05000322/LER-1987-035

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LER 87-035-02:on 871221,880106 & 0330,high Energy Line Break Logic Isolations of RWCU & Main Steam Line Drain Valves Occurred.Caused by Problems W/Temp Monitoring Units. Grounding Scheme Changed & Transformers Rewired
ML20055C501
Person / Time
Site: Shoreham File:Long Island Lighting Company icon.png
Issue date: 05/16/1990
From:
LONG ISLAND LIGHTING CO.
To:
Shared Package
ML20055C491 List:
References
LER-87-035, LER-87-35, NUDOCS 9005240081
Download: ML20055C501 (4)
v • d • e


LER-2087-035,
Event date:
Report date:
3222087035R00 - NRC Website
v • d • e

text

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On three separate occasions, High Energy Line Break Logic Isolations l

of the Reactor Water Cleanup (RWCU) and Main Steam Line (MSL) Drain Valves (iG33*MOV-034 and 1B21*MOV-032) occurred most likely due to l high amplitude electrical noise on the Isolated Ground System (IGS).  !

At the time of the events, the plant was in Operational Condition 4 l (Cold Shutdown) with theThe mode switch in Shutdown and all rods first two events occurred on December 21, inserted in the core.

1987 and January 6, 1988. An investigation discovered a grounding >

problem within the Primary Containment Monitoring Panel (PCM) where the Temperature Monitoring Units (TMU) are located. This particular l

problem was alleviated as a result of an engineering change which grounded the power supplies for the TMUs to the IGS. However, a third isolation occurred on March 30, 1988 due to a similar problem with the IGS. In all three events, operators verified the signals as false and returned the systems to their normal configurations. l Plant Management was notified of the events and the NRC was notified in accordance with 10CFR50.72. This Supplemental Report is being submitted to list the status of corrective actions taken.

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IDENTIFICATION OF THE EVENT Electrical Noise on the Isolated Ground System (IGS) resulted in High Energy Line Break Isolation Logic (IJ) initiations.

Event Date: 12/21/87, 01/06/88, 03/30/88 Report Date: 5/16/90 l t

CQHDlIl0HS_EB10R_IQ_IEE_EYEHI Operational Condition 4 (Cold Shutdown)

Mode Switch - Shutdown RPV Pressure : O psig RPV Temperature = 112, 113, 114 degrees F POWER LEVEL - 0 ,

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All rods' inserted in the core  !

I DESCRIPTION OF_IHE_EYEHI l On three separate occasions, High Energy Line Break Logic Isolations of the Reactor Water Clean-up (RWCU) System [CE) and Main Stean, Line

-(MSL) Drain Valves (1G33*MOV-034 and 1B21*MOV-032) occurred nost likely due to high amplitude electrical noise on the IGS. The first i two events occurred on December 21, 1987 and January 6, 1988 at 1633  :

cnd 0825 respectively. Investigation into the events at the time revealed that a grounding problem existed in the Primary Containment Monitoring Panel (1H11*PNL-PCM) where the system Temperature ,

Monitoring Units-(TMUs) (1G11*TMU-500A and 500B) are located. It was believed that the problem was due to the 24 VDC power supplies for the TMUs being grounded to the station ground, and the TMUs being grounded to the IGS. Voltage measurements of potential between the two grounds revealed that an electrical noise potential oxisted, which could have lead to the false signals being generated within the TMUs and in turn, initiate the isolations. An cngineering change was implemented which caused the 24 VDC power i supplies for the TMUs to be grounded to the IGS. This was discussed with the vendor (Rosemount) and was expected to alleviate the problem.

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ol ? nl2 m, m - . -.c , -nm On March 30,1988 at 0845, a third event occurred with identical results. In all three events, Operators verified the signals as .

false and returned the systems to their configurations prior to the '

event. Plant Management was informed of each event and the NRC was notified at 1753 on December 21, 1987, 1058 on January 6, 1988 and 1055 on March 30, 1988.

Similar ocevrrences in June of 1985 were reported via LER 85-023.

Those events were attributed to electromagnetic interferences due to work activities, but could not be repeated after testing. It is now believed that those isolations were also due to the same grounding problem.

CAUSE_DE_IHE_.EYEH.T The apparent cause of the event is that the electrical noise level on the IGS ground was and is such that periodic voltage spikes of large magnitudes trigger the misoperation of the TMUs (Rosemount Analog Trip Units), thereby causing the isolations. A contributing cause for the first two events was believed to be the grounding problem of the TMUs in the PCM panel, but this problem has since been corrected.

ANALYSIS OF THE_ EVENT These events were the unplanned actuations of an Engineered Safety Feature (ESF). There was no safety significance to the events. The high energy line break isolation logic performed as designed by initiating isolations of various systems. The improper grounding of the TMU power supply would affect only those valves that are open.

Had this event occurred under a more severe set of circumstances (5%

power), there would still be no safety significance.

CORRECTIVE ACTIONS

1. The grounding scheme for the TMU power supplies was changed to place the power supply and the instrument on the isolated instrument ground.
2. An inspection of other similar panels revealed that the grounding problem with the PCM Panel 24 VDC power supplies was an isolated case.

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3. Because of the sensitivity of the Analog Trip Units to plant fault currents, a modification to their grounding circuits was prepared which will provide a " cleaner" isolated ground path.
4. To further reduce noise and voltage transients on the Instrument Grounding System, modifications were prepared to rewire the neutrals for two transformers such that they would be connected to the Station Ground instead of the Instrument Ground System. Also a modification was prepared to connect the Instrument Ground System buried ground rod to the Station Ground Network, i

Implementation of the above modifications has been deferred indefinitely due to the New York State - LILCO Shoreham Settlement  :

Agreement. l, ADDITIONAL _lNFORMAI1QB

a. Manufacturer and_medel_ number _nLiailed_comnonent_Lal N/A
b. LEB_ numbers _of_nrexious similar_. events85-023 teaC ,orm ansa (6401

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