-0313 was a
major task in the staff long range plan to deal with BWR pipe cracking
that was presented to the Commission in SECY 84-301. This revision
includes the relevant recommendations of the Piping Review Committee Task
Group on Pipe Cracking issued as NUREG- 1061, Vol. 1, "Report of USNRC
Piping Review Committee", and consideration of public comments on that
document. NUREG-0313, Rev. 2 describes the technical bases for the staff
positions on materials, processes, and primary coolant chemistry to
minimize and control IGSCC problems. Inspection schedules and inspection
sample sizes are based on the susceptibility of weldments to initiation
and propagation of IGSCC. Inspection schedules are comparable to those
specified in Section XI of the ASME Boiler and Pressure Vessel Code in
cases where the piping material is IGSCC resistant. Varying amounts of
augmented inspections are specified for piping with a greater
susceptibility to cracking, where there is less certainty about the
effectiveness of mitigation measures used, or in cases where repairs have
been performed. When improved water chemistry control with hydrogen
additions is implemented, less augmentation of inspection schedules is
required.
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The purpose of this Generic Letter is to seek information regarding
implementation of the new staff positions covering these technical areas.
This Generic Letter supersedes Generic Letter 84-11, "Inspection of BWR
Stainless Steel Piping."
This Generic Letter applies to all BWR piping made of austenitic stainless
steel that is four inches or larger in nominal diameter and contains
reactor coolant at a temperature above 200xF during power operation
regardless of Code classification. It also applies to reactor vessel
attachments and appurtenances such as jet pump instrumentation
penetration assemblies and head spray and vent components.
This Generic Letter does not apply to piping made of carbon steel
classified as P-1 by the ASME Boiler and Pressure Vessel Code.
Staff Positions have been developed covering the following subjects:
1. Staff Position on Materials
2. Staff Position on Processes
3. Staff Position on Water Chemistry
4. Staff Position on Weld Overlay Reinforcement
5. Staff Position on Partial Replacement
6. Staff Position on Stress Improvement of Cracked Weldments
7. Staff Position on Clamping Devices
8. Staff Position on Crack Characterization and Repair Criteria
9. Staff Position on Inspection Methods and Personnel
10. Staff Position on Inspection Schedules
11. Staff Position on Sample Expansion
12. Staff Position on Leak Detection
13. Staff Position on Reporting Requirements
These Staff Positions are fully delineated in Attachment A to this letter.
The staff continues to believe that replacing susceptible piping with
IGSCC resistant materials will provide the greatest degree of assurance
against future cracking problems. Licensees may follow Generic Letter
84-07, "Procedural Guidance for Pipe Replacements at BWRs"; the staff
encourages programs to replace degraded piping so as to reduce the
potential for cracking and to minimize the need for augmented
inspections. However, the staff recognizes that, if the staff positions
of this Generic Letter are implemented, adequate levels of piping
integrity and reliability can be achieved. The staff believes this
Generic Letter, together with the revision to NUREG-0313, will be of use
to licensees in making sound decisions regarding IGSCC. Each weldment can
be evaluated considering its material, heat treatment history, stress
level, chemical environment and surveillance program. This will provide a
basis for a reasonable judgment regarding the long-term acceptability of
that weldment. Considering that each piping system has many weldments and
each plant has many piping systems, the entire problem must be evaluated
in an integrated way.
The Commission has determined that, unless appropriate remedial actions
are taken, BWR plants may not be in conformance with their current design
and licensing bases, including 10 CFR 50, Appendix A, General Design
Criteria 4, 14, and 31.
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Accordingly, pursuant to 10 CFR 50.54(f), you, as a BWR operating reactor
licensee or construction permit holder, are requested, to furnish, under
oath or affirmation, your current plans relating to piping replacement,
inspection, repair, and leakage detection. Your response should indicate
whether you intend to follow the staff positions included in this letter,
or propose alternative measures. This information is needed for the
Commission to determine whether an operating license should be issued, or
if you hold an operating license, whether it should be modified or
revoked.
An acceptable response to this letter would include the following specific
items:
1. Your current plans regarding pipe replacement and/or other measures
taken or to be taken to mitigate IGSCC and provide assurance of continued
long term piping integrity and reliability.
2. An Inservice Inspection (ISI) Program to be implemented at the next
refueling outage for austenitic stainless steel piping covered under the
scope of this letter that conforms to the staff positions on inspection
schedules, methods and personnel, and sample expansion included in this
letter.
3. A change to the Technical Specifications to include a statement in the
section on ISI that the Inservice Inspection Program for piping covered by
the scope of this letter will be in conformance with the staff positions
on schedule, methods and personnel, and sample expansion included in this
letter (see enclosed model BWR Standard Technical Specification). It is
recognized that the Inservice Inspection and Testing sections may be
removed from the Technical Specifications in the future in line with the
Technical Specifications Improvement programs. In this case, this
requirement shall remain with the ISI section when it is included in an
alternative document.
4. Confirmation of your plans to ensure that the Technical Specification
related to leakage detection will be in conformance with the staff
position on leak detection included in this letter.
5. In accordance with 10CFR50.55a(o), your plans to notify the NRC of any
flaws identified that do not meet IWB-3500 criteria of Section XI of the
Code for continued operation without evaluation, or a change found in the
condition of the welds previously known to be cracked, and your
evaluation of the flaws for continued operation and/or your repair plans.
Licensees and construction permit holders for BWR plants are requested to
respond to this generic letter within 180 days of receipt of this letter.
NRC review of your submittal of information in response to this letter is
not subject to fees under the provisions of 10 CFR 170. However, should
you, as part of your response or in a subsequent submittal, include an
application for license amendment or other action requiring NRC approval,
it is subject to the fee requirements of 10 CFR 170 with remittance of an
application fee of $150 per application (Sections 170.12(c) and 170.21)
and subsequent semi-annual payments until the review is completed or the
ceiling in Section 170.21 is reached.
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This request for information was approved by the Office of Management and
Budget under clearance number 3150-0011 which expires December 31, 1989.
Comments on burden and duplication may be directed to the Office of
Management and Budget, Reports Management Room 3208, New Executive Office
Building, Washington, D.C. 20503.
Sincerely,
Frank J. Miraglia, Jr.
Associate Director for Projects
Office of Nuclear Reactor Regulation
Enclosures:
1. Staff Positions on IGSCC In BWR Austenitic Stainless Steel Piping
2. Model BWR Standard Technical Specification for Item 3 of this Generic
Letter
3. NUREG-0313, Revision 2
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(ATTACHMENT A)
STAFF POSITIONS ON IGSCC IN BWR AUSTENITIC STAINLESS STEEL PIPING
Scope
These Staff Positions apply to all BWR piping made of austenitic stainless
steel that is four inches or larger in nominal diameter and contains
reactor coolant at a temperature above 200xF during power operation
regardless of Code classification. It also applies to reactor vessel
attachments and appurtenances such as jet pump instrumentation
penetration assemblies and head spray and vent components.
This Generic Letter does not apply to piping made of carbon steel
classified as P-1 by the ASME Boiler and Pressure Vessel Code.
Staff Position on Materials
Materials considered to be resistant to sensitization and IGSCC in BWR
piping systems are:
(1) Low carbon wrought austenitic stainless steel, which includes types
304L, 304NG, 316NG and similar low carbon grades with a maximum carbon
content of 0.035%. Type 347, as modified for nuclear use, will be
resistant with somewhat higher carbon content, the usual maximum of 0.04%
is adequate. These materials must be tested for resistance to
sensitization in accordance with ASTM A262-A or -EI or equivalent
standard.
(2) Low carbon weld metal, including types 308L, 316L, 309L and similar
grades, with a maximum carbon content of 0.035% and a minimum of 7.5%
ferrite (or 7.5 FN) as deposited. Low carbon weld metal especially
developed for joining modified type 347 Is also resistant as deposited.
Welds joining resistant material that meet the ASME Boiler and Pressure
Vessel Code requirement of 5% ferrite (or 5 FN) but are below 7.5%
ferrite (or 7.5 FN) may be sufficiently resistant, depending on carbon
content and other factors. These will be evaluated on an individual case
basis.
(3) Piping weldments are considered resistant to IGSCC if the weld heat
affected zone on the inside of the pipe is protected by a cladding of
resistant weld metal. This is often referred to as corrosion resistant
cladding (CRC).
(4) Cast austenitic stainless steel with a maximum of 0.035% carbon and a
minimum of 7.5% ferrite (or 7.5 FN). Weld joints between resistant piping
and cast valve or pump bodies that do not meet these requirements are
considered to be special cases, and are covered in the Staff Position on
Inspection Schedules below.
(5) Austenitic stainless steel piping that does not meet the requirements
of (1) above is considered to be resistant if it is given a solution heat
treatment after welding.
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(6) Other austenitic materials, including nickel base alloys such as
Inconel 600, will be evaluated on an individual case basis. Inconel 82 is
the only commonly used nickel base weld metal considered to be resistant.
It is the staff position that no austenitic material is resistant to
cracking in the presence of a crevice, such as formed by a partial
penetration weld, where the crevice is exposed to reactor coolant.
Staff Position on Processes
The processes considered to provide resistance to IGSCC in BWR piping
welds are:
(1) Solution Heat Treatment (SHT)
(2) Heat Sink Welding (HSW)
Either of these two processes will upgrade non-resistant material to IGSCC
Category A (see Table 1)
(3) Stress Improvement (SI)
Either of the following processes will upgrade non-resistant material to
IGSCC Category B or C (See Table 1)
a. Induction Heating Stress Improvement (IHSI)
b. Mechanical Stress Improvement Process (MSIP)
Last pass heat sink welding (LPHSW) is not considered to be fully
effective.
Staff Position on Water Chemistry
The use of hydrogen water chemistry, together with stringent controls on
conductivity, will inhibit the initiation and growth of IGSCC. However,
the responses of BWRs to hydrogen injection differs from plant to plant,
and the development and verification of a generic HWC specification is
not yet complete. For these reasons, reductions in piping inspection
frequencey based on the use of HWC will be considered on an individual
case bases at the present time. Staff criteria for evaluating the
effectiveness of water chemistry improvements are under development, and
will be available prior to general use of the HWC option. If fully
effective HWC is maintained, a factor Categories B, C, D, and E
weldments. (See Table 1)
Staff Position on Weld Overlay Reinforcement
Cracked weldments that are reinforced with weld overlay are acceptable for
short-term operation, and may be considered for longer term operation
provided:
(1) The overlayed weldments are in conformance with the criteria of IWB
3600 of Section XI of the 1986 Edition of the ASME Boiler and Pressure
Vessel Code, and
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(2) they are inspected in conformance with the Staff Position on
Inspection Methods and Personnel, by UT examiners and procedures
qualified to inspect overlayed welds.
Staff Position on Partial Replacement
If portions of cracked piping are replaced in the course of repair, the
replaced portions will be subjected to inservice inspection requirements
that will depend on the materials and processes used. All relevant staff
positions of this Generic Letter will apply.
Staff Position on Stress Improvement SI of Cracked Weldments
Stress Improvement is also considered to be an effective mitigation
process when applied to weldments with short or shallow cracks.
Specifically, welds with cracks that are no longer than 10% of the
circumference, and are no deeper than 30% of the wall thickness will be
considered to be mitigated by SI.
SI is only considered to be effective if it is followed by a qualified UT
examination, and if cracks are found, they must be sized both in depth and
length, by procedures and personnel qualified to perform sizing
evaluations:
Staff Position on Clamping Devices
Clamping devices may be used for temporary reinforcement of cracked
weldments. Each case must be reviewed and approved on an individual
basis.
Staff Position on Crack Evaluation and Repair Criteria.
Methods and criteria for crack evaluation and repair should be in
conformance with IBW-3600 of Section XI of the 1986 Edition of the ASME
Boiler and Pressure Vessel Code.
Evaluation of cracks for continued operation without repair requires that
crack growth calculation be performed. As some details are not yet
provided in the Code, the following will be acceptable to the staff.
The crack growth rate (da/dt) selected for use by the staff is expressed
as:
da/dt = 3.590 x 10E-8 x K(I)**2.161 inches per hour
where
K(I) is the applied stress intensity factor (Ksi * SQRT(in))
Linear elastic solutions for KI are required for crack growth
calculations. Any standard method is acceptable, for example, those
described in the ASME Boiler and Pressure Vessel Code, Section XI,
Appendix A. The axial residual stress distribution considered acceptable
by the staff for large diameter pipes (12 inches and larger) is described
by the following nondimensional expression.
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e/e(i) = SUM(from j=0 to j=4) of e(j) E**j
where
eo = 1.0
e1 = 6.920
e2 = 8.687
e3 = 0.480
e4 = -2.027
E = x/t
e(i) = stress magnitude at E = 0 (inner surface)
The above formula permits calculation of the residual stress value at any
point (x) through the vessel wall thickness (t) as a function of the peak
residual stress value at the inside diameter (ID), e(i).
Technical basis and additional discussion related to evaluation and repair
are given in NUREG 0313 Revision 2.
Staff Position on Inspection Methods and Personnel
Examinations performed under the Scope of this letter should comply with
the applicable Edition and Addenda of the ASME Code, Section XI, as
specified in paragraph (9), "Inservice Inspection Requirements" of
10CFR50.55a, Codes and Standards, or as otherwise approved by the NRC.
In addition, the detailed procedure, equipment and examination personnel
shall be qualified by a formal program approved by the NRC such as that
being conducted in accordance with the NDE Coordination Plan agreed upon
by NRC, EPRI, and the Boiling Water Reactor Owners Group for IGSCC
Research, being implemented at the EPRI NDE Center in Charlotte, North
Carolina.
A summary of the Staff Position on Inspection Schedules is given in Table
1. Additional details and definitions are provided below. NUREG-0313,
Rev. 2, Section 5 provides background information and technical bases.
(1) Welds of resistant material, IGSCC Category A, shall as a minimum be
examined according to an extent and frequency comparable to that specified
in applicable provisions of Section XI of the ASME Boiler and Pressure
Vessel Code, as reflected in Table 1, attached. The selection of specific
welds to be included in this sample is the responsibility of the
Licensee, and should include considerations of stress levels, piping
configurations, weld details, etc, and should represent his best judgement
regarding selection of a representative and meaningful sample.
The provisions of 10CFR50.55a, (b),(2),(ii) may be invoked if it is
determined necessary to use the 1974 edition of the Code to permit a
meaningful sample selection.
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(2) Although castings with higher carbon content than 0.035% are not
considered to be resistant to sensitization, welds joining such castings
(in the form of pump and valve bodies) to piping have been relatively
free of IGSCC. This may be attributed to a favorable residual stress
distribution, as calculations have indicated. For this reason, welds
joining resistant material to pumps and valves will be considered to be
resistant welds, and included in IGSCC Category A. If extensive weld
repairs were performed the residual stress may be unfavorable, in which
case such welds should be included in Category D.
(3) Welds that have been treated by SI or reinforced by weld overlay that
are classified as IGSCC Category F because they do not meet the
applicable staff positions may be upgraded to Category E if no adverse
change in crack condition is found after 4 successive examinations.
Staff Position on Sample Expansion
If one or more cracked welds in IGSCC Categories A, B, or C, are found by
a sample inspection during the 10 year interval, an additional sample of
the welds in that category shall be inspected, approximately equal in
number to the original sample. This additional sample should be similar
in distribution (according to pipe size, system, and location) to the
original sample, unless it is determined that there is a technical reason
to select a different distribution. If any cracked welds are found in
this sample, all of the welds in that IGSCC Category should be inspected.
If significant crack growth or additional cracks are found during the
inspection of an IGSCC Category E weld, all other Category E welds should
be examined.
a) Significant crack growth for overlayed welds is defined as crack
extension to deeper than 75% of the original wall thickness, or for
cracks originally deeper than 75% of the pipe wall, evidence of crack
growth into the effective weld overlay.
b) Significant crack growth for SI mitigated Category E welds is defined
as growth to a length or depth exceeding the criteria for SI mitigation
(either 10% of circumference in length or 30% of the wall in depth).
Staff Position on Leak Detection
Leakage detection systems should be in conformance with Position C of
Regulatory Guide 1.45 "Reactor Coolant Pressure Boundary Leakage Detection
Systems," or as otherwise previously approved by the NRC.
1. Plant shutdown should be initiated for inspection and corrective action
when, within any period of 24 hours or less, any leakage detection system
indicates an increase in rate of unidentified leakage in excess of 2 gpm
or its equivalent, or when the total unidentified leakage attains a rate
of 5 gpm or equivalent, whichever occurs first. For sump level monitoring
systems with fixed-measurement-interval methods, the level should be
monitored at approximately 4-hour intervals or less.
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2. Unidentified leakage should include all leakage other than:
(a) leakage into closed systems, such as pump seal or valve packing leaks
that are captured, flow metered, and conducted to a sump or collection
tank, or
(b) leakage into the containment atmosphere from sources that are both
specifically located and known either not to interfere with the operations
of unidentified leakage monitoring systems or not to be from a through
wall crack in the piping within the reactor coolant pressure boundary.
3. For plants operating with any IGSCC Category D, E, F, or G welds, at
least one of the leakage measurement instruments associated with each
sump shall be operable, and the outage time for inoperable instruments
shall be limited to 24 hours, or immediately initiate an orderly
shutdown.
If any cracks are identified that do not meet the criteria for continued
operation without evaluation given in Section XI of the Code, NRC approval
of flaw evaluations and/or repairs in accordance with IWB 3640 and IWA
4130 is required before resumption of operation.
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TABLE 1
SUMMARY OF INSPECTION SCHEDULES FOR BWR PIPING WELDMENTS
IGSCC INSPECTION
DESCRIPTION OF WELDMENTS NOTES CATEGORY EXTENT & SCHEDULE
Resistant Materials A 25% every 10 years
(at least 12% in 6
years)
Non-resistant Matls (1) B 50% every 10 years
SI within 2 yrs of (at least 25% in 6
operation (1) years)
Non-resistant Matls (1) C All within the next 2
SI after 2 years refueling cycles,
of operation then all every 10
years (at least 50%
in 6 years)
Non-resistant Matls (1) D All every 2 refueling
No SI cycles
Cracked (1)(2) E 50% next refueling
Reinforced by weld outage, then all every
overlay or mitigated 2 refueling cycles
by SI
Cracked (2) F All every refueling
Inadequate or outage
no repair
Non-Resistant (3) G All next refueling
Not Inspected outage
Notes:
(1) All welds in non-resistant material should be inspected after a stress
improvement process as part of the process. Schedules shown should be
followed after this initial inspection.
(2) See recommendations for acceptable weld overlay reinforcements and
stress improvement mitigation.
(3) Welds that are not UT inspectable should be replaced, "sleeved", or
local leak detection applied. RT examination or visual inspection for
leakage may also be considered.
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(Attachment B)
Model BWR Standard Technical Specification
for Item 3 of Generic Letter 88-01
APPLICABILITY
SURVEILLANCE REQUIREMENTS
4.0.1 Surveillance Requirements shall be met during the OPERATIONAL
CONDITIONS or other conditions specified for individual Limiting
Conditions for Operation unless otherwise stated in an individual
Surveillance Requirement.
4.0.2 Each Surveillance Requirement shall be performed within the
specified time interval with:
a. A maximum allowable extension not to exceed 25% of the surveillance
interval, but
b. The combined time interval for any 3 consecutive surveillance intervals
shall not exceed 3.25 times the specified surveillance interval.
4.0.3 Failure to perform a Surveillance Requirement within the specified
time interval shall constitute a failure to meet the OPERABILITY
requirements for a Limiting Condition for Operation. Exceptions to these
requirements are stated in the individual Specifications. Surveillance
requirements do not have to be performed on inoperable equipment.
4.0.4 Entry into an OPERATIONAL CONDITION or other specified applicable
condition shall not be made unless the Surveillance Requirement(s)
associated with the Limiting Condition for Operation have been performed
within the applicable surveillance interval or as otherwise specified.
4.0.5 Surveillance Requirements for inservice inspection and testing of
ASME Code Class 1, 2, & 3 components shall be applicable as follows:
a. Inservice inspection of ASME Code Class 1, 2, and 3 components and
inservice testing of ASME Code Class 1, 2, and 3 pumps and valves shall be
performed in accordance with Section XI of the ASME Boiler and Pressure
Vessel Code and applicable Addenda as required by 10 CFR 50, Section
50.55a(g), except where specific written relief has been granted by the
Commission pursuant to 10 CFR 50, Section 50.55a(g) (6) (i)
b. Surveillance intervals specified in Section XI of the ASME Boiler and
Pressure Vessel Code and applicable Addenda for the inservice inspection
and testing activities required by the ASME Boiler and Pressure Vessel
Code and applicable Addenda shall be applicable as follows in these
Technical Specifications:
ASME Boiler and Pressure Vessel Required frequencies
Code and applicable Addenda for performing inservice
terminology for inservice inspection and testing
inspection and testing activities activities
Weekly At least once per 7 days
Monthly At least once per 31 days
Quarterly or every 3 months At least once per 92 days
Semiannually or every 6 months At least once per 184 days
Every 9 months At least once per 276 days
Yearly or annually At least once per 366 days
GE-STS 3/4 0-2
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APPLICABILITY
SURVEILLANCE REQUIREMENTS (Continued)
c. The provisions of Specification 4.0.2 are applicable to the above
required frequencies for performing inservice inspection and testing
activities.
d. Performance of the above inservice inspection and testing activities
shall be in addition to other specified Surveillance Requirements.
e. Nothing in the ASME Boiler and Pressure Vessel Code shall be construed
to supersede the requirements of any Technical Specification.
f. The Inservice Inspection Program for piping identified in NRC Generic
Letter 88-01 shall be performed in accordance with the staff positions on
schedule, methods, and personnel and sample expansion included in this
generic letter.
GE-STS 3/4 0-3
