L-97-215, SG Secondary Side Loose Object Safety Evaluation
| ML20199J273 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 10/23/1997 |
| From: | WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP. |
| To: | |
| Shared Package | |
| ML20199J227 | List: |
| References | |
| SECL-97-215, NUDOCS 9802050273 | |
| Download: ML20199J273 (6) | |
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ATTACIBiENT 2
, SECL 97415 Page i of 6 WESTINGtlOUSE SAFETY EVALUATION CliECK LIST i) NUCLEAR PLANT (S): Scaumsh Unit 2
- 2) CIIECK LIST APPLICABLE TO: Jigm Generator Secondary Side lose Ohlect
- 3) The written safety evaluation of the rnised procedure, design change or modification required by 10 CFR 50 59 has been prepared to the extent required and is attached. If a safety evaluation is not equired or is incomplete for any reason, explain on Page 2. Pans A and B of this Safety Evaluation Cineck List are to be completed only on the basis of the safety evaluadon performed.
CHECK LIST.PART A 3.1) Yes_ No1 A change to the plant as described in the FSAR?
3.2) Yes. No1 A change to procedures as desenbed in the FSAR?
3.3) Yes_ NoX A test or experiment not desenbed in the FSAR7 4
3.4) Yes_ No1 A change to the plant technical specifications (Appendix A to the Operating License)?
- 4) CHECK LIST PART B (Justification for Part B answers must be included on page 2.)
4.1) Yes_ No1 Will the probability of an accident prniously evaluated in the FSAR be increased?
4.2) Yes_ No1 Will the consequences of an accident prniously evaluated in the FSAR be increased?
4.3) Yes_ NoX May the possibility of an accident which is different than any already evaluated in the FSAR be created?
4.4) Yes_ No1 Will the probability of a malfunction of equipment important to safety prniously evaluated in the FS AR be increased?
4.5) Yes_ No1 Will the consequences of a malfunction of equipment important to safety previously evaluated in the FS AR be increased?
4.6) Yes_ No1 May the possibility of a malfunedon of equipment important to safety different than any already evaluated in the FS AR be created?
4.7) Yes_ NoX Will the mergin of safety as defined in the bases to any technical specification be reduced?
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Page 2 of 6 If the answcrs to any of the above questions are unknown, indicate under $) REMARKS and explain below.
If the answer to any of the above questions in Part (3.4) or Part B cannot be answered in the negadve, the change review requires an applicadon for license amendment in accordance with 10 CFR $0.39 (c) and submitted to the NRC pursuant to 10 CFR 40.90.
- 3) REMARKS; The answers given in Section 3 Part A, and Secdon 4, Part B, of the Safety Evaluadon Checklist, are based on the attached Safety Evaluation.
Reference doeurnent(s):
See Section 5.0 FOR FSAR UPDATE Section: Pages: Tables: Figures:
Reason for / Descripdon of Change:
N/A SIGNATURES m/ Ac Prepared by: Name:
/#/> 3/f 7
,. Date:
9 Independendy Reviewed by: Narne: W OkA 2 Date: / V Licensing Review by: Name: / 6 LJ /
Date:
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SECL.97 315 Page 3 of 6 Sequoyah Unit 2 >
Steam Generator Secondary Side Loose Object Safety Evaluation l
1.0 INTRODUCTION
Dunng the recent outage at Sequoyah Unit 2, a foreign object search and retrieval (FOSAR) was perfonned and a foreign object was found on the hot leg, secondary side of steam generator 2. The object l appears to be a nail approximately 1/8 inches in diameter and 31/2 inches long. The object lies . early honzonta!!y between Rows 33 and 34, and the flat head of the object is reported between Columns 37 and 38, wlule the tip of the object is at about the Column 40 posidon. The oyect is bent slighdy upward, with the flat head closer to the face of the tubesheet and the tip pointed upward.
This evaluation will address the rie.st cycle operation of Sequoyah Urut 2 with the identified object remaining in the secondary side of steam generator 2. It will show that plant operation with the foreign object present in the secondary side of steam generator 2 will not adversely affect the pressure boundary integnty of the steam generator and do .s not represent an untniewed safety question per the criteria of 10 CFR $0.59, 2,0 REGULATORY BASIS In the worst case, a loose object on the steam generator secondary side could cause tube wear, pnmary to secondary leakage and possibly a potertial tube rupture event. loose paru and loose objects have caused two of the steam generator tube rupture (SGTR) estnts in the U.S. to date, however, the majonty of loose objects pose no threat to tube bundle integrity. He NRC staff has identified reconunended industry actions within Reference 1. These actions include.
Quality assurance procedures go trning all work within the steam generators Visual inspection of the secondary sides of steam generators
- Loose parts monitoring systems it is the NRC staff's position that loose pans or loose objects which are found from the above actions should be removed from the steam generators when possible. In addition, tubes observed to have sisible damage should be eddy current inspected and plugged if found to be defective. For objects that cannot be removed, entena should be dntioped to address the maintenance of steam generator tube integnty during subsequent plant operation.
The ASME Boiler and Pressure Vessel Code prmides criteria and requirements for evejustion of the stress levels in pressure boundary components for design, normal operating, and postulated accident conditions.
Any modifications, repair, or replacement of these components must also meet the applicable requirements of the Code, as required or modified by the NRC, to assure that the basis on which the unit was evaluated is unchanged. The margin of afety is provided by the inherent safety factors in the enteria and requirements of the ASME Code.
3.0 EVALUATION The object in the secondary side of steam generator 2 at Sequoyah Unit 2 is lapng on the top of the tubesheet. The object was not able to be moud laterally from its position dunng retneval efforts, however, it was able to be moved vertically appmimately 1/4 inch. The object appears to be wedged in place, and as such sigmficant relative motion and. therefore, significant tube wear, is not considered credible. There were no shiny spots reportal during the visual examination which may have indicated wear by the objcct. Tubes directly adiacent to the object were eddy current tested dunng the current I
SECL 97 215 Page 4 of 6 outage using the rotating pancake coil probe. None of these tubes showed any indication of wear (Refuence 2)
In the unlikely event that the obje:t should become dislodged and either remain at the current locadon, or move to another locadon, it is possible that tube wear may occur since relative modon between the object and tubes would now be possible. Calculadons have been performed to esumate the mirumum wear ume w hen the object is located at the tube that exiubits the linuting amplitudes of vibradon and cross flow fluid velocity, and the tube has existing 20% through wall degradation at this location. The naluation also assumes that the object rests upon a sludge pile approximately 6 inches deep (at this elevadon, tube vibradon amplitudes are larger than those at the top of the tubesheet). Addidonal conservauve assumpdons are made in that the object will remain in the same locadon (once tube wear begins) and that only the tube (s) will expenence wear. The evaluadon shows that the minimum estimated impacting-sliding wear time for the loose object is greater than 1.5 y ears to obtain a minimum wall thickness of 60%
(Reference 2). Therefore, this loose object is not expected to wear into a tube to the minimum acceptable wall thickness (40% through wa'l) dunng the next operating cycle (~ 1.5 year).
Also, due to the small mass of the foreign object. it isjudgea that the energy which the loose object would impart to a tube during a collision is sufficiently low that significant deformadon of a tube due to impacting is not expected during the next operating c3tle.
In summary. should the object remain at the current locadon, or become dislodged and more to a more limiting location, significant tube wear is not projmed to occu during the next operating cytle. The tubes adjacent to the object do not have indications 4 any wear and therefore will not require plugging during the current outage, llowever, at a minimurn, these tubes should be eddy cunent inspected during the next scheduled outage to monitor any potentia) wear that may develop.
4.0 ASSESSMENT 0F UNREVIEWED SAFETY QUESTION It has been evaluated and detennined that the next cycle of operation of Sequoyah Unit 2 with the object present in the secondary side of steam generator 2 does not represent an untniewed safety question per the enteria of 10 CFR 50.59, on the basis of the followingjustification.
- 1. Will the probability of occurrence of an accident prniously naluated in the FSAR be increased?
No. Based on rotating pancake coil probe examination of the tubes adjacent to the object along with the analyses results desenbed above, tube wear resulting in a steam generator tube n pture event is not expected during the upcoming period of operation. Steam generator tube integrity and leaktightness are calculated to be maintained consistent with RG 1.121 criteria.
- 2. Will the consequence of an accident previously evaluated in the FS AR be increased?
No. A postulated steam generator tube nipture event resulting from excessist wear due to contact with the object would be bounded by the existing analysis assumptions. Due to the size, shape, and fixity of the object involved, throt ghwall penetradon of active tubes would not be expected dunng the next operating cycle. In the unlikely nent that primary to secondary leakage should result due to tube wear, release of primary coolant and radioactive matenal into the secondary system and into the emironment would not be greater than that assumed for a steam generator tube rupture event in the Sequoyah Unit 2 FSAR.
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, 3. May the possibility of an accident of a different t>pe than already mlualed in the FSAR be created? '
I t No. Steam pnerator tube integnty is expected to be maintained during the next fuel cycle. :
The object in steam generator 2 csnnot interact with other components of the reactor coolant i
system. Migration of the object would tend to de emphasize its already negligible tube wear and interacuon potential. Should the object migrate to the tube of highest vibration ;
amplitude, and wear a tube through wall, the conseqwness of this mnt would still be '
bounded by the effects of an analped tube rupture mnt as desenbod in the FSAR, and therefore would not represent a potential accident different than pmiously analyzed.
- 4. Will the probability of a malfunction of equipmer.t important to safety pmiously evaluated in the FSAR be increased?
J No. Steam generster 2 is expected to meet all pmiously applieble design criteria during the
- next fuel cycle. Since tube integrity and lenktightness are expected to be maintained, there is no mechanism for the loose object to affect any other portion of the steam pnerator or any ,
i other component No other system connecting with the steam pnerator or other safety ;
rotated component could be adversely affected by the operation of the steam pnerator with j the looes object present.
l S. Will the consequences of a malfunction of equipment important to safety pmiously mluated in the FSM,be increased?
i j No. Steam pnerator 2 will continue to sneet all applicable de6ign criteria during all plant l l conditions. Ahhough considered to be an unlikely mot, the worst case consequence that l l could occur during subsequent plant operation is primary to escondary leakage less than a !
double ended tube rupture Pmicus discussion has shown that the object evaluated has a i' negligible potential to cause tube wear of any significant extent in active tubes. The presence of the object does not represent a mechanism which could offect any other safety related ,
equipawnt. ;
- 6. May the possibility of a malfuncdon of equipment important to safety different than that
{
j already evaluated in the FSAR be created? '
s
! No. As pmiously discussed, Sequoyah Unit 2 steam generator 2 is expected to continue to
. meet individual tube integrity and tube lenktightnes6 requimnents, therefore, the presence of
- this object will not act as an accident initiator.
- 7. Will the margin of safety as defined in the basis of any technical specification be reduced? i No. The margin of safety with respect to primary to secondary pressure boundary integrity is i
prmidad, in part, by the safety factors included in the ASME Code, and is not reduced.
i Safety margins would not be expected to be reduced since the potential for tube wear is considered negligible. Operation with the presence of the foreign object in the secondary side of steam generator 2 does not require a change to the Technical Specifications nor will it prevent inspections required by the Technical Specifications.
'l Therefore, operation of Sequoyah Unit 2 for the next fbel cycle is not expected to adversely affect steam generator tube integnty, and does not represent an unmiewed safety question in accordance with 10 CFR
$0.59.
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5.0 REFERENCES
1.
NUREG 0844, (NRC Integrated Program for the Resolution of Unresolved Safety issues A 3, A4, and A S Regarding Steam Generator Tube Integnty"). USNRC, Apnl 1983.
2.
NSD E.SGD A 97170," Final Disposition of Foreign Object in Sequoyah Unit 2 SG # 2",10/22/97
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