ML20043A784

LER 90-013-00:on 900416,manual Reactor Trip Initiated Due to Imminent Loss of Condenser Vaccum.Caused by Inadequate Administrative Guidance When Debris Collected on Trash Rack. Severe Weather Procedure revised.W/900516 Ltr

ML20043A784
Person / Time
Site:	Millstone
Issue date:	05/16/1990
From:	Hulme N, Scace S NORTHEAST NUCLEAR ENERGY CO., NORTHEAST UTILITIES
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-90-013, LER-90-13, MP-90-495, NUDOCS 9005230119
Download: ML20043A784 (4)

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'T2 II ac ay P.O. BOX 270 HARTFORD. CONNEoTICUT 06414-0270 L EI*I'YE'e*ar~'E'g*v twinent tw Enw co'mp*a*ny (2031666-6000 May 16, 1990 MP-90-495 Re: 10CFR50.73(a)(2)(iv)

U.S. Nuclear Regulatory Commission Document Control Desk Washington, D.C. 20555

Reference:

Facility Operating License No. NPF-49 Docket No. 50-423 Licensee Event Report 90-013-00 Gentlemen:

This letter forwards Licensee Event Report 90-013 required to be submitted within thirty manual actuation of the Reactor Protect) ion System (RPS).(30) days pursuant to 10C Very truly yours.

NORTHEAST NUCLEAR ENERGY COMPAh"t' lw occ -

Steph E. Scace Director, Millstone Station SES/NDH:mo ^

Attachment:

LER 90-013-00 cc: T. T. Martin, Region 1 Administrator W. J, Raymond, Senior Resident Ins 3ector, Millstone Unit Nos.1, 2 and 3 D. H. Jaffe, NRC Project Manager, Millstone ' Unit No. 3 p .

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NAME T E L E PHONE NUMBE A ARE A CODt Nelson D. Hulme, Senior Engmeer, X5398 :g0l3 4;4l7;-l1l7l9l1 COMPwETE ONE LINE FOA E ACH COMAONENT F Alt.JGE DESCA BED IN THis AEDOAT f13)

CAUSE SYSTEU COMPONENT ThC' [d CAUSE SYSTEM COMPONENT N h 0* 7g I II I I I I I II I I I I m

I II I I I I I II I I I I SUPPLEMENTAL RECOAT EXPECTED f144 MONTH DAY YEAA g,g g SUBM.SS ON

] YEs(ifyes compi.1, ExPE CTED SUBM;SSION D ATE 1 NO A 06) g g l AeS1n*CT iumuo ,.x .eac... i . . . aoo.os, mas.wie.n .,,-.oac. ivn.w.m., kn..i no On April 16,1990, at 1201 hours0.0139 days <br />0.334 hours <br />0.00199 weeks <br />4.569805e-4 months <br /> with the plant at 48G power in hiode 1, a manual reactor trip was initiated because of an anticipated turbine trip due to loss nf condenser vacuum. Circulating Water Pump 3CWS-P1B was providmg coohng water for Condenser Waterboxes " A" and "B" A rapid buildup of seaweed on "B" Travehng Screen resulted in an automatic trip of 3CWS-P!B due to high screen differential level. As the loss of cooling to two condenser bays would have resuhed in a low condenser vacuum, a reactor trip was initiated.

The reactor inp caused a Alam Turbine and Generator trip in accordance with design.

The root cause of the event was madequate administrative guidance in that debris was allowed to collect on the trash rack. During trash rack rakmp, seaweed broke free and clogged *B" Travehng Screen. Higher than normal seaweed influx and 1309 flow due to cross connecting A & B waterboxes contributed to the high differential level tnp. The severe weather procedure was revised to require each circulating water pump to supply only the associated waterbox during severe weather. For corrective action, the trash racks high differential level alarm point has been lowered from 15 inches to 6 inches. Personnel have been instructed to closely momtor the trash rack water levels and clean the racks before levels exceed 4 inches.

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1. Dewrindon of Evem On April 16,1990, at 1201 hours0.0139 days <br />0.334 hours <br />0.00199 weeks <br />4.569805e-4 months <br /> with the plant in Mode 1 at 48% power, at 572 degrees Fahrenheit and 2250 psia, a manual reactor trip was initiated because of an anticipated turbine tnp due to loss of condenser vacuum. The plant had five of six circulating water pumps in operation with Circulating Water Pump 3CWS-PI A and its associated traveling screen shut down for repairs. Circulating Water Pump 3CWS-PIB was providmg cooling water for Condenser Bays "A" and "B". A rapid buildup of seaweed on the "B" Travehng Screen resulted in an automatic trip of 3CWS-PIB due to high screen differential level. As the loss of cooling to two condenser bays would have resulted in a low condenser vacuum, a reactor tnp was initiated. The reactor trip caused a Main Turbine and Generator trip in accordance with design. All safety rystems were fully operable at the time of the tnp, and all plant systems performed as designed in response to the tnp.

Prior to the trip, reactor power was being increased at a rate of 3% per hour, in response to a "B" Trash Rack high differential level alarm (15"), an operator was sent to the intake to clear the debris from the rack. During the cleanng effort, large amounts of seaweed broke free and swept past the trash rack onto the "B" Traveling Screen. The buildup of debris on the screen was too fast for the screen wash system to clear, so a high differential level trip of "B" Circulating Water Pump resulted.

At the time of the trip, operators verified that the Reactor Trip and Bypass Breakers were open, that all control rods were fully inserted, and that neutron flux was decreasing. A Feedwater Isolation was received due to low Average Reactor Coolant System temperature following the trip. An Auxiliary Fredwater actuation occurred as a result of a steam generator low-low level signal. These are normal plant responses following a trip. No additional Engineered Safety Features were required or initiated.

Plant stability was achieved at 1216 hours0.0141 days <br />0.338 hours <br />0.00201 weeks <br />4.62688e-4 months <br />.

11. Cause of Etem The root cause of the event was inadequate administrative guidance in that debris was allowed to collect on the trash tack. When the operator proceeded to clean the rack, seaweed broke free and clogged "B" Traveling Screen.

There were two factors contributing to this incident. The first factor is the influx of seaweed which peaks in April - May and September - October. The Northeast Utilities Environmental Lab has reported the 1990 spring peak to be the highest in ten years. The second contributing factor was the increased flow through the "B" traveling screen. The *B" circulating water pump was aligned to supply flow to both the " A" and

• B" waterboxes. This results in approximately 130% flow across the "B" screen.

111. Annhsic of Evem This event is being reported in accordance with 10CFR50.73(a)(2)(iv), as an event or condition that resulted in manual actuation of an Engineered Safety Feature, immediate notifications were performed in accordance with 10CFR50.72(b)(2)(ii).

The safety significance of the event is minimal as the trip was manually initiated because a turbine trip due to loss of vacuum was imminent. Power level was at 56% just before the differential level increase was observed on the "B" Traveling Screen. When screen differential level reached the alarm setpoint l (18*), a turbine load reduction was initiated in order to reduce the transient that the impending reactor trip would have-on the plant. Power level was at 48G when 3CWS-PIB tripped on high differential level (30").

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Operation of the Service Water system was not jeopardized due to the ratio of Sersice Water System flow (approximately 1$,000 gallons per minute) to circulating water pump flow (approximately 150,000 gallons  ;

per minute) for one bay. When a circulating water pump trips, there is a reduction in flow resistance through the blocked screens, This allows differential level across the screens to return to an acceptable value, r

IV. Corrective Action The trash racks high differential level alarm point has been lowered from 1$ inches to 6 inches, Personrael have been instructed to closely monitor the trash rack water levels and clean the racks before levels exceed 4 inches. This will minimize the debns which can break through the trash racks during raking operations.

The procedure which governs plant response to severe weather has been revised to require each circulating water pump to supply only the associated waterbox if severe weather is predicted, This will ensure flow across the screen is limited to 100% flow, in addition, the efficiency of the cleaning rakes has been improved. In the original design, the rake tips were located flush with the rack bars, The tips have since been designed to extend about one inch in between each of the bars.

\'. Additional Information Licensee Event Report (LER) numbers $6-035,88-014, 88-004,69-006, and 90-011 are similar in that a reactor trip due to a turbine trip resulted when fouling of the intake screens caused circulating water

• pumps to trip causing condenser vacuum to decrease. LER 90-011-00 identified design changes and maintenance actions that were taken to improve screen wash effectiveness in removing debris from the

( traveling screens. In addition, procedures have now been established to mitigate the consequences of

( seaweed buildup if the weather and tides are unfavorable, or the debris removal systems cannot adequately clear seaueed. Actions to be taken include the assignment of additional personnel to clear debns from the trash racks, and the reduction of power to a point where loss of some circulating pumps and subsequent turbine generator trip will not cause a reactor trip.

E119 CODES Systems comnonente -

l_ Circulating Water System - KE Pumps - P l Traveling Water Screens - SCS Condenser - COND F

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