05000410/LER-1986-011, :on 861127 & 1208,automatic Initiation of Standby Gas Treatment Sys Divs I & II Occurred.Caused by Spurious Reactor Bldg Exhaust Ventilation Duct High Radiation Signals.Accumulator Added to Lines

:on 861127 & 1208,automatic Initiation of Standby Gas Treatment Sys Divs I & II Occurred.Caused by Spurious Reactor Bldg Exhaust Ventilation Duct High Radiation Signals.Accumulator Added to Lines

ML20204C399
Person / Time
Site:	Nine Mile Point
Issue date:	03/18/1987
From:	Lempges T, Randall R NIAGARA MOHAWK POWER CORP.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-86-011, LER-86-11, NMP23836, NUDOCS 8703250294
Download: ML20204C399 (5)

LER-1986-011, on 861127 & 1208,automatic Initiation of Standby Gas Treatment Sys Divs I & II Occurred.Caused by Spurious Reactor Bldg Exhaust Ventilation Duct High Radiation Signals.Accumulator Added to Lines

Event date:
Report date:
4101986011R00 - NRC Website

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NAME TELEPHONE NUMSER AREA CODE Robert G. Randall. Supervisor Technical Support 3l 11 5 a 419l -l 21 4 1 45 1

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..- - n.,,n Automatic initiation of the Standby Gas Treatment System divisions I & II occurred on November 27,1986 at 0645 hours0.00747 days <br />0.179 hours <br />0.00107 weeks <br />2.454225e-4 months <br /> and on December 8,1986 at 1442 hours0.0167 days <br />0.401 hours <br />0.00238 weeks <br />5.48681e-4 months <br />.

Initiation was caused by spurious Reactor Building exhaust ventilation duct high radiation signals. A root cause analysis has been completed per procedure S-SUP-1, " Root Cause Analysis Program".

Testing and investigations have isolated the following design deficiencies of electrical and mechanical components in the process radiation monitoring cabinets:

1.

A high frequency pressure variation in the sample lines.

2.

A low set point for the differential pressure switches.

3.

Ground Ioops.

4.

Externally mounted power transistors picking up electrical noise.

The corrective actions taken are as follows:

1.

Addition of accumulator in the sample lines.

2.

The set point for the dif ferential pressure switches was increased.

3.

thdesirable grounds were removed.

4.

A modification request has been initiated to shield the externally mounted transistors.

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I.

DESCRIPTION OF EVENT

On Novenber 27,1986 at 0645 hours0.00747 days <br />0.179 hours <br />0.00107 weeks <br />2.454225e-4 months <br /> and on December 8,1986 at 1442 hours0.0167 days <br />0.401 hours <br />0.00238 weeks <br />5.48681e-4 months <br /> with the mode switch in the shutdown position, the Nine Mile Point NJclear Station #2 experienced automatic initiation of the Standby Gas Treatment System divisions I and II.

These initiations were caused by spurious high radiation signals being transmitted from Radiation Monitoring Sample Cabinets 2HVR*328 (below refueling floor monitor) and 2HVR*148 (above refueling floor monitor).

II.

CAUSE OF EVENT

A root cause tnalysis has been completed per procedure S-SUP-1, ' Root Cause Analys is Program".

Testing and investigations have isolated the following design deficiencies of electrical and mechanical components in the process radiation monitoring cabinets:

1.

A high frequency pressure variation in the sample lines due to pump design was causing degradation of the charcoal filters.

2.

The set point for the pressure differential switches was too low resulting in frequent occurrences where the switch would hum or chatter as the set point was being approached introducing significant noise levels on the power supply of the microprocessor. The noise was resulting in spurious high radiation signals.

3.

Oscilloscope readings revealed die presence of significant ground loops which induce noise into process signal cabling typically resulting in spurious high radiation signals.

4.

Externally mounted power transistors on the microprocessors were picking up electrical noise and contaminating the power supply.

III.

ANALYSIS OF EVENT

This event is reportable per 10 CFR 50.73 part (a) (2) (iv) automatic actuation l

of any B1gineered Safety Feature.

The design basis of the Standby Gas Treatment System is to limit the release of radioactive gases from the reactor building to the environment within the guidelines of 10 CFR 100 in the event of a Loss of Cooling Accident (LOCA) and to maintain a negative pressure in the Reactor Building under accident conditions.

Since the Engineered Safety Feature initiated and functioned per design, there were no adverse safety consequences and would not have been even in a design basis accident circumstance.

The Secondary Containment Veltilation System was in normal operating condition within five hours for the 11/27/86 event and within two hours for the 12/8/86 event.

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I V.

CORRECTIVE ACTION

1.

Modification PN2Y86MX153 has been completed which adds a transient pressure accumulator between the sample pump and the charcoal filters (reference Sketch #1) for six category I process radiation monitor cabinets (2HVR* CAB 14A, B, 32A, B, and 2 CMS

• CAB 10A, B). This has greatly reduced charcoal filter degradation. A modification request (Issue

1. I20045) has been initiated for the balance of the cabinets.

2.

The alarm set point for the filter differential pressure switches was increased on all cabinets experiencing " chattering" to mitigate the effects of noise level produced by the switch on the power supply of the microprocessor.

3.

The undesirable grounds from the shield on process signal cables for all category I process radiation monitor cabinets have been removed per modification PN2Y86MX145.

4.

A modification request (Issue #I20046) has been initiated to shield the externally mounted power transistors on the microprocessors.

V.

ADDITIONAL INFORMATION

Identification of Components Referred to in this LER IEEE 803 IEEE 805 Component EIIS Funct System ID Standby Gas Treatment Systems N/A BH Sample Cabinet CAB IL Radiation Monitors MON IL Filter (on cabinet)

FLT IL Accumulator ACC IL Secondary Containment Ventilation System N/A VA Pressure differential switch PDS IL Power Supply RJX IL There are no previous similar events.

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c- ?, ~ g IMP 23836 NAGARA MOH AWK POWER CORPORATION ^ NIAGARA MOHAWK SYRACUSE,NY 13212

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f + March 18,'1987'

- thited States telear Regulatory Commission Document Control Desk Washington, DC 20555 RE:

- Docket No. 50-410 LER 86-11 Supplement 1 Gen tlemen:

In_ accordance.with 10 CFR 50.73, we hereby submit the following Licensee Event Report: LER 86-11 Which is being submitted in accordance with 10 CFR 50.73 Supplement 1 (a) (2) (iv), "Any event or condition that resulted in manual or automatic actuation of any Engineered Safety Feature (ESF), including the Reactor Protection System (RPS). However, actuation of an ESF, including the RPS, that: resulted from and was part of the preplanned sequence during ' testing or reactor operation need not be' reported." A telephone notification was made at 0700 hours on Noverrber 27,1986, and at 1442 on December 8,1986 per'10 CFR 50.72 part -(b) (2) (iv). This report was completed in the format designated in NtRES-1022, Supplement No. 2, dated September 1985. Very truly yours,, +744_.- Thomas E. Lempges Vice President mclear Generation TEL/DRG/mjd

Attachments cc:

Regional Administrator, Region 1 Senior Resident Inspector, W. A. Cook i t }}