05000423/LER-1990-019

LER 90-019-00:on 900606,automatic Reactor Trip from Negative Flux Rate Signal Occurred Due to Dropped Control Rod.Caused by Broken Connection in Stationary Coil Power Cable for Rod. Special EOP Performed & Connector replaced.W/900703 Ltr

ML20055E126
Person / Time
Site:	Millstone
Issue date:	07/03/1990
From:	Jensen K, Scace S NORTHEAST NUCLEAR ENERGY CO., NORTHEAST UTILITIES
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-90-019, LER-90-19, MP-90-653, NUDOCS 9007110039
Download: ML20055E126 (4)

LER-2090-019,

Event date:
Report date:
4232090019R00 - NRC Website

text

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• EN'a:NIn[ed IEc'Eany _ HAR1 FORD. CONNECTICUT 06414-0270 t*vineast Nue=ar Erwegy company (203)665-6000 W[ .

July 3. 1990 MP-90-653 i

, - Re:' 10CFR50.73(a)(2)(iv) .

' U.S. Nuclear Regulato'ry Commission Document Control Desk -

Washington, D.C. 20555

Reference:

Facility Operating License No. NPF-49

Docket No. 50-423 .

Licensee Event Report 90-019-00

- Gentlemen: 1

- This letter forwards Licensee' Event Report 90-019-00 required to be submitted n'ithinL 3~

. thirty _(30) days pursuant to 10CFR50.73(a)(2)(iv), any event or condition that resulted .

in automatic actuation of the Reactor Protection System (RPS). .

Very truly yours, NORTHEAST NUCLEAR ENERGY COMPANY h

Stephen . Scace cale. -

Director, Millstone Station e

SES/KHJ:ljs .

Attachment:

LER 90-019-00 m  ;

cc: T. T. Martin, Region 1 Administrator .

l' W. J. Raymond,- Senior Resident las 3ector, Millstone Unit-Nos.1, 2 and 3 '

D. H.:Jaffe, NRC Project Manager, vlillstone Unit No. 3

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the Paperwork Reouction P o#ect (3160-0104). Office of Management anc Esuaget. Washington. DC 20503 F ACILITY NAME (1) DMl NWUEh (2) W --*

Millstone Nuclear Power Station Unit 3 ol 61 of 01014 l213 1lOFl0l3 T!TLE (4) .

Reactor Trip Due to Dropped Rod Due to Broken Cable to Stationary Gripper EVENT DATE (51 L E A NUMBF A e64 AFPOAT DATF f 71 OTHER F AClllTIES INVOLVED fbi MONTF DAY YEAR YEAR > MONT H DAY YEAR F ACILITY NAMES 0lslololol l l

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LICENSEE CONT ACT FOR THIS LE A (121 TELEPHONE NUMBER l NAME AREA CODt Keith Jensen, Engineer, Ext. 5496 2l0l3 4l4l7l-l1l7l9l1 COMPLETE ONE LINE FOR E ACH COMPONENT F AILURF DESCA6 BED IN THfS REPORT (13i .i 1

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SUPPLEMENTAL AEPORT EXPECTED (141 p MONTH DAY YEAf4 f SUB SiON YES (if yes. compiete EXPECTED SUBMISSION DATE) ] NO DA E 06) 1l2 2l8 9l0 ABSTRACT (Limit to 1400 spaces. i.e...approximately fifteen single-space typewntion 1:nes) 06)

On June 6,1990, at 0618 hours0.00715 days <br />0.172 hours <br />0.00102 weeks <br />2.35149e-4 months <br /> with the plant in Mode I at 100% power,587 degrees Fahrenheit and 2250 -l psia, an automatic reactor trip from a negative flux rate signal occurred due to a dropped control rod, l The cause of this event was a broken connection in the stationary pripper coil power cable for rod G13. This' o single dropped rod resulted in a negative flux rate signal on two Power Range Detectors, thereby resulting in a reactor trip signal. The root cause of the broken connection could not be immediately determined, independent evaluation by a material testing facility is in progress to ascertain the failure mechanism. When the .i root cause is positively determined, a supplemental report will be issued.

1'

l. As immediate corrective action control room operators performed the actions required by the applicable .

j emergency operating procedure. The broken connector was replaced. A functional test was performed by fully }

l- withdrawing and then inserting the affected rod. Potential long term corrective actions will be evaluated when  ;

the root cause analysis is complete.

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1. Descrintion of Event On June 6,1990, at 0618 hours0.00715 days <br />0.172 hours <br />0.00102 weeks <br />2.35149e-4 months <br /> with .the plant in Mode 1 at 100re power, 587 degrees Fahrenheit and 2250 psia, an automatic reactor tnp from a negative flux rate signal occurred due to a dropped control rod.

At the time of the tnp, operators verified that the reactor trip and bypass breakers were open, that all control rods were fully inserted, and that neutron flux was decreasing. A Feedwater Isolation signal was :

received due to low Reactor Coolant System Average Temperature following the trip. An Auxihary Feedwater actuation. occurred as a result of a steam generator low-low level signal. These are normal- ,

9: plant responses following a trip from 1007c power. No additional engineered safety features were required or initiated. . There were no operational, maintenance, or construction activities in progress at - l the time which affected the event. Plant stabihty, based on Reactor Coolant System Average Temperature, was achieved at approximately 0638 hours0.00738 days <br />0.177 hours <br />0.00105 weeks <br />2.42759e-4 months <br /> on June 6,1990.

L II. Cnnse of Event  ;

The cause of the dropped rod was a broken connection in the stationary gripper coil p9wer cable for rod {

G13 in Shutdown Bank B. The single dropped rod resulted in a negative flux rate signal on two out of q four Power Range Channels, thereby meeting the required logic for a reactor trip signal, j The root cause of the broken connection could not be positively determined. The cable and connector

, assembly were sent to an independent materials testing facility to ascertain the failure mechanism. When this information is available, and the root cause has been positively determined, a supplemental report j will be issued. I The connector cable failure affected the Rod Control System in the following manner. To hold a control rod in a given position, a holding current is applied to the stationary gripper coil. This coil is mounted outside the Control Rod Drive Mechanism (CRDM) pressure housing. The coil is magnetically coupled i to the stationary gripper arm assembhes, which are inside the CRDM pressure housing. The gripper arms 0 engage with circumferercial grooves on the drive rod assembly, which is in turn connected to the affected control rod. If power to the stationary pripper coil is lost, the magnetic flux holding the gripper arms'in .

place will be interrupted. The gripper arms will disengage from the drive rod, and the control rod will be j released. Thus, when the G13 stationary gripper coil connector broke, power to the coll'was lost, and  !

the rod fell from its fully withdrawn position. I 1

Because rod G13 is located on the core periphery, excore Power Range Channels 42 and 44 detected the j rapid drop in nuclear power due to the falling rod. A reactor trip signal will be generated when two out i

l' of four Power Range channels generate a rate trip. As a result, a high negative flux rate trip signal was  ;

generated on both of these channels. The setpoint for this signal is a change of less than or equal to SFc ,

rated thermal power with a time' constant of greater than or equal to 2 seconds. :I N 111,  : Annivsis of Event

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] This event is being reported in accordance with 10CFR50.73(a)(2)(iv) as an event or condition that resulted in automatic actuation of any Engineered Safety Feature, including the Reactor Protection System. Immediate notifications were made in accordance with 10CFR50.72(b)(2)(ii).

There were no significant safety consequences due to this event. The intended design function of the negative rate trip is to mitigate the effects of a multiple rod drop event at high power. Multiple dropped rods, without a subsequent reactor trip, could cause local flux peaking. resulting in a localized, ,

non-conservative Departure from Nucleate Boiling Ratio (DNBR) values. By mitiating a reactor trip, and 4 thereby causing full insertion of all control rods, the negauve rate inp prevents these hmiting DNBR values from occurrir.g.

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!'! FACIUTY NAME n) DOCKET NUMBER (2) . t r A NUMpF A en PAGE 13n YEAR Q E L Millstone Nuclear Power Station

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Text o, mor in.c. is reaver.o. u.. aoii onei NRc Form assA si or) o Although this inp signal protecte against multiple dropped rods, discussions with the fuel vendor indicated that untier certain ecnditions, a single dropped rod could cause a reactor trip. Based on the reactivity worth of rod 013 and its geometrical relation to Power Range Channels 42 and 44, receipt of two negative rate signals upon the rod drop is a valid assumption. Further discussion.with the fuel vendor indicated that there have been several instances at similar plants where a single dropped rod caused a reactur trip. The response of the Reactor Protection System in generating a reactor trip signal due to rod 013 dropping is therefore conservative with respect to its design basis.

IV. Corrective Action in order to determine which control rods had dropped, a special procedure was performed in which each control rod was individually latched and withdrawn approximately 6 inches off the bottom.- Rod 013 in Shutdown Bank B was the only rod that would not move. Subsequent continuity checking from the Rod Control cabinets to the CRDM coil indicated an electrical fault in a section of cable inside Containment.

A Containment entry was made, and the faulted section was identified and removed. Bench testing and 1 inspection revealed that a stationary gripper coil power cable had broken in the connector. = The

connector was replaced, the cable was continuity checked, and the cable =was then reinstalled. The entire power loop,"from the Rod Control Cabinets to the CRDM coil stack, was then continuity checked.

As a final functional test, rod 013 was latched and withdrawn to its fully withdrawn position and then reinserted. No problems with rod 013 were experienced. As an additional test, the continuity of all d.

other control rods, from the Rod Control cabinets to the CRDM coil stacks, was verified to be l satisfactory. Furthermore, potential long term corrective actions will be evaluated when the root cause aneysis is complete. A supplemental report will be submitted by December 28, 1990, detailing the results of the root cause analysis and any additional corrective action deemed necessary.

V, ' Additionni Information .

x There have been no similar events with the same root cause and sequence of events.

Ells Codes -

System j i Control' Rod Drive System - AA L Reactor Coolant System - AB- 1 Auxiliary Feedwater System -BA j Plant Protection System - JC 1 Excore . Monitoring System - IG j f

Comnonent l

.i Cable, Low Voltage - Power - CBL4 Coll - CL-Rod - ROD ~

. Detector - DET go,m 3 6 4

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