ML19270F527

LER 79-009/03L-0:on 790112,power to Vital Bus Y2 Lost. False Integrated Control Sys Signal Resulted in Reactor & Full Steam & Feedwater Rupture Control Sys Trip.Caused by Blown 125 Volt DC Fuse to Inverter

ML19270F527
Person / Time
Site:	Davis Besse
Issue date:	02/08/1979
From:	Lingenfelter J TOLEDO EDISON CO.
To:	NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION III)
Shared Package
ML19270F522	List: ML19270F521 ML19270F527
References
FOIA-79-149 LER-79-009-03L, LER-79-9-3L, NP-33-79-13, NUDOCS 7902150382
Download: ML19270F527 (3)

Text

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60 61 DOCKET NUMBER t>8 69 EVENT DATE i4 75 REPORT DATE 80 EVENT DESCRIPTION AND PROB ABLE CONSEQUENCES h into the Action Statemenp ITTTl l On 1/12/79, power to Vital Bus Y2 was lost entering the unit j

Channel 2 of RPS, SFAS, and SFRCS lost AC power and caused a false pi77] l of T.S. 3.8.2.1.

4 Integrated Control Syctem signal which resulted in a rapid reduction of feed flow andg o l A reactor and full SFRCS trip occurred. No. 2 g loIsl l a rapid increase in reactor power.

Auxiliary feedwater re- l S/c level went below the minimum required by T.S. 3.4.5.

Io Is l l Power was restored to the Y2 Bus. There l lo l2l l turned the S/G level above the low limit.

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was no danger to the public or unit personnel. (NP-33-79-13) 80 SU8 CODE S E CODE SUBLO E COMPONENT CODE DE o 9 l E l Ej @ [B_J@ l A l@ f3 ClKlTlBlK]R]@ lXl@ W @ 18 19 20 9 10 11 12 REVISION 7 8 OCCURRENCE REPORT SEoUENTIAL TYPE NO.

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6 TOLEDO EDISON COMPANY DAVIS-BESSE NUCLEAR POWER STATION UNIT ONE SUPPLEMENTAL INFORMATION FOR LER NP-33-79-13 DATE OF EVENT: January 12, 1979 FACILITY: Davis-Besse Unit 1 1DENTIFICATION OF OCCURRENCE: Loss of Vital Bus Y2 and loss of Steam Generator (SG) level The unit was in Mode 1, with Power (MWT) = 2772, Conditions Prior to Occurrence: Reactor Protection Systen (RPS) Channel 3 and Auxi-and Load (Gross MWE) = 900.

11ary Feed Pump 1-2 were inoperable for surveillance testing.

Description of Occurrence: At 0933 hours0.0108 days <br />0.259 hours <br />0.00154 weeks <br />3.550065e-4 months <br /> on January 12, 1979, an accidental ground caused a loss of 120 VAC Vital Bus Y2 and a subsequent loss of SG level. The event was initiated by an accidental grounding of Containment Hydrogen Analyzer AE 5028 at 09:33:03 hours. The ground was large enough to blow both the 10 amp 120 volt AC fuse feeding the Hydrogen Analyzer and the 200 amp 125 volt DC fuse in the inverter feeding Bus Y2. The loss of Y2 Bus caused a loss of power to the 120 volt AC section of the Reactor Protection System (RPS) Channel 2, Safety Features Actuation System (SFAS) Channel 2 and Steam and Feedwater Rupture Control System (SFRCS) Channel 2.

The loss of RPS Channel 2, which was feeding the Reactor Coolant System (RCS) flow indication to the Integrated Control System (ICS), caused a low RCS flow reactor power runback. The loss of RPS Channel 2 combined with the bypassed RPS Channel 3 also caused the auctioneered reactor power signal to drop from 100% to approximately 50% of full power. This caused the ICS to increase reactor power and reduce feed-water flow. The loss of Channel 2 120 volt AC to the SFRCS caused a closure of the startup feedwater control valve to SG 2. The combination of these events caused the level of SG 2 to fall rapidly.

A reactor trip occurred at 09:33:16 hours due to a RPS flux / delta flux / flow trip and SG 2 level continued to drop until at 09:34:44 hours, the level fell low enough (17 inches) tt ;ause a full SFRCS trip and isolation of both SGs. Aux iliary f eedwater was supplied to SG 1 but Auxiliary Feed Pump 2 was in manual in prepara-tion f or surveillance testing, so no water reached SG 2 from Auxiliary Feed Pump 2 until the operator took manual control of Auxiliary Feed Pump 2 speed to control the level of SG 2.

The Control Room opera-SG 2 level remained below 18 inches for about five minutes.

tor assured the safety of test personnel and placed Auxiliary Feed Pump 2 in opera-tion to restore SG 1evel.

Power to Bus Y2 was restored at 0940 hours0.0109 days <br />0.261 hours <br />0.00155 weeks <br />3.5767e-4 months <br /> although it was not f ed from its normal source. The fuses were replaced and normal power was restored by 1445 hours0.0167 days <br />0.401 hours <br />0.00239 weeks <br />5.498225e-4 months <br />.

LER #79-009

TOLEDO EDISON COMPANY DAVIS-BESSE NUCLEAR POWER STATION UNIT ONE PAGE 2 SUPPLEMENTAL INFORMATION FOR LER NP-33-79-13 Designation of Apparent Cause of Occurrence; Although an accidental ground caused by station personnel initiated the event, the 10 amp fuse feeding the Containment Hydrogen Analyzer should have blown fast enough to clear the fault before the 200 amp inverter fuse blew. Investigation shows that the 10 amp fuse was a standard fuse, and the 120 volt AC equipment specification called for f ast-acting (current limiting) fuses.

Analysis of Occurrence: There was no danger to the health and safety of the public or to unit personnel. The loss of power to the Y2 Bus at 0933 hours0.0108 days <br />0.259 hours <br />0.00154 weeks <br />3.550065e-4 months <br /> resulted in unit entry into the Action Statement of Technical Specification 3.8.2.1 which requires in Modes 1-4, on loss of energization of a vital bus, the bus be restored to an operable status within eight hours or be in at least Hot Standby within the next six hours and in Cold Shutdown within the next thirty hours. At 1455 hours0.0168 days <br />0.404 hours <br />0.00241 weeks <br />5.536275e-4 months <br />, the unit was removed from the Action Statement by fuse replacement and restoration of normal power. The loss of Y2 Bus caused safety related instrumentation to fail to the safe condition.

The low SG level put the unit in Action Statement (b) of Technical Specification 3.4.5, which requires in Modes 1-4, that with the SG inoperable due to water level being .outside the limits, the unit be in Hot Standby within six hours and in Cold Shutdown within the next thirty hours. The unit was removed from the Action State-ment af ter five minutes by level restoratien using No. 2 Auxiliary Feedwater Pump.

The loss of level made one SG unavailable for reactor heat removal, but the other SG had sufficient capacity during the short time period for decay heat removal.

Corrective Action: The inverter fuses were replaced under Maintenance Work Order (MWO) 79-1272. MWO 79-1285 was issued to check all 120 VAC essential bus output fuses, and all other fuses were found to be fast-acting.

Failure Data: There have been no previous reported occurrences.

LER #79-009