ML20235W115

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Provides Background Info on Plant Steam Generator Svc Experience from 1978-1987 in View of Tube Leak in Steam Generator C on 870715
ML20235W115
Person / Time
Site: North Anna Dominion icon.png
Issue date: 07/16/1987
From: Cheng C
Office of Nuclear Reactor Regulation
To: Richardson J
Office of Nuclear Reactor Regulation
Shared Package
ML20235W119 List:
References
FOIA-87-461 NUDOCS 8707230440
Download: ML20235W115 (6)
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UNITED STATES l

'j'['ena\o*t, NUCLEAR REGULATORY COMMISSION l g . ,i W A$HINGTON, D. C. 206H i

,f JUL 16 M7 MEMORANDUM FOR: James E. Richardson, Assistant Director Division of Engineering and Systems Technology l 1

FROM: C. Y. Cheng, Acting Chief Materials Engineering Branch Division of Engineering and Systems Technology

SUBJECT:

BACKGROUND INFORMATION ON NORTH ANNA UNIT 1 STEAM GENERATOR SERVICE EXPERIENCE 1

In view of the tube leak in Steam Generator "C" at North Anna Unit 1 on July 15, 1987, we have compiled the following background infonnation.

We are tracking this event closely and will keep you informed of developments i as they occur.

North Anna Unit I went into commercial service in June of 1978. It has three (3) Model 51 Steam Generators (S/G) with 3388 tubes in each. -

Early Experience 1978 - 1982 Since start-up in 1978, the unit has been on All Volatile Treatment (AVT) secondary water chemistry. During February 1979, 200-300 pounds of Powdex .

resin were inadvertently ~ transferred into the steam generators. During the '

l resin intrusion event, the resin decomposed to form sulfuric acid, lowering the steam generator pH to 6.02. Two other less severe intrusion events occurred in l July and September of 1979. As the January 1980 refueling outage indicated that denting had occurred in the hot leg, boric acid treatment was initiated.

The unit has followed Westinghouse recommendations for use of boric acid since 1980. .

Virtually all of the seven hot leg support plate intersections exhibit some degree of denting. Recent comparisons of 1984 and 1985 data indicate that the growth in both the number of dented intersections and extent to which intersec-tions have been deformed has been arrested.

The other tube degradation experienced was primary side cracking of tae Row I U-bends (observed during the 1980 inspection). Preventive plugging of all Row I tubes appears to have resolved this issue.

FC A- 87 IM/

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6. 'M 3 n

J. Richardson 1

  • I January 1984 Outage The first significant indication of steam generator corrosion on North Anna I was in December 1983. Primary to secondary leakage in Steam Generators B and C increased to a maximum value of 396 gallons per day (GPD). The unit was shut down January 10, 1984, and a visual inspection of both steam generators during a hydrostatic test showed a total of five (5) leaking tubes and four (4) leaking explosive plugs between the two steam generators.

l A total of 579 tubes were inspected in Steam Generator B during the outage.

Four (4) tubes were identified with greater than 40% through-wall indications at the tube support plates while 13 tubes were identified as having significant distortion. All leaking tubes exhibited eddy current indications, but none of the significantly distorted indications were identified as leakers.

In Steam Generator C, a total of 552 tubes were inspected. Four tubes contained deep eddy current indications and were plugged. Additionally, two 1 tubes were identified as having distorted indications. The plant returned to power operation until t May 1984 refueling outage. 3 May 1984 Refueling Outage Steam generator activities during the May 1984 refueling outage included complete eddy current inspection in all 3 steam generators and an attempted tube removal effort. A total of eight tubes in Steam Generator A, 1 tube in Steam Generator B, and 4 tubes in Steam Generator C exhibited eddy current indications greater than 40% through-wall. Additionally, 2 tubes in A and 1 tube in C were plugged as a preventive measure. Due to the complex signal distortions arising from denting, copper, and magnetite, any presence of signal distortion was not recorded by the utility. The plant returned to service in September 1984.

August 1985 Outage Primary to secondary leakage was detected again in February 1985 in trace ,

amounts. Noticeable step changes occurred in April 1985. The leakage gradually increased to a maximum value of 213 GPD in Steam Generator A in late July. The Unit came off-line on August 1985 and an inspection of Steam Generator A was performed.

A video inspection of the tubesheet was performed while the steam generator was filled and pressurized., A total of 3 tubes were identified as leaking.

Subsequently, a total of 830 tubes were eddy current tested, twelve (12) of which had pluggable indications (eleven of these were greater than 90%

through-wall) . Although distorted indications were identified in this ,

inspection, these tubes were not separated from a larger class of 95 tubes which contained some degree of distortion. Consequently, only thirteen tubes were removed from service. The unit returned to service in mid-August 1985 until the November 1985 refueling outage.

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. v i J. Richardsoa

  • November 1985 Outage 1

The Unit returned to service in mid-August with trace leakage in Steam i

Generators B and C, The leakage suddenly increased after approximately five days on-line to approximately 90 GPD where it remained until the November refueling outage. Steam generator activities performed during the November )

outage included complete eddy current of all three steam generators and the removal of two tubes containing a total of four support plate intersections.

The ECT program resulted in the plugging of 43 tubes for indications greater j thanTech. Spec, limits (40%). Thirty additional tubes containing strong distorted l indications were also removed from service for preventive purposes. The Unit I' returned to service in January 1986 with no primary to secondary leakage.

Intermittent trace leakage was detected in Steam Generator A beginning in February 1986.

  • l Spring 1987 Outage l

j During the Spring 1987 refueling outage at North Anna Unit 1, eddy current inspection results at the hot leg support plate intersections of all three steam generators revealed 158 tubes with eddy current signals in which the interpretation of the condition of the tube wall was precluded due to the l level of signal distortion caused by tube denting. Virginia Electric and J Power Company had previously extracted two tubes from Steam Generator C and l metallographically examined them. As a result of the tube examination program, I the tube wall degradation has been characterized as outer diameter (OD) and 1 inner diameter (ID) initiated stress corrosion cracking (SCC), located within l and extending beyond the tube support plate thickness (3/4 inch) in conjunction with tube denting. All such tubes were plugged as well as 69 tubes with clear l indications exceeding the 40% plugging limit. The results of the Spring 1987 steam generator tube inspection are shown in Table 1.

Tube examination results indicated that the eddy current signals (" distorted indications") were from stress corrosion cracks, both outer diameter and l inner diameter 1,n the dented tube support plate region. Inner diameter crack locations correlated with tube ovality due to denting. Many of the cracks were in the 40% to 99% thru-wall range. Virginia Electric and Power Company presented a proposal to the Nuclear Regulatory Comission for accepting tubes having distorted E/C indications that could represent through-wall axial cracks that exceed the plugging limit at support date locations if the cracks did not extend a specific length beyond the support plate. This proposal was not approved by the NRC.

C. .

7-C. Y. Cheng, Acting Ch Materials Engineering Branch Division of Engineering and Systems Technology l

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Enclosure:

As stated cc: See next page

, .' s  !

-JUL 3 f 1997 J. Richardson - 4.-

cc: T. Murley J. Sniezek R. Starostecki L. Shao S. Varaa

  1. M G. Laines L. Rubenstein K. Wichman E. Murphy L. Frank W. Hazelton H. Conrad

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I DISTRIBUTION:

Docket File i EMTB RF EMTB PF l

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0FFICIAL RECORD COPY (5620 document name: Conrad-10)

TABLE 1 June 2, 1987

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North Anna Unit 1 '

1987 Steam Generator Plugging Summary Ind_ication A B C q

i 30 Clear Indications (2.40%) 25 14 Distorted Indications 43 37 78 l Tube Sheet Indications 15 11 9 Erroneously Plugged (R6 C22) -

0 0 1 l

.i Total 83 62 118 Previously Plugged Currently Current Plugged This Outage Plugged  % Plugged A 126 83 209 6.17 B 116 62 178 5.25 C 153 118 271 7.99 E E E q e

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f0PTH ANNA .I STEAM GENERATOR 'RJBE LEAK i

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JilLYJ_5,1987 PROBLEM: STEAM G9!ERATOR TUEE LEAK PES!!LTED IN SCRAM, SAFETf INJECTION, ATO DECLARATION OF ALEPT, i

CAllSE Lt!KfGt SIGNIFICANCE STEAM GENEPATOR TUBE RUPTUPE.

PADI0 ACTIVITY RELEASE OLITSIDE C0ffTAltfBTT, POSSIBILITY FOR MOPE SEVEPE ACCIDEffT.

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DISCUSS!nN l

"C" STEAM LINE RAD f0NITOR ALAPED HIGH I

OPERATOR NOTED PRESSURIZED LEVEL AND PPESSURE DECREASING OPERAIDR MANUALLY SCRAttED PEACTOR SAFETY INJECTION )

i EERGENCY PROCEDilRES INITIATED FOR POST SGTP C00LD04 STEAM JET AIR EJECTOR ( SJAE) RAD MONITOR WAS OLTT OF SERVICE (to AlfTD-  ;

TRANSFER OF SJAE EXHAUST FROM STACK TO CONTAINMENT) -

1DTAL PADi0 ACTIVITY PELEASE FROM SJAE AND AUX-FEED TERRY TURBINE O.265 CI, (ABOLIT 0.6% T.S. ALLOWABLE)

ALL STEAM GENERATOR TUBES EDDY CUPPENT ltsitu DURING REFUELING O!TTAGE, l

FOLLOWilP L!CENSEE ITNESTIGATION CONTINUING PEGION II RESPONSE TEAM UPGPADED TO AIT, CONTACT: SWENSON/NPR foIM-7?- W/

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