ML20028H440

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 Gripper Coil Power Cable.Connector Replaced & Power Loop checked.W/901224 Ltr

ML20028H440
Person / Time
Site:	Millstone
Issue date:	12/24/1990
From:	Clement C, Marshall F NORTHEAST NUCLEAR ENERGY CO., NORTHEAST UTILITIES
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-90-019, MP-90-1333, NUDOCS 9101070169
Download: ML20028H440 (4)

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December 24, 1990 MP-90-1333 U.S. Nuclear Regulatory Commission Document Control Dest Washincton, D.C. 20555

Reference:

Facility Operatin- ense No. NPF-49 Docket No. 50-Licensee Eveni ; ? ort 90-019-01 Gentlemen:

This letter forwards Licensee Event Report 90-019-01 required to be submitted by De.

cember 28,1990, in accordance with Licensee Event Report 90-019-00. Licensee Event Report 90-019-00 was submitted pursuant to 10CFR50.73(a)(2)(iv), any event or condi.

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

Very truly yours.

NORTHEAST NUCLEAR ENERGY COMPANY FOR: Stephen E. Scace Director, Millstone Station

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w,7 BY: carl H. Clement Millstone Unit 3 Director SES/FM:ljs

Attachment:

LER 90-019-01 cc: T. T. Martin Recion 1 Administrator W. J. Ravmond,' Senior Resident inssector, Millstone Unit Nos.1, 2 and 3 D. H. Jaffe, NRC Project Manager, .stillstone Unit No. 3 4* P 7oMo3 W 9101070169 901224 IEna PDR ADOCK 05000423 S PDR ///

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Reactor Trip Due to Dropped Rod Due to Broken Cabit to Stationary Gripper ivt.o D.ti + an,c.urc 4, orow mr+ o m< n

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On June 6,1940, 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 100'i power 567 degrees Fahrenheit and 2250 psia, an automatic reactor trip from a negative flux rate signal occurred due to a dropped control rod.

The cause of this esent was a broken connection in the stationary gripper coil power cable for rod G13. This single dropped rod resuhed 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 was corrosion at the conductor /pm mterface. l As immediate corrective acuan control room operators performed the actions required by the applicable emergenc) operatmp procedure. The broken connector was replaced. A functional test was performed by fully withdrawing and then insertmg the affected rod. Long term correcthe action will be to mspect and replace connectors as necessary during the third refueling outage.

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PAGE 0' YEAR Nh N muse osci hiil! stone Nuclear Power Stauon U"il 3 0l5l0l0lojal2l3 9l0 0l1l9 0l0 0l 2 OF 0l3 uxT ni me. um. i. ,.w.m use .ott.ono NRc Fom1 MA O Ne I, Deermtion of Es ent On June 6,1990 at 0616 hours0.00713 days <br />0.171 hours <br />0.00102 weeks <br />2.34388e-4 months <br /> with the plant in Niode 1 at 1009 power, $$7 degrees Fahrenheit and 2250 psia, an automaue reactor tnp from a negatise flux rate signal occurred due to a dropped control rod.

At the time of the trip, operators senhed that the reactor tnp and bypass breakers were open, that all control rods were fully inserted, and that neutron flux was decreasing. A Feedwater Isolation signal was recch'ed due to low Reactor Coolant System Average Temperature followmg the tnp. An Auxihary Feedweler actuation occurred as a result of a steam generator low-low lesel signal. These are normal plant responses following a top from 1009 power. No additional engineered safety features were required or initiated. There were no operational, mamtenance, or comtruction activities in progress at the time which affected the event, Plant stabihty, based on Reactor Coolant System Average Temperature, was achiesed at approximately 0636 hours0.00736 days <br />0.177 hours <br />0.00105 weeks <br />2.41998e-4 months <br /> on June 6,1990.

11. Came_.nLfrent The cause of the dropped rod was a broken connection in the stationary gnpper coil power cable for rod G13 in Shutdown Bank B. The smgle dropped rod resulted m a negative flux rate signal on two out of four Power Range Channels, thereby meeung the required logic for a reactor trip signal.

The root cause of the broken connection was corr.sion at the conductor / pin interface, caused by semi-fluid matenal found inside the connm The intended matenal inside the connector is a hard cured epoxy resm potung compound to seal out moisture. The cable and connector assembly were sent to an independent matenals testirj facihty for analysis, The rnatenal found was not conclusively identified, but it was determmed to be corrosive upon contact with bare metal, such as the conductor wire.

The connector cable failure affected the Rod Control System in the following manner, To hold a control rod in a gisen position, a holding current is applied to the stationary gnpper coil. This coil is mounted outside the Control Rod Drhe Mechanism (CRDM) pressure housing. The coil is magnetically coupled to the stauonary gripper 1rm assemblies, which are inside the CRDM pressure housing. The gnpper arms engage with circumferential grooves on the drive rod assembly, which is in turn connected to the affected control rod. If power to the stationary gnpper 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 released Thus, when the G13 stationary gripper coil connector broke, power to the coil was lost, and the rod fell from its fully withdrawn position, Because rod G13 ts located on the core periphery, excore Power Range Channels 42 and 44 detected the rapid drop m nuclear power due to the falhng rod. A reactor trip signal will be generated when two out 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 setpomt for this signal is a change of less than or equal to 59 rated thermal power with a time constant of greater than or equal to 2 seconds.

111. Anahs of Event 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 notif cations were made in accordance with 10CFR50.72(b)(2)(ii).

There were no signincant safety consequences due to this esent. 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 Depanure from Nucleate Boiling Ratio (DNBR) values. By iniuating a reactor tnp, and i thereby causing full insertion of all control rods, the negative rate tnp prevents these limiting DNBR values from occurring.

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ol 6l 0] ol ol4 l2 l3 9l0 0l1l9 0l0 0l 3 OF 0l3 Text of mo, ste. .s e. woo. ,se sopw. Nr: svm mA : m Although this tnp ngnal prettetS agamst multiple dropped rods, discussions with the fuel vendor indicated that under cenmn cond:tions, r= single dropped rod could cause a reactor trip. Based on the reactivity worth of rod 013 and its geomeincal re!auon to Power Range Channels 42 and 44, receipt of two negative rate signals upon the rod crop is a vahd essumption Further discussion with the fuel vendor indicated that there have been several msttnces rit similar plants where a single dropped rod caused a reactor trip. The respor.se of the Reactor Protection System in generating a reactor trip signal due to rod 013 droppmg is there!cte conser.tuve w'th respect to its design basis.

IV. Conectwe Action in order to determine which control tods had dropped a special procedure was performed in which each control rod us individually jatched and wnhdrawn approximately B inches off the bottom. Rod 013 in Shutdown Bank B was Int' only rod thrt would not move. Subsequent continuity checking from the Rod Control cabinets to the CRDM coil indicated an eiectrical fault in a section of cable mside Containment.

A Containment entry wts made, and the fauhed section was identified and removed. Bench testing and inspection revealed that a stationary gnpper coil power cable had broken in the connector. The connector was replaced, the cable was continuity checked, and the cable was then temstalled. The entire power loop, from the Rod Control Cabmets to the CRDM coil stack, was then continuity checked.

As a final functior.al test, rod 013 was latched and wahdrawn to as fully withdrawn position and then reinsened. No problems with rod 013 were experienced. As an additional test, the continuity of all other control rods, from the kod Control cabmets to the CRDM coil stacks, was verified to be satisfactory. Long term action to prevent recurrence will be to inspect a minimum of 10 connecters during the third reluchng outage, if neither the semi-fluid material nor camage to the connectors is found, no further action will be taken. If either similar semi-fluid material or damage to the connectors is found, all 61 connectors will be inspected. At that time, the severity of the damage will be assessed and the appropnate action will be taken, from single pin replacement to multiple connector replacement.

V. AdditionM informatmn There have been no similar events with the same toot cause and sequence of events, Ells Codes System Control Rod Drne Sys;em - AA Reactor Coolant System - AB Auxiliary Feedwater System -BA Plant Protection System - JC Excore Monitoring System - 10 Comnonent Cable, Low Yohage - Power - CBL4 Coil - CL Rod - ROD Detector - DET 4

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