ML19210D628

LER 79-104/03L-0:on 791025,reactor Coolant Pump 2-2 Tripped While Pump 1-1 Shut Down.Caused by Blown Fuse in Pump Interlock Circuit Due to Double Grounding on Dc Buses.Maint Work Orders & Facility Change Requests Completed

ML19210D628
Person / Time
Site:	Davis Besse
Issue date:	11/21/1979
From:	Beeler T TOLEDO EDISON CO.
To:	NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION III)
Shared Package
ML19210D611	List: ML19210D610 ML19210D628
References
LER-79-104-03L, LER-79-104-3L, NUDOCS 7911270408
Download: ML19210D628 (3)

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60 61 DOCKET NUMBER 68 69 EVENT DATE 74 l l1 2l1 75 REPORT DATE 7 9l@ 80 EVENT DESCRIPTION AND PROBABLE CONSEQUENCES h 3 l At 1258 hours0.0146 days <br />0.349 hours <br />0.00208 weeks <br />4.78669e-4 months <br /> on 10/25/79, Reactor Coolant Pump (RCP) 2-2 tripped and with RCP l-1 l gl shutdown previously because of seal staging dif ficulties, the reactor tripped on a 4 l 'Tlux to number of RCPs" actuation. This placed the unit in the Action Statement of l 3 [ Technical Specification 3.4.1 and in violation of the Operating License Condition 2C l Ts-} l (3)(a) which requires three RCPs in operation while in Modes 1 or 2. There was no E danger to the health and safety of the public or station personnel. One RCP in each l 8 l loop was operable during this trip. (NP-33-79-121) 8 9 80 C DE CODE SUBC E COMPONENT CODE SUBCOD'E S E E 8 9 l Cl Bl@ (_Bjh l A lh l R l E l L l A l Y lX lh Jh J h to 11 12 13 20 18 19

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43 44 C l6 l4 9[g 47 CAUSE DESCRIPTION AND CORRECTIVE ACTIONS h 3-] l RCP 2-2 tripped from a blown fuse in an RCP interlock circuit. The fuse blew because l lof a double grounding problem on the DC buses. One circuit was grounded through the 'a p case on the Couch (Deutsch) relays. Several additional grounds were found in the DC g l lighting distribution panels. Six maintenance work orders and two facility change re-;

g l quests were completed to correct the problems. By 11/14/79 the problems were resolvedj 8 9 80 SA S  % POWE R oTHER STATUS Dis O RY DISCOVERY DESCRh' TION

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R 79-158 NAME OF PREPARER Thomas P. Beeler PHONE:

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TOLEDO EDISON COMPANY DAVIS-BESSE NUCLEAR POWER STATION UNIT ONE SUPPLEMENTAL INFORMATION FOR LER NP-33-79-121 DATE OF EVENT: October 25, 1979 FACILITY: Davis-Besse Unit 1

, IDENTIFICATION OF OCCURRENCE: Reactor trip from loss of Reactor Coolant Pump (RCP)

,,. 2- 2 .

Conditions Prior to Occurrence: The unit was in Mode 1, with Power (MWT) = 1996, and Load (Gross MWE) = 660.

Description of Occurrence: At approximately 1256 hours0.0145 days <br />0.349 hours <br />0.00208 weeks <br />4.77908e-4 months <br /> on October 25, 1979, station personnel de-energized the 480 volt Lighting Distribution Switchboard Bus E5 to re-move station transformer STl from service to install a new cable on the South Outdoor Distribution Center BY2 per a facility change request. At approximately 1258 hours0.0146 days <br />0.349 hours <br />0.00208 weeks <br />4.78669e-4 months <br />, RCP 2-2 (in Reactor Coolant System (RCS) Loop 2) tripped. RCP l-1 (in RCS Loop 1) had been shutdown previously because of seal staging difficulties. The Reactor Pro-tection System (RPS) tripped the reactor on a " flux to number of RCPs" actuation.

The turbine Dnmediately tripped from the reactor trip.

This placed the unit in Action Statement (b) of Technical Specification 3.4.1 for Modes 1 and 2. The Technical Specification requires the operability of both reactor coolant loops and both RCPs on each loop. Action Statement (b) states that startup and porer operation may be initiated and may proceed provided thermal power is re-stricted to less than 58.6% of rated thermal power.

The setpoints for high flux and flux- d flux-flow trip must be reduced within four (4) hours to the values specified in the Technical Specification 2.2.1.

In addition, this occurrence is not in accordance with the Operating License Condi-tion 2C(3)(a) which requires at least three RCPs in operation while in Modes 1 or 2.

Designation of Apparent Cause of Occurrence: The cause of the reactor trip was the loss of RCP 2-2 from a blown fuse in the DC supply circuit feeding the non-safety related RCP auxiliary relay interlock circuit. This was verified on October 27, 1979, when the sequence of events was repeated several times, verifying that the loss of the 480 volt Lighting Distribution Switchboard Breaker BE510 caused the loss of RCP 2-2. The cause of the blown fuse and loss of RCP 2-2 has been determined to be a double grounding problem on the DC buses which was caused by an unexpected inductive surge in the circuit when the contacts open causing a shorting of the relay contacts to ground. The initial design did not anticipate the surge. Several other grounds were found in the DC lighting distribution panel, caused by improper initial installa-tion. There is an interrelationship between breaker BE510, which automatically transfers to Dil2, a DC supply to feed DC lighting distribution panel L57El on a loss of AC (which is strictly non-essential) and DC motor control center 1 (for which the RCPs interlock circuits are powered).

LER #79-104

4 TOLEDO EDISON COMPANY DAVIS-BESSE NUCLEAR POWER SIATION UNIT ONE SUPPLEMENTAL INFORMATION FOR LER NP-33-79-121 PAGE 2 A separate deficiency discovered revealed Lnproper initial wiring of the non-safety related RCP auxiliary relay interlock circuit in Relay Cabinet RC3717. The flow switch for component cooling water and seal makeup for RCP 1-1 was wired to RCP 2-2

_,and the flow switch for component cooling water and seal makeup for RCP 2-2 was

• ,~ wired to RCP l-1 (positive leads only) . This wiring error would not have caused misoper.ation of the RCPs. The only misoperation that would occur is if the control power fuse blew, the incorrect RCP would trip.

"...alysis of Occurrence: There was no danger to the health and safety of the public or to station personnel. One other RCP in each loop was operable during the period that RCP 1-1 and RCP 2-2 were inoperable.

Corrective Action: Under Maintenance Work Order (MWO) 79-3346, the first occurrence was investigated. After testing and troubleshooting at the station, it was found that the RCP interlock circuit was grounded through the case on the Couch (Deutsch) relays by an inductive surge in the circuit. Later tests by the manufacturer proved this statement to be correct. As a followup, MW0s 79-3460,~79-3472, and Work Request IC-093-70-79 were issued to troubleshoot for additional problems. Several additional grounds were found in the DC lighting distribution panels and DCMCC2. These grounds have all been cleared under the above MW0s.

Under Facility Change Request (FCR)79-378, arc suppression diodes for the RCP inter-lock circuits were added to suppress an inductive voltage caused by the interruption of time delay coil circuits on 125 volt DC. Arc suppression diodes were also added to several other circuits under this FCR.

Under MW0s 79-3431 and 79-3508, the non-safety related relay cabinet (RC 3719) wiring was scheme checked. The wiring deficiency was a result of original field installa-tion work. Under the above work orders, the wiring was corrected and FCR 79-375 was prepared to correct drawings to an as-built condition.

As a followup, MWO 79-3509 replaced all the Couch (Deutsch) relays for the component cooling water (seal cooling water) and reactor coolant seal injection flow switches for all four pump interlock circuits.

On November 14, 1979, all four RCP interlock auxiliary relay circuits were picked up in their normal configuration for running pumps. At this time, the 480 volt lighting distribution switchboard ES (breakers BE508 and BE510 and F5 (breakers BF503 and BF510)) were opened and automatically transferred to DC lighting distribution panels . -

L37El, L57El, L49El and L41El. No fuses blew in the RCP interlock circuits.

Failure Data: There have been previous reports of loss of RCPs from blown fuses, see Licensee Event Reports NP-33-78-129 and NP-32-79-11.

LER #79-104

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