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United States Nuclear Regulatory Commission - Protecting People and the Environment

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 
.

     (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. 
.
                                    - 3 -  

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 
.

                               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. 
.
                                    - 2 -

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. 
.
                               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. 
.
                                    - 2 - 

     (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. 
.
                                    - 3 -  

          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. 
.
                                    - 4 - 

               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 
.
                                    - 5 - 

     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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