05000237/LER-1983-012, Updated LER 83-012/01X-1:on 830210,while Conducting Routine Mechanical Snubber Test,Main Steam Line Mechanical Snubbers 44,46,50,51 & 53 Found Inoperable.Thorough Investigation Revealed No Cause.Snubbers replaced.W/840302 Ltr

Updated LER 83-012/01X-1:on 830210,while Conducting Routine Mechanical Snubber Test,Main Steam Line Mechanical Snubbers 44,46,50,51 & 53 Found Inoperable.Thorough Investigation Revealed No Cause.Snubbers replaced.W/840302 Ltr

ML20087J804
Person / Time
Site:	Dresden
Issue date:	03/02/1984
From:	Mirochna R, Schroeder B, Scott D COMMONWEALTH EDISON CO.
To:	James Keppler NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION III)
References
LER-83-012-01X, LER-83-12-1X, NUDOCS 8403230096
Download: ML20087J804 (6)

LER-2083-012, Updated LER 83-012/01X-1:on 830210,while Conducting Routine Mechanical Snubber Test,Main Steam Line Mechanical Snubbers 44,46,50,51 & 53 Found Inoperable.Thorough Investigation Revealed No Cause.Snubbers replaced.W/840302 Ltr

Event date:
Report date:
2372083012R00 - NRC Website

text

,CRO PCRM 386 U. S. NUCLEAR REGULATCOY COMMIS$10N

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U P dated Report LICENSEE EVENT REPORT Previous report dated 3-9-83 CONTfCOL BLOCK: l i

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$ 80 81 DOCK ET NUMB ER 69 EVENT QATE 75 REPCRT QATE EVENT DESCRIPTION AND PRO 8ASLE CONSEQUENCES h lo l2l l During Unit 2 refueling outage, while conducting routine mechanical snubber l f6oTTl I functional test (Tech. Spec. 4.6.I.2), Main Steam Line (MSL) mechanical snubbers l lo 141 1 44,46,50,51,53 were found to be inoperable. Safety significance is minimal because l 1oisl~l analysis showed that even without the snubbers, or with one snubber failed rigidly l lo is 1 l oer line. the piping stresses were less than vield stress. A similar event was l l o 171 l renorted by R.O. 82-16/03L on Docket 50-249. I l o is i l l T 8 9 80

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g3 lol l The cause of snubber failure is not known even though a thorough investigation was ;

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ATTACHMENT TO LICENSEE EVENT REPORT 83-012/01X-1 COMMONWEALTH EDISON COMPANY (CWE)

DRESDEN UNIT (ILDRS2)

DOCKET # 050-237 This supplemental-report concerns the inoperable snubbers found during the routine mechanical snubber functional tests during the previous Unit-2 refueling outage (Jan. 1983). Main Steam Line (MSL) snubbers #44,46,50,'

51,53 were identified as being inoperable and/or in questionable condition.

Please refer to the initial D(R/LER submittals.

The Station Nuclear Engineering Depeartment (SNED) performed a comprehensive investigation (assisted by Sargent & Lundy, Pacific Scientific, and Dresden Station)to determine the mode and cause of snubber inoperability.

The conclusion of this investigation forms the remainder of this supplemental report.

Conclusion of SNED's report is as follows:

As a result of the Main Steam line snubber failures on Dresden 2, an intensive effort was initiated to assess the safety significance and to identify the cause of s'nubber failure. The results of the safety significance assesement would determine whether the safe operation of Dresden Unit 2 had been in jeopardy while operating with the failed snubbers. It would also provide assurance that continued operation of D-3, QC-1, and QC-2 as well as D-2 would not jeopardize plant safety.

Several actions were taken to verify safe plant operation. To begin with additional inspections were performed on Dresden 2 to quantify the extent of damage. Snubbers surveillances were also condheted on D-3, QC-1 and QC-2. No other damage was found on D-2 and no snubber failures were found on the other units.

A variety of operability assessments assuming differing failure configurations <

and a different SRV opening time were performed. First, these indicated that D-2 was operable while operating with the failed snubbers. Second, considering the results of the different postulated failure conditions analyzed, these operability assessments indicated thet the D-2, D-3, QC-1 and QC-2 Main Steam Lines will remain operable even in the event of similar snubber failures. Since no snubbers failures were found on the other units, additional D-2 pipe and steel inspections showed no failures, and the results of the operability analyses indicated that the piping was operable; safe operation of the plants was ensured.

This conclusion is reinforced by the existence of the Technical Specification surveillance which originally identified the failures. These requirements delineate an augmented snubber inspection to be performed in the event of snubber failures. The safety significance of operating without knowing Page 1 of 3

_m . _.. _ _ . __ _ _ . _. _ _ _ . _ _ _

l ATTACHMENT TO LICENSEE EVENT REPORT 83-012/01X-1 (CONTINUED) -

the cause of a snubber failure was assessed when these requirements were j instituted. Therefore, the Technical Specification escalating surveillance requirements ensure safe operation.

Concurrent with the above actions a comprehensive investigation was being

, pursued to determine the cause of snubber failure. All possible evidence concerning the D-2 failures was gathered. As previously stated additional

inspections were performed to quantify the extent of the D-2 damage. Snubber >

autopsies were performed to determine the nature of the snubber failures.

Finally, an action plan was pursued which concentrated on items which seemed most likely to have contributed to snubber failures. The areas investigated were:

1) Adequacy of snubbers materials

2) Vibration

3) Overload

4) Installation

! 5) Design The original autopsies indicated that the snubbers had been subjected l to an overload condition. The investigation, however, to identify a possible

overload condition proved negative. A load high enough to fail the snubbers i could not be identified by either in plant testing or by analysis. The conclusion drawn from the D-2 steel inspection also indicated that a high ,

, load did not exist.

+

The in plant tests which were performed monitored snubber loads for the highest

[ known load producing transient (the SRV actuation) having occurred while j the snubbers were installed. In fact the SRV actuation was determined to be the highest load producing transient of those which could occur to the subject

, lines. Also the plant startup was monitored to seek out unanticipated transients. As a follow-up, provisions have been made to continuously monitor the remainder of the D-2 operating cycle. (Note: This is already ,

installed and operating). "This will aid in detecting unknown transients causing snubber failure.

l A detailed review of the snubber as-built configurations was also done to determine if an installation problem existed. Binding of snubber components had been identified early on as a possible mode contributing to snubber failure. The as-built configuration was modeled with the actual (as i determined by field test data) pipe movements input to simulate the field conditions.~ In no case did binding occur. Therefore, binding wcs ruled l

out as a cause of these specific failures.

One of the PSCo (Pacific Scientific Company) field trips revealed that there could be'a field installation technique which degrades the' snubber. This places the snubber in a condition in which.it may lock up. This' installation

. problem has been determined not to.be the cause of the extensive snubber.

- damage occurring on D-2. This conclusion.is based on two supporting' facts.

i i l

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ATTACHMENT TO LICENSEE EVENT REPORT 83-012/0lX-1 (CONTINUED) -

First, the investigation into the magnitude of thermal loads created due to snubber lock-up have resulted in loads which have been determined not to be severe enough to have caused the additional snubber damage. Second, only four of the five failed snubbers exhibited this installation prompted condition. This area will be pursued further, but considering the above, the results of this action are not significant to this failure investigation.

Since overicad was not the cause of failure, a snubber testing program was initiated to delineate the PSA-10 snubber capabilities. It was determined that the snubber performed adequately when subject to a cyclic high magnitude dynamic load. The results did indicate that the snubber performance would be drastically degraded with the snubber subjected to a bending moment.

As previously stated, though, binding did not occur on the Dresden 2 snubbers. Therefore, this information is not applicable to this failure investigation.

The failure investigation also included a review of the Main Steam support system design. This included a review of the analyses performed and the assumptions used in performing the analysis. Comparisons of other support system designs for similar piping s'ystems were also performed. The results of these reviews indicated there was no item which would result in the analyses producing loads near the snubber failure point. -

Finally, investigations were performed to verify snubber material adequacy and to determine if vibration was degrading the snubber performance. The material investigations were performed considering the extent of snubber damage. The vibration concern was raised because of prior knowledge of Main Ste am Line vibration. The issults of these efforts indicated that the materials were adequate and that the vibration was not severe enough to affect snubber operation.

In summary, the data is contradictory and, therefore, inconclusive.

The snubber autopsies indicate that the snubbers were overloaded. The review of the possible transients and of the analyses, and the results of the inplant testing program and the pipe and steel inspections all indicate that an overload con 2ition did not exist. This would lead to the conclusion that other factors r ch as component binding must be degrading snubber performance. But, again, the investigations performed have indicated that binding did not occur even with the mismatched components.

No conclusion delineating the cause of snubber failure can be drawn; however continued operation of the plants has been shown to be justified

. even considering the lack of a definitive cause of snubber failure.

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JM

. x Commonwealth Edison

_c ) Dresden Nucle:r Power Stitlin

( m ~ R.R. #1 Morris, Illinois 60450

\N Telephone 815/942-2920 March 2, 1984 l l

DJS Ltr. #84-197 James G. Keppler, Regional Administrator Region III U.S. Nuclear-Regulatory Commission 3

799 Roosevelt Road Glen Ellyn, IL 60137 Updated Reportable Occurrence Report #83-12/01X-1, Docket #050-237 is being submitted to your office in accordance with Dresden Nuclear Power Station Technical Specification 6.6.B.1.(i), performance of structures,

systems, or components that requires remedial action or corrective measures to prevent operation in a manner less conservative than assumed in the accident analyses in the safety analysis report or technical specifications bases; or discovery during plant life of conditions not specifically considered in the safety analysis report or technical specifications that require remedial action or corrective measures to prevent the existence or developemnt of an unsafe condition. This updated report is provided in order to report further investigation regarding the cause of the event and the corrective actions taken.

D. . Scott Station Superintendent

Dresden Nuclear Power Station i

DJS/jmt i

Enclosure I

cc: Director of Inspection & Enforcement Director of Management Information & Program Control U.S. NRC Document Management Branch File /NRC I-MAR'fg393,.

l JEao\

. SUPPLEMENT TO DVR '

DVR No. .

STA UNIT. YEAR NO.

D - 12 -

2 - 83 - 17 PA'(T I i TITLE OF EVENT OCCURRED . ._

1 2/.10/83

~

0800  !

Inoperable Main Steam Line Snubbers DATE TIME i REASON FOR SUPPLEMENTAL REPORT Tn vannre on He results of an investication into reasons for the failure of the MSL snubbers detected during the spring 1983 refuel . outage. This supplettental report is incorporated with Deviation Reports 12-2-83-19, 12-2-83-20 and 12-2-83-27.

PART 2 l ACCEPTANCE BY STATION REVIEW

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DATE . 3l 13l8Y 1

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7 SUPFLEMENTAL REPORT AFFROVED

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