ML20235E790

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Supplemental Ro:Updates Listed Util Repts Re Defective Main Steam Line High Flow Switches,Eccs Pump Start Permissive Switch (Ps) & ECCS Valve Operating Ps,Respectively. Instrumentation Problems Diagnosed & Corrected
ML20235E790
Person / Time
Site: Monticello, 05000000
Issue date: 02/21/1972
From: Mayer L
NORTHERN STATES POWER CO.
To: Morris P
US ATOMIC ENERGY COMMISSION (AEC)
Shared Package
ML20235B311 List: ... further results
References
FOIA-87-111 NUDOCS 8709280271
Download: ML20235E790 (4)
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NSF NORTHERN STATES POWER COMPANE i NONTICELLO WCLEAR GENERATIfG PLANT Monticello, Minnesota 55362

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February 21, i972 S g W.,)k~ 4 c . ,

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  • c z i Dr. Peter A. Morris, Director FE820197& 5  % Vhadd!

j Division of Reactor Licensing -

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United States Atomic Energy Commissio g grenosOk' cm e g g rg

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Washington, D. C. 20545 -

Dear Dr. Morris:

as N0f#1 CELLO WCLEAR GENERATIFG PLAfd Docket No. 50263 License No. OPR-22 ADDITIONAL I NFORMATION ON If6TRUMEfRATION Pf0BLEM3 This letter is to inform you of additional information obt ined concerning several plant instrumentation problems which were the subject of previously submitted reports.

1. Main Steam Line High Flow i instrumentation Problems (See report to Dr. Morris dated November 23, 1971)

Sixteen switches with a range of 0-70 psid were installed in the main steam line high flow isolation system on September 1,1971, to replace the original 0-200 psid switches. The lower range switches were  ;

installed because of lower than expected differential pressure measure- J ments from the main steam line flow nozzles. During surveillance {

testing on November 11 and 12,1971,. three of the switches were found 1 to exhibit signs of high friction or binding in their mechanisms and l another switch was found to exhibit non-repeatable trip settings. The i four defective switches were immediately replaced by 0-200 psid l switches previously used for the protective function. The remaining I twelve swiiches were replaced on November 13, 1971. )I One of the defective switches was returned to the factory for inspection. }

The vendor's inspection revealed that the torque tube jewel bearing )

was contaminated by a lead paste compound that was used to seal the '

joint between the instrument case and the di fferential pressure unit.

The contamination which occurred during switch assembly at the factory, caused excessive hysteresis in the torque tube movement. The vendor '

l indicated that the jewel bearing contamination was a random occurrence in switches of this type and model, and 'therefore there was no assurance that other similar instruments did not have, the potential for similar  ;

failures. A review of the instrumentation used in the plant protection system was conducted to identi fy all instruments with the potential for l

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-a contaminated jewel bearing. A total of 39 instruments were so identified and inspected following the vendor's recommended proce-du ro. No jewel- bearing contamination was found.

.The switch which was found to exhibit _ a non-repeatable trip setting was investigated at the site. Tests disclosed that the erratic -

operation'was caused by a loose pivot pin on the cam follower assembly and was not due to the snap action switch, as previously reported.

The loose pivot- pin caused the trip point to vary between 109% steam flow and 127% steam flow. This problem also appears to have resulted a

from a manufacturing error. All plant instruments of this t inspected for excessive deadband (greater that 5% of range) ype and were:

were found to be satisfactory.

2. Failure of ECCS Pumo Start Permissive Switch and Relav (See report i to Dr. Morris dated December 20, 1971) x .1 i

On November 20, 1971, while performing a surveillance test, the trip  ;

setting of ECCS Pump Start Permissive Switch PS 2-3-53 A, switch #2, '

was found to be 12 psi below the required setting of >_ 450 psig.

During this surveillance test it was also discovered that a relay in I the ECCS pump start permissive logic failed to energize when pump  ;

start pernnssive switch PS 2-3-53 A, switch #1 was closed. l An investigation of the setpoint problem revealed that the two switches contained in PS 2-3-53 A were not designed for the 125 V DC application for which they were being used, but were designed for 115  !

V AC service. The wrong switches were initially specified for this application. An analysis performed by the instrument manufacturer revealed that the microswitch contacts in the instrument were burned due to excessive current. The vendor believes the burned contacts  ;

may have caused the change in the instrument setpoint. J A check of plant instrumentation disclosed that 22 instruments with microswitches rated for 115 V AC were being used in 125 V DC ci rcui ts.

These switches (listed below) have all been' replaced with switches i

rated for 125 V DC.

Insirument Funetion PS 2-3-49 A & B LPCI Recire Loop Selection PS 2-3-50 A & B LPCI Recire Loop Selection PS 2-3-52 A ECCS Valve Open Permissive PS 2-3-53 A & B ECCS Pump Start Permissive PS 23-68 A through D HPCI Steamline Low Pressure Isolation PS 2-128 A & B RHR Shutdown Cooling Isolation PS 14-47 A & B Core Spray Header High Pressure Alarm PS 14-44 A through D Core Spray "AC Interlock" l PS 13-78 RCIC Turbine Exhaust Diaphram i

High Pressure Alarm l

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Instre ent Function  !

PS 13-72 A & 8 RCIC Turbine High Pressure Alarm The investigation of the relay which failed to energize when the

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switch #1 of PS 2-3-53 A was closed, revealed that the pull-in voltage was set too high., This resulted in an intermittent failure of the relay to energize. The pull-in voltage of the relay was found to be 110 V DC (the operating voltage available at the relay was measured to be 125 V DC). The pull-in voltage was reset to 80% of the operating voltage and the relay ai r gap was reduced. All of the DC relays of this typeintheplanthadbeenpreviouslysetandtestedat80% pull-in voltage in accordance with a field engineering memo in August 1970.

Additional investigation revealed that the 80% setting is applicable only to AC relays. All DC relays of this type were reset to the factory recommended 60% pull-in voltage. The relay which failed to operate during the surveillance test was the only relay of this type to experience a failure since the DC relays were set and tested in August 1970.

3. Failure of ECCS Valve Ooenino Permissive Switch (See report to Dr.

Morris dated December 21, 1971.)

On November 24, 1971, while performing a surveillance test, the trip setting of ECCS Valve Opening Permissive Switch PS 2-3-53 B, switch

  1. 1, was found to be 42.5 psi below the required setting of >_ 450 psig. An investigation revealed that the instrument lacked a setpoint

" locking" device, a modification recommended by the instrument manu-facturer. The locking device was innediately installed and the instrument was calibrated to trip at 460 psig.

Locking devices were also installed on all sim-ilar instruments with snap action switches used in the plant protection systems.

The main steamline high flow instruments have mercury type switches and are not designed for setpoint locking devices. Since past-experience has shown the setpoints of these mercury switches to be quite stable, the switches have not been redesigned or modified to include a setpoint locking device.

Locking devices were installed on the following instruments:

I ns t runeni Function DPIS 23-76 A & B HPCI High Steam Flow Isolation DPIS 23-77 A & B DPIS 13-83 RCIC High Steam Flow isolation DPIS 13-84 DPIS 14-43 A & B Core Spray Line Break Detection PS 2-52 B ECCS Valve Opening Permissive r

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I ns t ru ent Funct ion DPIS 2-129 A through D LPCI Loop Selection

? DPIS 2-136 A & B " " "

DPIS 2-137 A & B " " -"

DPI S 2-138 A & B - " " "

DPIS 2-139 A & B " " "

DPS 10-92 A &' B RHRHeatEschanger/P.

All the above i-strumentation problems were corrected prior to plant startup on-January 24, 1972. Detailed reports on these problems -

have been prepa ed at -the plant and are svailable for the Compliance-I nspecto r.

Iours very trul.,

a" o.

L. O. Mayer Director of Nuclear Support Services .

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