ML20096C589

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Requests Temporary Relief from Requirements for Code Repair of Degraded Cooling Water Sys Pipe,Based on Flaws in Sys Identified Via Ultrasonic Insps.Further Insps Performed Per Generic Ltr 90-05 Revealed Addl Flaws
ML20096C589
Person / Time
Site: Prairie Island  Xcel Energy icon.png
Issue date: 05/07/1992
From: Parker T
NORTHERN STATES POWER CO.
To:
NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
GL-90-05, GL-90-5, NUDOCS 9205140105
Download: ML20096C589 (8)
v • d • e


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Northem States Power Company 414 N cot let,'Aall Minneapohs, Minnesota 55401 1927 Telephone (612) 3345500 May 7,1992 10 CFR Part 50 Section 50.55a(g)(6)(1)

U S N". clear Regulatory Commission Attn: Document Control Desk Washington, DC 20555 PRAIRIE ISLAND NUCLEAR GENERATING PLANT Docket Nos. 50-282 License Nos. DPR-42 50-306 DPR-60 Request for Temporary Relief from the Requirements for Code Renair of a Degraded Cooling Water SysLem Pipz Piping flaws have been identified in the Cooling Water System by ultrasonic inspections.

Due to the length of time required to repair the pipe and the Tecanical Specification Limiting Condition for Operation Action Statement, we cannot perform repairs without a two unit shutd.wn.

The pipe flaws present no problem to the safe operation of the plant as shown by analyses which demonstrate that the s t rue tt,ral integrity of the piping is sound.

Therefore, per the guidance in Generic Letter 90-05, we request Temporary Relief from the ASME Section XI requirements for code repair.

We found the first of the indications on March 17, 1992 and communicated to Armand Masciantonio of the NRC staff on March 19, 1992.

Further inspections performed per Generic Le t te r 90-05 revealed additional flaws which we communicated to Bill Long and Jim Davis of the NRC staif on Ap.il 8, 1992.

We completed our inspections and analyses and discussed these with Long onsite on April 22, 1992.

Our request fo:

afief is attached to this letter.

Please contact us if you require additional.nfnrmation related to this request.

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fktfb Thomas M Parker Manager Nuclear Support Services 9205140105 920507 PDR ADOCK 05000282

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1 Northem States Power Company

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- - May 7, 1992 Page 2_.

ReS onal' AQninistrator - Region III, NRC i

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Senior Resident Inspector, NRC NRR : Proj ect Manage r, NRC J E Silberg Attachments:

1.

Request for Relief 2.

Figure 1 - Isometric of the cooling water system, showing inspected areas i

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'k, REQUEST for RELIEF Cooling Water System Indications Below Code Minimum Wall FIAW DETECTION and IMPRACTICALITY OF REPAIR r

On March 17, 1992, three indications below ASNI B31.=1 Code minimum wall thickness were identified by ultrasonic _ inspection (UT). On March 18, 1992, the size of.

each indication waa determined, and a stress evaluation was completed. UT of the

. piping' characterized the flaws as microbiologically influenced corrosion' (MIC).

MIC typically occurs in stagnant'or low flov areas. This portion of the cooling water system has-been a low flow area for much of plant operation.

The MIC is

. a localized attack, evidenced _ by a conical depression down to a pinhole-type _

defect.

- Per the guidance of Generic Letter 90-05, the indications identified on March 17, 1992 required augmented inspection of additional. sections of piping.

Five additional'similar areas were inspected and additional flaws were identified.

These flaws were physically similar to the prcviously identified flaws.

The-initial scope of the _UT inspections, even prior to detection of _ flaws, was 100%

irspection of the _24" pipe at each branch connection. This scope satisfies the augmented' inspection requi sments.

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The -_ scope ; of the - inspections was-broadened to non-similar areas, since all similar areas had been. inspected. Areas with high flow.s were inspected and flaws were identified. :Since MIC attack is influenced by many interacting factors, it is :important; to - note ' that _the attack in these _ higher flow areas - differed significantly-from hver flow areas. -In the areas of higher flow (e.g., between K

CL-36 and G wall; see Figure 1), no conical c'epressions were found, The-flaws.

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were characterized as individual " worm holes",' originating at the _ interior Tsurface of the pipe. The pipe surrounding the flaw was near nominal-thickness.

Additional inspection, using tangential radiography, supported the UT findings.

_ Due to) the. difficulty in.specifically defining these flaws, the flaw was very conservatively estimatad as occupying all of the 1/4" by 1/4" _ square in which a single indication was;found.

The size of all flaws was determined and a str'.ctural evaluation perforced per -

the-- guidance provided in Generic Letter 90-05.

All flaws satisfied - the acceptance, criteria of the "Through Vall Approach" Therefore, the piping-_

associated with the flaws-is considered operable.

At this time, code repair is impractical. In order to perform the necessary code repairs, both units-need to be removed from operation.

Train A supply header serves. Unit 1 and Unit 2 safeguards components.

Train B supply header also serves Unit 1 and Unit 2 safeguards components. A Technical Specifications (TS)

Limiting Condition for Operation (LCO) allowed out of-service time of 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> e

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Recpest for Relief Page 2 of 5

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s in effect if either header is removed from service.

A work plan has been-i outlined to determine if the scope of the code repair could reasonably be completed within the allowed out-of-service time. Our conclusion is that there is not adequate time to perform the code repair <1 thin the allowed out-of-service j

time period.

Therefore, the repairs must be dene during a 'wo unit outage.

I A two unit outage is planned for the Fall of 1992.

Since the piping -is structurally sound and considered operable, we request relief from the ASME Section XI requirements for code repair to delay the cooling water piping repairs until the two unit outage scheduled for this fall.

The locations of the inspected arens ara shown on Figure 1.

This piping was designed and constructed to ANSI B31.1 and later classified ASME Code Class 3 for

-Section XI inspection purposes. Design temperature is 100 ' F and design pressure is 150_psig.

Therefore, this piping is considered moderate energy piping per Generic-Letter 90-05.

ROOT CAUSE DETEPMINATION AND FIAW CHARACTERIZATION The flaws were identified by UT inspection. The inspection was being performed as part of the continuing inspection program set forth by commitments in Generic Lettor 89 13. The scope of _ this inspection was to evaluate the 24" headers. The

'24" heacar was to be inspected 12" either side of each branch connection.

The UT examiner characterized the flaws as resulting from MIC.

The flaws were localized indications, surrounded by structurally sound pipe. The UT examiners have determined MlC attack in the part and are familiar with its appearance. The flaws were mapped by location, and evaluated to determine the af fec ted area. The sin of the flaw was used to perform the stress evaluation.

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

Examiners are certified in accordance with a program that meets the j'

requirements.of ASNT-TC-1A.

FLAW EVALUATION The inspection r#sults are summari e d in Table 1.-

This evaluation concludes that the flaws identified do not exceed the limitations of Generic Letter 90-05.

The evaluation methodclogy was the "Through Wall Crack" approach in Get cic Letter 90-05.-

The pipe minimum wall thickness (t-min) in the equations was replaced by adjusted wall thickness (t-adj), as suggested in the draf t Code case transmitted l

to us by Mr Dilanni of the NRC staff on November 15, 1990 l

-The length of the flaw was determined by either direct mapping of the flaw (using the_ longest dimension) or bounding the flaw axially and circumferentially and i

using _ the diagonal measureme.nt.

The allowable stress is 35 kst/in, All indications are below the allowable limit.

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.Requee for Relief Page 3 of 5 AUGMENTED' INSPECTION 1)' The structural-integrity of the pipe was verified by analysis.

No.through wall. leakage was identified.

2) A qualitative assessment of the piping will be performed weekly by a walkdown. The integrity of the pipe will be verified once per three months by NDE (typically UT), in accordance with Generic Letter 90 05,
3) Five additional areas we: e initially inspected to comply with Generic Letter 90 05.

Both. Loop A and B were inspected at each branch connection as part

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of the original scope.of inspection.

This constitutes 100% inspection of similar areas.

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Attachmmt 1 Re@mst for Relief Page 4 of $

TABLE 1

1. C2 cling Wate_r Pipe Data 24" Diameter Mean radius 11.8125" Nominal wall thickness 0.375" B31.1 minimum wall 0.120" Design temperature 100
  • F Design pressure 150 psig TOTAL STRESS 10242 psig 30" Diameter Mean radius 14.8125" Nominal wall thickness 0.375" B31.1 minimum vall 0.149" Design temperature 100
  • F Design pressure 150 psig TOTAL STRESS 13027 psi underground 13283 psi turbine building (limiting) 2893 psi locally near hanger C'JH-108 A

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Attactweent 1 Re4*st for Relief Pap 5 of 5

. TABLE _1 (continued)

2. Flaw Evaluation Summary acceptable Report #
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<35 ksi Comments P92-0001 none

'P92-0002 3

yes

'P92 0003 none

-P92 0004 rene

P92-0005 tone P92-0006 1.

yes P92-0007 1-yes P92-0008 4

yes P92-0009 13 yes vorm hole type P92-0010-

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-yes vorm hole type P92-0011 1

yes worm hole type

-P92-0012 2

yes worm hole type P92 0013 2

yes j'

P92-0014 2_

yes worm hole type P92-0015 3

yes worm hole type P92-0016.

1 yes worm hole type P92-0017 11 yes worm hole type P92-0018 5

yes worm hole type j'

P92-0019 6

yes worn hole type P92 0020 none P92-0021 1

yes worm hole type l.

l P92 0022-7 yes worm hole type

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