Inspections of BWR Stainless Steel Piping (Generic Letter No. 84-11)
UNITED STATES
NUCLEAR REGULATORY COMMISSION
WASHINGTON, D. C. 20555
April 19, 1984
TO ALL LICENSEES OF OPERATING REACTORS, APPLICANTS FOR OPERATING LICENSE,
AND HOLDERS OF CONSTRUCTION PERMITS FOR BOILING WATER REACTORS
Gentlemen:
SUBJECT: INSPECTIONS OF BWR STAINLESS STEEL PIPING (Generic Letter 84-11)
Inspections conducted at several boiling,water reactors (BWRs) revealed
intergranular stress corrosion cracking IGSCC) in large-diameter
recirculation and residual heat removal piping. These inspections were
conducted pursuant to IE Bulletins 82-03, Revision 1 and 83-02, and the NRC
August 26, 1983 Orders. The Commission believes that the results of these
inspections mandate an ongoing program for similar reinspections at all
operating BWRs. Where IGSCC is discovered, repairs, analysis and additional
surveillance may also be required to ensure the continued integrity of
affected pipes.
Staff efforts to date on this issue include review of the Electric Power
Research Institute (EPRI) report dated August 4,,1983, establishment of a
pipe crack study group within the staff, evaluation of the results of IGSCC
inspections already conducted, and discussions with licensees and industry
groups. As a result of these considerations, the staff has concluded that
the following actions would be considered an acceptable response to the
current IGSCC concerns:
1. A reinspection program of piping susceptible to IGSCC should be
undertaken. The reinspection should commence within about two calendar
years, adjusted to coincide with the next scheduled outage, from the
previous inspection performed under IE Bulletins 82-03, 83-02, or our
August 26, 1983 Order.
2. These reinspections should include the following stainless steel welds,
susceptible to IGSCC, in piping equal to or greater than 4" in
diameter, in systems operating over 200F, that are part of or
connected to the reactor coolant pressure boundary, out to the second
isolation valve as follows.
(a) Inspection of 20% of the welds in each pipe size of IGSCC
sensitive welds not inspected previously (but no less than 4
welds) and reinspection of 20% of the welds in each pipe size
inspected previously (but not less than 2 welds) and found not to
be cracked.
This sample should be selected primarily from weld locations shown
by experience to have the highest propensity, for cracking.
(b) All unrepaired cracked welds.
(c) Inspection of all weld overlays on welds where circumferential
cracks longer than 10% of circumference were measured. Disposition
of any findings will be reviewed on a case-by-case basis. Criteria
for operation beyond one cycle with overlaid joints are under
development.
8404230029
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(d) Inspection of any weld treated by induction heating stress
improvement which has not been post treatment UT acceptance
tested.
(e) In the event new cracks or significant growth of old cracks* are
found, the inspection scope should be expanded in accordance with
IEB 83-02.
NOTE: [Results of inspections conducted to date under IEB 82-03 and
83-02 indicate that all stainless steel piping welds in systems
operating over 200F are susceptible to IGSCC. In addition,
field data shows that the cracking experience does not correlate
well with the Stress Rule Index (SRI) and the carbon content.
Therefore, the primary index for sample selection should be field
experience, where other factors such as weld preparation,
excessive grinding, extensive repairs, or high stress locations
are known to exist, they should also be considered in the sample
selection.]
3. All level 2 and level 3 UT examiners should demonstrate competence in
accordance with IEB 83-02 and level 1 examiners should demonstrate
field performance capability.
4. Leak detection and leakage limits should be sufficiently restrictive,
to ensure timely investigation of unidentified leakage. See Attachment
1.
5. For crack evaluation and repair criteria see Attachment 2.
Accordingly, pursuant to 10 CFR 50.54(f), BWR operating reactor licensees
and applicants for an operating license (this letter is for information only
for those utilities that have not applied for an operating license) are
requested, in order to determine whether your license should be modified or
suspended, to furnish, under oath or affirmation, no later than 45 days from
the date of this letter, your current plans relative to inspections for
IGSCC and interim leakage detection. Your response should indicate whether
you intend to follow the above staff recommended actions or to propose an
alternative approach to resolving IGSCC concerns. In either case, your
response should address:
(a) Scope and schedule of planned inspections
(b) Availability and qualification of examiners
(c) Description of any special surveillance measures, in effect or
proposed, for primary system leak detection, beyond those measures
already required by your Technical Specifications
(d) Results of the Bulletin inspections not previously submitted to
NRC
(e) Remedial measures, if any, to be taken when cracks are discovered
* Significant growth of the old crack is defined as Growth to a
new crack size that, cannot be accepted without repair for the
remaining period of the current or a new cycle of operation, in
accordance with Attachment 2.
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The staff considers the IGSCC problem to be generic for all BWRs. Therefore,
your response may incorporate by reference materials furnished by an Owners'
Group. To the extent practicable, Owners' Group and EPRI participation in
the IGSCC effort is encouraged.
Licensees and applicants may request an extension of time for submittals of
the required information. Such a request must set forth a proposed schedule
b and justification for the delay. Such a request shall be directed to the
Director, Division of Licensing, NRR. Any such request must be submitted no
later than 15 days from the date of this letter.
This request for information was approved by the Office of Management and
Budget under clearance number 3150-0011 which expires April 30, 1985.
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.
Darrell G. Eisenhut, Director
Division of Licensing
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ATTACHMENT 1
LEAK DETECTION AND LEAKAGE LIMITS
The reactor coolant leakage detection systems are operated in accordance
with the Technical Specification requirements to assure the discovery of
unidentified leakage that may be caused by throughwall cracks developed in
austenitic stainless steel piping.
A. The leakage detection system shall be sufficiently sensitive to detect
and measure small leaks in a timely manner and to identify the leakage
sources within practical limits. Particular attention should be given
to upgrading and calibrating those leak detection systems that will
provide prompt indication of an increase in leakage rates.
Other equivalent and/or local leakage detection and collection systems
will be reviewed on a case-by-case basis.
B. Plant shutdown shall be initiated for inspection and corrective action
when any leakage detection system indicates, within any period of 24
hours, an increase in rate of unidentified leakage in excess of 2 gpm
or its equivalent, whichever occurs first. For sump level monitoring
systems with a fixed-measurement interval method, the level shall be
monitored at 4-hour intervals or less.
C. 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.
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D. Unidentified leakage should include all leakage other than
(1) leakage into closed systems, such as pump seal or valve packing
leaks that are captured, flow metered, and conducted to a sump or
collecting tank, or
(2) 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.
E. A visual examination for leakage of the reactor coolant piping shall be
performed during each plant outage in which the containment is
deinerted. The examination will be performed consistent with the
requirements of IWA-5241 and IWA-5242 of the 1980 Edition of Section XI
of the ASME Boiler and Vessel Code. The system boundary subject to this
examination shall be in accordance with IWA-5221.
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ATTACHMENT 2
CRACK EVALUATION AND REPAIR CRITERIA
1. Background
(a) Code Requirements
The ASME Boiler and Pressure Vessel Code Section XI has rules for
evaluating the acceptability of flaws for further operation. Table
IWB 3514-3 provides rules for acceptability of flaws without
further evaluation; although the specific dimensions of such
acceptable flaws depends on both the length and depth of the
flaws, the practical effect is that flaws less than about 10% of
the wall thickness are acceptable for further operation without
analysis or repair.
A new section has recently been added to the Code, IWB 3600. This
extends the Code flaw evaluation rules for piping to include
specific rules whereby flaws deeper than those allowed by IWB
3514-3 can be accepted for further operation without repair.
Section IWB 3600 also requires that these acceptable flaw sizes
include considerations of crack growth by stress corrosion and
fatigue. In other words, if a crack is to be considered acceptable
for further operation without repair, it must be shown that it
will not grow to be larger than the IWB 3640 limits during the
time period for which the evaluation is performed.
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(b) Crack Growth Assessment
IGSCC at welds in BWRs is primarily initiated by very high tensile
residual welding stresses on the inside surface at the heat
affected (sensitized) zones of the base metal very near the welds.
This tensile residual stress changes to a compressive stress
toward the middle of the pipe wall; this reduction in stress
reduces the crack growth rate through the center portion of its
pipe wall. As the crack progresses further through the wall, the
relative effect of the pressure and bending stresses increases,
and the crack growth rate will increase.
The residual stress patterns and calculational methods for crack
growth rates are fairly well established by considerable research
and correlations with service experience. The staff has selected
parameters that should lead to overprediction of growth. This is
intended to compensate for uncertainties discussed in more detail
below.
(c) Staff Treatment of Uncertainties
One of the main uncertainties associated with the evaluation of
pipe cracks is the uncertainty of crack sizing, both depth and
length of IGSCC cricks. Although this technology is being
improved, the uncertainty in crack sizing will likely remain.
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The staff has used a relatively simple approach to cover sizing
uncertainties. In practice, the staff approach permits operation
of unrepaired cracks but only if calculations show that they would
not exceed Code limits even if the crack at the start of operation
were actually twice as large as reported.
2. Staff Acceptance Criteria
(a) Criterion for Operation without Repair
Plant operation is permitted with cracked welds only for the time
period that the cracks are evaluated to not exceed 2/3* of the
limits for depth and length provided in ASME Code Section XI,
Paragraph IWB-3640. Crack growth analyses must include any
additional stress imposed on the weld by other weld repair
operations, and each analysis must be approved by the NRC.
(b) Criteria for Cracked Repairs
(i) If cracked welds are repaired by weld overlay, the thickness
of the overlay must be sufficient to provide full IWB-3640
margin during the proposed operating period, assuming that
the cracks are or will grow completely through the original
pipe wall and the first overlay layer to the low carbon and
low ferrite portion of the overlay, unless it is demonstrated
that the crack(s) are shallow enough to be arrested by the
weld overlay.
* This criterion allows for an uncertainty of up to 100% in crack depth
sizing for reported cracks up to 25% of wall thickness.
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Effective overlay thickness is defined as the thickness of
overlay deposited after the first weld layer that clears
dye-penetrant testing (PT) inspection.
(ii) The minimum effective overlay thickness permitted, even for
very short cracks in either longitudinal or circumferential
direction, is two weld layers after the first layer to clear
PT inspection.
(iii) Full structural strength weld overlays must be provided for
long cracks with total circumferential, extent approaching
the length that would cause limit load failure if they were
actually throughwall.
(iv) Multiple short circumferential cracks are to be treated as
one crack with a length equal to the sum of the
circumferential lengths.
3. Discussion of Staff Acceptance Criteria
Since the period of operation between inspections could vary from plant
to plant and the applied stress level varies from location to location,
use of a fixed simplified repair criterion established on the bases of
crack size prior to the period of operation would be difficult. In Any
case, however, flaws less than about 10% of the wall thickness are
acceptable for further operation without repairs. For a typical 18
month operating cycle, the staff criteria would generally require that
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cracks greater than 30% of the circumference and,cracks with reported
depth of 25% or greater of the thickness will likely need some form of
repair. For the same 18 month cycle, cracks of smaller size down to 10%
of wall thickness may be acceptable without repair but would require
evaluation in accordance with the staff criteria.
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