Guidance for Performing Temporary Non-Code Repair of ASME Code Class 1, 2, and 3 Piping (Generic Letter 90-05)
June 15, 1990
TO: ALL HOLDERS OF OPERATING LICENSES FOR NUCLEAR POWER PLANTS
SUBJECT: GUIDANCE FOR PERFORMING TEMPORARY NON-CODE REPAIR OF ASME CODE
CLASS 1, 2, AND 3 PIPING (GENERIC LETTER 90-05)
INTRODUCTION
Section XI of the ASME Boiler and Pressure Vessel Code (hereafter called the
code) specifies code-acceptable repair methods for flaws that exceed code
acceptance limits in piping that is in service. A code repair is required
to restore the structural integrity of flawed ASME Code piping, independent
of the operational mode of the plant when the flaw is detected. Those
repairs not in compliance with Section XI of the ASME Code are non-code
repairs. However, the required code repair may be impractical for a flaw
detected during plant operation unless the facility is shut down. Pursuant
to 10 CFR 50.55a(g)(6)(i), the Commission will evaluate determinations of
impracticality, and may grant relief and may impose alternative
requirements. The staff has developed a position on temporary non-code
repairs depending on the ASME Code class of the piping. The staff continues
to find temporary non-code repairs of code Class 1, 2 and 3 piping
unacceptable without specific written relief granted by the NRC. However,
this generic letter provides guidance that will be considered by the NRC
staff in evaluating relief requests submitted by licensees for temporary
non-code repairs of code Class 3 piping.
Temporary non-code repairs are applicable until the next scheduled outage
exceeding 30 days, but no later than the next scheduled refueling outage.
This guideline applies when a flaw is detected during plant operation. If a
flaw is detected during a scheduled shutdown, a code repair is required
before plant restart.
Code Repair Versus Temporary Non-Code Repair
Article IWA-4000 of Section XI of the ASME Code describes the code repair
procedures. A code repair requires the removal of the flaw and a subsequent
weld repair. The repair weld is subject to post-repair nondestructive
examination and a post-repair pressure test may also be required. A code
repair is practical during a scheduled shutdown. If a flaw is detected
during plant operation, the plant may have to be shut down to perform a code
repair. To avoid a plant shutdown and to limit the leakage from a
through-wall flaw, some licensees have used temporary non-code repairs such
as clamps with rubber gasketing, encapsulation of leaking pipes in cans
using liquid sealants, or weld overlays. Temporary non-code repairs are not
permitted on ASME Code piping without prior relief from the NRC.
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STAFF POSITION
This staff guidance on temporary non-code repairs depends on the ASME Code
class of the piping. Safety-related piping for recent plants is classified
as code Class 1, 2, and 3, according to Regulatory Guide 1.26. For older
plants, safety-related piping is reclassified as code Class 1, 2, and 3 for
the purpose of inservice inspection specified in Section XI according to
Regulatory Guide 1.26. Piping in the reactor coolant pressure boundary is
code Class 1. Typical examples of code Class 2 piping are those in
engineered safety feature systems connected to the reactor coolant pressure
boundary that are designed for emergency core cooling, residual heat
removal, reactor shutdown, and containment heat removal. Typical examples
of code Class 3 piping are those in the cooling water, seal water, and
auxiliary feedwater systems.
ASME Code Class 1 and 2 Piping
For code Class 1 and 2 piping, a licensee is required to perform code
repairs or request NRC to grant relief for temporary non-code repairs on a
case-by-case basis regardless of pipe size. Temporary non-code repairs of
code Class 1 and 2 piping must have load-bearing capability similar to that
provided by engineered weld overlays or engineered mechanical clamps.
Licensee requests based on repairs such as encapsulation of leaking pipes in
cans using liquid sealants, clamps with rubber gasketing, or non-engineered
weld overlays (patches) will not be approved by the staff.
Engineered weld overlays or engineered mechanical clamps are designed to
meet the load-bearing requirements of the piping, assuming that the flaw is
completely through the wall for 360 o, that is, all around the pipe
circumference, at the location of the flaw. Engineered weld overlays and
engineered mechanical clamps are discussed in Generic Letter 88-01, "NRC
Position on IGSCC in BWR Austenitic Stainless Steel Piping."
ASME Code Class 3 Piping
For code Class 3 piping, a licensee is also required to perform code repairs
or request NRC to grant relief for temporary non-code repairs on a
case-by-case basis regardless of pipe size. Because of the rather frequent
instances of small leaks in some Class 3 systems, such as service water
systems, the staff is providing guidance in Enclosure 1 that will be
considered by the staff in evaluating relief requests for temporary non-code
repairs of code Class 3 piping. The guidance for code Class 3 piping in
Enclosure 1 consists of assessing the structural integrity of the flawed
piping by a flaw evaluation and assessing the overall degradation of the
system by an augmented inspection. In addition, licensee evaluation should
consider system interactions such as flooding, spraying water on equipment,
and loss of flow. Furthermore, temporary non-code repairs should be
evaluated for design loading conditions.
Temporary non-code repairs of code Class 3 piping in high energy systems,
that is, the maximum operating temperature exceeds 200 o F or the maximum
operating pressure exceeds 275 psig, must have load-bearing capability
similar to that provided by engineered weld overlays or engineered
mechanical clamps. Licensee requests for high energy Class 3 piping based
on repairs such as
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encapsulation of leaking pipes in cans using liquid sealants, clamps with
rubber gasketing, or non-engineered weld overlays (patches) will not be
approved by the staff. For temporary non-code repairs of code Class 3
piping in moderate energy systems, that is, other than high energy systems,
the licensee may consider non-welded repairs. Furthermore, the structural
integrity of the temporary non-code repair of code Class 3 piping should be
assessed periodically.
For code Class 3 piping, two specific flaw evaluation approaches as
discussed in Enclosure 1 should be considered, namely, the "through-wall
flaw" and the "wall thinning" approaches. If the flaw is found acceptable
by the "through-wall flaw" approach, a temporary non-code repair may be
proposed. If the flaw is found acceptable by the "wall thinning" approach,
immediate repair is not required but the licensee should comply with the
guideline for repair and monitoring. An augmented inspection is a part of
the relief acceptance criteria. The extent of the augmented inspection is
more stringent for high energy lines than for moderate energy lines because
of the potential for more severe failure consequences.
CONCLUSIONS
The staff concludes that adherence to the guidance provided in this generic
letter will reasonably assure structural integrity and protect public health
and safety. The staff has determined that an ASME Code repair is required
for code Class 1, 2 and 3 piping unless specific written relief has been
granted by the NRC. However, the staff has determined that temporary
non-code repair of Class 3 piping that cannot be isolated without a plant
shutdown is justified in some instances. The rather frequent instances of
small leaks in some Class 3 systems, such as service water systems, could
lead to an excessive number of plant start-up and shutdown cycles with undue
and unnecessary stress on facility systems and components if the facilites
were to perform a code repair when the leakage is identified. For the
purpose of this generic letter, impracticality is defined to exist if the
flaw detected during plant operation is in a section of Class 3 piping that
cannot be isolated for completing a code repair within the time period
permitted by the limiting condition for operation (LCO) of the affected
system as specified in the plant Technical Specifications, and performance
of code repair necessitates a plant shutdown. Pursuant to
10 CFR 50.55a(g)(6)(i), the Commission may grant relief for temporary
non-code repair of code Class 3 piping, where impracticality exists in
performing an ASME Code repair while the facility is operating, based on a
staff evaluation considering the guidance in this generic letter.
Backfit Discussion
The objective of this generic letter is to maintain structural integrity of
repaired ASME Code piping. The staff is not imposing a new or different
position. However, this generic letter provides guidance that will be
considered by the NRC staff in evaluating relief requests submitted by
licensees for temporary non-code repairs of code Class 3 piping. Compliance
with the staff guidance is not required. Because the implementation of the
guidance for Class 3 piping is voluntary, 10 CFR 50.109 does not apply.
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This generic letter consists of guidance and does not require a response.
Therefore, an OMB clearance number is not necessary.
If you have any questions about this matter, please contact one of the NRC
technical contacts listed below.
Sincerely,
James G. Partlow
Associate Director for Projects
Office of Nuclear Reactor Regulation
Enclosures:
1. Staff Guidance in Evaluating Relief Requests for Temporary Non-Code
Repair of ASME Code Class 3 Piping
2. Listing of Recently Issued Generic Letters
Technical Contacts:
S. Lee, NRR
(301) 492-0771
R. Hermann, NRR
(301) 492-0768
K. Wichman, NRR
(301) 492-0757
.
Enclosure 1
STAFF GUIDANCE IN EVALUATING RELIEF REQUESTS
FOR TEMPORARY NON-CODE REPAIR OF ASME CODE CLASS 3 PIPING
A. INTRODUCTION
The guidance provided herein will be considered by the NRC staff in
evaluating relief requests submitted by licensees for temporary non-code
repairs of ASME Code Class 3 piping. The guidance is restricted in scope
and has limitations and specific considerations. The guidance consists of
assessing the structural integrity of the flawed piping by a flaw evaluation
and assessing the overall degradation of the system by an augmented
inspection. For a relief request prepared according to criteria different
from those set out in this guidance, the staff will evaluate case-by-case
the basis provided by the licensee.
B. SCOPE, LIMITATIONS, AND SPECIFIC CONSIDERATIONS
1. Scope
Only ASME Code Class 3 piping fabricated from ferritic steel or austenitic
stainless steel are within the scope of this guidance. However, leakage
through a flange gasket is not considered to be a flaw in the piping by
Section XI of the ASME Code and is excluded. Furthermore, pumps, valves,
heat exchangers, and components other than piping are excluded. For
materials other than ferritic steel and austenitic stainless steel, a
licensee should justify the material properties used in the flaw evaluation
of Section C.3.a below.
2. Limitations
This guideline for temporary non-code repair of code Class 3 piping applies
when a flaw, which originates in the inner diameter of the pipe, is detected
during plant operation. If a flaw is detected during a scheduled shutdown,
a code repair is required before plant restart. A temporary non-code repair
is applicable until the next scheduled outage exceeding 30 days, but no
later than the next scheduled refueling outage. The temporary non-code
repair should then be replaced with a code repair.
3. Specific Considerations
System interactions such as the consequences of flooding and spraying water
on equipment should be considered. The potential significance of a loss of
flow to the system should also be considered. Furthermore, temporary
non-code repairs should be evaluated for design loading conditions, such as
deadweight, pressure, thermal expansion, and seismic loads.
The integrity of the temporary non-code repair of code Class 3 piping should
be assessed at least every 3 months by a suitable nondestructive examination
(NDE) method. This examination should involve the application of ultrasonic
testing (UT) or radiographic testing (RT). Furthermore, a qualitative
assessment of leakage through the temporary non-code repair should be
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performed at least every week during plant walkdown inspections to determine
any degradation of structural integrity. The licensee should perform an
engineering evaluation to assess the rate and extent of the degradation to
determine what remedial measures are required. A temporary non-code repair
is no longer valid if the structural integrity is not assured.
ASME Code Class 3 piping encompasses both high energy systems, that is, the
maximum operating temperature exceeds 200 F or the maximum operating
pressure exceeds 275 psig, and moderate energy systems, that is, other than
high energy systems. Temporary non-code repairs of code Class 3 piping in
high energy systems must have load-bearing capability similar to that
provided by engineered weld overlays or engineered mechanical clamps.
Licensee requests based on repairs such as encapsulation of leaking pipes in
cans using liquid sealants, clamps with rubber gasketing, or non-engineered
weld overlays (patches) will not be approved by the staff.
Engineered weld overlays or engineered mechanical clamps are designed to
meet the load-bearing requirements of the piping, assuming that the flaw is
completely through the wall for 360 o, that is, all around the pipe
circumference, at the location of the flaw. The staff position on
engineered weld overlays is provided in Generic Letter 88-01, "NRC Position
on IGSCC in BWR Austenitic Stainless Steel Piping." For engineered weld
overlays of ferritic steel piping, the calculation method described in ASME
Code Case N-463 is recommended. Furthermore, overlay welding on ferritic
piping may be performed according to the "half bead" technique described in
Section XI or the "temper bead" technique described in ASME Code Case N-432
without the specified post-weld heat treatment (PWHT) requirements of
Article NB-4622 of Section III of the ASME Code. The staff position on
engineered mechanical clamps is also provided in Generic Letter 88-01, and
such devices require staff review on an individual case basis.
For temporary non-code repairs of code Class 3 piping in moderate energy
systems, the licensee may consider (1) non-welded repairs, and (2) leaving
the piping as-is if there is no leakage and the flaw is found acceptable by
the "through-wall flaw" approach discussed in Section C.3.a below.
C. EVALUATION GUIDELINE
Figure 1 shows a flow chart for the staff evaluation guideline on temporary
non-code repairs of code Class 3 piping. The flow chart consists of (1)
flaw detection during plant operation and impracticality determination, (2)
root cause determination and flaw characterization, (3) flaw evaluation, and
(4) augmented inspection.
1. Flaw Detection During Plant Operation and Impracticality Determination
The initiating event is the detection of a flaw in code Class 3 piping
during plant operation. An example would be the discovery of a leak in a
service water system pipe by maintenance personnel during plant operation.
The licensee should determine the existence of any impracticality in
performing a code repair. If practical, that is, if the affected section of
piping can be isolated for completing a code repair within the time period
permitted by the limiting condition for operation (LCO) without a plant
shutdown, the licensee is required to perform a code repair.
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2. Root Cause Determination and Flaw Characterization
The root cause of the piping degradation should be determined. The flaw
evaluation criteria in the staff guidance assume a localized flaw. The flaw
geometry should be characterized by a suitable NDE method for subsequent
flaw evaluation. This examination should involve the application of UT or
RT techniques. The flaw geometry should be suitably bounded to account for
NDE uncertainties and limitations. Figure 2a shows a schematic of a
generalized flaw in a pipe wall originating in the inner diameter of the
pipe. The flaw may or may not be through-wall.
3. Flaw Evaluation
The structural integrity of the flawed piping should be assessed by a flaw
evaluation. Two specific flaw evaluation approaches as discussed below
should be considered, namely, the "through-wall flaw" and the "wall
thinning" approaches. The flawed piping should satisfy the criteria of
either of these two approaches. The licensee may select either approach for
flaw evaluation, except that the "wall thinning" approach is not applicable
to (1) a through-wall flaw, including a pinhole leaking flaw, and (2) a
crack-like flaw. It is noted that the "through-wall flaw" approach may be
applied to a flaw that is not through-wall.
a. "Through-Wall Flaw" Approach
This approach assumes a through-wall flaw and evaluates the flaw
stability by a linear elastic fracture mechanics methodology.
Figure 2b shows some geometric parameters used in the evaluation.
The code-required minimum wall thickness "t min" should be
determined. The maximum length of the portion of the flaw that
extends beyond "t min", independent of orientation with respect to
the pipe, is the through-wall flaw length "2a". As shown in
Figure 2b, the flaw does not have to be through-wall for the
application of this approach. The length "2a" can be determined
according to Figure 2b for a flaw that may or may not be
through-wall.
If the length "2a" exceeds either 3 inches or 15 percent of the
length of the pipe circumference, the flaw is not acceptable by
this approach.
The stress "s" at the flawed location should be determined from
the combination of deadweight, pressure, thermal expansion, and
safe-shutdown earthquake (SSE). For evaluation purposes, the
through-wall flaw length "2a" should be conservatively assumed to
be in the circumferential direction and the stress "s" should be
assumed to be a bending stress. A safety factor of 1.4 should be
applied to the stress as shown in equation (1) below. This safety
factor is consistent with the factor of a square root of two on
the stress intensity for flaw evaluation under faulted loads in
Article IWB-3600 of Section XI of the ASME Code.
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Based on linear elastic fracture mechanics and assuming a pipe
thickness of "t min", the stress intensity factor "K" resulting
from
the flaw under the applied load is given in Reference 1 as
K = 1.4 s F ( 3.1416 a ) 0.5 (1)
where the geometry factor "F" is
F = 1 + A c 1.5 + B c 2.5 + C c 3.5 (2)
where
c = a / (3.1416 R) (3)
R = mean pipe radius
A = -3.26543 + 1.52784 r - 0.072698 r 2 + 0.0016011 r 3 (4)
B = 11.36322 - 3.91412 r + 0.18619 r 2 - 0.004099 r 3 (5)
C = -3.18609 + 3.84763 r - 0.18304 r2 + 0.00403 r 3 (6)
r = R / t min (7)
For flaw stability, linear elastic fracture mechanics methodology
specifies "K" to be less than the critical stress intensity factor
which represents the fracture toughness of the material.
For ferritic steel, the value of "K" from equation (1) should be
less than 35 ksi(in) 0.5, which is consistent with the lower-bound
fracture toughness property in ASME Code Case N-463.
For austenitic stainless steel, the value of "K" from equation (1)
should be less than 135 ksi(in) 0.5, which is consistent with the
lower-bound fracture toughness property used in Article IWB-3640
of Section XI of the ASME Code.
If the flaw satisfies the criteria of this evaluation approach, a
temporary non-code repair of the code Class 3 piping may be
proposed. It is noted that the rate of degradation is not
considered in this approach because the flaw is assumed to have
grown through the pipe wall and the temporary non-code repair is
applicable, at maximum, until the next scheduled refueling outage.
b. "Wall Thinning" Approach
This approach assumes wall thinning and evaluates the structural
strength of the flawed piping based on the acceptance standards in
Article 3000 of ASME Code Case N-480. Although ASME Code Case
N-480 addresses wall thinning as a result of erosion/corrosion,
the acceptance standards in ASME Code Case N-480 are extended by
the
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staff to all wall thinning mechanisms such as microbiologically
induced corrosion (MIC) for applications within the scope of this
generic letter.
Figure 2c shows some geometric parameters used in the evaluation.
The code-required minimum wall thickness "t min" should be
determined. The minimum measured wall thickness "t meas" should
be determined by NDE. Based on an estimated wall thinning rate
and "t meas", the minimum predicted wall thickness "t p" projected
to the next inservice examination should be determined. ASME Code
Case N-480 provides rules for determining the allowable local wall
thickness "t aloc" for the measured length of the flaw. Local
wall thinning is acceptable if "t p" exceeds "t aloc".
If the flaw satisfies the criteria of this evaluation approach,
immediate repair of the code Class 3 piping is not required.
However, the licensee should comply with the repair and monitoring
guideline in ASME Code Case N-480.
c. Single Versus Multiple Flaws
If multiple proximate flaws are detected, they may have to be
considered in the flaw evaluation as a single flaw. The guideline
discussed in this section is based on Article IWA-3330 of Section
XI of the ASME Code.
Figure 3a shows the geometric parameters used in the evaluation
for the "wall thinning" approach. The minimum spacing "S",
independent of orientation relative to the pipe, between two flaws
of depths "d 1" and "d 2" are shown. For "d 2" larger than "d 1",
the two flaws should be treated as a single flaw if "S" is less
than or equal to two times "d 2".
Figure 3b shows the geometric parameters used in the evaluation
for the "through-wall flaw" approach. The difference between
Figure 3a and Figure 3b is that the parameters are measured from
"t min" in Figure 3b. The minimum spacing "S *", independent of
orientation relative to the pipe, between two flaws of depths
"d 1 *" and "d 2 *" is shown. For "d 2 *" larger than "d 1 *",
the two flaws should be treated as a single flaw if "S *" is less
than or equal to two times "d 2 *".
4. Augmented Inspection
If the flaw is evaluated and found acceptable by one of the above evaluation
approaches, the licensee should perform an augmented inspection via UT or RT
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to assess the overall degradation of the affected system. The augmented
inspection, performed within 15 days of detection of the flaw which results
in a temporary non-code repair, is a part of the relief acceptance criteria
of the temporary non-code repair of code Class 3 piping.
From the root cause determination, the most susceptible locations should be
identified. The extent of the augmented inspection depends on whether the
line is high energy or moderate energy. The failure of a high energy line
may have more severe consequences than the failure of a moderate energy line
because of the energy content. Thus, a more extensive augmented inspection
should be performed for high energy lines. As shown in Figure 1, the
inspection of at least 10 most susceptible (and accessible) locations for
high energy lines and at least 5 most susceptible (and accessible) locations
for moderate energy lines should be performed. Flaws detected in the
augmented inspection should be characterized and evaluated. If any flaw is
detected having a minimum measured wall thickness "t meas" less than the
code-required minimum wall thickness "t min" in the augmented inspection
sample, inspection of an additional sample of the same size should be
performed. This process should be repeated within 15 days of each other
until no flaw having "t meas" less than "t min" is detected in the
additional inspection sample or until 100 percent of susceptible (and
accessible) locations have been inspected.
D. REFERENCES
1. "NRC Leak-Before-Break (LBB.NRC) Analysis Method for Circumferentially
Through-Wall Cracked Pipes Under Axial Plus Bending Loads,"
NUREG/CR-4572, May 1986.
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NOTE: The figures on pages 7, 8, and 9 attached to Enclosure 1 of this
generic letter are not transferable electronically.
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