United States Nuclear Regulatory Commission - Protecting People and the Environment

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
.                                                                 IN 88-39 
                                                                 June 15, 1988
                                                                 Page 2 of 4


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. 
.                                                                 IN 88-39
                                                                 June 15, 1988
                                                                 Page 3 of 4


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.
.                                                                 IN 88-39
                                                                 June 15, 1988
                                                                 Page 4 of 4


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 
Page Last Reviewed/Updated Tuesday, November 12, 2013