Information Notice No. 88-39: Lasalle Unit 2 Loss of Recirculation Pumps with Power Oscillation Event
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
June 15, 1988
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 requirements; 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
unstable (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 required 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
8806130168
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June 15, 1988
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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 clear. 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. Approximately 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.
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June 15, 1988
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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 core
will be stable under all operating conditions during a fuel cycle.
(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, con-
taining recommendations regarding BWR core thermal hydraulic stability.
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 the LaSalle-calculated DR was low and the
perception of margin to instability was substantial, these
recommendations were not incorporated into the LaSalle abnormal operating
procedures. Since the event, these recommendations have been
incorporated. In addition, some improvements to the existing procedures
were made:
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
on power distribution and future rod movement. In addition,
APRM/LPRM 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.
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June 15, 1988
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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
Page 1 of 1
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
the Model Spec 2-T States and NRC
Radiographic Exposure licensees
Device authorized to
manufacture,
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
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