ML20205T123

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Steam Generator Tube W/55% Through Wall Defect Safety Evaluation for Structural Integrity
ML20205T123
Person / Time
Site: Haddam Neck File:Connecticut Yankee Atomic Power Co icon.png
Issue date: 05/08/1986
From: Ely J, Galler M, Kupinski M
CONNECTICUT YANKEE ATOMIC POWER CO.
To:
Shared Package
ML20205T117 List:
References
NUDOCS 8606130059
Download: ML20205T123 (4)
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Page 1 CONN 8CTICUT YANEEE

' STEAM GENERATOR TUBE WITH 554 THRODGE WALL DEFECT SAFETY EVALUATION FOR STRDCTURAL INTEGRITY 1

INTRODUCTION This safety evaluation assessos the structural integrity of Connecticut Yankee's steam generator tube Row 37, Column 73 which contains a 554 through wall defect. It describes the defective tube and evaluates this tube against the plant design bases with respect to maintaining reactor coolant system integrity. 'The criteria of 10CFR50.59a.2 is applied to determineiwhether an unreviewed safety question is created by continued operation of the plant.

DBSCRIPTION OF DEFBCTIVE TUBE on May 6, 1986, during preparation of the Maddam Neck Steam Genera-tor Tube Inservice Inspection Final Report, it was discovered that a tube at Row 37, Column 73 (Tube 37-73) of the 42 Steam Generator should have been plugged during the recently completed refueling outage. A. transcription error during the evaluation of the eddy-current test data for this tube resulted in the tube at Row 37, Column 33 (Tube 37-33) being plugged by mistake. Tube 37-73 has a small diameter pit that is 55% of the nominal tube wall thickness.

The plugging limit, as defined in Specification 4.10.1.D.1.f is greater than or equal to 50% of the nominal tube wall thickness.

Further data verification has shown this to be a unique error.

DESIGN BASES The original design bases for Connecticut Yankee steam genera-tors include the ASME Boiler and Pressure Vessel Code,Section VIII, 1962 Edition; the Safety Technical Specification, Facil-l ity Description and safety Analysis, Vestinghouse Electric Corp. Technical Manual 1440-C78 for Model 27 Steam Generator, l n December 1966; Westinghouse Report WNEP-8521: Connecticut l .oEE Yankee Steam Generators Stress Report, 4-1-85 and ASME Code 380- Cases No. 1270N and 1273N.

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Basically, the tube design considers pressure retention capabi-g lity af ter accounting for anticipated numbers of normal, acci-no dont and test loading cycles. Also, tubes are designed for 88 vibrational stability in -the fluid environment.

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5 The safety Technical Specification (Section 4.10.1.D.1.f)

S$ requires that defective tubes containing flaws 2 through m a.e wall be plugged. This specification, however, does, 504not provide definitive engineering basis that a 50% throughwall flaw thres-hold constitutes the required margin .of safety beyond which 1

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" " * " * * ' " ~ , Page 2 CONNECTICUT YANKEE STEAM GENERATOR TUBE WITE 55% THROUCE WALL D8FBCT SAFETY EVALUATION FOR STRUCTURAL INTEGRITY corrective action is re qu ired'. Regulatory Guide 1.121, " Bases for Plugging Degraded PWR Steam Generator Tubes", does provide the criteria to judge acceptability of degraded or defective tubes.

PROPOSED CHANCES The requirements of the Safety Technical Specification must be changed to allow the defective tube, Row 37, Column 73 of No. 2 steam generator to remain in service until the end of operation of cycle fourteen.

In support o'f this change to the technical specifications is NUSCO Calculation No. 85-053-685GP, " Evaluation of a 554 through wall Defect in a Steam Generator Tube". This calculation demonstrates the following:

1. The allowable end-of-cycle pit depth for Connecticut Yankee steam generator tubes is 894 through wall for a single large (.187" diameter) pit.
2. The projected maximum end-of-cycle pit depth for the subject pit is 834.

7 l 3. This allowable pit depth satisfies Regulatory Guide 1.121

. and provides margins of safety which exceed the requ ire--

ments of the ASME Code Section III.

POSTULATED FAILURES The existence of an unplugged defective tube in the No. 2 steam generator has negligible effect on the probability of tube rupture.

This failure is reviewed with respect to effect, and preventative safeguards are noted.

The safety hazard arising from a potential rupture of the defective tube is that primary cooling water could leak into the secondary side with resultant dispersion of radioactivity. Such failure could encompass a pin-hole leak to a completely severed tube. The latter situation is a worst case scenario in which the severed tube is able to whip around damaging adjacent tubes. Past failures of steam generator tubes demonstrate that it is very unlikely that tubes containing pit type defects will sever. Instead, tubes will rupture in a ductile manner, leaking before exhibiting a complete break.

The design basis for the plant adequately addresses such an event by r- __. _ __.__ _ _ A

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p.g 3 CONNBCTICDT YANQE STEAM GENERATOR TUBE WITH 55% THRODCE IIALL DEFECT SAFETY EVALUATION FOR STRUCTURAL INTEGRITY requiring in-service radiation monitoring and primary to secondary leakage monitoring.

In summary, the worst credible postulated failure resulting from this defect is a ductile rupture preceded by leakage. This leakage would be readily detected and the plant shut down before additional tubes would rupture. Since the single rupture is a design basis for the plant, postulated failures are not significant with respect to public health and safety.

UNREVIEWED SAFETY QUESTION DETERMINATION

1. Effect of Proposed Change on the Probability of Occurence or The Consequences of an Accident or Malfunction of Equipment Important to Safety Previously Evaluated in the Safety Analysis Report.

o A single defective steam generator tube has a negligible effect on the probability of occurence of previously analyzed acci-dents (e.g., tube rupture). Furthermore, this defective steam generator tube meets the original and current structural design requirements for normal and faulted loading conditions. Thus, the probability of occurence of an accident or malfunction of equipment important to safety is not increased.

o A single defective steam generator tube has no effect on the

,' consequences of previously analyzed accidents as a tube rupture has been considered in the design basis of the plant. Thus, the consequences of an accident or malfunction of equipment impor-tant to safety is not increased.

2. Etf act of Proposed Change with Respect to Creating Accident or Malfunction Possibilities of a Different type than Previously Evaluated in the Safety Analysis Report.

o The possibility that a defective tube might create an accident not previously analyzed was investigated. Tube rupture is an analysed accident, and no other credible failure can be postu-lated from this degradation. Thus, the original component design criteria are met and no new accidents would be created.

3. The Effect of the Proposed Change on the Margin of safety as Defined in the Basis for Any Technical specification The Connecticut Yankee Technical specification Basis' section

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Page 4 CONNECTICUT YANKEE STEAM GENERATOR TUBE WITE 554 THRODGE WALL DEFECT SAFETY EVALUATION FOR STRUCTURAL INTEGRITY 4.10.1.D.1.f requires that steam generator tubes with imperfec-tions with depths 50% of the nominal wall thickness and larger be plugged. The technical basis for the 50% plugging limit and the associated safety margin is not defined in the Safety Technical Specification. In the absence of a specific technical basis supporting the 50% plugging limit, a technical evaluation was made in accordance with Regulatory Guide 1.121, " Bases for Plugging Degraded PWR Steam Genertor Tubes"' to assess the impact of the specific flaw in steam generator tube 37-73 on the reactor coolant pressure boundary integrity / safety margins.

The indication has been characterized as a pit with a diameter of .080 inches and 554 throughwall. Using the worst case pro- l grossion rate and inspection error the projected end-of-cycle depth will be 834. Based on NUSCO Calculation 85-053-685GP, it is . concluded that the limiting defect depth for the pit type indication which will assure the necessary margin against burst

, for the normal and/or f aulted design conditions is a pit-type 1

defect of 894 throughwall. Therefore, it is concluded that the safety margin against burst required by Regulatory Guide 1.121 would not be compro=ised by operating with the 55% deep pit-type indication during the upcoming cycle and the required reactor coolant pressure boundary integrity would be main-tained.

Previous burst testing on steam generator tubes (Reference 1 from Calculation 85-053-685GP, Revision 0), clearly illustrates

. that single large pits up to 92% through wall have a greater l margin to burst than 50% through wall uniformly thinned tubes.

Based on the above evaluation, it is concluded that the margin of safety is not reduced.

In conclusion, continued operation through the end of cycle 14 as proposed by this safety evaluation, does not constitute an unreviewed safety question as defined in 10CrR50.59.

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CONNECTICUT YANKEE STRAM GENERATOR TUBE WITH 55% THRODGE WALL DEFECT SAFETY EVALUATION FOR STRUCTURAL INTEGRITY Prepared by:

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