ML20011F383

LER 90-002-00:on 900126,reactor Tripped from 73% Power. Caused by Spurious Reactor Coolant Pump Monitor Signal Indicating Only One Pump Running in Each Loop.Pump Seal to Be Replaced During Sixth Refueling outage.W/900226 Ltr

ML20011F383
Person / Time
Site:	Davis Besse
Issue date:	02/26/1990
From:	Storz L, Stotz J TOLEDO EDISON CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-90-002, LER-90-2, NP33-90-002, NP33-90-2, NUDOCS 9003050264
Download: ML20011F383 (4)

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t EDOISON .

EDISON PLAZA I

333 MADISON AVENUE '

TOLEDO. OHIO 43C520301 February 26, 1990 ,

Log No.: BB90-00319 l NP33-90-002 Docket No. 50-346 License No. NFF-3 l

United States Nuclear Regulatory Commission a Document Control Desk vashington, D. C. 20555 l Gentlemen:

LER 90-002 Davis-Besse Nuclear Power Station, Unit No. 1 Date of Occurrence - January 26, 1990 -

l Enclosed please find Licensee Event Report 90-002, which is being submitted to provide 30 days written notification of the subject occurrence. This LER is being submitted in accordance with 10CFR50.73(a)(2)(iv).

Yours truly, i j' ,

h' f(Circ.o I Louis F. Storr.

Plant Manager Davis-Besse Nuclear Power Station .

LFS/plf Enclosure cc Mr. A. Bert Davis

\- Regional Administrator

/ USNRC Region III Hr. Paul Byron DB-1 NRC Sr. Resident Inspector

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"T] ves tier . = rxPactro sUsanimon oArts "'"]No Ol6 0l1 910 Ae:T A Act m-, M := .- . ,. . ,, rui . ,-. ,r ,,. ..e., nei On January 26, 1990, at 0846 hours0.00979 days <br />0.235 hours <br />0.0014 weeks <br />3.21903e-4 months <br />, during performance of Surveillance Test DB-MI-03205, RCP Current Honitor Inputs to SFRCS ACill and RPS Channel 1, the Station experienced a reactor trip from 73 percent full pover. Prior to the trip, the Station had been operating with three of four RCPs running. Plant response to the trip vas normal with key parameters remaining in the normal

, post-trip band. Steam generator header pressure was intentionally reduced to approximately 970 psig to aid in fully reseating two main steam safety valves solidly. After stabilization of the plant, the Letdown Isolation Valve, HU2B, vould not reopen. The inability to restore the letdown flow resulted in slower than normal plant cooldown and Mode 5 was achieved on January 27, 1990, at 0345 hours0.00399 days <br />0.0958 hours <br />5.704365e-4 weeks <br />1.312725e-4 months <br />. An action plan is being prepared to determine the cause of the RCP current monitor circuit transient which caused the Reactor Protection System (RPS) to sense that only one RCP was running in each of two loops.

HU2B valve vill be replaced this outage. The cause of its failure vill be investigated.

Immediate notification was made per 10CFR50.72(b)(2)(ii) on January 26, 1990, at 1007 hours0.0117 days <br />0.28 hours <br />0.00167 weeks <br />3.831635e-4 months <br />. The reactor trip is reportable as an LER per 10CFR50.73(a)(2)(iv).

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On January 26, 1990, at 0846 hours0.00979 days <br />0.235 hours <br />0.0014 weeks <br />3.21903e-4 months <br />, during performance of DB-HI-03205 Reactor Coolant Pump (RCP) Current Monitor to Steam and Feedvater Rupture Control l System (SFRCS-JB) Channel 1 and Reactor Protection System (RPS-JC) Channel 1 surveillance test, the Station experienced a reactor trip from 73 percent thermal power. Prior to the trip, the Station had been operating with three of the four RCPs running. RCP 2-2 vas shut down January 22, 1990, as a .

precautionary measure due to high indicated vibrations.

Plant response to the reactor trip was normal. Steam Cenerator pressure was reduced to approximately 970 psig to aid in fully reseating two main steam safety valves (HSSVs). One of the valves that did not initially resent fully was SPl7A7, which also did not fully resent after the January 18, 1989, event.

The identification of the other HSSV that did not fully resent is not knovn.

After stabilization, the operators found that Letdown Isolation Valve, HU2B, vould not re-open. The inability to restore letdown flow resulted in the need .

to decrease RCP seal injection to slow the rate of pressurizer level increase.

RCP 2-2 (the shutdovn RCP) seal return temperature increased when seal injection flow was reduced. This resulted in seal return flov being isolated from RCP 2-2.

l The reactor trip (RPS actuation) is reportable under 10CFR50.73(a)(2)(iv).

l Immediate notification was made to the NRC via the Emergency Notification System (ENS) at 1007 hours0.0117 days <br />0.28 hours <br />0.00167 weeks <br />3.831635e-4 months <br /> on January 26, 1990, per 10CFR50.72(b)(2)(ii).

l Mode 5 vas achieved on January 27, 1990, at 0345 hours0.00399 days <br />0.0958 hours <br />5.704365e-4 weeks <br />1.312725e-4 months <br /> to start the Sixth Refueling Outage.

Apparent Cause of Occurrencet The plant tripped when RPS sensed that reactor power was above the trip limit

! for operation with only one RCP running in each reactor coolant loop. RPS

! received a signal that indicated there was only one pump running in each loop l because RCP l-2 test restoration induced a current transient in the RCP l current monitor circuit making it look like RCP l-2 vas off. Vith RCP 2-2 l actually off and RCP l-2 appearing to be off, RPS reduced the high flux / number of RCPs trip setpoint to approximately 55 percent of full thermal power as designed. Since the plant was operating at 73 percent, RPS Channels 2, 3, and 4 tripped causing a reactor trip. RPS Channel 1 did not trip because it had l been placed in manual bypass per the test procedure. The root cause of the induced current transient in the RCP monitor circuit for RCP l-2 is still being investigated. The trip occurred while restoring the test setup that i functionally checks the high and low current setpoints for RCP l-2. RCP l-1 l circuits had been similarly tested just minutes earlier without incident.

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1 Davis-Besse Unit No. 1 o p jo jo lo l3 l 4l6 9 l0 0 l 0l 2 3 l0 0l3 or 0 l3 von . s anc a m wim The cause of the HU2B failure is not known yet. The valve vill be inspected and cause of the failure determined during the ongoing refueling outage. ,

Analysis of Occurrence:

5 There were no challenges to the Safety Features Actuation System (SFAS-JE) or the Steam Feedvater Rupture Control System (SFRCS-JB). Key parameters remained in the normal post-trip band. Minimum RCS pressure was 1790 psig and maximum was 2175 psig. Steam generator pressures ranged from a maximum of 1076 psig to a minimum of 970 psig on SG l-1 and a maximum of 1053 psig and minimum of 970 psig on SG l-2. The transient from 73 percent vould be expected to be less severe than a transient from 100 percent thermal pover.

The MSSVs not fully resenting did not rignificantly affect proper SG pressure response as pressure was being controlled at approximately 1025 psig immediately post-trip. The operators lovered turbine header pressure to approximately 970 fully seat two MSSVs solidly. >

Corrective Actions to Prevent Recurrence:

A troubleshooting plan is being developed to determine the cause of the apparent electrical transient in the RCP l-2 current monitor circuit. Once the cause is knovn, appropriate corrective actions vill be taken.

Previously scheduled inspections and maintenance are being performed on the MSSVs this outage.

Once the HU2B valve is inspected and the cause determined, appropriate corrective action vill be taken.

RCP 2-2 seal was one of the three RCP seals previously scheduled to be replaced during the Sixth Refueling Outage with an improved seal cartridge design.

This LER vill be revised upon availability of updated information.-

Failure Data g

Until the cause of transient in the RCP current monitor circuitry is

/ determined, relevant failure data cannot be assembled. This is the first LER (since all reactor trips became reportable in January 1984) where RPS reduced the high flux / number of pumps trip setpoint even though there vere actually three of four pumps running.

REPORT NO.: Np33-90-002 PCAO NO.: 90-0036

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