ML20057B263
| ML20057B263 | |
| Person / Time | |
|---|---|
| Site: | Farley  |
| Issue date: | 09/14/1993 |
| From: | Dennis Morey SOUTHERN NUCLEAR OPERATING CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 9309210089 | |
| Download: ML20057B263 (4) | |
Text
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Southern Nuc!aar Operating Company Post O'fice Box 1295
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Birmingham, Alabama 3S201 Telephone (205) 868 5131 L
Southern Nuclear Operating Company
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Vice Presidert Far!ey Project September 14, 1993 the southem electnc system i
Docket No. 50-364 U.S. Nuclear Regulatory Commission l
ATTN: Document Control Desk Washington, D.C.
20555 l
Joseph M. Farley Nuclear Plant - Unit 2 Steam Generator Tube Support Plate Interim Pluaaina Criteria Gentlemen:
On September 1, 1993, the NRC Staff and Southern Nuclear Operating Company discussed several concerns associated with the request for an interim plugging criteria (IPC) for Unit 2 for the upcoming cycle.
Contained in is information requested concerning one of the issues discussed during the conference call.
If there are any questions, please advise.
Respectfully submitted, brD 2? i v Dave Morey REM:maf IPCRAI. REM Attachment SWORN TO AND SUBSCRIBED BEFORE ME cc: Mr. S. D. Ebneter THIS
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F Responses to NRC Concerns
4 NRC Ouestion 1.
The submittal permits bobbin indications greater than 1.0 volt but less than 3.6 volts to remain in service if a rotating pancake coil (RPC) probe inspection does not detect a flaw, and it requires flaw indications with a bobbin voltage greater than 3.6 volts to be plugged or repaired.
The staff notes that the 3.6 volts reflects an alternate plugging criteria (APC) voltage limit that was derived in WCAP-12871, Revision 2.
Since the issuance of WCAP-12871, Revision 2, in February 1992, additional data has been added to the data base used in the development of this APC voltage limit and several of the existing data points in the data base have been updated as a result of additional analysis. The staff believes that the voltage limit at which tubes can remain in service if an RPC probe inspection does not detect a flaw and the voltage limit for flaw indications requiring repair regardless of RPC probe confirmation should be updated to reflect the additional / modified data.
In addition, the eddy current data analysis guidelines employed during the inspection should be consistent with the guidelines used in the development of the burst pressure correlation (e.g.,
calibration procedure).
SNC Response During the last two steam generator inspections at Farley Nuclear Plant, bobbin indications at tube support plates with bobbin voltage greater than 1 volt were repaired with an allowance for intersections with bobbin indications less than 3.6 volts to remain in service if an RPC inspection did not detect a flaw.
During the last Unit 2 outage, 17 tubes remained in service using the RPC NDD, less than 3.6 volt guidance.
Although insignificant from a leakage standpoint, these 17 tubes are significant for a unit losing generating capacity due to steam generator tube plugging.
While it is true that additional tube pulls have been performed throughout I
1 the industry and incorporated in the data base since the 3.6 volt limit was developed, when used in conjunction with a 1 volt interim plugging criteria i
(IPC), the 3.6 volt limit remains conservative. TR-100407, Revision 1, "PWR Steam Generator Tube Repair Limits - Technical Support Document for Outside Diameter Stress Corrosion Cracking at Tube Support Plates," was submitted to the NRC in September 1993. Figure 3-2 of the TR-100407, provides an updated curve applicable to 7/8 inch tubes.
As can be seen from the curve, the updated structural limit for 3 times normal operating differential pressure is 4.5 volts.
This corresponds to a 2.6 volt plugging limit using the l
previous Farley assumptions concerning growth rate and NDE uncertainty.
However, the 3.6 volt limit is considered acceptable for the following reasons-l t
a.
An indication which is NDD (no detectable degradation) by RPC will not burst during normal operation.
The 3.6 volt limit is based on a RG l
1.121 requirement that "the margin of safety against tube rupture under normal operating conditions -should be not less than 3 at any tube location where defects have been detected." During normal operation, all flaws subject to IPC are within the bounds of the tube support plate.
There is no credible mechanism by which a tube constrained by a tube support plate could rupture during normal operation. Therefore, the 3.6 vol t limit from a tube rupture standpoint during normal operation is extremely conservative.
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Page 2 b.
A tube with a bobbin voltage of 3.6 volts which is NDD by RPC has significant margins to tube rupture during a steam line break (SLB).
RG-1.121 requires that the margin of safety against tube failure under postulated accidents should be consistent with the margin of safety contained in the ASME Boiler and Pressure Vessel Code. As can be seen in Figure 3-2 of the EPRI report, for a safety margin of 1.43 times SLB differential pressure (1.43 x 2560 = 3660), the structural limit is greater than 9 volts.
Using the same Farley assumptions, a 9 volt structural limit corresponds to a plugging limit of over 5 volts. With the conservatisms already included in the burst' curve, a tube would not be expected to burst at voltages less than 5 volts during a SLB.
Again, the 3.6 volt limit provides significant conservatism.
c.
Leakage from tubes which are NDD by RPC will not be significant during a SLB. As can be seen in the EPRI topical report, TR-100407, Table D-2 for 7/8 inch tubing, the lowest voltage flaw to leak at 2650 psi is 2.81 volts.
The 2.81 volt indication leaked at a rate of 0.15 liters / hour or 0.0007 gallons / minute. To leak at 1 gallon per minute, t
over 1,500 intersections leaking at 0.15 liters / hour are required. The next lowest voltage to leak at SLB differential pressure is over 6.5 volts.
In summary, of all the model boiler specimens and pulled tubes which have been burst tested, only one tube has leaked at a voltage less than 3.6 volts. Furthermore, the leakage from that tube is insignificant in comparison to the total allowable leekbe for implementation of the IPC.
d.
The latest tube pulls from Farley Unit 1 indicate that for flaws with voltages in the range of interest (approximately 3 volts), flaws which i
are detectable by both bobbin and RPC do not leak at SLB differential pressures. As previously reported to the NRC Staff, indications with voltages of 3.3 volts and 3.2 volts burst at differential pressures of 6,600 psig and 8,100 psig, respectively.
Neither flaw leaked at SLB differential pressures. This provides a Farley specific example where i
flaws which were detectable by RPC in the range of interest did not i
leak at SLB differential pressures.
Again, allowing flaws less than 3.6 volts which are NDD by RPC to remain in service is demonstrated to be conservative, i
Based on these arguments, Southern Nuclear believes that allowing bobbin indications which are NDD by RPC which have bobbin voltage less than 3.6 volts continues to be a conservative approach to the IPC and that revision of the 3.6 volt value due to new data from pulled tube testing and analysis is not necessary.
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