ML20029A951

Requests Temporary Relief from Requirements for Code Repair of Degraded Cooling Water Sys Piping

ML20029A951
Person / Time
Site:	Prairie Island
Issue date:	02/27/1991
From:	Parker T NORTHERN STATES POWER CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
NUDOCS 9103050142
Download: ML20029A951 (10)

Text

i Northem States Power Company 414 Nicollet Matt Minnenr..M. Minnesota 65401 1977 Telephone (012) 330 5500 February 27, 1991 10 CPR Part 50 Sectis 50.55a(g)(6)(1) i U S Nuc1 car Regulatory Commission Attn:

Document Control Desk Washington, DC 20555

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l PRA1RIE ISTAND NUCLEAR CENERATING PLANT Dochet Nos. 50 282 License Nos. DPR 42 50 306 DPR 60 Request for Temporary Relief from the Requirernents for Code Rennir of a Degraded Coolinr Vater Svotem P(pn A small through wall pipe leak has been identified in the Cooling Water System.

Due to the length of timo required to repair the pipe and the l

Technical Specification Lituiting Condition for Operation Action Statement, we cannot perform a repair without a two unit shutdown.

The Icak itself presents no problem to the safe operation of the plant and an analysis has been performed which demonstrates that the structural integrity of the pipe is sound.

Therefore, per the guidance in Cencric Letter 90 05, we request Temporary Relief fro. the requirements for code repair until the leak can be repaired during a scheduled t.wo unit outage in 1992.

This condition was discovered October 29, 1990, the pipe was inapped by ultrasonic !nspection techniques, and a fracture analysis was completed by l

November 2, 1990. We comtunicated our evaluation to our NRR Project Manager, Dominic Dilanni, on Noveinber 13, 1990. This was followed by a conference call between NSP, Mr Dilanni, and Messrs Wichman and Lee of the Materials &

Chemical Engineering P, ranch.

Ongoing discussions with members of the NRC staff regarding appropriate methodology to analyze for structural integrity

esulted in the reennt completion of a reanalysis. Our reanalysis utilizes the inethodology outlined in Generic Letter 90 05, inodified by a draf t Code Case transinitted to us by Mr Dilanni on Novenier 15, 1990.

Our request for relief is attached to this letter.

In the request, locations designated 4, 5 6, and 7 were analyzed using the " Branch Reinforcement" approach and all ot.her locations were analyzed per the " Fracture Mechanics" I

approach discussed by the draft Code Case.

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9103050142 910227 PDR ADOCK 05000082 P

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.UShaC February 27, 1991 l' age 2 Northem Statet her Company Please contact us if you require additional information > lated to this request.

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//f)V.9 Thomas M Parker Manager 11uelcar Support Serviceo c: Regional Adtninistrator Region III, NRC Senior Resident Inspec:or. NRC

!!RR Project Manager, NRC J E Silberg Attachments:

Request for Relief for Cooling Water Syrten Leakage Figure 1 Cooling Water System

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f pago j_ of j7 REQUEST for RELIEF for Cooling Water System Loakago FLAW DETECTION & IMPRACTICAJ gf of REPAIR On October 29, 1990, a throug4 wall leak was identified on Cooling Water Loop A, near MV-32144. The loak rato was 4-5 drops per minute. Ultrasonic (UT) inspection of the lonk and surrounding area characterized this flaw as microbiological 1y influenced corrosion (MIC).

MIC typically occurs in stagnant flow areas. This portion of the Cooling Water system has been a stagnant area throughout most of plant operation. The MIC is a locslized attack, and is evidenced by pinhole defocts.

The location of the leak is shown on Figure 1, Cooling Water System. The 24" line (24-CL-20) is the supply lino for Unit 1 and Unit 2 Train A safeguards equipment and somo Unit 1 non-safeguards equipment. This piping is ANSI B31.l(1967), later classified ASME Code Class 3 for Section XI inspection purposes. Design temperature is 100 F and design pressure is 150 psig.

Thorofore, this piping is considered moderato energy piping per Generic Letter 90-05.

On November 2,

1990, indications of flaws below allowable minimum wall thickness were identified on Cooling Water Loop B, near MV-32159. This area is the mirror image of the leak on Loop A.

At this time, a code repair is impractical. The section of piping that is required to be isolated and drained to complete the repair would rendor Train A safeguards components on both units inoperable. A 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> LCO is then in effect. A work plan has been outlined to determino if the scope of code repair could reasonably be done within the LCO timo frame. Our conclusion is that there is not adequate time to perform the Code repair within the LCO period. The codo repair will be dono during the 1992 two unit outage. A two unit outage is necessary because one Cooling Water loop supplies one train of safeguards components on each unit.

During a single unit outage, the other unit is still under the Technical Specification LCO.

ROOT CAUSE DETERMINATION & FLAW CHARACTERIZATION Identification of the through wall leak in the Cooling Water piping is considered a flaw that exceeds-code acceptable limits. The pipe (24-CL-20) nominal diameter is 24" and nominal thickness is.375". The ANSI B31.1 minimum acceptable wall thickness is.120".

i After identification of this flaw, UT' inspection was l

performed on the flaw location and 54" axially along the upstream pipe length. The results of this inspection

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page 2 of f identified six additicnal indications below minimum allowable wall thickness were identified. The UT examinor characterized each of these indicatione as MIC, rather that extensive thinning. The indications were mapped by location, and further evaluated to determine the affected area. An initial value of half wall (.188") was used to bound the affected area.

Thoro are no other components in the area of the leak that are affected by the leakage. An increase in leakage up_to a full spray could be tolerated without affecting the operability of nearby components.

rLAW EVALUATIO!1 Those results were transmitted to Structural Integrity Associates, Inc. to perform a stress analysis.

Design stress values woro provided by Fluor Daniel Inc. The evaluation of Loop A is summarized in Table 1 and 2. This evaluation concludes that the flaws identified do not exccod the limiting dimensions calculated.

Table 1 providos a description of the flawed piping, including the details of the measurements taken for each of the thinned areas. The piping loadings are also defined. In performing the ovaluation, it was determined that the largest stress is that due to thermal expansion binding about the piping vertical axis. Therefore, location unique thermal stresses woro determined based on the most highly stressed location within each of the thinned areas.

The ovaluation methodology was in accordance with Generic Lettor 90-05, except that the pipo minimum wall thickness (t-min) in the equations was replaced by an adjusted wall thickness (t-adj) as suggestod in the draft code caso. The adjusted wall thickness for each location was the examination wall depth used in defining the extent of thinning..Decause

'f the sensitivity to this paramotor in the equations, the allowable extent of thinning increases rapidly as t-adj is 11ercased.

Taolo 2 provides the results of the ovaluation. The maximum ext int of thinning has been determined based either on actual measuromonts of the area, or diagonal of an area bounded by axial and circumferential measuromonts. The allowable extent of ttinning is that where the computed stress intensity would be 35 ksidin, as required by-Generic Letter 90-05.

The ovaluation of the indications on Loop B are presented in Table 3.

The maximum dimension of the thimned area is the diagonal of an area bounded by-axial and-(.rcumferential measurements. The calculated stress is that obtained using the "Through Wall crack" approach in Generic Letter 90-05.

i page iL of jL The allowable stress in 35 kai/In. All indications are below the allowable limit.

The UT technique used was a basic pulse-ocho procedure, in accordance with ASTM E-797.

Examinors are certified in accordance with a program that meets the requirements of ASNT-TC-1A.

AUGMENTED INSPECTION LOOP A

1) The structural integrity of the pipo has been verified by analysis. No patch will be applied to the pipe. The leakage will be collected and routed to a naarby floor drain. This allows continuous monitoring of the leakago rate.

2) A qualitativo assessment of leakage will be performed wookly by a walkdown. The integrity of the pipo will be verified once por three months by NDE (typically UT), in accordanco with Generic Lotter 90-05. This inspection was performed as scheduled on January 24,1991. There was no detectable increase in any flaw size.

3) Generic Letter 90-05 requires 5 additional inspections of areas that are both accessible and susceptibio to MIC. These inspections were completed. One additional area with indications below allowable minimum wall thickness was discovered.

LOOP B

1) The structural integrity of the pipe was verified by analysis. No through wall leakago was identified.

2) A qualitative assessment of leakage will be performed weekly by a walkdown. The integrity of tho= pipe will be verified once per three months by NDE-(typically UT), in accordance with Generic Letter 90-05. This inspection was included in the January 24 inspection.

3) As described for Loop A, one of the five areas of the augmented inspection identified additional areas of MIC.

Fivo additional areas (for a total of 10) were then inspected. None of these areas had indications below-allowable minimum wall thickness.

CONCLUSION NSP has complied with the guidance for relief stated in Generic Letter 90-05, Enclosure 1.

Relief is requested to continue power operation with the identified flaws in the

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1 pago d of 1 cooling Water system piping. The intpoetion plan differs somewhat from the g';idancs providod-duo to the unique piping configuration at the prairic Island Nuclear Gonorating plant.

3 However, the scope of the insp?ction and code repair plan addressos all of the safoguards branch connections from the

loops, i

Based on the above, we conclude that t.as relief does not; l

a.croato a possibility for an accident or millfunction of a different type than ovaluated previously jn the USAR or 4

l uubsequent commitments, l

b. increase the probability of occurrence of an accident or

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malfunction of equipment important to safety previously 1

analyzed in the USAR or subsequent commitments, c.increaso the consoquences of any accident or inalfunction l

of equipment important to safety previously analyzed in j

the USAR or subsequent commitmonts,-

d. reduce the margin of safety definod in the bases for any Technical Specification.

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..Page4 AFD 91-007, SIR 91005 l

Table 1 Description of Thinned Service Water Piping Pipe

Description:

24" Schedule 20, ASTM A 106, Grade B (carbon steel), with nominal wall thickness of 0.375 inches.

Pressure and 71mperature: Design Pressure of 150, Design Temperature of 100*F.

Pipe Stresses:

Ixngitudinal Pressure (P) =

2.288 ksi (computed)

Dead Weight (W) 0.772 ksi

=

Seismic (SSE) = OBE x 2 =

2.166 ksi 5.132 ksi (maximum)

Thermal (TH)

=

Thermal Moments My = 69,232 ft-lb (about vertical axis)

M, =

322 ft lb (about horizontal axis) i M, =

117 ft lb (about pipe axis)

Description of 77 tinned Areas:

1 Location Coordinates (inches) t,ay.o Minimum i

(inches)

Thickness Axial Circumferential

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1 1.50 -

2.00 46.75 48.25 0.1875 Throughwall

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10.75 11.75 46.75 - 48.00 0.1875 0.111 3

12.50 13.75 49.37 50.37 0.1875 0,144 i

4 38.25 - 39.50 53.75 - 55.25 0.25 0.088 l

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6.125 23.0 24.6 0.25 0.091 7

7.5 9.37 22.6 24.0-0.25 -

0.080 8

25.50 - 27.00 32.5

- 33.75 0.1875 0.063 l

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• Imcations 5 and 6 were bounded e a single thinned location based on UT readings for thinned extent with wall thickness less than 0.25 inches.

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-Page4 AFD 91-007, SIR 91005 Table 2 Results of Evaluation Based on C trrent Measurements Location Stress (ksi)

Extent of Thinning (inches)

Actual Allowable 1

9.19 1,58 (diagonal) 1.68 2

9.13 1.60 (diagonal) 1.70 3

9,71 1.00 (diagonal) 1.60 4

10.33 1.68 (meas.)

2,09 5-6 10.06 1.81 (meas.)-

2.15 7

10.12 1.87 (meas.)

2.14 8

7.40 1.95 (diagonal) 2.04 i

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page j7 of-j7 Table 3 Results of Loop B Evaluation INDICATION 1 AXIAL (in)

CIRC (in)

FLAW (in)

CALC K 1

0.500 0.375 0.625 16.39 2

0.375 0.375 0.530

.14.66 3

0.375 0.376 0.530 14.66 4

0.500 0.500 0.707 17.89.

5 0.625 0.500 0.800 19.62 G

0.750 0.500' O.901 21.55 7

1.000 0.250 1.031 24.10 8

0.250 0.250 0.354 11.40 9

0.625 0.250-0.673 17.27 10 0.500 0.250 0.559 15.18 K Allowable = 35 ksidTn 4

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