Bulletin 88-07: Supplement 1: Power Oscillations in Boiling Water Reactors (BWRs)

                                                  OMB No.:  3150-0011
                                                  NRCB 88-07, Supplement 1

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

                                December 30, 1988

                                       REACTORS (BWRs)


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


The purpose of this supplement is (1) to provide additional information con-
cerning power oscillations in BWRs and (2) to request that addressees take 
action to ensure that the safety limit for the plant minimum critical power 
ratio (MCPR) is not violated.

Description of Circumstances:

Licensees were requested in NRC Bulletin No. 88-07 to take actions to prevent 
the occurrence of uncontrolled power oscillations during all modes of BWR 
operation.  In addition, as mentioned in the bulletin, the NRC, as a separate 
action, requested that the BWR Owners Group (BWROG) perform generic 
evaluations of the BWR plant response to core thermal hydraulic instabilities.
The preliminary results of these evaluations and the NRC's ongoing independent 
study indicate that when regional power oscillations become detectable on the 
average power range monitors (APRMs), the MCPR safety margin may be 
insufficient under some operating conditions to ensure that manual actions 
taken in response to APRM signals would prevent violation of the MCPR safety 
limit.  In addition, design features of many reactor protection systems (e.g., 
filtered APRM signals to provide a Simulated Thermal Power Monitor in the 
power/flow scram circuit) render them ineffective for automatic scram 
protection during regional thermal hydraulic instabilities.

In November 1988, General Electric Company (GE) issued a letter entitled 
"Interim Recommendations for Stability Actions" to the BWR licensees.  The 
interim corrective actions recommended by GE were reviewed by the NRC and have 
been found acceptable for those plants which have effective automatic scram 
protection for regional oscillations.  For plants which do not have effective 

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                                                  December 30, 1988 
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automatic scram (such as flow biased scram) protection for regional oscilla-
tions, the staff believes that the procedures recommended by GE and adopted by 
the BWROG may not provide sufficient, reliable protection.  Therefore, the 
staff is requesting that plants without effective automatic scram protection 
for regional oscillations manually trip the reactor in response to every 
situation in which two recirculation pumps are tripped with the reactor in the 
RUN mode.  In addition, the BWROG recommendations were addressed to GE-fueled 
reactors.  The staff is addressing the applicability of these recommendations 
to reactors with fuel supplied by other vendors and for new fuel designs which 
have not been previously approved.  The BWROG recommendations and provisions 
of this supplement are not applicable to Big Rock Point (Docket No. 50-155) 
because of unique design features involving a lack of flow control capability, 
and because existing operating limitations enforced by technical 
specifications address the stability concerns which are the subject of this 

The NRC is currently working with the BWROG to develop a generic approach to 
long-term corrective actions.  The BWROG is planning to evaluate hardware 
modifications and/or additions and complementary procedure revisions needed to 
facilitate stability monitoring and to improve the manual and/or automatic 
protective actions as needed to avoid neutron flux oscillations and to 
suppress them if they occur.  The BWROG study is expected to be completed 
within 6 months of the issuance of this supplement and to result in generic 
recommendations.  The BWROG has indicated that these recommendations will be 
transmitted to addressees in parallel with the NRC review in order to 
facilitate development of plant-specific solutions within 12 months.  The 
staff expects to issue another generic communication that will provide 
guidance for long-term resolution of the stability issue.

Actions Requested:

Operating Reactors:

(1)  Within 30 days of receipt of this supplement, all BWR licensees should 
     implement the GE interim stability recommendations described in Attach-
     ment 1.  However, for those plants that do not have effective automatic 
     scram protection in the event of regional oscillations, a manual scram 
     should be initiated under all operating conditions when two recirculation 
     pumps trip (or "no pumps operating") with the reactor in the RUN mode.  
(2)  The boundaries of Regions A, B, and C shown in Figure 1 of the GE recom-
     mendations (Attachment 1) were derived for those BWRs using NRC approved 
     GE fuel.  For BWRs using fuel supplied by other vendors, these regions 
     should be adopted in principle, but the power/flow boundaries should be 
     based on existing boundaries that have been previously approved by the 
     NRC.  For proposed new fuel designs, the stability boundaries should be 
     reevaluated and justified based on any applicable operating experience, 
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                                                  December 30, 1988 
                                                  Page 3 of 4 

     calculated changes in core decay ratio using NRC approved methodology, 
     and/or core decay ratio measurements.  There should be a high degree of 
     assurance that instabilities will not occur under any circumstances of 
     operation in Region C.

(3)  The BWROG recommendations of Attachment 1 are ambiguous with respect to 
     permissible conditions for entry of Regions B and C.  In addition to 
     entry of these regions for purposes of fuel conditioning during rod 
     withdrawal startup operations, intentional entry is allowable in 
     situations where rod insertion or a flow increase is required by 
     procedures to exit Regions A and B after unintentional entry.  Licensees 
     should ensure that the procedures and training employed for 
     implementation of these recommendations avoid any similar ambiguity which 
     could lead to operator confusion.

Construction Permit Holders:

All construction permit holders should complete the requested actions above 
before the date scheduled for fuel loading.

Reporting Requirements:

Within 60 days of receipt of this supplement, pursuant to 10 CFR 50.54(f), all 
holders of OLs shall advise the NRC by letter whether the requested actions 
have been completed and implemented.  Prior to fuel loading, CP holders shall 
advise the NRC by letter that the requested actions have been completed and 
implemented.  The staff will evaluate these responses to determine whether any 
additional action by the staff is necessary.

The required written reports 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 of the report shall be submitted to the 
appropriate Regional Administrator.

This request is covered by Office of Management and Budget Clearance Number 
3150-0011 which expires December 31, 1989.  The estimated average burden hours 
is approximately 100 to 200 person-hours per response, including assessment of 
the new recommendations, searching data sources, gathering and analyzing the 
data, and preparing the required reports.  These estimated average burden 
hours pertain only to these identified response-related matters.  Comments on 
the accuracy of this estimate and suggestions to reduce the burden may be 
directed to the Office of Management and Budget, Room 3208, New Executive 
Office Building, Washington, D.C.  20503, and to the U.S. Nuclear Regulatory 
Commission, Records and Reports Management Branch, Office of Administration 
and Resource Management, Washington, D.C.  20555. 

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                                                  December 30, 1988
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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:  L. Phillips, NRR
                     (301) 492-3235

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

1.  "Interim Recommendations for Stability Actions," GE, November 1988.
2.  List of Recently Issued NRC Bulletins

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