ML19207C012

Responds to Telcon & Verbal Request for Description of Events Re 3-minute Interruption of Forced Circulation on 790817.Circulator Trips on Circulator Speed/Feedwater Flow Program Experienced as Result of Cable Resistance

ML19207C012
Person / Time
Site:	Fort Saint Vrain
Issue date:	08/29/1979
From:	Warembourg D PUBLIC SERVICE CO. OF COLORADO
To:	Kuzmycz G Office of Nuclear Reactor Regulation
References
P-79189, NUDOCS 7909060314
Download: ML19207C012 (5)

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public service company oe osc1nde P.O. Box 361, Platteville, Colorado 80651 August 29, 1979 Fort St. Vrain Unit No. 1 P-79189 Mr. George Kuznycz U. S. Nuclear Regulatory Commission Division Project !bnagement Special Projects Washington, D.C. 20555 Docket No. 50-267

Subject:

Fort St. Vrain Unit No. 1 Plant Upset, August 17, 1979

Dear Mr. Kuzmycz:

Per our telecon and your verbal request, please find enclosed a description of the events which occurred on August 17, 1979.

You will note that several recommendations have resulted from the event review committee. These recocmendations are preliminary in nature and will be evaluated further by plant operations, our Technical staff and our engineering department. It is requested, therefore, that you treat this information as pre-liminary until necessary Technical evaluations can be completed.

Very truly yours, Wws.t Don Warembo Shnager, Nuclear Production DW:dkm Enclosure S i t

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THREE MINUTE INTERRUPTION OF FORCED CIRCULATION AUGUST 17, 1979 B_a_ck gro und Circulator trips on circulator speed /feedwater flow program had been experienced as a result of circulator speed cable resistance changes at the circulator conrcc-tor. As the resistance changed, an imbalance of the speed modifier output would appear as a low circulator speed signal to the control system. As the speed would appear to decrease, the control sys tem, through the speed controller (SC), would call for further opening of the circulator steam turbine speed valve (SV) . As the SV would open to correct the apparent low speed condition, the actual circulator speed would increase even though the circulator speed had actually been at the proper setpoint for the existing plant conditions. The plant protective system (PPS) circulator speed /feedwater flow program trip was unaffected by the loss of speed signal to the control system. As a result of the actual circulator speed in-crease, the PPS functioned to trip the circulator as designed.

To avoid circulator trips, it was decided to connect an electrical high selector

(>) on the output of the steam turbine and water turbine speed pulse to current t rans mi tte rs . This provides a speed signal to the control system from either of the identical speed elements (SE) on the circulator so that the steam turbine SE becoming imbalanced would not cause a programmed circulator overspeed trip.

The recuired approvals were received and preparations were made to wire the high selector (>) into the I-36B instrument panel. The SC for ID circulator was placed in manual control duing the installation.

Plant Conditions Prior to Event Re acto r 68% Power Circulator Speeds lA 6,700 RPM 1B 6,700 RPM 1C 6,400 RPM 1D 6,400 RPM Primary Coolant Flow 77%

Reactor Pressure 645 psia Average Circulator In-let Temp erature 666*F Average Co re Outlet Temp e ra ture 1,334*F Turb ine / Gene rato r 212 MW S team Pressure 2,400 psig Te mp e rat ure 1,000*F Flow 1,400,C00 pounds / hour ey } r n .sc, J.L tb] ruf .*

Sequence of Eventc Time Event The high selector (>) was being wired into I-36B when a wire was inadvertently shorted to ground. This interrupted AC power to I-36B and I-36A and resulted in a voltage dip on in-strument Bus 2 which is the AC power supply to I-36B and I-36A.

1522 55 55 The voltage dip on instrument Bus 2 caused a spurious trip of the Loop 1 steam generator penetration pressure high circuitry.

This initiated a Loop 1 shutdown which initiated 1A and 1B cir-culator steam turbine trips and a half-load turbine runback.

There was no steam water dump due to duration time of the pene-tration pressure high trip signal.

1522 56 26 The Loop 1 steam generator penetration high trip returned to no rmal.

1523 2 38 Reactor scrammed due to spurious hot reheat temperature high signals in scram channels B and C as observed by reactor opera-tor.

1523 2 38 Loop 2 circulator speeds decreased due to a decrease in cold reheat s team pressure. This decrease was due to loss of con-trol signal power to PV-2230 (Loop 2 main steam bypass pres-sure control valve) which prevented normal steam flow to the bypass flash tank during the turbine runback. The auxiliary boiler was in manual at a low flow rate which prevented use of steam from the 150 pound header supply. NOTE : At this time the turbine was in the runback mode, circulator speed valves were apparently closing due to :oss of control signal power, and circulator speed was decreasing as was feedwater flow.

This indicates that with ISS in POWER, Loop 2 circulator speed /

flow trip setpoints were being approache'd.

1523 9 21 1D circulator steam turbine trip from circulator speed /feedwater flow program.

1523 10 54 1C circulator steam turbine trip from circulator speed /feedwater flow program.

1523 10 54 Two loop trouble scram and turbine trip initiated by last and final circulator trip.

1523 36 Loop 2 feedwater flow isolated for less than 5 seconds. Loop 2 feedwater flow was isolated when the reactor operator closed FV-2206 (Loop 2 feedwater flow control valve). The reactor operator closed FV-2206 by selecting manual control on FC-2206.

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_ Sequence of Events (continued)

Time Event 1524 19 37 ISS hand switch placed in the low power position as indicated by alarm p rinter output (alarms retutning to normal status) .

1525 35 1C circulator speed manually increased on steam turbine to re-store primary coolant flow.

1525 40 1D circulator speed manually increased on steam turbine.

NOTE: Secondary coolant flow on Loop 2 decreased during this incident but feed-water temperature (TT-2206) indicates feedwater flow remained in service. Since PV-2230 (Loop 2 main s team bypass valve) failed to open, the Loop 2 main steam safety relief valves lif ted to vent pressure and maintain Loop 2 secondary flow.

Primary coolant flow was lost for 2 minutes and 26 seconds. The reactor opera-tor manually res tarted "Cf' circulator as noted above. No automatic water tur-bine s tart occurred because the ISS switch had been placed in low power position.

It should be noted that "C" circulator inlet helium temperatures exceeded 800*F for approximately 12 minutes. The general design limit for circulator inlet he-lium temperatures is 800*F.

_Pos t Trip Discussion The reactor operator noted white and green indicating lights lit on HV-2202 hand swit ch during this even t. This would indicate valve position disagreement with hand switch position. HV-2202 hand switch was in the automatic position and with-out a red indicating light, it would indicate HV-2202 was closed. No failure method can be determined which would cause HV-2202 to close; i.e., if hydraulic pressure is lost to HV-2204, operator, process pressure will open or hold open HV- 2202.

Due to the loss of Loop 2 feedwater flow indication on FR-2206, the reactor opera-tor concluded that Loop 2 feedwater had been isolated. In order to reestablish the apparent loss of feedwater flow in Loop 2, the operator manually ran FC-2206 to the lower limit. This action isolated flow to Loop 2 for less than 5 seconds.

Recommendation _s_

1. Power I-36A and I-36B from separate fuses. Also review o ther cabinets for similar problem.

2. Review instrument fusing for possible additional fuses to prevent total loss due to failure.

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Recommendations (con tinue d) *

3. Program data logger for a five second resolution on the post trip review.

4. Add circulator helium inlet (two loop) and a core outlet temperature indica-tion to the post trip review.

5. Remove boiler feed pump alarms on trip f rom ala rm p rinte r or ge t a mo re re-liable source (boiler feed pump overspeed) .

6. Enginee ring evaluate circulator inlet temperatures above design implications.

7. Position indication on the primary loop shutdown, circulator trip, and steam water dump valves placed on alarm printer as digital contacts readout:

HV-2201,11V-2202, HV-2203, HV-2204 FV-2205, FV-2206 HV-2223, HV-2224 HV-2215, HV-2216, HV-2217, HV-2218 HV-2249, HV-2250, HV-2251, HV-2252 HV-2241, HV-2242

8. Review circulator trip requirements ; i.e. , why let the last and final circu-lator trip on program feedwater/ speed trip when the plant is coming down around your ears if you still have bearing water and other ancilliries to the circulator.

9. Tune "C" boiler feed pump throttle p ressure con troller PIC-5253. See Frame 10.

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