05000029/LER-1986-007, Forwards LER 86-007-01,per 10CFR50.73(a)(2)(v)

Forwards LER 86-007-01,per 10CFR50.73(a)(2)(v)

ML20205H317
Person / Time
Site:	Yankee Rowe
Issue date:	03/19/1987
From:	St Laurent N YANKEE ATOMIC ELECTRIC CO.
To:	Murley T NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I)
References
NUDOCS 8704010021
Download: ML20205H317 (1)

LER-2086-007, Forwards LER 86-007-01,per 10CFR50.73(a)(2)(v)

Event date:
Report date:
0292086007R00 - NRC Website

text

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YANKEE ATOMIC ELECTRIC COMPANY r.i.a,c,,. tmi m.m, Star Route. Rowe, Massachusetts 01367

, NKEE March 19, 1987 U.S. Nuclear Regulatory Commission Region I 631 Park Avenue King of Prussia, Pennsylvania 19406 Attention: Dr. Thomas E. Murley, Regional Administrator

Subject:

Licensee Event Report 50-29/86-07, Rev. 1 480 VAC Busses Cross-Tie Electrical Loading Problem -

Supplement 1

Dear Sir:

In accordance with 10 CFR 50.73(a)(2)(v), the attached revised Licensee Event Report is hereby submitted.

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Very trul ,y yours, h/KY . .At?n Y '

VYN Normand N. St. L3 nt Plant Superintendent BLD/nm Enclosure cc: [3] NSARC Chairman (YAEC)

[1] Institute of Nuclear Power Operations (INP0) 8704010021 870319 PDR ADOCK 05000029 S PDR

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• GIRC Pere 308 U s. NUCLEAle RESULATORY -

M3 APP 8tOVE) Oess NO.3190-4104

'"REs. einises LICENSEE EVENT REPORT (LER)

DOCKET psWASER 12) PAGE(E FACILITY 8tAsst m Yankee Nuclear Power Station, Rowe, Massachusetts o l5 l0 to l o l0 l 2l9 1 lOFl0 l5 TITLS (46 480 VAC Busses Cross-Tie Electrical Loading Problem - Supplement 1 EVEff? OATE(S) LER NUMBER 161 REPORT DATE 17) OTHER F ACILITIES INVOLVED IS)

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J. A. Kay ,,,, coo , "" ""

Technical Services Manager 4l1l3 4 l 2 l 4I-l 512l6 l1 COMPLETE ONE Llht FOR EACM COMPONENT FAILURE DESCRIBED IN TMit F.EPORT (133

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SUPPLEMENTAL REPORT EXPECTED (14) MONTM DAY Y[AR YES far ven, comnween IXPECTEO SventsSSION DATil NO l l l ASSTAACT (Limte se f 400 uneocs, La, apswesameresy hrssen smpe-spece typeerrrten haess 061 On June 18, 1986, with the plant in Mode 1 at 68% power, Yankee completed its review of the recent electrical system evaluation performed by Stone & Webster.

The analysis identified a concern in meeting the minimum starting voltage requirements for the HPSI pump motors when 480 volt busses No. 6-3 and 4-1 are cross-tied during heatup and cooldown operations. The study also indicated that Station Service Transformer (SST) No. 6 which feeds the two tied busses, could exceed its rated capability if a Safety Injection Actuation Signal (SIAS) occurs when the plant is in this configuration.

In order to resolve the above concerns, Yankee intends to install a new 2400 volt circuit breaker to connect SST No. 4 directly to 2400 volt bus No. 1. The new 2400 volt breaker will be installed during the next refueling outage, scheduled for May, 1987.

In the interim, Yankee is stationing a dedicated operator in the Control Room to trip specific plant breakers if a SIAS is received when the 480 volt busses are cross-tied. These actions assure a normal plant response to a SIAS signal.

! There was no adverse effect to the public health and safety as a result of this

! finding. A similar occurrence was reported in LER 79-26.

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NRC form 36dA U 5. NUCLEA5: LE!ULATM7 COMMISSION LICENZEE EVENT REPORT (LER) TEXT CONTINUATION ursovEo ous No asso-oio4 e . EXfl7.ES: 8/31/85 FACILITY NAME (1) DOCKET NUMSER (2) LER NUMBER (6) PAGE131 Yrnkee Nuclear Power Station ,E., saou m .o

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On June 18, 1986, with the plant in Mode 1 at 68% power, Yankee completed its review of the recent electrical system evaluation performed by Stone and Webster. The analysis identified a concern in meeting the minimum starting voltage requirements for the HPSI pump motors when 480 volt busses No. 6-3 and No. 4-1 are cross-tied as shown in Attachment No. 1. This cross-tied configuration is limited by Technical Specifications to less than or equal to 30 MWe). The study also indicated that Station Service Transformer (SST) No. 6 which feeds the two tied busses, could exceed its rated capability if a Safety Injection Actuation Signal (SIAS) occurs when the plant is in this configuration.

We have reviewed and analyzed the results regarding the configuration in which 480 volt Busses 4-1 and 6-3 are tied together and fed from SST No. 6. Since the electrical characteristics of SST No. 5 are identical to those of SST No. 6, a similar sequence of events would happen for the mirror image configuration of SST No. 5 feeding 480 volt Busses 5-2 and 4-1. We have determined that if a SIAS occurs with heavy loaded busses, LPSI Pumps 1 and 2 motors will successfully start and operate. However, due to the initial heavy loading and the addition of LPSI pump loads, a low voltage level at 480 volt Emergency Busses 1 and 2 will not allow starting of HPSI Pumps 1 and 2. The circuit breakers which tie the 480 volt normal busses to their associated emergency busses (BTIA, B and BT2A, B) would open on degraded voltage and the safety loads would be restarted by the diesel generators. Although this would result in the successful operation of the safety loads, the sequencing of the loads would be delayed due to the starting and loading of the diesels, and the loads would depend upon only the emergency diesel generators as a power source.

Bus-tie operation is prohibited by the Technical Specifications at power levels greater than 30 MWe such that a LOCA occurring during bus-tie operation would have considerably lower fuel temperatures and decay heat than for the full power analyzed LOCA case. An evaluation was performed to determine if the full power LOCA analysis results bounded the bus-tie operation case where ECCS flow is delayed. The results of that evaluation demonstrated that the full power LOCA case was the bounding transient. Small break LOCAs and steam line break during bus-tie operation were also found to be bounded by Licensing analysis.

In order to resolve the above concerns, Yankee intends to install a new 2400 volt circuit breaker to connect SST No. 4 directly to 2400 volt bus No. 1 as shown in Attachment No. 2. This modification will result in each 480 volt bus being fed directly from its own 2400 volt bus. This eliminates the need to tie the 480 volt busses together and thus alleviates the HPSI pump motor and transformer loading concerns. The new 2400 volt breaker will be installed during the next refueling outage, scheduled for May, 1987.

In the interim, Yankee is stationing a dedicated licensed operator in the Control Room near the EDG control board prior to and whenever the 480 volt busses are tied together and the Safety Injection System is required to be

NRC Perm 3 sea U.S. NUCLEAR r.EIULt. TORY Commission LICENSEE EVENT REPORT (LER) TEXT CONTINUATION m.oveo on No asso-om EXPIRES 8/31/85 FACILITY NAME (1) DOCKET NUM8ER (2) LER NUM8tR (g3 PAGE(3)

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0 l1 0 l3 oF 0l5' TEXT W more space a requwod, one aMDonel NRC form 366Kal(17) operable. The dedicated operator's sole function is to open breakers BT2A and BT2B if a Safety Injection Actuation Signal (SIAS) is received. Furthermore, if the plant is operating at a power level greater than 30 MWe with the electrical busses normal and a plant trip occurs, the 480 volt electrical busses may be tied together prior to stationing the dedicated operator provir 6d that:

a. The #2 Emergency Diesel Generator has started and is providing power to the

1. 2 Emergency Bus with the Bus tie breakers, BT2A and BT2B open and,.

b. The load on the 480 volt Station Service Transformer providing power to the tied 480 volt busses is maintained s 900 amps.

The above information was reported in YAEC letter to USNRC dated June 19, 1986 (FYR 86-056). A summary (dated June 27,1986) of a meeting held with NRC on June 23, 1986, also documents additional discussion on this issue and concludes that the above actions for resolving the undervoltage problem are acceptable.

There was no adverse effect to the public health and safety as a result of this finding. A similar occurrence was reported in LER 79-26.

The root cause of the problem was two fold. First, an incorrect motor voltage was assumed for the HPSI Pumps in an earlier voltage study of the plant and was found when the nameplate voltages were verified by the engineers working on the recent study. Second, the additions of new loads and more conservative load assumptions were made in the recent study than evaluated in the earlier study.

To assure that the plant remains within its rated electrical capability in the future, we have developed in-house computer capability to evaluate new loads.

As loads are changed or added within the plant, engineering can now evaluate the effects on the electrical components and systems. Also, the voltage study will be periodically updated. Prior to formal issuance of study, the engineer will verify any new loads by inspection of actual nameplates.

The combination of ongoing evaluations of load changes, periodic updates of the voltage study and verification of nameplate ratings of new loads will assure that the plant remains within its rated capabilities.

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