ML12110A085

Response to Request for Additional Information for Relief Request RR-ENG-3-08 (Essential Cooling Water System)

ML12110A085
Person / Time
Site:	South Texas
Issue date:	04/10/2012
From:	Rencurrel D South Texas
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
NOC-AE-12002829, STN-50-499, TAC ME8159
Download: ML12110A085 (16)

Text

Nuclear Operating Company South Texas Pro/ect Electric Generating Station PO Box 289 Wadsworth, Texas 77483 April 10, 2012 NOC-AE-12002829 File No. G25 10 CFR 50.55a U. S. Nuclear Regulatory Commission Attention: Document Control Desk One White Flint North 11555 Rockville Pike Rockville, MD 20852-2738 South Texas Project Unit 2 Docket No. STN 50-499 Response to Request for Additional Information for Relief Request RR-ENG-3-08 (Essential Coolinq Water System) (TAC No. ME8159)

Reference:

D. W. Rencurrel, STP Nuclear Operating Company, to NRC Document Control Desk, "Request for Relief from ASME Boiler and Pressure Vessel Code, Section Xl Requirements for the Essential Cooling Water System (Relief Request RR-ENG-3-08)", dated March 12, 2012 (NOC-AE-12002807) (ML12079A034)

In accordance with the provisions of 10 CFR 50.55a(a)(3)(ii), the South Texas Project requested relief from IWA-5250 of Section Xl of the ASME Boiler and Pressure Vessel Code to allow deferral of code repair of flaws found in the Unit 2 Essential Cooling Water (ECW) Class 3 piping (reference above). Attached are responses to questions from NRC reviewers dated March 22, 2012. Code repair of the affected valves at this time is not possible because the necessary parts are not currently available.

The work will be performed following receipt of the replacement parts.

Compliance with the requirements of 10 CFR 50.55a would result in a hardship without a compensating increase in the level of quality and safety.

Replacement parts are not currently available; consequently, resolution of the valve flaws will not occur before July 28, 2012, when the parts are expected to arrive. Repairs are expected to be completed by October 28, 2012.

A list of commitments is attached.

If there are any questions, please contact either Mr. P. L. Walker at (361) 972-8392 or me at (361) 972-7867.

D. W. Rencurrel Chief Nuclear Officer PLW

Attachment:

1.

Response to Request for Additional Information for Relief Request RR-ENG-3-08 (Essential Cooling Water System) (TAC ME8159)

2.

List of Commitments A04l STI: 33431345

(,A A

NOC-AE-1 2002829 Page 2 of 3 cc:

(paper copy)

(electronic copy)

Regional Administrator, Region IV U. S. Nuclear Regulatory Commission 1600 East Lamar Blvd Arlington, Texas 76011-4511 Balwant K. Singal Senior Project Manager U.S. Nuclear Regulatory Commission One White Flint North (MS 8 B1) 11555 Rockville Pike Rockville, MD 20852 Senior Resident Inspector U. S. Nuclear Regulatory Commission P. 0. Box 289, Mail Code: MN1 16 Wadsworth, TX 77483 C. M. Canady City of Austin Electric Utility Department 721 Barton Springs Road Austin, TX 78704 John Ragan Chris O'Hara Jim von Suskil NRG South Texas LP A. H. Gutterman, Esquire Morgan, Lewis & Bockius LLP Balwant K. Singal U. S. Nuclear Regulatory Commission Kevin Polio Richard Pena City Public Service Peter Nemeth Crain Caton & James, P.C.

C. Mele City of Austin Richard A. Ratliff Alice Rogers Texas Department of State Health Services NOC-AE-12002829 Page 1 of 13 SOUTH TEXAS PROJECT UNIT 2 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION FOR RELIEF REQUEST RR-ENG-3-08 (ESSENTIAL COOLING WATER SYSTEM) (TAC NO. ME8159)

1. General The NRC and the ASME Code provide guidance concerning temporary repairs in NRC Generic Letter 90-05, "Guidance for Performing Temporary Non-Code Repair Of ASME Code Class 1, 2, and 3 Piping," (ADAMS Accession No. ML031140590) and ASME Code Case 513-3, "Evaluation Criteria for Temporary Acceptance of Flaws in Moderate Energy Class 2 or 3 Piping, Section Xl, Division 1," with condition specified in NRC Regulatory Guide 1.147, Revision 16, "Inservice Inspection Code Case Applicability, ASME Section Xl, Division 1" (ADAMS Accession No. ML101800536).

The NRC staff notes that this guidance has not been included in the RR under consideration. The NRC staff believes that failure to consider all the available guidance may result in temporary repairs which do not meet appropriate safety criteria. For each of the requirements contained in both the code case and GL 90-05, please identify whether the current RR a) meets the requirement; b) the requirement is not applicable; or c) how the current RR provides an acceptable level of safety.

RESPONSE

Code case N-513-3 is not applicable to valves.

Staff guidance in the scope of Generic Letter 90-05 evaluation criteria specifically excludes valves. Consequently, it is not applicable to this relief request. The specific evaluation criteria and their relevance to the relief request are as follows:

1. Flaw Detection Durinq 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.

Comparison: The repair can be performed on-line. However, replacement parts are not available, and are not expected to be received before July 2012. STP experience shows that valve replacement is preferred over weld repairs on aluminum-bronze.

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 NOC-AE-12002829 Page 2 of 13 generalized flaw in a pipe wall originating in the inner diameter of the pipe. The flaw may or may not be through-wall.

Comparison:

The root cause of dealloying of aluminum-bronze castings is well-known and has been extensively studied from previous instances. A flaw can only be examined visually: UT and RT can not determine flaw geometry.

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.

Comparison: Although there is no apparent leakage in progress, the dealloyed flaws are treated as "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 "tmin" should be determined.

The maximum length of the portion of the flaw that extends beyond "tmin", 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.

Comparison: The enclosure to the generic letter states that the scope of the staff guidance excludes valves. Consequently, this approach for evaluating through-wall flaws is not applicable to this relief request.

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.

Comparison: This approach is not used in evaluating this condition.

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 Xl 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 "dl" and "d2" are shown. For "d2" larger than "dl", the two flaws should be treated as a single flaw if "S" is less than or equal to two times "d2".

NOC-AE-12002829 Page 3 of 13 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 "tmin" in Figure 3b. The minimum spacing "S", independent of orientation relative to the pipe, between two flaws of depths "dl*" and "d2*" is shown. For "d2*" larger than "d1*", the two flaws should be treated as a single flaw if "S*" is less than or equal to two times "d*".

Comparison:

The flaw evaluation accounts for multiple flaws by assuming 100%

dealloying.

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 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.

Comparison:

Neither UT nor RT are suitable for assessing dealloyed aluminum-bronze piping. Only visual examination is useful.

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 "tmeas" less than the code-required minimum wall thickness "tmin" 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 "tmeas" less than "tmin "

is detected in the additional inspection sample or until 100 percent of susceptible (and accessible) locations have been inspected.

Comparison: Visible areas of ECW piping are visually examined (VT2) for evidence of dealloying at six-month intervals. This frequency is sufficient for timely identification of flaws due to dealloying.

NOC-AE-12002829 Page 4 of 13

2. Section 1. - Component for Which Relief is Requested 1.) Section 1.(d) discusses the indications found and describes these indications as through-wall dealloying with no cracks and no measurable leakage. Was any leakage noticeable and if so, please describe the leakage?

2.) Section 1.(d) describes the five indications found on Valve 2-EW-FV-6936 and multiple indications found on valve 2-EW-FV-6937.

Please discuss if these indications were treated as a single defect and how they compared to the 3-inch critical flaw size stated in Section 6.4 of your request. If they were treated independently, please discuss the basis for making that assumption.

RESPONSE

1.) The de-alloying indications were identified by residue buildup on the outside surface of the valve body. This residue consists of non-volatile material from the water (sediment, minerals, and corrosion products). Once liquid seeping through the de-alloyed area reaches the surface of the valve body, it evaporates faster than the seepage rate through the metal. The leakage is only noticeable, therefore, as residue buildup on the outside surface of the valve body without any apparent wetness.

2.) There is only one indication of dealloying on 2-EW-FV-6937.

The various flaws are not treated independently. 100% dealloying of the component is assumed to have occurred for calculational purposes. The allowable stress is revised accordingly. There are no cracks present.

NOC-AE-1 2002829 Page 5 of 13

3. Section 4 - Reason for Request

1) This section discusses the reason for this request and states that the valves will be replaced following receipt of the replacement parts. The proposed duration presents regulatory ambiguity because it is not clear to the NRC staff the exact length of time the relief is requested. The licensee needs to provide the exact date beyond which the RR will be expired and no longer be effective.

It is not clear to the NRC staff that the flaw evaluation in Section 6.4 of the RR demonstrates that the through wall flaw in the valve at the end of proposed duration would still be within the critical crack length of 3.0 inches. Please demonstrate that the flaw size in the subject valves at the end of duration period will be within the critical crack length of 3.0 inches.

2) Please demonstrate that appropriate actions have been taken to procure a replacement valve and address the concept of using a valve constructed from an alternate material on a temporary basis. If practical, please state the delivery dates for such valves.

RESPONSE

1.) The relief request was requested to be applicable for the length of time required for the replacement valves to arrive.

The valves are expected to be replaced within three months of delivery of the new valves, currently expected to occur July 28, 2012.

The areas of dealloying are monitored for changes in appearance, including size, on a frequency consistent with the slow development of dealloying.

Aspects of the monitoring are as described in response to question 4-1. This monitoring ensures that the piping remains within the design limits.

2) STPNOC has issued a purchase order with Flowserve Corporation for two replacement valves manufactured out of aluminum-bronze material as specified in the original design. Estimated date of receipt of the replacement valves at the South Texas Project is July 28, 2012.

Staying with the original valve design is compatible with performing a long-term fix and ensures equipment compatibility, as well as avoiding the unnecessary expense of purchasing new valves for use only as temporary replacements.

NOC-AE-1 2002829 Page 6 of 13

4. Section 6.2 - Specific considerations 1.) This section describes how structural integrity of these valves will be monitored.

It states that structural integrity and monitor frequency are re-evaluated if significant changes in the condition of the dealloying occur. Please provide the criteria that define significant changes in the condition of the dealloying.

2.) This section states that dealloying flaws are only detectable by visual examination once they have reached the piping surface. Please describe how the evaluation procedures account for the inside diameter (ID) degradation and its effect on the overall valve degradation. Please describe any inspection procedures that may be used to verify the ID degradation.

If current inspection procedures cannot detect the extent of the ID dealloying, justify why a single linear indication that is part through wall for the complete circumference, and through-wall for the extent of the OD defects should not be assumed in the fracture analyses.

3.) Section 6.2, page 4, states that the affected valves will be inspected by a monthly walkdowns.

ASME Code Case N-513-3, paragraph F requires that for through-wall leaking flaws, leakage shall be observed by daily walkdowns to confirm the analysis conditions used in the evaluation remain valid. Please explain why a monthly inspection is sufficient to ensure that the changes in flaw size or leak rate will not affect the structural integrity of the valves.

4.) The licensee stated that leakage from Essential Cooling Water (ECW) piping in the subject valve location flows to the mechanical auxiliary building sumps. The licensee further stated that sump level alarms are available to warn operator if leakage exceeds the sump pump capacity. This seems that the leak rate would have to be significant before the sump level alarms will annunciate in the control room.

By such time, the crack may have been grown to a significant size such that the structural integrity of the pipe would be in question. Please discuss the leak rate and associate crack size in the ECW piping that would cause sump level alarm to initiate.

RESPONSE

1.)

The flawed piping is inspected on a monthly basis for signs of continued degradation.

Structural integrity and the monitoring frequency are reevaluated if a significant change in the condition is found. Significant changes are indicated by:

Residue buildup over an area greater than twice the original area of residue buildup Residue buildup or leakage in a new area on the valve body A change from residue buildup to measurable liquid leakage Indication of crack development.

Inspection frequency is increased to weekly if the flaw growth is projected to compromise the integrity of the valve prior to its scheduled replacement.

The condition is addressed for resolution under the STP corrective action program should reevaluation determine that structural integrity of the valve is no longer assured.

NOC-AE-12002829 Page 7 of 13 In addition, these valves are located in areas that are subject to observation by plant personnel on a daily basis. Gross changes in the valve condition should be apparent and reported as part of the corrective action program.

2.) Evaluation procedures account for inside diameter degradation, i.e. where presence of dealloying may be more extensive than is visible on the surface, by assuming that the material of the affected piping has experienced 100% dealloying. Inspection procedures are not suitable for determining the degree of dealloying without destructive testing. By assuming 100%

dealloying when calculating

stresses, such determination is unnecessary.

3.) As stated in 6.5 of the submittal, the dealloyed areas have not shown sufficient change from the time of discovery to warrant accelerated implementation of corrective measures. The experience at the South Texas Project is that the dealloying process progresses very slowly. Changes observed in flaw conditions over a period of months have been inconsequential or non-existent. Any changes in flaw parameters would be identified well before operability of the affected ECW train is challenged. Consequently, monthly inspections are appropriate.

The provisions of Code case N-513-3 specifically do not apply to valves.

4.) A switch turns the pumps on when the sump level reaches 24 inches above the bottom of the tank and off at 18 inches above the bottom of the tank. Another switch at 36 inches actuates an alarm. However, flow through the valve is normally throttled from upstream to 300 gpm. Each of the two ECW sump pumps (in each unit) has a capacity of 390 gpm. Loss of structural integrity of the pipe will not result in excessive flow to the sump.

NOC-AE-12002829 Page 8 of 13

5. Section 6.4 - Flaw Evaluation 1.) This section describes the flaw evaluations completed in this effort. A critical crack length of 3.0 inches was determined for the through wall crack. Please provide details on the analyses conducted including the fracture mechanics procedures used and the material properties assumed.

2.) Please provide details on the basis for the 100 percent dealloyed tensile strength (30 ksi). Please include the number of specimens tested, the data used to determine this value, and describe the experimental procedure used in its development.

In addition, please discuss how 100% dealloyed material can leak yet have residual strength.

3.) A summary of fracture results from 4-inch nominal pipe sizes was used in the fracture analysis. Please provide additional details on the experiments used in the comparison and their applicability to the dealloyed valves in this request.

Include details on the materials, geometry, loading conditions and flaw type.

4.) Please provide details on the allowable moment and stress calculations for both end loading and postulated pipe break stress.

5.) This section states that significant margin exists compared to the fracture analysis calculations.

Please provide details on the fracture margin.

The beginning of this section describes a 3-inch critical flaw size derived from experiments but does not estimate a margin.

RESPONSE

1.) The critical bending stress for failure by plastic collapse or by fracture of aluminum-bronze (Al-Bronze) casting as a function of through-wall de-alloying length and/or through-wall crack length was established by APTECH report AES-C-1964-1, "Calculation of Critical Bending Stress for Dealloyed Aluminum-Bronze Castings in the ECW System," dated December 1, 1993. This report was submitted as an attachment to "Request for Relief from ASME Boiler and Pressure Vessel Code Section XI Requirements (Dealloying) (Relief Request RR-ENG-35) (Supplement 2)," (August 10, 2000) (ML003742174).

The critical bending stress is the failure stress (i.e. ultimate bending load capacity) without a safety factor.

2.) The phenomenon of de-alloying of aluminum-bronze castings for flanges, valves and fittings, grade CA 952 and CA 954, in the ECWS has been identified and discussed in depth with the NRC (reference: HL&P Letter ST-HL-AE-2748, dated November 1, 1988).

Table 2.5 of ST-HL-AE-2748, Attachment 2, provides Tensile Test Results on De-alloyed Sample of CA 954 Material from fittings 3.) APTECH calculation # AES-C-1964-1 Rev. 0 was used as a reference for comparison.

No additional experiments were conducted.

4.) The calculations for allowable moment and stress are attached.

5.) APTECH calculation # AES-C-1964-1 Rev. 0 was used as a reference for comparison; no additional experiments were conducted.

Based on the calculation for a 4-inch nominal diameter pipe with a 3-inch long circumferential through-wall crack, the critical bending (i.e. the failure stress) is 18.19 ksi compared to the calculated stress of 1.7 ksi from weight and seismic loads.

NOC-AE-12002829 Page 9 of 13

6. Section 6.5 Augmented Inspection 1.) Indications have been found in similar valves in two of the three ECW loops. Please discuss the rationale for not implementing the augmented inspection on the similar valve in the third ECW loop.

RESPONSE

1.) Dealloying in the second valve was found during a walkdown at a six-month interval, and continuing that practice will enable identification of instances of dealloying in similar valves well before there would be adverse consequences to plant operability and safety.

The six-month interval is consistent with the slow progress demonstrated by the dealloying process.

Application of augmented inspection to additional valves will not enhance the structural integrity of the ECW piping.

In addition, the valves are located in areas that are subject to observation by plant personnel on a daily basis. Gross changes in the valve condition should be apparent and reported as part of the corrective action program.

NOC-AE-12002829 Page 10 of 13 SOUTH TEXAS PROJECT JOB,NO. 14926 CALCULATION SHEET SEE THE COVER SHEET CALC NO.

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PAGE 79 PIPE BREAK LOCATION DETERMINATION TITLE ESSENTIAL COOLING WATER PROJECT NUMBER 14926001 PROBLEM NUMBER 3MI59RC6370-USER YOGI TRIVEDI LOAD CASES ALL CODE SC3W75, CLASS 2 ME 1Oi/K1I DATE 101084 ELEMENT FROM TYPE TO TITLE 130 251 8 251 B 251 M 251 M 251 E 251 E 255 255 260 260 265 265 270 265 275 275 276 276 277 277 278 278 279 279 280 TNGT BEND BEND TNGT TNGT TNGT TNGT TNGT TNGT TNGT TNGT TNGT TNGT EON 9 PSI 1865 1460 2073 2709 2709 3485 2429 4063 4063 4571 368 212 61 B6 212 206 206 140 140 169 169 199 147 152 152 158 EON 10 PSI 2368 1229 2399 2270 2270 2090 1071 809 809 1425 100 48 0

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PIPE BREAK LOCATION DETERMINATION PAGE so MEIOI/Kl DATE 101084 ELEMENT FROM TYPE To TITLE 280 TNGT 281 278 CMPT 285 285 CNPT 286 286 CMPT 287 2,87 CMPT 288 231 TNGT 295 B 295 B BEND 295 M 295 M BEND 295 E 295 E TNGT 300 300 TNGT 305 EXCEEDED ALLOWABLE EON 9

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NOC-AE-12002829 Page 1 of 1 LIST OF COMMITMENTS The following table identifies the actions in this document to which the STP Nuclear Operating Company has committed. Statements in this submittal with the exception of those in the table below are provided for information purposes and are not considered commitments.

Please direct questions regarding these commitments to Philip Walker at (361) 972-8392.

Commitment Expected CR Action No.

Completion Date 2-EW-FV-6936:

Replacement parts are not currently available; consequently, resolution of the valve flaws will not 10/28/2012 11-12309-9 occur before July 28, 2012, when the parts are expected to arrive.

Repairs are expected to be completed within three months of receipt of the parts.

2-EW-FV-6937:

Replacement parts are not currently available; consequently, resolution of the valve flaws will not 10/28/2012 12-1044-20 occur before July 28, 2012, when the parts are expected to arrive.

Repairs are expected to be completed within three months of receipt of the parts.