ML19095A228
| ML19095A228 | |
| Person / Time | |
|---|---|
| Site: | Surry  |
| Issue date: | 07/27/1978 |
| From: | Stallings C Virginia Electric & Power Co (VEPCO) |
| To: | Harold Denton, Schwencer A Office of Nuclear Reactor Regulation |
| References | |
| Serial No. 430 | |
| Download: ML19095A228 (3) | |
Text
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RICHMOND, VIRGIN IA 23261 July 27,: 1978:
- Mr. Harold R. Denton, Director Office of Nuclear Reactor Regulation Attention: Albert Schwencer U. S. Nuclear Regulatory Commission Washington, D. C.
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Docket No. ~-281 License No. D]R-37
Subject:
Steam Generator Tube Plugs Surry Unit No. 2
Dear Sir:
During the recently completed steam generator inspection on Surry Unit No. 2, two steam generator tube plugs were found to be missing from their original loca-tions anci were located loosely inserted in other tubes.
This letter summarizes this incident and provides additional information on the tube plugging and inspec-tion process.
Sununary of Occurrence At the beginning of the recently completed steam generator inspection program on Surry Unit No. 2, two loose plugs were found, one in the hot leg of steam genera-tor B (Row 4, Column 4). and one in the hot leg of stecim generator C (Row 35, Column 24).
Photographs taken in April 1978,, at the end of the previous inspection and plugging program, showed that these locations had never been plugged, indicating that the plugs had migrated from other locations.
A further review of the photo documentation from the March-April outage identified 2 tubes which were plugged but were now empty.
All other plugs were intact.
The two empty tubes were probed with eddy current units to verify that the plugs were not elsewhere in the tubes.
The tubes were clear.
Based on this information it has been concluded that the 2 plugs found were from the hot leg of steam generator B.
To support this conclusion it is necessary to identify a mode of transport by which one of the loose plugs could travel from the hot leg of steam generator B to the hot leg of steam generator C.
This transfer could only have occurred dur-ing a period of coolant backflow through coolant loop B.
A review of operations logs since the March-April outage indicates that there were several periods during which backflow in loop B occurred.
For example, during the April start-up, reactor coolant pump C was started first.
(Pump A or C is always started first since the pressurizer spray scoops are located in A and C loops).
This caused backflow in loop B.
One of the plugs in loop B, which was either already out or was dislodged by the pump start, could have been carried through loop B hot leg, into the reactor vessel and out into the hot leg of loop C where it remained.
A similar transfer could have occurred during unit shutdown, when loop B reactor coolant pump is the first to be secured.
Since the Yiarch-April outage, there ha~~e_b£en 2 unit start-ups ffo\\
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VIRGINIA ELECTRIC AND POWER COMPANY TO Mr. Harold R. Denton Page ijo *. 2 and 2 shutdowns for a total of over 30* hours of loop* B backflow..We have conclud-ed that the plug transfer occurred*during one of these.periods.
A visual inspection of steam generator Band C tubesheets was made for evi-dence of damage caused by the loose plugs.
No evidence was fou!).d.
No inspection of the upper internals of the reactor vessel was mac].e.
However, based on analysis performed by Westinghouse. for another utility of* similar.design,.. the mass and velo-city of the loose plug would not have been sufficient to cause daniage to the upper internals during the transit of the reactor vessE!l.
Identification of Missing Plugs Identification of missing plugs is made on.the basis of "as found" and "as left" tube sheet photographs.* This photo.documentation has been in use since June 1977., A review of the current photographs against plugging maps and docu-mentation has shown that no plugs are missing or unaccounted for.
This.review was conducted on July 25, 197s:.
- The plugs are numbered and Westinghouse provides Vepco with.lists of which plug, by Serial Number, went into which channel hec1-d.
The loose plugs.that were recovered form Band C steam generator during the recent inspection were battered such that the. serial numbers were not legible.
Loose Plugs -.Mode of Failure The only credible mechanisms. for plugs becoming loose is either no detona-tion or a partial detonation.
The two loose plugs found this*.outage appeared to have been shot with insufficient force to achieve a we],.d **:
One plug was asymmetri-cally deformed and one was expanded symmetrically.but only.slightly expanded. It appears that the problems lies either in. the loading of the'. explosive or. the un-detected pulling out. of the primacord.
Recent procedural improvements*. by Westing-house have reduced.the probability of these types of errors.
Potential.for Recurrence We have reviewed the histroy of.tube plug failures and.the implications of such failures to determine if continued operation would pose an undue risk to the general.public or to the station.
Our-findings are summarized below.
The incidence of loose plugs is extremely small.
Operating history at Surry Power Station contains only 2 other instances of loose plugs.
In July of 1977-,:
in Unit 2 steam generator B, a plug from hot leg row 1 columri 42 dropped out and was found loosely inserted in.the same channel head in row 29 column lL Another loose plug was found in.the hotleg of steam generator B in March of 1977,.: No original location was determined.
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VIRGINIA ELECTRIC AND POWER COMPANY TO Mr. Harold.R. Denton Page lil"o. 3
.Thus a.total of 4 plugs have*become loose*throughout,the tube.plugging history of Surry Power Station. This.is from a total of 8490 plugs installed,' for an over-all dropped plug rate of less than one* twentieth of one percent.
With the accumu-lation of tubeplugging experience and.recent inprovements*in.techniques and proce-dures, the incidence of failures has decreased.
- We have reviewed* recent efforts by Westinghouse to improve the quality. and reliability of their tube plugging ser-vices and are.confident that the incidence.of loose plugs will be reduced or eli-minated in.the.future.
In addition toan extremely lowprobabilityof occurrence, the consequences of a loose plug are not severe.
The plug itself weighs less* than one pound.
If a plug came loose it would most likely come to rest in an area of low coolant flow.
The most probable ultimate locations for a loose plug are the bottom of.the reactor vessel or in a tube in a hot leg, as occurred in the subject incident.
Analysis performed by Westinghouse for another utility of similar design has shown that operation with a loose plug in the reactor.vessel will not result in any adverse structural effects on the.reactor vessel or its cladding or the reactor internals.
Additionally, no adverse thermal hydraulic or other core conditions would result.
Summary A review of photo.documentation has established that no tube plugs are missing or unaccounted for~
The probability of loose plugs occurring in the future is ex-tremely small.
Should a loose plug occur, analysis has shown that there would be no adverse effect on plant safety or operation.
Accordingly, we have concluded that continued operation poses no undue risk to the health and safety of the gen-eral public.
Very truly yours, ----~-
,,;t:-z!1~A{/i C.. M. Stallings 7'h-'-="
Vice President - Power Supply and Production Operations cc:
Mr. James P. O'Reilly