Information Notice 1988-39, Loss of Recirculation Pumps with Power Oscillation Event
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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 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 operatihg procedures. Since the event, these recom- mendations 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.
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
June 15, 1988 LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
Information
Date of
Notice No.
Subject
Issuance
Issued to
88-38
88-37
88-36
88-35
88-34
Failure of Undervoltage
Trip Attachment on General
Electric Circuit Breakers
Flow Blockage of Cooling
Water to Safety System
Components
Possible Sudden Loss of RCS
Inventory During Low Coolant
Level Operation
6/15/88
6/14/88
6/8/88
All holders of OLs
or CPs for nuclear
power reactors.
All holders of OLs
or CPs for nuclear
power reactors.
All holders of OLs
All holders of OLs
or CPs for nuclear
power reactors.
All holders of OLs
or CPs for nuclear
power reactors.
Inadequate Licensee Performed 6/3/88
Vendor Audits
Nuclear Material Control
and Accountability of
Non-Fuel Special Nuclear
Material at Power Reactors
5/31/88
87-61,
Supplement 1
Failure of Westinghouse
W-2-Type Circuit Breaker
Cell Switches
5/31/88
All holders of OLs
or CPs for nuclear
power reactors.
88-33
Recent Problems Involving
the Model Spec 2-T
Radiographic Exposure
Device
Promptly Reporting to
NRC of Significant
Incidents Involving
Radioactive Material
5/27/88
5/25/88
All Agreement States
and NRC licensees
authorized to manu- facture, distribute
or operate radio- graphic exposure
devices and source
changers.
All NRC material
licensees.
88-32 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
PCWen
LEPhillips
06/10/88
06/10/88 dg!/88
- C/SRXB:DEST:NRR
MWHodges
06/10/88
- SAD/DEST:NRR
ACThadani
06/10/88
- PPMB:ARM
TechEd
06/10/88
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
PCWen ;VWX
061io/88 SR MB
. NRR
^ :EST:NRR
LEPhillips
i WHodges
06/ho/88 u 06/l a/88 SAD DEST RR
ACThadai5*"
06/ ¢/88 PPMB:ARM
TechEd
0611ol88