ML20236P043

Requests That NRC Reevaluate Conclusion on Acceptability of Facility Recirculation Pump Trip Design Based on Info Provided,Per 10CFR50.62.Commitment or Schedule to Upgrade Sys Will Not Be Submitted Until Issue Resolved

ML20236P043
Person / Time
Site:	Brunswick
Issue date:	11/13/1987
From:	Zimmerman S CAROLINA POWER & LIGHT CO.
To:	NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM)
References
RTR-NUREG-CR-4126 NLS-87-244, TAC-59075, TAC-59076, NUDOCS 8711170119
Download: ML20236P043 (7)

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7-i CD&L Carolina Power & Light Company NOV 13 BQ SERIAL: NLS-87-244 10CFR50.62

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i United States Nuclear Regulatory Commission ATTENTION: Document Control Desk Washington, DC 20555 BRUNSWICK STEAM ELECTRIC PLANT, UNIT NOS.1 AND 2 DOCKET NOS. 50-325 & 50-324/ LICENSE NOS. DPR-71 & DPR-62 ATWS-RPT SYSTEM

References:

1. CP&L letter from S. R. Zimmerman dated April 14,1987, Serial No. NLS-87-074, "ATWS - System Descriptions"

2. CP&L letter from S. R. Zimmerman dated June 18,1987, Serial No. NLS-87-128. "ATWS - Additional Information"

3. CP&L letter from S. R. Zimmerman dated July 22,1987, Serial No. NLS-87-151, "ATWS-RPT -- Additional Information"

4. CP&L letter from A. B. Cutter dated July 24,1987, Serial No. NLS-87-155, "ATWS - RPT/ARI Testability"

5. NRC letter from E. D. Sylvester dated September 18,1987, " Compliance with ATWS Rule 10 CFR 50.62 Relating to ARI and RPT Systems, Brunswick Steam Electric Plant, Units 1 and 2 (TAC Nos. 59075 & 59076)"

Gentlemen:

1 By letter dated April 14,1987 (Reference 1), Carolina Power & Light Company (CP&L) submitted information to demonstrate the adequacy of the standby liquid control (SLC),

alternate rod injection (ARI), and recirculation pump trip (RPT) systems for compliance with the requirements of 10CFR50.62 for the Brunswick Steam Electric Plant (BSEP),

Units 1 and 2. j The CP&L analysis included a Probabilistic Risk Assessment (PRA) which used generic data to demonstrate that the redundant trip coils used in the Monticello RPT design did not appreciably improve reliability. This analysis produced an estimated unavailability of 2.4 E-3 for both the BSEP and Monticello designs.

The Reference i submittal was reviewed by the Staff's consultant, EG&G. While the fault trees and logic were deemed appropriate, the results were concluded to be incorrect because CP&L used the same probability of failure for both types of circuit breakers (MG set field breaker and 4160V breaker) and did not include the alleged dominant coil failure (" failure to operate").

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'NLS-87-244 / Page 2 i Based on BSEP experience, NRC IE Notice 87-12, General Electric Service Information Letter (SIL) 488, and INPO Safety Evaluation Report 48-84, CP&L's analysis contained the assumption that the probability of failure for the AKF-2-25 generator field breaker  !

(the breaker tripped in the Monticello design) was greater than that of the 4160V circuit breaker. Therefore, CP&L considered the use of the same unavailability for the two breakers to be conservative. This assumption was explicitly stated in the Reference I submittal, although supporting detail was not provided.

The EG&G review demonstrated that the dominant coil failure (failure to operate)is not significant. EG&G used equal values for each component in each design and computed the unavailability. The resulting unavailabilities were 2.5 E-3 for the Monticello design and 2.7 E-3 for the BSEP design. This coil dominant failure accounts for only an 8 percent difference in the results.

Af ter discussing the first analysis and the EG&G review with the Staff, CP&L performed an additional analysis using plant-specific data for the circuit breakers under discussion.

Data was gathered from BSEP trouble tickets and information contained in an NRC IE Notice on the Pilgrim Plant. This analysis further demonstrated that the BSEP RPT design was equivalent to or better than the Monticello RPT design. The Company also i determined that one of the data sources used by EG&G, NUREG/CR-4126 (draf t), was j inappropriate for the MG set field breaker AKF-2-25.

On July 15,1987, the second PRA analysis was discussed with the NRC Staff in a meeting in Bethesda, Maryland. The EG&G consultant / reviewer participated by telephone. At that time, the consultant was informed he had misused the data in NUREG/CR-4126 (draf t) for the AKF-2-25 breaker. The EG&G consultant agreed to review this matter and went on to state that acquiring data specific to the components in question was a good approach. He also agreed that if CP&L had sufficient backup data to support the contention that the 4160V breaker was more reliable than the AKF-2-25 breaker, then the BSEP and Monticello RPT designs were equivalent. From these discussions, " backup" was understood by CP&L to mean specific AKF-2-25 data from BSEP records. In addition, the NRC Staff stated that a PRA was an acceptable method for proving the equivalency of the RPT c'esigns and that single failure criteria did not have to be met for ATWS systems. This statement is consistent with the ATWS rule published in the Federal Register (Vol. 49, No.124, page 26040), which states:

"In view of the redundancy provided in existing reactor trip systems, the equipment provided by this amendment does not have to be redundant within itself."

Following the July 15,1987 meeting, CP&L provided another submittal (Reference 3) for review by the NRC Staff and EG&G. In the EG&G review of the Reference 3 submittal, ,

EG&G states that severalitems appear to have the potential to impact the CP&L analysis. These items were identified as:

1. The limited failure data chosen for estimating the failure probability of the field circuit breaker, model AKF-2-25.

2. The majority of the AKF-2-25 breaker failures can be attributed to 1 improper lubrication and/or misadjustment of the breaker internal parts. l 1

Relative to Item 1, CP&L believes there is nothing " limited" about the failure data l 4

provided for the AKF-2-25 breaker. The data used covers three nuclear units (BSEP-1, BSEP-2, and Pilgrim) over a broad period of time (approximately 10 years).

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' Document Control Desk NLS-87-244 / Page 3' .s Relative to Item'2, the Staff's statement at the July 15,1987 meeting and the ATWS rule information redundancy) )

are published not required infor the ATWSFederal systemsRegister mean thatthat singletrip redundant failure criteria coils are not (i.e., '

necessary, even if improper lubrication and/or misadjustments of the breaker internal parts were addressed.

L EG&G also describes their search of a licensee event report data base to obtain better RPT field circuit breaker data (34 plants over a 7i year period). This data resulted in the following:

Field Circuit Breaker: 3.6 E-3/ failures / demand (Monticello) 4160V Circuit Breaker: 9.8 E-4/ failures / demand (BSEP)

From this failure data, EG&G calculated the following unavailabilities:

BSEP RPT Design: 4.6 E-3 Monticello RPT Design: 7.2 E-3 This again supports CP&L's position that the BSEP RPT design is equivalent to or better than the Monticello RPT design. EG&G then focused on the ranges of unavailabilities from the Reference 1 and Reference 3 analyses to conclude that the range estimates indicate a "potentially better" design for the Monticello RPT system compared to the BSEP RPT system (see Enclosure 1).

BSEP Range: 0.00022 to 0.0126 (4160V breaker)

Monticello Range: 0.000058 to 0.0082 (field circuit breaker)

EG&G states on page 6 of their review:

"The lower-valued estimates for both the 480V and the 4160V breakers rewcsent the data extracted from NUREG/CR-4126 (draf t). As pointed c., LTS CP&LOhe low voltage data is not a representative of the metal cidd breaker, AKF-2-25."

When EG&G recog(nized NUREG/CR-4126 draft) data,and thensupported CP&L's position it was inappropriate for EG&Gthat theyused to have misapplied the the NUREG/CR-4126 (draf t) data in their range comparison. Not using the NURLG/CR-4126 (draf t) data would eliminate the Case 2 data from consideration and raise the lower range on the Monticello case to 0.0025. This results in the Monticello case being worse than the BSEP lower range.

In addition, CP&L has reviewed the Millstone i data as presented by EG&G and has determined that the data is not used appropriately. On page 4 of the first EG&G review, the following is stated:

" Reference 4,(Millstone Ul P.S.S.) which evaluates the unavailability of RPT, identifies an unavailability estimate of 2.95 E-3 for the field circuit breaker compared to an estimate of l 6.2 E-3 for the 4kV drive motor breaker."

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NLS-87-244 / Page 4 From Figure 3.2.21-1 of the referenced Millstone PRA, Fault Tree for Failure to Trip Recirculation Pumps given ATWS, the 6.2 E-3 value quoted above is for " Control Switch Failure or Drive Motor Breaker Fails to Open on Recirc A." Because the number is j stated on the fault tree to be an "or" combination of a control switch failure and the breaker, CP&L sought clarification from the Northeast Utilities analyst. The analyst stated that the 6.2 E-3 was derived as follows:

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2.75 E-4 Control Switch Falls to Operate 2.97 E-3 Control Switch Contacts Fail to Close Given Switch l Operates  !

2.95 E-3 Circuit Breaker Fails to Open-  !

6.2 E-3 The same value of 2.95 E-3 was used for the drive motor circuit breaker failure and the generator field breaker. Therefore, the EG&G range analysis based on the Millstone data is meaningless. This eliminates the Case 3 data from consideration and places the BSEP high value below the Millstone high value. -The Company did nat have the Millstone PRA available for review and reference until recently; therefore, this information was not-commented on in our previous submittals.

The above information supports CP&L's position that the ranges stated by EG&G are incorrect. The EG&G analyses that use the NUREG/CR-4126 (draft) data for Monticello-should be discarded, as should the analyses for BSEP and Monticello that use the -

Millstone data. Doing so results in the following unavailability ranges (also refer to Enclosure 2):

Monticello: 0.0025 to 0.0082 BSEP: 0.00022 to 0.0046 Using the EG&G argument, this again demonstrates that the BSEP RPT design is equivalent to or better than the Monticello design.

The NRC Safety Evaluation Report dated September 19,1987 (Reference 5) made the following statement concerning reliability:

"Given equal mean time to failure for the field breakers and the 4kV breakers, the Monticello design was considered inherently more reliable due to the redundant trip coils."

This conclusion appears to be based on the fact that the redundant trip coils would provide less potential for single failures. However, as noted previously, the NRC Staff has stated that ATWS systems are not required to meet single failure criteria.

Furthermore, EG&G has performed a review using equal failure data for the breakers, l including the alleged dominant failure of the trip device. The system unavailabilities calculated from this analysis (stated on the fourth page of Reference 5) are as follows:

Monticello: 2.5 E-3 BSEP: 2.7 E-3

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NLS-87-2% / Page 5 _

These results demonstrate only an 8 percent difference between the single trip coil and dual trip coil RPT designs.

-Moreover, the qualitative analysis based on redundant trip coils is addressing only one of the many potential single failures in circuit breaker. NUREG/CR-4126 (draf t) lists the -

following summary of circuit breaker faults by subcomponent:

Bearing Contacts Latch Mechanism Spring Bushing Drive Pawl Motor Switch Cable Fuse O/L Device Trip Mechanism CB Case Interlock Plunger T/D Element I CKT Board Indicator Relay Wiring Coil Locking Device Stud Holder Any one of the above subcomponents represents a potential single failure affecting the functioning of a circuit breaker. The coil failure and trip mechanism contribute only 15 percent of the faults of the total mechanism according to NUREG/CR-4126 (draf t),

Figure 13, page 48. Therefore, making one fractional source of single failure redundant  ;

(i.e., the trip coil) is not rational. .

Furthermore, the BSEP has had an RPT system installed for 12 years. We are not aware l

of any events where the RPT system failed to trip when called upon to do so. Obviously,

tripping the 4kV breaker is a reliable method of providing an RPT function. .

Therefore, CP&L requests the the Staff re-evaluate its conclusion concerning the acceptability BSEP RPT design based on the information provided herein. The Company will not be providing a commitment or schedule for upgrading the ARI and RPT systems until this matter is resolved. In the interim, CP&L plans to install the previously proposed upgrade to the RPT system for BSEP-2 during the upcoming refueling outage scheduled to begin January 2,1988.

Please refer questions concerning this matter to Mr. Sherwood R. Zimmerman at (919) 836-6242.

Yours very truly, ititwn ----

S. . Zi erman nager Nuclear Licensing Section WRM/vtn (5328WRM)

Enclosures cc: Dr. J. Nelson Grace Mr. W. H. Rutand Mr. E. D. Sylvester

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, ENCLOSURE 1

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' CASE 3 CASE 4 '1

-BASE' CASE 2 (MILL- . (B&PIL . -CASE 5 PLANT CASE CASE 1 (4126)' STONE) GRIM) . LER BRUNSWICK ;2.5 E-3 2.7 E-3 4.22 E-4 1.26 E-2 2.2 E 4.6 E-3 MONTICELLO 2.5 E-3 2.5 E 5.8 E-5 5.9 E-3 8.2 E-3 . 7.2 E ~

BASE CASE:. . ORIGINAL CP&L ANALYSIS OF THE TWO DESIGNS USING WASH 1400 DATA.

CNSE1:- EG&G ANALYSIS USING WASH 1400 DATA AND " DOMINANT TRIP .

COIL FAILURE - FAILURE TO OPEN." r CASE 2: - EG&G AN^ LYSIS USING NUREG/CR-4126 (DRAFT) DATA. CP&L '

POINTED OUT THIS DATA IS NOT' APPROPRIATE FOR THE MONTICELLO DESIGN AND THE EG&G ANALYST AGREED (PAGE 6, AUGUST REVIEW).

CASE 3: EG&G ANALYSIS USING DATA FROM THE MILLSTONE UNIT 1 PRA, .,

-VOL II. LCP&L BELIEVES THIS ALSO INAPPROPRIATE DATA FOR 1 MONTICELLO DESIGN AS WELL AS BRUNSWICK DESIGN.

. CASE 4: SECOND CP&L ANALYSIS USING PLANT SPECIFIC FIELD BREAKER DATA FOR MONTICELLO DESIGN AND NUREG/CR-4126 (DRAFT) DATA FOR BRUNSWICK DESIGN.

CASE 5: EG&G ANALYSIS UTILIZING LER DATA FROM 34 PLANTS OVER 7i YEARS FOR BOTH DESIGNS.

RANGES:-

BRUNSWICK: 0.00022 TO 0.0126 MONTICELLO: 0.000053 TO 0.0082 (5328WRM/vtn)

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ENCLOSURE 2 PLANT;- BASE CASE ' CASE 1 CASE 4 CASE 5 BRUNSWICK - 2.5 E-3 2.7 E-3 .' 2.2 E-4 4.6 E-3

- MONTICELLO 2.5 E 2.5 E-3 8.2 E-3. - 7.2 E-3 RANGES:

BRUNSWICK:' O.00022 TO 0.0046 '

MONTICELLO: 0.0025 TO 0.0082 l

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