Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
OMB No.: 3150-0011
NRCB 88-07, Supplement 1
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
December 30, 1988
NRC BULLETIN NO. 88-07, SUPPLEMENT 1: POWER OSCILLATIONS IN BOILING WATER
REACTORS (BWRs)
Addressees:
All holders of operating licenses (OLs) or construction permits (CPs) for
boiling water reactors (BWRs).
Purpose:
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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. NRCB 88-07, Supplement 1
December 30, 1988
Page 2 of 4
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
supplement.
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,
. NRCB 88-07, Supplement 1
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.
. NRCB 88-07, Supplement 1
December 30, 1988
Page 4 of 4
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
Attachments:
1. "Interim Recommendations for Stability Actions," GE, November 1988.
2. List of Recently Issued NRC Bulletins
.
