ML20210C182

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Forwards Rept Providing Results of Insp of Eggcrate Tube Supports Done on Secondary Side of Sgs,Using Remote Controlled Visual Equipment
ML20210C182
Person / Time
Site: San Onofre  Southern California Edison icon.png
Issue date: 07/22/1999
From: Scherer A
SOUTHERN CALIFORNIA EDISON CO.
To:
NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
NUDOCS 9907230360
Download: ML20210C182 (6)


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sOUiHIRN CAlllORNIA A. Edward Scherer EDI. SON Nuclear Regulatory Affairs 1

An Ll)laON INTLRN4TlONM** Company I l July 22,1999 l

l U. S. Nuclear Regulatory Commission Document Control Desk Washington, D.C. 20555

Subject:

Docket No. 50-361 and 50-362 Special Report: Steam Generator Secondary Side is .tlon San Onofre Nuclear Generating Station, Units 2 ar-

Reference:

1. Southern California Edison Letter from A. E. Scherer to U. S. I l

Nuclear Regulatory Commission Document Control Desk, dated February 9,1999

2. Southern California Edison Letter from A. E. Scherer to U. S.

Nuclear Regulatory Commission Document Control Desk, dated April 28 1999

3. Steam Gene,rtor Program Guidelines, Nuclear Energy Institute Document Number NEl 97-06 (Original), dated December 1997 Gentlemen:

The attached report provides the results of the inspection of eggerate tube supports done on the secondary side of the steam generators, using remote-controlled visual equipment. Southern California Edison (SCE) provided the required Technical Specification 5.7.2.c reports for inservice inspection of steam generator tubes at San Onofre Nuclear' Generating Station in reference 1 for Unit 2 and in reference 2 for Unit 3. This report was prepared in accordance with the guidance in reference 3 and

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contains supplemental information above those required by Technical Specifications.

230069 h

1 9907230360 990722 DR ADOCK0500g1 P. O. Box 128 San Clemente. CA 92674-0128 949 368-7501 Fn 949-368-7575

Document Control Desk July 22,1999 San Onofre Nuclear Generating Station Units 2 and 3 Special Report This report coi tains no new commitments, if you require any additional information, please contact Mr. Clay E. Williams at (949) 368-6707.

Sincerely,

Attachment:

cc: E. W. Merschoff, Regional Administrator, NRC Region IV L. Raghavan, NRC Project Manager, San Onofre Units 2 & 3 J. A. Sloan, NRC Senior Resident, San Onofre Units 2 & 3 Institute of Nuclear Power Operations (INPO)

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1 SPECIAL REPORT SECONDARY SIDE INSPECTION OF THE STEAM GENERATOR TUBE SUPPORTS

) Page 1 of 4

Background

The ABB/CE model 3410 steam generator tubes are supported at ten different locations by eggerate type support arrangement. There are seven full supports numbered one through seven and three partial supports, eight through ten, with ten being the upper most support. The eggcrate support is composed of metal flat bars that surround each tube in a square arrangement to restrict tube movement.

Visual inspections conducted in the cycle 9 refueling outage (RFO) on the secondary side revealed significant degradation of eggerate tube support structures at the peripheral regions of the SONGS Unit 3 steam generators. Failure analysis determined deposit accumulation created conditions leading to Flow Accelerated Corrosion (FAC) of the eggerate supports, To arrest the degradation, chemical cleaning was performed ,

during RFO 9 to remove deposits in the steam generators. Visual inspections in the i oggerate tube support structures were performed at Unit 3 on March 1998 during the 4 mid cycle 9 outage (MCO 9) and on April 1999 during tne cycle 10 refueling outage (RFO 10) to monitor degradation in the steam generators.

Folicwing the cycle 9 refueling outage discovery of significant degradation in Unit 3, an inspection of Unit 2 was conducted at its next outage, mid cycle 9 in February of 1999.

As predicted, only minor levels of degradation were observed. Similar to Unit 3, the deposit accumulation had been removed from the Unit 2 steam generators in the 1998 cycle 9 refueling outage.

Inspection Scope The eggerates around approximately 100% of the peripheral hot leg tubes in both of the Unit 3 steam generators were inspected and assessed during U3C10. The inspections were conducted from the sixth to the uppermost eggcrate at each location.

Supplemental information was obtained from the fourth and fifth eggcrates at most locations. Unit 2 visual inspections were conducted on approximately 20% of the periphery tubes in both steam generators. The purpose of the inspection was to verify the cessation of previously noted FAC and develop appropriate recommendations regarding the scope of future eggcrate inspections.

g SPECIAL REPORT I

' SECONDARY SIDE INSPECTION OF THE STEAM GENERATOR TUBE SUPPORTS Page 2 of 4 l

Assessment Method The following grading system is utilized to express the level of degradation at a given lattice bar position. The visual inspections of the eggerates are conducted by lowering 1 remote cameras into the secondary side of the tube bundle. The resulting camera i images are captured onto high-resolution SVHS videotapes, which are subsequently reviewed by engineers qualified to assess the level of eggerate degradation or thinning.

Grade A: ' Nominal thickness of the eggcrate lattice bar is greater than 50% of the original thickness Grade B: Nominal thickness of the eggcrate lattice bar is greater than 10%

and less than 50% of the original thickness Grade C: Nominal thickness of the eggcrate lattice bar is less than 10% of the original thickness, but the bar is not severed Grade D: The lattice bar is severed Tube Reoair Criteria The conservative criterion at SONGS is to plug a tube if there are two consecutive uncredited eggerate supports for a given tube location, although analysis indicates that the majority of tubes should only be plugged when there are greater than three consecutive uncredited eggerate supports. A non-credited eggerate is defined as when two or more lattice bars around a tube are each less than 10% (i.e., received "C" or "D" grados) because the support is conservatively assumed to be unsatisfactory for mitigation of flow-induced vibration (FIV).

Inspection Results A review of the previously thinned areas revealed that these areas now have a surface of rough magnetite deposits, indicating that the thinning is no longer active. Active regions of FAC would be expected to exhibit (1) a specular surface finish due to lack of particulate magnetite attachment and (2) scalloping of the affected surfaces. Neither of these attributes was seen in Unit 2 or 3.

i Since the camera image evaluation is subject to interpretation by the analysis, thickness grading caused three tubes to require plugging rather than as a result of FAC. These three tubes on the periphery of the tube bundle in " Unit 3 Steam Generator EO89" (SG 3E089) were preventively plugged and stabilized due to " grade changes" assigned to an eggerate tube support.

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SPECIAL REPORT i SECONDARY SIDE INSPECTION OF THE STEAM GENERATOR TUBE SUPPORTS Page 3 of 4 The distribution of degradation grades assigned to the inspected Unit 3 eggerates ,

during RFO 10 is shown in Table 1. As shown,7-8% of the lattice bars have thinned to less than 10% of the original thickness (i.e., received a grade of "C" or "D") in each of the Unit 3 steam generators.

Table 1 Units 2 and 3 RFO 10 Eggcrate Grades

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SG 2EO88 SG 2EO89 SG 3EO88 SG 3E0089 Grade A: 99.2 % 96 % 68.9 % 73.4 %

Grade B: 0.7% 2.6% 23.9% 19.3%

Grade C: 0.1% 0.9% 4.9% 5.1%

Grade D: 0.0% 0.5% 2.4% 2.2%

SUMMARY

Visual inspection data collected in both Units 2 & 3 steam generators during MCO 9-and RFO 10 revealed no evidence that FAC is progressing. The lack of evidence that FAC is occurring in combination with the aggressive steps taken to limit future deposit accumulation in the steam generators indicate FAC is not likely to recur. This is supported by the following observations:

(1) The overall level of eggcrate degradation, as reflected by the distribution of eggerate grades that are assigned to individual lattice bar positions during visual inspections, was comparable at RFO 9, MCO 9 and RFO 10.

(2) The secondary side inspection during the RFO 10 did not reveal significant deposit buildup at eggcrates since RFO 9. Heavy corrosion deposit accumulation prior to RFO 9 was fundamental in the occurrence of FAC at SONGS. Because of the lack of heavy deposits, it is unlikely that the thermal hydraulic (THD) conditions required for FAC exist in the steam generators at this time. Furthermore, evaluations of the root cause of FAC performed independently by ABB suggest that it would be unlikely that the Units 2 & 3 steam generators would accumulate sufficient deposits through the remainder of the operating license to reestablish unfavorable THD conditions.

(3) The chemical cleaning of steam generators during RFO 9 removed essentially all of the corrosion deposits, and SCE has taken effective steps (e.g., " tailored amine injection") which have reduced corrosion product ingress into the steam generators. These steps reinforce the belief that adverse THD conditions leading to FAC should not recur in the Unit 3 steam generators.

(4) Comparisons of MCO 9 and RFO 10 inspection videotapes of select degraded lattice bar locations at a number of eggerate elevations and azimuthal positions I

I SPECIAL REPORT

' SECONDARY SIDE INSPECTION OF THE STEAM GENERATOR TUBE SUPPORTS Page 4 of 4 along the steam generator periphery did not reveal discernible feature changes at these locations. This is considered strong evidence that FAC has been arrested.

J To date, a total of 145 tubes (115 at RFO 9,27 at MCO 9, and 3 at RFO 10) in the Unit 3 steam generators have been plugged. No tubes were plugged as a result of the eggerate inspection on Unit 2.

Future inspections.

Unit 2 Eggerate tube support inspections at Unit 2 are not planned during the next refueling outage, cycle 11. Following cycle 11, SCE plans to evaluate the need for visual inspections of the steam generator eggerates at Unit 2 based on the considerations provided below: I (1) Any observations / insights regarding the nature of the FAC mechanism at SONGS which are derived from future inspections of the Unit 3 eggerate supports, (2) Any relevant future industry experience which alters the current understanding of the FAC mechanism at SONGS and indicates that additional Unit 2 inspections might be warranted, (3) Any changes in plant operation (e.g., water chemistry changes) at SONGS Unit 2 which could potentially affect the progression of FAC at Unit 2, and )

(4) Any indications at Unit 2 which suggest that FAC is ongoing or that the potential risk of FAC has increased, such as evidence of significant deposit accumulation to levels comparable to those prior to the first SONGS chemical cleaning at RFO 9, altered fluid flow patterns in the steam generators, or unexplained reductions in steam generator pressures, i Unit 3 The inspections of the steam generator eggcrates at SONGS Unit 3 have provided  !

evidence that the FAC, which caused the eggerate thinning prior to RFO 9, has abated. {

However, while the overall inspection data and compensatory measures taken by SCE indicate that FAC is not occurring in Unit 3, changes in grades did occur between MCO 9 and RFO 10.

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Therefore SCE plans to inspect during Cycle 11 selected regions of the eggcrates where grade changes may result in tube plugging.

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