B13375, Responds to Request for Addl Info Re Electrical Distribution Sys Design Associated W/New Switchgear Bldg.New Switchgear Bldg Provides Opportunity to Minimize Dependence on Motor Control Ctr 5 & Further Reduce Level of Risk

Responds to Request for Addl Info Re Electrical Distribution Sys Design Associated W/New Switchgear Bldg.New Switchgear Bldg Provides Opportunity to Minimize Dependence on Motor Control Ctr 5 & Further Reduce Level of Risk

ML20248G130
Person / Time
Site:	Haddam Neck File:Connecticut Yankee Atomic Power Co icon.png
Issue date:	09/29/1989
From:	Mroczka E CONNECTICUT YANKEE ATOMIC POWER CO., NORTHEAST UTILITIES
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
B13375, NUDOCS 8910100100
Download: ML20248G130 (7)

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HARTFORD. CONNECTICUT 06141-0270 L L T; "","2'%','OC'. (203) ces-sooo September 29, 1989 Docket No. 50-213 B13375 Re: 10CFR50, Appendix R ISAP Topic 1.64 U.S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, DC 20555 Gentlemen:

Haddam Neck P1 ant Nce: Switchgear Building Response to Reouest for Additional Information On August 29, 1989, the NRC Staff and Connecticut Yankee Atomic Power Company (CYAPC0) conducted a telephone conference to discuss certain aspects of the electrical distribution system design associated with the New Switchgear Building at the Haddam Neck Plant. Specifically, this conference was held to discuss NRC Staff concerns related to the Semi-Vital AC Auto Bus Transfer (ABT) scheme between train "A" MCC 5 and train "B" MCC 12-11. At the conclusion of this conference, the NRC Staff requested that CYAPC0 provide a written response to the concerns addressed during this conference c all .

CYAPC0's response to this request for additional information is included in Attachment 1.

We trust you will find this information satisfactory and we remain available to answer any questions you may have.

Very truly yours, CONNECTICUT YANKEE ATOMIC POWER COMPANY r J E. J./Ir6czka // ~

Seni6r Vice President cc: W. T. Russell, Region I Administrator A. B. Wang, NRC Project Manager, Haddam Neck Plant J. T. Shedlosky, Senior Resident Inspector, Haddam Neck Plant f00I o

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, Docket No. 50-213 B13375 Attachment 1 Haddam Neck Plant New Switchgear Building Response to Request for Additional Information September 1989

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Attachment 1 g B13375/Page 1

-September 29, 1989 Haddam Neck Plant Resoonse to Reauest for Additional Information NRC Ouestion The semi-vital AC distribution system modifications associated with the New Switchgear Building at the Haddam Neck Plant, include the installation of an Auto Bus Transfer (ABT) device between train "A" of MCC 5 and train "B" of MCC 12-11 which could potentially cross-tie these two trains during a faulted condition. This feature appears contrary to Safety Guide 1.6 guidelines, which disallow such a switching scheme.

CYAPC0 Response:

The original design of the Haddam Neck Plant did not include complete redun-dancy of electrical systems, and MCC 5 is the sole power supply to the 120-volt semi-vital AC distribution panels. In addition, MCC 5 is not train "A" dedicated, but can be aligned to either train "A" or "B" at the discretion of plant operators. Should the selected supply to MCC 5 fail, the MCC will auto transfer to the other division. This feature has been reviewed by the NRC Staff and found acceptable in the Staff's July 1, 1971 Safety Evaluation Report for the Haddam Neck Plant. This feature compensates for the lack of redundant load groups within certain systems.

The semi-vital AC (SVAC) system is being modified under the auspices of improvements being implemented to satisfy 10CFR50, Appendix R requirements.

The SVAC modifications of concern to the NRC Staff, although being accom-plished under this program, is not actually required to satisfy Appendix R but rather was initiated by CYAPC0 concurrently with the Appendix R' modifications to reduce core melt frequency via a reduction in transient initiation fre-quency.

The Haddam Neck Plant Probabilistic Safety Study (I) identified the loss of motor control center 5 (MCC 5) as a risk outlier. MCC 5 is the sole power supply to the 120V semi-vital AC distribution panels. Subsequently, a design modification to preclude damage to the centrifugal charging pumps given loss of MCC 5 or semi-vital power was implemented. An Emergency Operating Proce-dure (EOP) for the loss of MCC 5 was also implemented. An E0P for Total Loss of Semi-Vital Power had been in effect for some time.

(1) J. F. Opeka letter to C. I. Grimes, "Haddam Neck Plant Probabilistic Safety Study - Summary Report and Results," dated March 31, 1986. 1 l

1 Attachment 1 B13375/Page 2 September 29, 1989 l

l While loss ofthese MCC improvements 5, some residualhave risksignificant remains.f g) reduced the level of risk from Two major consequences of the loss of semi-vital power include:

o trip of both main feedwater pumps after 14 second delay on low suction pressure signal.

o loss of control power for the pressurizer PORVs necessary for feed and bleed.

Therefore, two of three means for decay heat removal would be lost. The third means, Auxiliary Feedwater, would be potentially available.

The new switchgear building provides the opportunity to minimize the depen-dence on MCC 5 and further reduce the level of risk. Under Integrated Safety Assessment Program (ISAP) Topic 1.64 " System Dependencies on MCC 5," an assessment is on-going of potential design changes whereby redundant equipment would be repowered from sources other than MCC 5.

For the reasons noted above, the semi-vital power system has been identified as a very important system which is totally dependent on MCC 5. It is desir-able to provide a source of power which is physically and electrically inde-pendent of MCC 5. An alternate power supply for the semi-vital power system would increase the system reliability for random failure of MCC 5, tornado wind and missile concerns, and fire in the old switchgear room or cable spreading area.

Since MCC 12-11 is in the new switchgear building and is tornado protected, it is the logical choice for the alternate power supply for semi-vital power.

With careful consideration to the routing of electrical cables, substantial risk reduction for fire and tornado can be realized from the proposed alter-nate power supply. By repowering additional equipment (now on existing load centers and/or MCC 5) from the new 480-V load center and/or MCC 12-11, the capability to " feed and bleed" from equipment powered independently of MCC 5 could be realized.

In order to realize the full benefit of an alternate power source for semi-vital power, an essentially continuous power supply is necessary. The plant transient response to a total loss of semi-vital power is so rapid that there is insufficient time for operator action to prevent a total loss of main (2) E. J. Mroczka letter to U.S. Nuclear Regulatory Commission, " Response to Generic Letter 88-20--Individual Plant Examinations for Severe Accident Vulnerabilities," dated July 27, 1989.

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1 Attachment 1 B13375/Page 3 September 29, 1989 feedwater and reactor trip. The proposed auto bus transfer will preclude loss of semi-vital power as a result of a number of single failures in the 480V AC distribution system, such as a loss of MCC 5.

Based on insights from the existing Probabilistic Safety Study for internally initiated events, fire, and tornado winds and missiles, the proposed modifica-tion to the semi-vital power system is estimated to provide a 2 to 6 x 10 5/ year core melt frequency reduction. The benefit realized would depend on the extent to which other equipment currently on MCC 5 is repowered from the 480 VAC distribution center in the new switchgear building.

CYAPC0 has performed a failure analysis on the proposed modification to the SVAC system, and has concluded that the likelihood of a failure occurring which could cross-connect and result in failure of both train "A" and "B" components is extromely unlikely. In fact, five different failures would need to occur to tie MCC 5 and MCC 12-11 together. The following is a discussion of the failure analysis in support of this conclusion. A simplified one-line diagram is attached to aid in your review of this analysis.

o Failure #1 is a catastrophic failure of the electrically operated two pole-double throw ABT device. This failure can be assumed to result in faults to ground, bolted faults or a very improbable tying together of both of the ABT input sources of power. The probability of this failure is low since maximum fault current from the two sources is limited to 167 amps each , a level not likely to cause failures of a catastrophic nature. Furtner, due to the design of the device, it is not possible to manually tie the two sources of power together via the ABT as a result of operator error.

o Failure #2 is a failure of the MCC 12-11 connected Category 1E Regulating ,

Transformer to perform as specified, i.e., its inability to continuously I maintain its electrical integrity with a bolted fault on its secondary  !

terminals while limiting primary current to less than full load. This i failure is considered highly unlikely because the transformer is a current limiting device. This has been documented by the manufacturer in the vendor supplied documentation as follows: l "when the load is increased beyond the transformer's rated  !

value, a point is reached where the output voltage suddenly collapses and will not regain its normal value  ;

until the load is at least partially released. Under a l direct short circuit, the load current is limited to l approximately 150-200% of the rated full load value, the input wattage to less than 10% of normal. A CVS will protect both itself and its load against damage from excessive fault currents. Fusing of load circuits is not necessary."

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A Attachment 1 l B13375/Page 4'

- September 29,~1989 ll o Failure #3 is that.of the MCC 12-11 30 ampere circuit breaker to remove

-the fault-precipitated by Failure #2 above. This breaker is coordinated with the MCC 12-11 supply breaker and, functioning as designed, would remove-the fault with minimal upset to the bus; however, we will assume that it fails and results in the loss of MCC 12-11, the "B" division MCC.

Note - (Thus.far, with three (3) failures, one division of safety related MCC power is lost.)

o Failure #4 is identical to #2 above except that it is experienced by the regulating transformer powered via MCC 5.

o Failure #5 is identical to #3 above except that it is experienced by the MCC 5 30 ampere circuit breaker.

The series of failures (5) required to tie the two buses (MCC 5 and 12-11) together is not considered credible. The benefits gained by this improvement to the SVAC system far outweigh the potential for reducing the availability of redundant MCC's. CYAPC0 viewed this issue more as a Regulatory Guide 1.75

" qualified. isolator" issue rather than a Safety Guide 1.6 issue, but agrees that a case could be reasonably made for either or both Guides being the governing criteria.

CYAPC0 is preparing a Regulatory Guide 1.75 Users Guide for the purpose of capturing design details and ensuring continued compliance with this Regu-latory Guide for future modifications using the new switchgear building and its connecting pathway as a source of power and/or cable routing. As request-ed by the NRC Staff during our telephone conference, CYAPC0 will provide a copy of this Users Guide to the Staff upon its completion.

Additionally, during our telephone conference the NRC Staff requested a write-up of the switching scheme for vital AC buses C and D along with supporting diagrams. CYAPC0willbeprovidetheNRCStaffwithtbisrequested information in a future submittal, expected to be submitted to the NRC by October.31, 1989, i

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• SIMPLIFIED DIAGRAM OF SVAC SCHEME ,

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489 Y MCC (A/B) 489 Y MCC 11 (B) b 30 A b 30 A W 19 WA W, la WA Q M Constant Uoltage Transformer Q Constant Voltage transformer l

V AUTOMATIC-BUS TRANSFER (ABT) DEVICE 4

v LEGEND SEMI-VITAL o A/B - A or B Train '

AC PANELS Operator 1 & 2 Selectable o B - B Train

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