ML20006E229

LER 89-018-01:on 891008,HPCI High Steam Flow Signal Closed HPCI Outboard Steam Supply Isolation Valves,Causing Isolation of Logic Circuit.Caused by Overly Conservative Value for Flow Isolation Signal.Sensor vented.W/900208 Ltr

ML20006E229
Person / Time
Site:	FitzPatrick
Issue date:	02/08/1990
From:	Fernandez W, Fish H POWER AUTHORITY OF THE STATE OF NEW YORK (NEW YORK
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
JAFP-90-0117, LER-89-018, NUDOCS 9002220344
Download: ML20006E229 (7)

Text

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? James A.Pitupetrick t

94 .j p* e Nuolest Power Plant .

" P.O. Box 41 p, Lycoming, New York 13093 ..

316 342-3640 E William Femander 11 -

Resident Manager F

L' February 8,1990' JAFP-90-0117

. j United States Nuclear Regulatory Commission L' DocumentsControl Desk -t E . Mail Station PI-137 Washington, D.C. 20555 l i

SUBJECT:

DOCKET NO. 50'-333 j 89-018-01 LICENSEE EVENT REPORT:

High Pressure Coolant Injection i Turbine .

Dear Siri.

This-is a revision to the' Licensee Event Report which was t submitted in accordance with 10 CFR 50.73(a)(2)(iv) on u

November 30, 1989.

-This revision reflects changes to the cause and corrective actions resulting from the knowledge gained from-subsequent extensive testing;of the HPCI system in December 1989

.(LER-89-025).  !

c Questions concerning this report may be addressed to-Mr. Hamilton Fish at (315) 349-6013.

.Very'truly yours, IL FERN IZ WF:HCF:lar I

. Enclosure ec: USNRC, Region I (l-USNRC Resident Inspector INPO Records Center 3 // ikp  !

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Update Report - Original Report Date 11/06/89 - Change to Cause and Corrective Action A routine surveillance test of the High Pressure Coolant Injection (HPCI) [BJ) system l

was in progress on 10/08/89 at 14 percent power during start-up after a planned

! three-week maintenance outage. At 10:26 A.M., a HPCI high steam flow signal closed the HPCI outboard steam supply isolation valves. Operators verified the absence of steam leakage. The surveillance tests required to be performed when HPCI is

. inoperable were initiated.

( Inspection of the differential pressure transmitter, which provides the high steam flow signal, found the calibration was accurate. During recalibration a small quantity of air.was observed to vent from the pressure instrument sensing lines.

Initially, it was incorrectly believed that presence of non-condensible, but compressible air in the sensing lines, combined with the fast start transient, resulted in oscillations and a false high steam flow signal. Subsequently I I

l (LER-89-025), it was dircovered that the high steam flow signal was valid and caused by use of a more conservative test procedure and overly conservative Technical Specification high steam flow and FSAR activation time limits.

1 Upon satisfactory performance of the surveillance test, HPCI was returned to service at 6:30 P M the same day.

LER-89-025 and LER-89-002 are related.

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Revised. February- 8, 1990 to reflect changes to cause and corrective actions. Previous report date 11/06/89.

Description j On October 8, 1989 the plant was engaged in power ascension and had achieved a level of 14 percent of full power following start-up from a '

planned three-week maintenance outage. Performance of Surveillance '

. Test ST-4N. "HPCI Flow Rate and In-Service Test (IST)" for the High-Pressure Coolant Injection (HPCI) system [BJ) was in progress. Tais test is performed at low pressure during start-up and agzin at normal operating pressure in accordance with Technical Specifications and Section XI of the ASME code. The purpose is to demonstrate pump and turbine-flow capacity, cycle isolation valves, and collect data for the '

IST: program. '

At 10:26 A.M. during HPCI turbine start-up, the "High Steam Flow" ,

annunciator alarmed and outboard steam supply isolation valve 23MOV-16 '

and steam line warming isolation valve 23MOV-60 automatically closed to isolate the HPCI steam line.

Operators immediately inspected the HPCI steam line and turbine' areas._ .

There was no evidence of steam leakage. HPCI was-declared inoperable, '

, Required HPCI inoperability surveillance testin; of alternate systems was initiated. Instrumentation and Control technicians checked the high flow instrument _ system calibration which was satisfactory. A small quantity of air was observed to vent from the sensing line during recalibration. The cause for the high steam flow signal was initially and incorrectly believed to have been the presence of this air in the sensingtline to the steam flow differential pressure transmitters.

The HPCILsurveillance test was then performed successfully. HPCI was restored to service at-6:30 P.M.- A subsequent high steam flow isolation of HPCI on November 30, 1989 (LER-89-025) led-to extensive testing over a period of'several weeks which established other root

-causes.

Cause The cause of the automatic isolation was the initiation of the HPCI steam line high flow isolation logic circuit. Air in the sensing system leading to potentially unstable oscillation of the differential pressure signal was initially and incorrectly believed to be the cause offthe high steam flow signal which isolated the HPCI turbine. The function of this circuit is to stop steam flow from the reactor in the event of-a steam line break outside the primary containment, g,, o.. u.,

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When steam flows to HPCI, a measurable differential pressure is created between the instrument taps on the inner and outer radii of the elbow

'in the steam supply piping system inside the primary containment.

Combined differential pressure sensors and transmitters send signals o to a master trip unit (MTU). The signal strength is proportional to the magnitude of the differential pressure. The trip point is adjusted to a differential pressure signal strength which is equivalent to a steam flow of 300 percent of normal full load flow.

The pressure sensing line to the transmitter is filled with liquid to provide a relatively incompressible transmission medium. During the inspection and recalibration of the sensor transmitter (following the isolation) a small quantity of air was observed to vent from the liquid-filled sensing line at the transmitter. The presence of compressible air pockets in the small diameter sensing line or pressure chambers may have created a condition conducive to unstable compression and oscillation in the sensing line during periods of rapid change in the steam flow which exist during fast start of the HPCI turbine. A '

similar isolation of the Reactor Core Isolation Cooling (RCIC) [BN) .i system occurred in 1986 (LER-86-015). The source of air entry for that event was the previous-replacement of a transmitter unit followed by inadequate venting. However, the transmitter for the HPCI system had not been opened for testing or corrective maintenance for approximately a year. On November 30, 1989 the HPCI system was again isolated by a

-high steam flow signal during surveillance testing (LER-89-025).

Following this isolation extensive testing and research of documentation established that the isolation signal was electrically-correct but the setpoint, limits, and assumptions upon which the setpoint was based were overly conservative. The HPCI system was able to be operated within this conservatism for 14 years until several changes were made within the HPCI system including test methodology, new hydraulic actuator,_and correction of a construction error in wiring of the turbine stop valve (see LER-89-002). These changes, combined with an unnecessarily conservative design basis for response L time,.resulted in measured transient steam flows occasionally exceeding l the existing isolation setpoint. There was also a failure to fully appreciate the integrated nature of the control system which led to a failure to sufficiently test and analyze HPCI transients following procedural changes and component replacement.

A summary of these four causes follows:

1

1. Overly conservative value for high steam flow isolation signal:

Although the FSAR Section 7.4.3.2.7 uses an analytical limit for determination of a break in the HPCI steam line of 300% rated flow, testo have shown that the Technical Specification setpoint isolates at a differential pressure equivalent to only 200% of rated flow.

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2. Change to make test procedure more conservative For fourteen years HPCI had been tested by fully opening the -

discharge test valve (23MOV-21) to the CST prior to starting the turbine. In March 1989, acting on INPO recommendations, the i discharge test valve was manually pre-positioned to simulate the discharge head against reactor pressure during the pump speed ramp rather than first opening the valve and then closing down on it to obtain the required discharge pressure. In addition, a requirement to measure the HPCI response time from initiation to full rated flow against an acceptance criteria of 25 seconds was added to the procedure. The result of these changes was a demand for higher initial steam flow to provide the energy required for higher initial discharge pressures and to meet the response time criteria.

3. Overly conservative FSAR design basis for HPC1 response time:

The FSAR actuation time for HPCI was defined as 25 seconds from receipt of reactor vessel low low water level or high drywell ,

pressure signals to achievement of rated flow. This requirement i was in the original vendor design specification. More recent plant specific analysis for 10CFR50.46, Appendix K, and based on vendor SAFER /GESTR LOCA application methodology, redefines HPCI response time parameters. The assumed value in the analysis is 30 seconds. The current fuel reload analysis also assumes this value. Other documentation clearly defines this as the time to achieve rated flow and does not require that the discharge valve be in the full open position for HPCI to be considered to be fully actuated. The result of the 25 second requirement was a higher initial steam flow demand and resulting potential for system isolation.

4. Failure to appreciate the integrated nature of the HPCI control system:

This led to the associated failure to test and analyze the test start-up transient stability in sufficient depth following changes to the test procedure, adjustment of cemponent controls, and replacement of individual components in the control system.

) Adjustment to a single et st r o l c o m p o n et.*. may require readjustment i of all other interfacing controls. An integrated system test is required to establish and readjust each enterfacing control system  !

component in the start-up and speed control system following i significant maintenance.

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in sufficient volume to maintain core coverage through a broad spectrum of hypothetical accident conditions. The principal component is a turbine-driven high pressure, high volume multi-stage centrifugal pump.  ;

The steam supply to the turbine comes directly from the reactor vessel  !

thus ensuring availability regardless of the condition of AC electric power supplies.

Because the HPCI system was inoperable due to an isolation signal, it i qualifies as an event reportable under 10 CFR 50.73(a)(2)(v) as a ,

condition that alone could have prevented the fulfillment of the safety function of a system needed to remove residual heat or mitigate the consequences of an accident. It is also reportable under 10 CFR 50,73(a)(2)(iv) as an activation of an engineered safety feature ,

for isolation.  ;

Surveillance tests of back-up emergency core cooling systems were successfully completed or in progress during the eight-hour investigation of possible causes of the isolation.

If the HPCI system had continued to be unavailable, core coverage would still have been assured by the automatic depressurization system together with low pressure emergency core cooling systems including the two core spray systems [BM) and four residual heat removal (Lew Pressure Coolant Injection) systems [B0].

Furthermore, the HPCI system remained operable ct the manual mode at '

all times. The SAFER /GESTER LOCA sensitivity analysis shows that the HPCI response time has little effect on peak clad temperature.  !

Corrective Action:

Short-Term: j The aressure sensor and transmitter units were immediately vented and calibrated. The system was restored to service within eight hours of the isolation event.

Following the extensive testing of the HPCI system (LER-89-025) and determination of the true root causes of the HFCI high steam flow isolation signal, the following corrective actions were accomplished:

1. The HPCI turbine start-up speed control ramp speed was adjusted to a new recommended value of 15 seconds.

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2. Technical Specification Tabic 3.2-2, Item 13 was amended to reflect a maximum mermitted high steam flow differential pressure e setting of 160 inches of water.

3. The HPCI steam supply line high steam flow differential pressure instrumentation setpoint was changed to 148 inches of water which correctly corresponds to approximately 300% of rated steam flow. i

4. Surveillance Test 4N, "HPCI Flow Rate and Inservice Test (IST)"

and 4B, "HPCI pump and MOV Operability Test" were revised to reflect the 30 second limit on response time for HPCI and closer control in determination of that response time.  :

5. The FSAR was revised to reflect a 30 second response time for HPCI i as the design basis.

Long-Term: *

1. A detailed engineering review of the complete HPCI system and its operating history will be performed. Further corrective actions may be recommended by the task force assigned to this review.

2. Existing surveillance procedures will be revised or new procedures will be developed to monitor and trend the HPCI start-up transient performance stability including peak flow measurements. This will facilitate identification of potential 1 degrading of the HPCI control system prior to entering a condition where the HPCI system would not fulfill its design requirement.

Additional Information This supplement revises the original LER report of November 30, 1989 to reflect the knowledge gained during the extensive testing of HPCI in December 1989 (LER-89-025). This supplement revises the causes and corrective actions.

LER-89-002 HPCI wiring error between steam stop valve and speed control ramp generator LER-89-025 HPCI isolated due to high steam flow

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