Information Notice 1988-39, Loss of Recirculation Pumps with Power Oscillation Event

Loss of Recirculation Pumps with Power Oscillation Event

ML031150269
Person / Time
Site:	Beaver Valley, Millstone, Hatch, Monticello, Calvert Cliffs, Dresden, Davis Besse, Peach Bottom, Browns Ferry, Salem, Oconee, Mcguire, Nine Mile Point, Palisades, Palo Verde, Perry, Indian Point, Fermi, Kewaunee, Catawba, Harris, Wolf Creek, Saint Lucie, Point Beach, Oyster Creek, Watts Bar, Hope Creek, Grand Gulf, Cooper, Sequoyah, Byron, Pilgrim, Arkansas Nuclear, Three Mile Island, Braidwood, Susquehanna, Summer, Prairie Island, Columbia, Seabrook, Brunswick, Surry, Limerick, North Anna, Turkey Point, River Bend, Vermont Yankee, Crystal River, Haddam Neck, Ginna, Diablo Canyon, Callaway, Vogtle, Waterford, Duane Arnold, Farley, Robinson, Clinton, South Texas, San Onofre, Cook, Comanche Peak, Yankee Rowe, Maine Yankee, Quad Cities, Humboldt Bay, La Crosse, Big Rock Point, Rancho Seco, Zion, Midland, Bellefonte, Fort Calhoun, FitzPatrick, McGuire, LaSalle, 05000000, Zimmer, Fort Saint Vrain, Shoreham, Satsop, Trojan, Atlantic Nuclear Power Plant
Issue date:	06/15/1988
From:	Rossi C Office of Nuclear Reactor Regulation
To:
References
IN-88-039, NUDOCS 8806130168
Download: ML031150269 (7)

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UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555 June 15, 1988 NRC INFORMATION NOTICE NO. 88-39: LASALLE UNIT 2 LOSS OF RECIRCULATION

PUMPS WITH POWER OSCILLATION EVENT

Addressees

All holders of operating licenses or construction permits for boiling-water

reactors (BWRs).

Purpose

This information notice is being provided to alert addressees to potential

problems resulting from the thermal hydraulic instability of BWR cores when

the plant is operating at certain unstable power/flow regions. It is expected

that recipients will review the information for applicability to their facili- ties and consider actions, as appropriate, to avoid similar problems. However, suggestions contained in this information notice do not constitute NRC require- ments; therefore, no specific action or written response is required.

Background:

BWR thermal hydraulic stability was the subject of Generic Issue B-19. Early

BWRs were designed to maintain decay ratios (DRs) of neutron flux perturbations

at less than 0.5 (DR = 1.0 corresponds to an undamped limit cycle oscillation).

Later core designs tended toward DR of 1.0 because of higher power density cores

and changing fuel design characteristics.

In the resolution of Generic Issue B-19, core designs that were potentially un- stable (DR = 1.0) under natural circulation operating conditions were approved, provided that operating procedures would ensure that neutron flux oscillations

indicative of core Instability would be readily detected and suppressed as re- quired by 10 CFR, Part 50, Appendix A, General Design Criterion 12, 'Suppression

of Reactor Power Oscillations." Licensees were informed by Generic Letter 86-02 that operating limitations must be implemented for new reload cores unless they

could demonstrate by approved calculational methods that the core was stable

throughout permissible operating regions of the power/flow map. A calculated

core DR of less than 0.8 using General Electric (GE) methods was approved as

acceptable evidence of core stability.

Description of Circumstances

On March 9, 1988, LaSalle Unit 2 underwent a dual recirculation pump trip event.

After the pump trip, the unit experienced an excessive neutron flux oscillation

8~06130168

IN 88-39 June 15, 1988 while it was on natural circulation. A detailed description of the event is

documented by the NRC augmented Inspection team (AIT) in Inspection Report

Nos. 50-373/88008 and 50-374/88008.

Before the event, the LaSalle Unit 2 reactor (a GE BWR-5) was operating at

steady-state conditions at approximately 84 percent power, with 76 percent

rated core flow using both recirculation pumps, and with the control rods

withdrawn to the 99 percent flow control line (FCL).

The initiating transient was caused by an instrument maintenance technician who

was performing a surveillance test of a wide range level instrument to check its

reactor core isolation cooling initiation function. During the performance of

this surveillance test, the technician opened a wrong valve. This produced a

high "indicated" level signal to the feedwater control system, and the response

eventually led to a trip of both recirculation pumps.

The trip of the recirculation pumps resulted in a flow decrease to natural

circulation while the control rods remained in the 99 percent FCL position.

The power-to-flow condition after the pump trip was known to be a susceptible

condition for instabilities in some BWRs. In addition, as a result of the

rapid power decrease, the feedwater heater level control system was unable

to control the level in the feedwater heaters and began isolating extraction

steam from the heaters. This resulted in a positive reactivity addition be- cause cooler feedwater was being supplied to the reactor. It, in turn, caused

an increase in power, further reducing the margin to instability.

Approximately 5 minutes after the recirculation pump trip, operators observed

that the average power range monitor (APRM) indications were oscillating be- tween 25 and 50 percent power (25 percent peak-to-peak) every 2 to 3 seconds, and the local power range monitor (LPRM) downscale alarms began to annunciate

and clec-. During this period, the operators recognized that they were in the

region of core instability. They attempted to restart a recirculation pump in

order to increase flow to prevent instability, but this action was unsuccessful

as all of the pump start permissive conditions had not been satisfied. Approxi- mately 7 minutes after the recirculation pump trip, as operators attempted to

restore forced flow (but failed again) and were preparing to perform a manual scram, the reactor automatically scrammed on APRM neutron high flux (118 percent

trip). The scram shut down the reactor and recovery from the scram proceeded

normally.

The review performed after the event, from the STARTREC trace (a high-speed

multi-channel recording system), revealed APRM peak-to-peak oscillations ranging

from 20 percent to about 75 percent power. Extrapolation of the traces to the

time of the scram indicated that the oscillations were at least 100 percent

peak-to-peak when the scram occurred. The staff estimated that the peaking

factor increased from 2.11 before the event to 2.65 at the time of the LPRM

alarm. The magnitude of oscillation and the shifting in power peaking were

unexpected on the basis of previous operating experience. Although the power

oscillations were larger than expected, no fuel thermal or mechanical limits

were exceeded during the event.

IN 88-39 June 15, 1988 Discussion:

The event indicated the following:

(1) Decay Ratio

The predicted DR for LaSalle licensing conditions was 0.60, yet limit

cycles were observed in this event. In response to NRC questions, GE

performed the thermal hydraulic stability analysis again on the basis

of the actual plant data recorded during the event. The analysis showed

a strong sensitivity to modeling and input conditions, especially

the

modeling of power distributions, and transient conditions of power/flow

and subcooling. This reanalysis result Indicates that the DR determined

by the licensing calculations Is not a reliable indicator that a

will be stable under all operating conditions during a fuel cycle.core

(2) Instrumentation

Instrumentation on many BWRs make detection and suppression of neutron

flux oscillations difficult.

(3) Operating Procedures

In 1984, GE Issued Service Information Letter (SIL) 380, Revision 1, taining recommendations regarding BWR core thermal hydraulic stability. con- This document discusses recirculation pump trip and recommends: "Imme- diately reduce power by Inserting control rods to or below the 80

percent

FCL using the plant's prescribed control rod shutdown Insertion-sequence."

Further, the SIL recommends that the operation of restarting recirculation

pumps should be performed below the 80 percent FCL. However, because

LaSalle-calculated DR was low and the perception of margin to Instability the

was substantial, these recommendations were not incorporated into

LaSalle abnormal operatihg procedures. Since the event, these recom- the

mendations have been incorporated. In addition, some Improvements

the existing procedures were made: to

High-worth "CRAM" rods have been designated for Immediate insertion

after recirculation pump(s) trip from power levels above the 80 per- cent FCL. Control rods in the CRAM array are preselected to achieve

a 10 percent reduction in the rod line while minimizing the effect

power distribution and future rod movement. In addition, APRM/LPRM on

signals will be monitored concurrently. The reactor is to be tripped

if instability is suspected.

CRAM rods are immediately inserted, followed by insertion of rods

in

sequence to get below the 80 percent FCL, if instability is indicated

while operating in a stability surveillance region with one or two

pumps. APRM/LPRM signals are concurrently monitored and the reactor

is scrammed if the instability has not been terminated within two

minutes.

IN 88-39 June 15, 1988 No specific action or written response is required by this information notice.

If you have any questions about this matter, please contact one of the techni- cal contacts listed below or the Regional Administrator of the appropriate

regional office.

Charles E. Rossi, Director

Division of Operational Events Assessment

Office of Nuclear Reactor Regulation

Technical Contacts: Laurence E. Phillips, NRR

(301) 492-3235 Peter C. Wen, NRR

(301) 492-1172 Attachment: List of Recently Issued NRC Information Notices

Attachment

IN 88-39 June 15, 1988 LIST OF RECENTLY ISSUED

NRC INFORMATION NOTICES

Information Date of

Notice No. Subject Issuance Issued to

88-38 Failure of Undervoltage 6/15/88 All holders of OLs

Trip Attachment on General or CPs for nuclear

Electric Circuit Breakers power reactors.

88-37 Flow Blockage of Cooling 6/14/88 All holders of OLs

Water to Safety System or CPs for nuclear

Components power reactors.

88-36 Possible Sudden Loss of RCS 6/8/88 All holders of OLs

Inventory During Low Coolant or CPs for PWRs.

Level Operation

88-35 Inadequate Licensee Performed 6/3/88 All holders of OLs

Vendor Audits or CPs for nuclear

power reactors.

88-34 Nuclear Material Control 5/31/88 All holders of OLs

and Accountability of or CPs for nuclear

Non-Fuel Special Nuclear power reactors.

Material at Power Reactors

87-61, Failure of Westinghouse 5/31/88 All holders of OLs

Supplement 1 W-2-Type Circuit Breaker or CPs for nuclear

Cell Switches power reactors.

88-33 Recent Problems Involving 5/27/88 All Agreement States

the Model Spec 2-T and NRC licensees

Radiographic Exposure authorized to manu- Device facture, distribute

or operate radio- graphic exposure

devices and source

changers.

88-32 Promptly Reporting to 5/25/88 All NRC material

NRC of Significant licensees.

Incidents Involving

Radioactive Material

OL = Operating License

CP = Construction Permit

IN 88-39 June 15, 1988 No specific action or written response is required by this information notice.

If you have any questions about this matter, please contact one of the techni- cal contacts listed below or the Regional Administrator of the appropriate

regional office.

Charles E. Rossi, Director

Division of Operational Events Assessment

Office of Nuclear Reactor Regulation

Technical Contacts: Laurence E. Phillips, NRR

(301) 492-3235 Peter C. Wen, NRR

(301) 492-1172 Attachment: List of Recently Issued NRC Information Notices

• SEE PREVIOUS PAGE FOR CONCURRENCES

• OGCB:DOEA:NRR *SRXB:DEST:NRR *C/SRXB:DEST:NRR *SAD/DEST:NRR *PPMB:ARM

PCWen LEPhillips MWHodges ACThadani TechEd

06/10/88 06/10/88 06/10/88 06/10/88 06/10/88 dg!/88

IN 88-XX

June xx, 1988 * CRAM rods are immediately inserted, followed by Insertion of rods in

sequence to get below the 80 percent FCL, if stability is indicated

while operating in a stability surveillance region (see Figure 1)

with one or two pumps. APRM/LPRM are concurrently monitored and the

reactor is scrammed If the Instability has not been terminated within

two minutes.

Several additional concerns with regard to TS adequacy and the validity of

previous safety analyses were reviewed by the NRC. The staff will consider the

need for an additional Generic Letter on this matter to strengthen the imple- mentation of core stability detection and suppression provisions for operating

BWRs.

No specific action or written response is required by this information notice.

If you have any questions about this matter, please contact one of the techni- cal contacts listed below or the Regional Administrator of the appropriate

regional office.

Charles E. Rossi, Director

Division of Operational Events Assessment

Office of Nuclear Reactor Regulation

Technical Contacts: Laurence E. Phillips, NRR

(301) 492-3235 Peter C. Wen, NRR

(301) 492-1172 Attachment: List of Recently Issued NRC Information Notices

OGCB:DOEA: RR SRMB . NRR ^ :EST:NRR SAD DEST RR PPMB:ARM

PCWen ;VWX LEPhillips i WHodges ACThadai5*" TechEd

061io/88 06/ho/88 u 06/l a/88 06/ ¢/88 0611ol88