ML19327C243

LER 89-026-00:on 891013,reactor Trip Occurred Due to High Neutron Flux Negative Rate on Two of Four power-range Neutron Monitoring Channels.Caused by High Resistance Connection on Gripper Diode.Diode replaced.W/891113 Ltr

ML19327C243
Person / Time
Site:	South Texas
Issue date:	11/13/1989
From:	Ayala C, Chewning R HOUSTON LIGHTING & POWER CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-89-026, LER-89-26, ST-HL-AE-3292, NUDOCS 8911210289
Download: ML19327C243 (5)

Text

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0 The Light L

t company P.O. Ilox 1700 llouston, Texas 77001 (713) 228 9211

. Houston IJghting Ae Power .__ . ,

November 13, 1989 ST-IIL-AE- 3292 File No.: G26 10CFR50.73 U. S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, DC 20555 South Texas Project Electric Generating Station Unit 2 Docket No. STN 50-499 Licensee Event Report 89-026 Regarding a Reactor Trip Due to a Dropped Control Rod Pursuant to 10CFR50.73, Houston Lighting & Power (liL&P) submits the attached Licensee Event Report 89-026 regarding a reactor trip due to a dropped control rod. This event did not have any adverse impact on the health and safety of the public.

If you should have any questions on this matter, please contact br. C. A. Ayala at (512) 972-8628. ./

. A -M R. W. Chewning .

Vice President Nuclear Operations RWC/BEM/eg

Attachment:

LER 89-026, South Texas, Unit 2 8911210289 891113

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Regional Administrator, Region IV Rufus S. Scott i

. Nuclear Regulatory Commission Associate General Counsel [

'611 Ryan Plaza Drive, Suite 1000 Houston Lighting & Power Company Arlington,-TX 76011 P. O. Box 1700  ;

Houston, TX 77001

- George Dick. . Proj ect Manager >

U. S. Nuclear Regulatory Commission INPO p Washington, DC 20555 Records Center

1100 Circle 75 Parkway i i J.'I. Tapia Atlanta, GA 30339-3064 Senior Resident Inspector c/o U. S. Nuclear Regulatory Commission Dr. Joseph M. Hendrie  !

P. O. Box 910 .

50 Be11 port Lane  ;

Bay City, TX 77414 Be11 port, NY 11713 I i J. R. Newman, Esquire D. K. Lacker Newman & Holtzinger, P.C. Bureau of Radiation Control  ;

1615 L Street, N.W. Texas Department of Health Washington, DC 20036 1100 West 49th Street Austin, TX 78756-3189 R. L. Range /R. P. Verret Central Power & Light Company

'P. O. Box 2121 Corpus Christi, TX 78403 ,

R. ' John Miner (2 copies) ,.

Chief Operating Officer City of Austin Electric Utility 721 Barton Springs Road Austin, TX 78704 R. J. Costello/M. T. Hardt City Public Service Board P. O. Box 1771 San Antonio, TX 78296 Revised 10/30/89 NL.DIST

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On October 13, 1989, Unit 2 was in Mode 1 at 100 percent power. At 1745 hours0.0202 days <br />0.485 hours <br />0.00289 weeks <br />6.639725e-4 months <br />, a reactor trip occurred due to the detection of high neutron flux negative rate on two of four power range neutron monitoring channels. The plant was brought to a atabic condition in Mode 3 with no unexpected post-trip transients. The cause of the event is believed to be an intermittent high resistance connection on a stationary gripper diode in the rod control system which caused rod F-8 in control bank A to drop. The diode has been replaced.

The remaining stationary gripper diodes on both units will be inspected during the next scheduled maintenance outage on each unit.

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0l0 0l2 OF 0 l3 rentu . o c, mu vnn DESCRIPTION OF EVENT:

On October 13, 1989, Unit 2 was in Mode 1 at 100 percent power. At 1745 hours0.0202 days <br />0.485 hours <br />0.00289 weeks <br />6.639725e-4 months <br />, a reactor trip occurred from two of four power range neutron monitoring ,

channels due to detected high neutron flux negative rate. The turbine tripped on the reactor trip and the feedwater isolation valves closed on low reactor coolant system average temperature. Auxiliary feedwater was initiated after the reactor trip from low steam generator water level as expected.

Approximately 4 minutes following the reactor trip, the operators closed the main steam isolation valves to prevent excessive cooldown. No safety injection actuation occurred and the plant was stabilized in Mode 3. The NRC was notified pursuant to 10CFR50.72 at 2021 hours0.0234 days <br />0.561 hours <br />0.00334 weeks <br />7.689905e-4 months <br />.

The sequence of events report and other computer alarm logs were examined to determine the cause of the negative rate trip. Due to computer scan frequency limitations, no conclusive evidence was found to confirm whether one'or more control rods had dropped. The rod control system power supplies (two motor-generator sets) were inspected by operations personnel. Both were found

, running normally and supplying the required voltage with no indicated faults.

The rod control system power cabinets were then inspected and no blown fuses or abnormal conditions were detected. The reactor trip breakers were reclosed and all nine rod banks were sequentially withdrawn to six steps (approximately 4 inches) and reinserted in an effort to isolate any dropped rods due to rod control system failure. All rods responded as indicated on the rod position indication (DRPI) system. Since an intermittent f ailure affecting one or more control rods was suspected, the rod control system power cabinets were inspected for loose connections. When no loose connections wers found, the resistance cf all stationary gripper coils was measured from the power cabinets. No abnormal readings were found. The power cabinet DC power supplies were tested. One of the backup power supplies was found inoperative and replaced, and was determined later to be unrelated to the reactor trip since the primary power supply was operative and did not lose its AC supply.

A comprehensive test of the rod control system was performed to obtain DC current profiles of the stationary gripper, moveabio gripper, and lift coils for each mechanism. No abnormal conditions were detected from the resulting data.

A recorder was installed to monitor stationary gripper circuits to isolate the intermittent failure in the event of another rod drop. At approximately 0609 hours0.00705 days <br />0.169 hours <br />0.00101 weeks <br />2.317245e-4 months <br /> on October 15, 1989, while withdrawing Control Bank A rods, Rod F-8 dronped from 21 steps (approximately 13 inches). All withdrawn rods were reinserted. Troubleshooting revealed an oren diode in the stationary gripper circuit for rod F-8. The diode was replaced and Mode 2 entered at 1853 nours.

l The reactor neutron flux was mapped at approximately 10 percent power to confirm the DRPI system indication (no toda at bottom).

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Text er ==. === = = ison, . ass w mac e== man w on CAUSE OF EVENT:

The cause of the reactor trip was high neutron flux negative rate detection on two of four power range neutron monitoring channels. The exact cause of the detected negative rate could not be conclusively determined, but has been '

attributed to Control Bank A. Rod F-8, dropping due to an intermittent high resistanco connection in its stationary gripper circuit diode. The cause of the high resistance connection could not be determined.

ANALYSIS OF EVENT:

Reactor trip and Engineered Safety Features actuation are reportable pursuant to 10CFR50 ?3(a)(2)(iv). The reactor tripped as required and plant equipment operated as expected.- No unexpected post-trip transients occurred and there was no safety injection actuation. There were no adverse radiological or safety consequences as a result of this event.

CORRECTIVE ACTION:

The following corrective actions are being taken as a result of this event:

1. The faulty diode has been replaced.

2. The stationary gripper circuit diodes in the rod control system will be inspected for similar conditions during the next scheduled outage on each unit.

ADDITIONAL INFORMATION:

l There have been no previous events reported regarding reactor tripe due to dropped control rods.

e The part number on the diode described above is IN1206AR and the diode was manufactured by Westinghouse, l

L NL.LER89026.U2 g, eon. 3 .

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