ML20215G833

Application for Amend to License NPF-11,allowing Performance of Snubber Surveillance for Second Fuel Cycle to Correspond to Scheduled Second Refueling.Fee Paid

ML20215G833
Person / Time
Site:	LaSalle
Issue date:	06/16/1987
From:	Allen C COMMONWEALTH EDISON CO.
To:	NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM)
Shared Package
ML20215G837	List: ML20215G833 ML20215G868
References
3212K, NUDOCS 8706230405
Download: ML20215G833 (9)

Text

• Y . Commonwealth Edloon M m ,, One First N-$onal Plan. Chicago, Illinois

\ . Address Heply to: Post Offee Box 767

\ Chicago,Illinoit 60690 0767 June 16, 1987 U.S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555

Subject:

LaSalle County Station Units'1 and 2 proposed Amendments to Technical Specifications for Facility Operating License NpF-11 NRC Docket No. 50-373 References (a): Federal Register Vol. 51 No. 44 dated Thursday, March 6, 1986.

Dear. Sir:

pursuant to 10CFR 50.90 Commowealth Edison proposes to amend Facility Operating License NpF-11. This one time only amendment is being submitted for your staff's review and approval and is in accordance with reference (e).

The amendment requested will allow performance of the snubber surveillance for LaSalle Unit 1, second fuel cycle to correspond to the scheduled second refuelling. This change is required due to the extended interval between completion of snubber testing and unit 1 startup.

Attachment A provides an introduction and discussion. Attachment B provides a discussion of the major causes of snubber failure during LaSalle Unit.1.first refueling. Attachment C provides copies of the changes to be made to the Facility License. Commonwealth Edison has reviewed this document and finds that no significant hazards exists. This review is

. documented in Attachment D.

We request approval for this change as soon as possible. The change is required prior to October. If for any reason the change would not be approved prior to that date Commonwealth Edison would need to know as early as possible to schedule a unit outage and obtain vendor support necessary to perform snubber surveillances. The effective date of this amendment when approved should be the date of issuance.

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PDR ADOCK 05000373 p PDR

Commonwealth Edison is notifying the State of Illinois of our l request for this amendment by transmitting a copy of this letter and attachments to the designated state official.

In accordance with 10 CFR 170, a fee remittance in the amount of I

$150.00 is also enclosed.

If you have any additional questions regarding this matter, please contact this office.

I V tr y yours, C. M. Aller' Nuclear Licensing kdministrator Attachments cc; Paul Shemanski - NRR Regional Administrator - RIII NRC Resident Inspector - LSCS M. C. Parker - IDNS l

l 3212K

4 ATTACHMENT A TECHNICAL SPECIFICATION CHANGE REQUEST-  ;

LASALLE COUNTY STATION UNIT.1 l

INTRODUCTION-Commonwealth Edison performed the initial' snubber functional test surveillance for Technical Specification 3/4.7.9 at' LaSalle County Nuclear Station Unit 1 (LSCS-1), during the, Unit 1 first refueling outage. The surveillance was of extended length, lasting from December 1985 until May 1986. Power operation. )

sof LSCS-1 following its first refueling did not' commence until September 1986.- The surveillance schedule requirements of Technical Specification .

3/4.7.9, when coupled with the~ extended length of the snubber surveillance and' .l first' refueling outage, dictate that the next snubber surveillance for LSCS-1  !

be done during the course of.the second fuel cycle. j i

' Based on the following discussion, commonwealth Edison'(LcSalle County Nuclear {

Station) is proposing to make a one time amendment to Unit 1 Technical l Specification 3/4.7.9 (operating license NPF-ll). The amendment would allow  !

the second LSCS-1 snubber functional test surveillance to be scheduled 18 q months from the September 1986 startup date. This amendment would bring the snubber functional test surveillance back into synchronization with the refueling schedule, and allow the second required surveillance.to be done at .

the.second refueling c.itage, rather than during the course of the second fuel 4 cyclo.

DISCUSSION The LSCS-1 safety related snubbers were initially functionally tested during the Unit 1 first refueling outage. This surveillance covered all of the j unit's 1235 safety related snubbers, and 104 snubbers failed to meet test acceptance limits for operability under Technical Specification 3/4.7.9. The

's snubber. population is made up exclusively of mechanical type units, manufactured by pacific Scientific Company (pSA). The following dates are associated with the initial LSCS-1 snubber surveillance:

LSCS-1 shutdown for first refueling-- October 18, 1985,  :

First snubber tested for surveillance - December 5, 1985, Snubber surveillance completed - May 20, 1986, l LSCS-1 restart following first refueling outage - September 18, 1986. J l

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Technical Specification 3/4.7.9 requires that all snubbers required to l maintain the structural integrity of reactor coolant system and other safety  !

related piping be functionally tested at first refuel, and every 18 months j thereafter during shutdown. In addition, the bases of the Technical )

Specification assumes that the frequency of snubber failures is constant over l time. These two factors dictate that the next LSCS-1 snubber test j surveillance be performed within 18 months of the date the first snubber was l tested at first refuel, in order for the bases of the Technical Specification to be met for the snubber population at all times. The extended length of the first refuel surveillance, coupled with the four month time period between surveillance completion and unit restart, resulted in having a significant portion of the 18 month interval (for the second rnubber surveillance) expire, while the unit was still in cold shutdown for its first refueling outage. The following dates are associated with the upcoming second snubber surveillance for LSCS-1:

First snubber tested - initial surveillance - December 5, 1985, 18 months after first snubber tested - June 5, 1987, 22.5 months after first snubber tested - October 19, 1987, LSCS-1 second refueling outage shutdown date - March 15, 1988.

The LSCS-1 snubber functional test surveillance is no longer in synchronization with the unit refueling schedule, even when the 25% extension (4.5 months) allowed by Technical Specifications is applied. Technical Specification 3/4.7.9 presently requires that all 104 test failures from the first surveillance be tested during this second surveillance, as well as an additional random sample. LSCS-1 is in a position where the second snubber functional test surveillance will be required to be done during the course of the second fuel cycle. Although a short mid-cycle outage began in June of 1987 for Unit 1, it is of insufficient length to cover the anticipated functional test duration. Therefore, if the snubber surveillance is performed during the second cycle, it will require extending the present outage or create another outage, which is highly undesirable.

Commonwealth Edison proposes to amend Technical Specification 3/4.7.9 on a one time basis for the second LSCS-1 snubber functional test surveillance. This amendment would schedule the second snubber functional test surveillance 18 months from the commencement of power operation for Cycle 2. The revised due dates for the affected surveillance are as follows:

LSCS-1 restart following first refueling outage - September 18, 1986, 18 months following restart - March 18, 1988, 22.5 months following restart - August 1, 1988.

This proposed amendment has been discussed in general terms in conversations (on March 24, 1987 and April 6, 1987) between Commonwealth Edison personnel and members of the Nuclear Reactor Regulation (NRR) staff.

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TECHNICAL JUST1FICATIONE j The_ technical justification for the' proposed technical specification amendment is based on the following:

- the safety related snubber population of LSCS-1 was fully operable-at the start of the second fuel cycle,

- there is a greatly reduced failure potential for snubbers when the unit is in cold shutdown conditions,

- in a majority of cases, the mechanical snubber will still be able to perform its primary function (limit piping motion and stress during dynamic events) while in a degraded / failed condition,

- all safety related mechanical snubbers in LSCS-1 are to be visually I inspected during the second fuel cycle, and

- a snubber reduction program is being implemented at LSCS-1.

1. All safety related snubbers were tested during the surveillance performed at the LSCS-1 first refueling outage, with 104 snubbers failing to meet j acceptance limits. Details of this surveillance can be obtained in Licensee Event Report (LER) 373/86-036. . Completion of the surveillance, along with the associated corrective actions for the test failures, returned the unit's snubber population to a baseline one hundred percent operable condition. This is the same condition the population was in at the beginning of first cycle operation.

i Technical Specification 3/4.7.9 only requires the initial snubber  !

functional test surveillance to be done at the unit's first refueling outage, regardless of the length of the first cycle. The proposed amendment would extend the maximum time interval between the first and second snubber surveillance from approximately 22 months (18 months plus-25% extension / to 26 months, a time frame which is still less than the length of the first cycle for LSCS-1. Since the entire snubber population on LSCS-1 was restored (during the initial surveillance) to an operable condition equivalent to what it was at the start of the first fuel cycle, the requested amendment would be consistent with what Technical Specification 3/4.7.9 allows for a plant with a fully tested and operable snubber population for a first fuel cycle.

2. The bases for Technical Specification 3/4.7.9 assumes that the frequency of snubber failures, and the initiating events which require them to protect plant piping, are constant over time. While these assumptions are useful in establishing surveillance schedules which assure a high level of snubber protection for piping, they do not reflect the actual conditions of snubber service.

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The occurrence of initiating events are not constant over time, since it is not possible for many of the events to occur while the unit is in cold shutdown. _ Examples of such initiating events for piping response, given in Section 3.9 of the Updated Final Safety Analysis Report (UFSAR),

include: loss of coolant accidents, main steam safety relief valve operation, turbine trips, and rupture of high energy piping. It is very conservative to assume that snubbers which are designed to help plant piping accommodate loads from these events, or in combination with the seismic events as postulated in Section 3.9 of the UFSAR, can potentially see such loads while the unit is in cold shutdown.

The frequency of snubber. failures is also not constant over time, since the factors which cause most of the LSCS-1 mechanical snubber failures are absent, or greatly reduced while the unit is in cold shutdown. Attachment B details the four prevailing causes of snubber failures for LSCS-1 during its first refueling outage, and the potential for inducing snubber failures from the same mechanisms during cold shutdown conditions. The prevailing causes of snubber failures were:

-low frequency steady state vibration of system piping,

-transient forces due to system startup/ shutdown,

-overheating of snubber which causes lubricant breakdown, and

-corrosion of snubber internals.

Since a significant portion of the snubber surveillance interval (between the first and second surveillance) was spent with the unit in cold shutdown, which is a period of reduced failure potential for the snubber population, the proposed amendment to extend the surveillance interval should not have un adverse impact on snubber operability or their ability to maintain the structural integrity of critical piping systems.

3. The mechanical snubbers which make up the LSCS-1 population, manufactured exclusively by Pacific Scientific Company (pSA), fulfill their required role of maintaining the structural integrity of critical piping systems by performing two functions. The primary function of each snubber is to activate and become rigid, at or below a specified acceleration level, to limit piping motion and stresses during the dynamic events postulated in the UFSAR. Each snubber also fills a secondary role of moving freely in the absence of dynamic loads, to allow normal system thermal expansion to occur without restrictions, and the accompanying bending stresses.

4 The majority of the snubbers that failed to meet test criterion during the LSCS-1 first refueling outage did not meet drag force limits, which is a measure of their ability to perform their secondary function. Test results from LSCS-1 show that mechanical snubbers tend to become more difficult to stroke as they fail in service, due to degradation of their close tolerance internal parts, and eventually they become rigid assemblies. Snubbers which exceeded drag force criterion still met activation acceleration limits in tension and compression, and would have done so even if locked rigid. Therefore, the snubbers which failed test limits for their secondary role could have fulfilled their primary function of protecting plant piping systems during design bases dynamic events, should they have occurred during the first fuel cycle.

The tendency of pSA mechanical snubbers to fail in a rigid mode, rather than to become free moving is supported by snubber tests which have been previously completed at LaSalle Station (1235 tests on Unit I and 1027 tests on Unit 2) and at other commercial nuclear power facilities.

Analysis of the effects the failed snubbers had on plant piping in LSCS-1 included fatigue and usage calculations, which evaluated the stresses induced when snubbers exceeded drag force limits or went rigid during piping system thermal cycles. None of the failures were found to have safety significance alone or in combination, and all associated piping j systems were still able to meet the ASME Code thermal stress and fatigue '

limits specified in Chapter 3.0 of the UFSAR.

Extending the time interval between the first and second snubber surveillance does not appear to pose a significant increase in the risk of losing the structural integrity of safety related piping systems. The tendency of pSA snubbers to fail in a rigid manner when degraded in service, means that failed snubbers will still be likely to restrain pipe motions during dynamic events. The additional pipe thermal stresses and fatique considerations associated with snubbers which fail to stroke freely generally require multiple system thermal cycles over an extended period of years to approach piping system design limits. The proposed amendment will delay the next LSCS-1 snubber surveillance for a few months during reactor power operation, so their should be minimal fatigue / usage impact of plant piping systems, even if some snubbers exceed drag force limits during the period prior to the test.

4. All safety related mechanical snubbers in LSCS-1 are due to be visually inspected per surveillance requirement 4.7.9.b during the second fuel cycle. The snubbers in the primary containment, which are normally inaccessible during reactor power operation, will be inspected during a short unit outage in June 1987. The remaining snubbers are scheduled to be inspected in the fall of 1987. Visual inspection of the snubber j population gives assurance that all snubbers are in good overall condition >

and operable, and gives confidence tnat widespread failures of snubbers due to events such as unanticipated system water hammer has not occurred.

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5. Commonwealth Edison is presently implementing a snubber reduction program at LSCS-1 and 2, to remove snubbers via system reanalysis, which employs revised design criteria. The current schedule has snubber reduction being completed prior to the end of the second refueling outage for each unit. ,

Modifications are now being prepared to reduce the LSCS-1 anubber l population to approximately 300 units. Snubbers which failed testing at first refuel, and those in known areas with the greatest potential for failure, are being given priority for removal under the program.

The proposed amendment would allow the station to implement the snubber i '

reduction program prior to performing the next snubber surveillance, thereby reducing the number cf possible test specimens. The reduced snubber population will limit the length of future snubber surveillances, even if all snubbers in the population must be tested during a given surveillance. This should insure that the snubber surveillance remains in synchronization with plant refueling outages in the future, without further Technical Specification amendments.

CONCLUSION Based on the previous discussion, commonwealth Edison (LaSalle County Nuclear Station) proposes to make a one time amendment to Unit 1 Technical Specification 3/4.7.9, as indicated in Attachment C. The amendment would allow the second LSCS-1 snubber functional test surveillance (surveillance requirement 4.7.9.e) to be conducted 18 months from the commencement of power operation for Cycle 2. Since the bases for Technical Specification 3/4.7.9 can be expected to be maintained with this amendment, Commonwealth Edison requests NRC review and approval of the proposed Unit 1 operating license amendment.

1 ATTACHMENT B MAJOR CAUSES OF SMUBBER FAILURES DURING INITIAL LSCS-1 SURVEILLANCE l

cause-Low frequency steady state vibration of system piping.

Description - prolonged exposure of the snubber to vibration leads to erosion and wear of close tolerance internals over time. The forces generated are due to turbulent flow in piping during i system full flow operation. Most LSCS-1 systems were in layup or under maintenance activity during the first refuel outage. Snubbers exposed only briefly to vibration during j system flow tests.

Cause - Transient forces due to system startup/ shutdown.

Description - Normal low level transient loads in systems associated with )

pump starts / stops, valve opening / closure, or trapped air.

The forces are usually below snubber capacity rating, and have negligible impact on system piping. These forces do not occur in cold shutdown for the same reasons as the low frequency steady state vibration case. Repeated application of these loads over time caused wear and cracking of snubber internals at LSCS-1.

Cause- Overheating of snubber which causes lubrication breakdown.

Description- Exposure of mechanical snubbers to temperatures above 250*F leads to breakdown of the snubber lubricant, causing unit to fail drag tests, or go completely rigid. This can occur if the snubber is installed in an area where high ambient temperatures exists, or if it is near a large piece of equipment, such as a valve bonnet, and sees high radiant heat loads. This failure mechanism is absent in cold shutdown with the unit depressurized and all systems at temperatures less than 200*F.  :

Cause - Corrosion of snubber internals.

Description - Mechanical snubbers installed in areas where they were soaked or '

sprayed with water had internals corrode, causing failure.

Water sources include valve bonnet and packing leaks, pipe flange leaks and pump seal leaks. In many cases, plant systems  ;

are depressurized and drained for maintenance while the unit is {

in cold shutdown, and leakage sources are eliminated.

Additional information on Unit 1 snubber surveillance may be obtained from Licensee Event Report 373/86-036. ,

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