Bulletin 88-07: Power Oscillations in Boiling Water Reactors (BWRs)

                                                       OMB No.: 3150-0011
                                                       NRCB 88-07

                                  UNITED STATES
                          NUCLEAR REGULATORY COMMISSION
                             WASHINGTON, D.C.  20555

                                  June 15, 1988



All holders of operating licenses or construction permits for boiling water 
reactors (BWRs).


The purpose of this bulletin is to request that holders of operating licenses 
or construction permits for boiling water reactors ensure that adequate 
operating procedures and instrumentation are available and adequate operator 
training is provided to prevent the occurrence of uncontrolled power 
oscillations during all modes of BWR operation.  

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 while it was on natural circulation.  The event is described in 
NRC Information Notice No. 88-39, "LaSalle Unit 2 Loss of Recirculation Pumps 
With Power Oscillation Event" dated June 15, 1988.  Additional details of the 
event have been documented by the NRC augmented inspection team (AIT) in In-
spection Report Nos. 50-373/88008 and 50-374/88008.  

After investigation of the event by the licensee and further review by the 
NRC, the staff concluded that this event has significant generic safety 
implications because (1) past licensing calculations are not a reliable 
indicator that a core will be stable under all operating conditions during a 
fuel cycle; (2) instrumentation for detection and suppression of neutron flux 
oscillations and recording instrumentation for evaluation of limit cycle flux 
oscillations may not be adequate; (3) operating procedures and training may 
not be adequate to respond to this kind of transient; (4) the magnitude and 
power peaking of the resultant neu-tron flux oscillations may be greater than 
previously experienced for in-phase limit cycle oscillations during special 
stability tests and foreign operating reactor events; and (5) there is a 
question of compliance with General Design Criterion 12, "Suppression of 
Reactor Power Oscillations," 10 CFR Part 50, Ap-pendix A. 

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                                                                 June 15, 1988
                                                                 Page 2 of 3

Foreign reactor experience has indicated that there are conditions other than 
the natural circulation condition that existed during the LaSalle event, such 
as loss of feedwater heaters, which can lead to power oscillations.  Further-
more, the power oscillations may be asymmetric and lead to high local neutron 
flux levels without detection and automatic scram.  Therefore, power 
oscillations are of safety concern because they may result in fuel damage. 

Some characteristics of the LaSalle neutron flux oscillation have led to con-
cerns about the applicability of previous safety analyses.  The magnitude of 
in-phase limit cycle oscillations previously observed on the APRMs during 
special stability tests and operating reactor events were typically in the 
range of 5% to 15% (peak-to-peak) of rated power, and as high as 25%.  This 
compares to peak-to-peak values of about 100% at the time of the 118% neutron 
flux trip for LaSalle.  The implications, if any, on the bounds for neutron 
flux oscillations considered in previous safety analyses, including peak power
resulting from asymmetric oscillations and the effects of recirculation pump 
trip actions for ATWS events, require further study.  The NRC will pursue 
these issues with the BWR Owners Group in parallel with utilities taking the 
actions specified below.

Requested Actions:

Operating Reactors: 

1)   Within 15 days of receipt of this bulletin, all BWR licensees should 
     ensure that any licensed reactor operator or Shift Technical Advisor 
     performing shift duties has been thoroughly briefed regarding the March 
     9, 1988 LaSalle Unit 2 event.

2)   Within 60 days of receipt of this bulletin all BWR licensees should 
     verify the adequacy of their procedures and operator training programs to 
     ensure that all licensed operators and Shift Technical Advisors are 
     cognizant of: 
     a)   those plant conditions which may result in the initiation of un-
          controlled power oscillations 
     b)   actions which can be taken to avoid plant conditions which may 
          result in the initiation of uncontrolled power oscillations 
     c)   how to recognize the onset of uncontrolled power oscillations, and 
     d)   actions which can be taken in response to uncontrolled power oscil-
          lations, including the need to scram the reactor if oscillations 
          are not promptly terminated.

Addressees should also verify the adequacy of the instrumentation which is 
relied upon by operators within their procedures. 

It is expected that all BWRs will have procedures and operator training 
programs to address uncontrolled power oscillations regardless of calculated 
decay ratios. 

.                                                                 NRCB 88-07
                                                                 June 15, 1988
                                                                 Page 3 of 3

Construction Permit Holders:

Prior to scheduled fuel load date, construction permit holders should complete 
actions 1 and 2, above.  

Reporting Requirements:

Within 30 days of completion of the actions requested above, all holders of 
operating licenses and construction permits shall confirm by letter that the 
requested actions have been completed and implemented.  Licensees and permit 
holders shall document and maintain at the plant site for a period of at least
two years an evaluation of the adequacy of their procedures, training programs 
and the instrumentation relied upon within their procedures.  

The written reports required above shall be addressed to the U. S. Nuclear 
Regulatory Commission, ATTN:  Document Control Desk, Washington, D.C.  20555, 
under oath or affirmation under the provisions of Section 182a, Atomic Energy 
Act of 1954, as amended.  In addition, a copy shall be submitted to the appro-
priate Regional Administrator.  

This requirement for information was approved by the Office of Management and 
Budget under blanket clearance number 3150-0011.  Comments on burden and 
duplications should be directed to the Office of Management and Budget, 
Reports Management, Room 3208, New Executive Office Building, Washington, D.C. 

If you have any questions about this matter, please contact one of the 
technical 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:  L. Phillips, NRR
                     (301) 492-3235

                     P. C. Wen, NRR
                     (301) 492-1172

Attachment:  List of Recently Issued NRC Bulletins

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