United States Nuclear Regulatory Commission - Protecting People and the Environment

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.



9006120310 
.

                                     -2-

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 
.

                                     -3-

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.

.

                                     -4-

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 
.

                                     -2-


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.

.

                                     -3-


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.

.

                                     -4-


          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 
          
.

                                     -5-


          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 
.

                                     -6-


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.

.


NOTE:     The figures on pages 7, 8, and 9 attached to Enclosure 1 of this 
          generic letter are not transferable electronically.
.ENDEND
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