ML20148C105

Forwards Summary of Microbiologically Induced Corrosion (MIC) Program & Technical Basis for Structural Integrity Evaluations,Per NRC Request Made at 871215 Meeting.Plant Acceptable for Restart

ML20148C105
Person / Time
Site:	Sequoyah
Issue date:	01/20/1988
From:	Gridley R TENNESSEE VALLEY AUTHORITY
To:	NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM)
References
NUDOCS 8801250120
Download: ML20148C105 (37)

Text

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TENNESSEE VALLEY AUTHORITY CH ATTANOOGA, TENNESSEE 37401 SN 1578 Lookout Place

'JAN 201988 U.S. Nuclear Regulatory Commissiop ATTN: Document Control Desk Washington, D.C. 20555 /

Gentlemen:

In the Matter of ) Docket Nos. 50-327 Tennessee Valley Authority ) 50-328 SEQUOYAH NUCLEAR PLANT (SQN) UNITS 1 AND 2 - MICR0 BIOLOGICALLY INDUCED CORROSION (MIC) PROGRAM On December 15, 1987, TVA presented to the NRC Staff the program elements and technical bases supporting our current initiatives for addressing the effects of MIC at Sequoyah. At the conclusion of the meeting, the Staff requested a formal submittal on the SQN MIC program including the technical basis for the structural integrity evaluations / conclusions supporting this program.

Enclosure 1 provides a summary of the SQN MIC Program. Enclosure 2 provides the technical basis for the structural integrity evaluations. Commitments made by TVA in this submittal are identified in enclosure 3.

Our conclusions are 1) SQN is acceptable for restart; and 2) if a MIC induced leak occurs during operation, appropriate operability evaluations will be made in accordance with the methodology outlined in enclosure 1.

If you have any questions concerning this issue, please call 0. L. Williams at (615) 632-7170.

t Very truly yours, TENNESSEE VA EY AUTHORITY i

R. .idley, 01 ector Nuclear Licen,ing and Regulatory Affairs Enclosures cc: See page 2 8901250120 080120 PDR ADOCK 05000327 P PDR An Equal opportunity Employer

s Mr. Stewart D. Ebneter r cc (Enclosures):

Mr. K. P. Barr, Acting Assistant Director for Inspection Programs Office of Special Projects U.S. Nuclear Regulatory Commission Region II 101 Marietta Street, NH, Suite 2900 Atlanta, Georgia 30323 Mr. G. G. Zech, Assistant Director for Projects Division of TVA Projects Office of Special Projects U.S. Nuclear Regulatory Commission Mail Stop 7E23 7920 Norfolk Avenue Bethesda, Maryland 20814 Sequoyah Resident Inspector Sequoyah Nuclear Plant 2600 Igou Ferry Road Soddy Daisy, Tennessee 37379

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ENCLOSURE 1 L

SEQUOYAH N1JCLEAR PLANT MICR0810 LOGICALLY INDUCED CORROSION (MIC) PROGRAM l

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l SEQUOYAH NUCLEAR PLANT MICR0 BIOLOGICALLY INDUCED CORROSION (MIC) PROGRAM TABLE OF CONTENTS I.

SUMMARY

r II. BACKGROUND III. LEAK DETECTION IV. PLANT RESPONSE TO LEAKAGE V. REPAIR VI. FURTHER EVALUATION b

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SEQUOYAH NUCLEAR PLANT (SQN) MIC PROGRAM I.

SUMMARY

The structural attack of stainless steel butt welds in the Essential Raw Cooling Water (ERCW) system has been the most significant problem associated with MIC during inspections at SQN. To address this problem, SQN has developed a program to inspect for leakage, evaluate the damage, and repair as required. Also, SQN will perform an ongoing investigation of corrosion damage in these welds to monitor this damage and subsequently verify the effectiveness of water treatment when the new water treatment program is implemented.

II. BACKGROUND 1

SQN management is committed to addressing identified MIC damage. These actions and timeframes for action are based on our understanding of MIC damage at SQN and its effect on the operability of the ERCH, as determined by our testing and analysis program. The following statements summarize our understanding and assumptions of MIC effects on the system.

1. The ERCH piping can tolerate extensive damage without compromising its ability to retain structural integrity throughout a seismic

, event. To date, the damage associated with these welds has not been found to encroach on structural integrity. This has been substantiated by radiographic (RT) evaluation and structural

nalysis of 61 welds which determined that significant structural margin existed based on original design allowables. The critical flaw size used to evaluate new leaking welds is a conservative value determined for worst case stresses.

2. The low leakage rates associated with this damage are readily '

tolerated by the system without affecting operability. Leakage identified by inspection must be corrected in a timely manner but does not affect the status of the operating unit (s).

3. Although calculations have demonstrated that the effects of water leakage resulting from this damage during a seismic event are insignificant, electrical equipment required to shut down the unit (s) during a seismic event has been protected by booting the

welds in the established leakage zone.

In suanary, the identification of a leak does not require that the system (or loop) which has experienced the leakage event be considered inoperable or that a limiting condition for operation (LCO) be declared until the weld joint can be fully analyzed.

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s III. LEAK DETECTION Leak detection is performed on a routine basis in the plant under normal housekeeping requirements and by daily walkdowns of the assistant unit operators. The housekeeping procedure (SQA-66) requires that all accessible areas of the plant be inspected for cleanliness at least monthly. Additionally, the work area of each work request issued is required to be cleaned at the completion of the activity. In either of these instances, the presence of leakage (water, oil, etc.) requires immediate notification of the Operations Section, which will initiate repairs. The assistant unit operators (AVO) are instructed by Operations Section Letter OSSLA99, which specifies the duties of the #

AUO. Attachment C to that letter specifies that, during the routine inspection of the auxiliary building (each shift), the AVO will inspect for leakage of the piping systems.

Each of these instructions is a plant internal mechanism that will identify leakage on any plant system and is not confined to just ERCW.

It is important to note that the affected ERCH headers are very accessible so that any significant leakage (i.e., greater than a few drops per minute) would be quickly identified by these mechanisms in any mode of operation.

In addition to normal procedures, a semiannual walkdown has been initiated. Preventative Maintenance (PM) Instructions (PMs 2220, 2221, 2222, and 2223 "Walkdown of ERCH System for Damage in Stainless Steel Butt Welds Resulting from MIC") specify a weld-by-weld inspection of the stainless steel ERCH butt welds. The PM instructions require close visual inspection of each weld to identify minute amounts of leakage.

IV. PLANT RESPONSE TO LEAKAGE PMs 2220, 2221, 2222, and 2223 and TI-109 will be revised to state that, when a leak is discovered, the following actions will be taken:

During Modes 5 and 6, a work request will be written to repair the damaged area, and that repair work will be a restart requirement.

During Modes 1, 2, 3, and 4. Technical Instruction (TI) 109, "Non-Dastructive Testing of Stainless Steel Butt-Welds to Assess Damage Resulting From Microbiologically Induced Corrosion (MIC),"

will be used to evaluate the corrosion damage. This instruction will be revised to specify that RT will be completed within seven days after leak discovery. The RT data will be corrpared against a preestablished screening criteria. If a weld is found with MIC damage that exceeds the screening criteria, that weld will receive further detailed seismic analysis within an additional seven days.

If the detailed seismic analysis determines a weld to be

, structurally inadequate, appropriate ERCW Technical Specification (TS) actions will be taken. Sequoyah experience indicates that all welds will be within the screening criteria and, therefore, TS actions should not be required.

o IV. PLANT RESPONSE TO LEAKAGE (Continued)

If the weld is considered to be structurally sound, the leakage from tr.:-

weld is insignificant (i.e., characterized as a weeper or does not pose a personnel hazard), and does not have the potential for leaking on safe shutdown equipment, the leak will be scheduled for repair at the next available outage. The weld will be entered into a monitoring program under PM 2240 and 2241 to monitor any type of MIC growth. This process will be implemented in the revision to TI-109.

The plant management may, at their discretion, elect to put the system into an outage and repair the leaking joint rather than perform radiography.

The above procedure revisions will be in place before the next monitoring period begins in March 1988.

V. REPAIR Currently, the only repair that is considered to be permanent :s the replacement of the affected weld with a pipe spool piece. This practice adds additional welds to the system; however, it allows upgrading of the weld material and total removal of the established MIC colonies.

TVA is also in the process of establishing a full structural sleeve repstr for these joints. This method is intended for repairs that must be completet in the timeframe of a 72-hour LCO. This sleeve would constitute a replacement for the degraded weld in structure and pressure boundary considerations. This repair method will be established before unit 2 restart.

TVA is also considering other options (such as structural fixtures) for repairing welds which are inaccessible for RT or full structural sleeve repair. These options are considered long-term and, if shown to be feasible, would be implemented at TVA's discretion.

VI. FURTHER EVALUATION A sample of 6-10 welds will be selected to monitor grc ' ,. cx' sting MIC indications and to identify development of color '

u welds.

This will be continued until the water treatment pr.. o'. to be effective and will be implemented under PMs 2240 and .'41. ..,>.

monitoring will begin in March 1988.

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ENCLOSURE 2 MICR0 BIOLOGICALLY INDUCED CORROSION TECHNICAL BASIS FOR STRUCTURAL INTEGRITY EVALUATION l

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SEQUOYAH NUCLEAR PLANT MICR0 BIOLOGICALLY INDUCED CORROSION TABLE OF CONTENTS I. List of Figures II. List of Tables

, III. Abstract IV. Technical Evaluation

1. Damage to ERCH Stainless Steel Piping

2. Leaking Welds Removed and Replaced

3. NDE and Destructive Examinations

4. Current Piping Margin Calculations

5. Study of MIC Pitting Growth

6. Correlation of HIC Growth Data and Structural Integrity

7. Conclusions V. References

MICR08I0 LOGICALLY INDUCED CORROSION LIST OF FIGURES Figure-1 Six-Inch ERCH Supply Line From 8-Inch Header to Electrical Board Room Figure 2 MIC Structural Attack Figure 3 MIC Structural ieck on Held 121968 Figure 4 RTs for MIC Damage

' Figure S MIC Structural Attack on Held 12203 Figure 6 MIC Pit Indications Figure 7 RF Histogram - Upset Condition Figure 8 RF Histogram - Faulted Condition i Figure 9 RF vs. MIC Total Length - Upset Condition Figure 10 RF vs. MIC Total Length - Faulted Condition Figure 11 Pit Depth as a function of Flaw Area Fi]ure 12 RT from Weld 12203 F'qure 13 Weld 12203 - Pit 1 Figure 14 RT from Weld 121968 Figure 15 Weld 121968 - Pit 8 Figure 16 Pit Depth as a function of Flaw Area Figure 17 No of Pits vs. Total MIC Length Y

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i MICR0 BIOLOGICALLY INDUCE 0 CORROSION LIST OF TABLES Tablo 1 Weld Examinations Table 2 MIC Void length Per Weld l

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III. ABSTRACT A review of sixty-one (61) welds taken from six-inch-diameter stainless steel piping in the Essential Raw Cooling Water (ERCW) System at ,

Sequoyah unit 2 has established the effect of Microbiologically Induced Corrosion (HIC) on the piping. This stainless steel piping has been in service at Sequoyah (SQN) unit 2 plant for nine (9) years. During this nine years of service, MIC has resulted in structural attack at the butt-welded joints in the stain' ss steel piping. The sixty-one (61) welds have been nondestructively examined and two (2) welds have been destructively examined to determine the amount of MIC damage on each weld.

Using the nondestructive examination data, reouced section properties were calculated for each pipe butt-weld. These reduced section prcperties were used to calculate pipe stresses for upset and faulted loadings. The worst cases of damage were shown to have a factor of 2 margin over acceptance criteria. The acceptance criteria was based on Reserve Factor (RF) calculations with the acceptance criteria being RF =

1.0.

A correlation of MIC pit growth data was used to establish an  !

examination program based on visual examination for leakage. The correlation data provides methodology to establish that a MIC-damaged weld will go through wall and leak before structural integrity margin is reduced below a Reserve Factor of 1.0. As a result of this correlation, an examination program based upon leakage detection is proposed.

IV. TECHNICAL EVALUATf0N

. 1. Damage of Stainless Steel Piping MIC damage in stainless steel piping has primarily been localized to the piping butt-welds and the heat affected zones of these welds.

This damage is observed as pits which, after entering the weld metal, produce volds in the weld and can progress through wall to produce pinhole leaks, normally measured in drops per minute.

Because of its localized nature, this form of attack, if left uncorrected, could impact the structural integrity of the affected piping system.

The major MIC damage to stainless steel butt welds has been shown to be located in 6-inch-diameter piping. This establishment of damage was based upon known leaks found in the piping during visual inspection. A total of 335 of 405 butt welds were visually

inspected in the ERCW system. These inspections identified 28 leaking welds. All of these welds are butt welds in 6-inch-diameter piping.

The piping isometric shown in Figure,1 provides a typical area where significant MIC damage has been found in ERCW piping. The piping in Figure 1 is a 6-inch-diameter ERCW supply line to the electric board ;

room air-conditioning condenser A. The supply line has been shown to have MIC damage at numerous stainless steel butt welds. These welds have been in service at SQN for approximately nine (9) years.

Two of the damaged weld locations are identified on Figure 1. These welds (121968 and 12203) will be examined in detail in the remaining sections of this report. Weld 121968 had a through wall leak and is the most damaged weld to be studied. Weld 12203 did not have a through wall leak but showed significant circumferential MIC damage based upon RT examinations.

A cross section view showing typical MIC damage to a stainless steel butt weld is shown in Figure 2. The MIC tubercule is shown on the pipe inside diameter. The wall damage initiates between this tubercule and the pipe wail. This damage starts as a pinhole into the wall. A subsurface void develops due to acid attack resulting from bacteria growth.

Figure 3 shows a cross section of MIC damaged piping (weld 121968).

As shown in Figure 3, ten (10) areas of HIC attack were identified 3 on this weld. The through wall leak was at pit number 8.

2. Leaking Welds Removed and Replaced The 28 leaking welds in the ERCW piping were removed and replaced with new welds. In several cases the easiest way to remove a leaking weld was to remove a whole section of piping. When sections

of piping were removed, more butt welds than just the leaking welds were removed. For the 6-inch-diameter pipin9, a total of 67 welds were removed. Table I shows the damage associated with these welds. 7

3. NDE and Destructive Examinations From the 67 welds examined, the RT data from 61 welds , used to ,

provide a flaw characterization approach. The RT examic. tion method is shown in Figure 4. This methodology provides for shooting through wall on the pipe and requires four RT shots to obtain a 360-degree coverage on each weld.

In order to provide data to the structural analysis, each MIC indication on the RT film was identified and its circumferential length measured. When two areas of MIC damage were closer to each other than the pipe wall thickness (0.28 inch), the two indications were considered to be together and assumed as one pit area. The example in Figure 4 then had eight (8) MIC pit areas although 13 MIC damage sites were identified in the radiograph. For each radiograph, the total MIC pit void length was recorded. The data -

from the 61 welds examined is shown in Table 2.

4. Current Piping Margin Calculations A method was established to determine :urrent ERCH piping margins based upon the actual damage as determined from RT examinations.

This method was based upon performing a str'>ctural analysis using reduced section properties. The methodology is as follows:

' Determine Bounding Moments In Pipe

- Upset Condition

- Faulted Condition

' Determine Reduced Section Properties

' Perform Structural Analysis When evaluating the RT examination results and determining reduced section properties, all MIC volds were assumed to be through wall and closely adjacent volds assumed to be continuous. These assumptions provide a conservative calculation of reduced pipe section properties.

The cross sectional damage of weld 12203 is shown in Figure 5. The i Figure 5 damage was identified by destructive examination. The RT examination of weld li203 identified 10 pit locations with a total MIC void length of 6.47 inches. A comparison of the actual damage to the estimated damage is shown in Figure 6. The comparison .

Illustrates the conservative assumptions used in development of the reduced section properties.

Figure 6 identifies the shift in centroid for the reduced section and provides the location of the minimum moment of inertia. The reduced section modules (Z.c oucio) is computed by dividing the minimum moment of inertia by the distance from shifted centroid to

, the outermost fiber (C) as shown on Figure 6.

P

The stress (S) in the MIC damage pipe is computed as follous:

- UPSET CONDITICN (U)

Su = M(U) + P x A(FLOH)

Z(REDUCED) A(REDUCED)

S46towa.or - 1.2 S.

- FAULTED CONDITION (F)

S, - M(F) + P x A(FLOH)

Z(RE00CED) A(REDUCED)

S4ttowastr = 2.4 S.

where: P - Pressure S - Stress A - Area S. - Allowable Stress In these equations the bounding moments (M) are Mu of 39,132 inch pounds and M, of 62,544 inch pounds, the value of S. used is 18,800 psi for SA-312, TP 316 stainless steel. The highest piping stress is at weld 121968 where stress levels are Su of 11,089 psi, and S, of 16,974 psi.

In order to establish margins, the FSAR allowables of 1.2 S. and 2.4 S. are used to calculate a reserve factor (RF).

The reserve factor is a ratio of allowable stress divided by calculated stress as follows:

RESERVE FACTOR (RF) - S(ALLOWABLE)

S(ACTUAL)

ACCEPTANCE CRITERIA RF > 1.0 As long as the RF is greater than 1.0, then margin is provided to FSAR allowables. For weld 121968 (highest stress weld), the reserve factors are as follows:

UPSET FAULTED RF = 2.03 RF - 2.66 Therefore, a factor of 2 is available in the worst case relative the acceptance criteria. The details of these calculations are documented in Reference 1. A plot of reserve factors of upset faulted condition; is shown cn figures 7 and 8.

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0 The RF is plotted versus total MIC length on Figures 9 and 10 for upset and faulted conditions respectively. The darkened squares >

indicate leaking welds. A regression line curve is plotted through i the data. Using dashed lines, upper and lower bound lines have been  ;

included on the plots. As shown in Figure 9, the lower bound line would predict an RF = 1.0 when total MIC length is approximately 9

, inches (based on RT data). .

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5. Study of MIC Pitting Growth l In an effort to study MIC pit growth, a review of NDE data from thirteen (13) welds was completed. For these thirteen welds, the circumferential length of each MIC pit was measured by using RT data; and the through wall pit depth was measured by using ultrasonic testing (UT) minimum wall data above the MIC void. For each MIC pit, a flaw area was calculated by multiplying the flaw length times the pit depth. Figure 11 shows a plot of pit depth versus flaw area. A study of this plot would indicate a possible correlation except for pits which have large flaw areas. In  !

reviewing the data points that have large flaw areas, it was '

established that each point had a pit length in excess of 1 inch based upon the RT data.

For welds 121968 and 12203, destructive examination data was available for numerous pits. Each of these pits was evaluated for ,

flaw area using the cross section data. Based on RT data for these  !

welds, the largest pits were evaluated as shown in Figures 12 through 15. Based on RT data, Figures 13 and 15 show that the pit '

lengths actually encompass several smaller pits. The flaw areas for ,

each of these smaller pits were calculated as shown in Figures 13  ;

and 15. i Based upon destructive examinations of welds 121968 and 12203, the actual pit flaw area, is plotted on Figure 16. This data can be ,

shown to have a lower bound line. The lower bound line provides a flaw area of 0.2 square inch when the pit depth equals 0.28 inch or through wall.

6. Correlation of MIC Growth Data and Structural Intecrity Figure 17 plots the number of pits versus total MIC pit length.

Based on a study of 61 welds, this plot shows the average number of pits per weld to be 4.3. The largest number of pits can be included by an upper bound number of 12 pits per weld. Using this upper i

bound number of pits per weld and a flaw size limit of 0.2 square inch, a maximum leakage void length can be determined as follows

MAX VOIO LENGTH = ( 0.2 SQ. INCH )(12 PITS) i

( PIT ) ~

O.28 INCH f MAX VOIO LENGTH - 8.57 INCHES [

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As discussed in Section 6.0, the Figure 9 extrapolation eould shoe that a total MIC flaw length of 9 inches would be required before RF would equal 1.0. When compared to a worst case void length for through wall leakage of 8.57 inches, the 9-inch MIC length provides that a MIC damaged weld will develop a leak before the structural integrity limit (RF-1.0) is reached.

7. Conclusions The results of the structural analysis and MIC pit growth data provide for the following:

• The current piping has significant margin above FSAR allowables under design basis earthquake conditions.

• Data and analysis substantiate leakage before the loss of structural integrity.

• Data shows that margin exists, even after leakage.

' Leak detection is an acceptable method of monitoring degradation V. REFERENCES

1. TVA-DNE Calculation CEB-COS-355. RO, Stress Evaluation of MIC Damage in ERCH 6-Inch Stainless Steel Girth Butt-Welds, December 15, 1987 (B41 871215 006).

Q TABLE 1 MICR0 BIOLOGICALLY INDUCED CORROSION HELD EXAMINATIONS Examined Total of 67 Welds Welds leaking - 28 Helds not leaking - 33 Welds with no damage - 6

s TABLE 2 MICR0 BIOLOGICALLY INDUCE 0 CORROSION MIC VOID LENGTH PER WELD HELD HIC VOID WELD HIC VOIO IDENTIFIER LENGTH IDENTIFIER LENGTH 12194 7.41 14450 2.20 121968 7.35 14340 2.10 12203 6.47 14403 2,07 12261 6.13 14350 1.84 12190 6.03 14445 1.69 12202 5.54 12197 1.60 14402 4.58 12316 1.56 14336 4.34 14351 1.44 121S2 4.23 14377 1.37 14427 4.02 12280 1.37 14449 3.97 143898 1.35 14349 3.84 14426C 1.15 14337 3.79 14422 1.13 14358A 3.77 14358 1.06 14401 3.77 14388 0.92 12191 3.71 14404 0.78 12263 3.57 14389 0.76 12198 3.51 14357 0.73 14425 3.48 12310A 0.69 12281 3.44 14376 0.62 14360 3.42 14444 0.31 12199 3.42 14360A 0 12196 3.37 123108 0 14426 3.24 12310C 0 12198A 3.23 14428 0 12279 3.09 14348 0 14389A 2.94 14339 0 14402A 2.66 14375 2.63 14361 2.57 122828 2.56 14425A 2.45 12282 2.40 12317 2.30

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lil0R0 BIOLOGICAL.LYINDUCEDCORROSION(MIC) 2 STRUCTURALINTEGRITY i o

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RESERVE FACTOR HISTOGRAM UPSET CONDITION 1 14 13 -

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coMPLLANCS 11 -

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, WICROBIOLOGICALLYINDUCEDCORROSION(WIC)

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o l

. t ENCLOSURE 3 i

LIST OF COMMITMENTS t

1. For Modes 5 and 6, revise PMs 2220, 2221, 2222, and 2223 before the next l; monitoring period begins (March 1988) to state that, when a leak is  !

discovered, a work request will be written to repair the damaged area and  :

that repair work will be a restart requirement, i i  ;

2. For Modes 1, 2, 3, and 4 revise TI-109 before the next monitoring period ,

, begins (March 1988) to specify that RT will be completed in 7 days after ,

leak discovery, compared against preestablished screening criteria; and,  ;

if weld exceeds that criteria, a detailed seismic analysis will be performed in an additional seven days and that leaking welds if not l repaired will be monitored under PMs 2240 and 2241.

(

3. Select 6-10 welds to monitor growth of existing MIC indications, i development of new colonies, and water creatment effectiveness and (

, implement under PMs 2240 and 2241 to begin in March 1988.

l i

+

4. Establish a full structural fixture for repairs of damaged welds before  !

unit 2 restart. ,

j  !

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f 4