Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON D.C. 20555
July 11, 1994
NRC GENERIC LETTER 94-02: LONG-TERM SOLUTIONS AND UPGRADE OF INTERIM
OPERATING RECOMMENDATIONS FOR THERMAL-
HYDRAULIC INSTABILITIES IN BOILING WATER REACTORS
Addressees
All holders of operating licenses for boiling water reactor except Big Rock
Point.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this generic letter to
request that each addressee (1) take the appropriate actions to augment its
respective procedures and training for preventing or responding to
thermal-hydraulic instabilities in its reactors and (2) submit to the NRC a
plan describing the long-term stability solution option it has selected and
the implementation schedule it proposes for the modification of plant
protection systems to ensure compliance with General Design Criteria (GDC) 10
and 12 in Appendix A to Part 50 of Title 10 of the Code of Federal
Regulations (10 CFR Part 50).
Background
The possibility of power oscillations due to thermal-hydraulic instabilities
in boiling water reactors (BWRs) and the consequences of such events have been
of concern for many years. The staff evaluated thermal-hydraulic stability as
Generic Issue B-19 and recommended closure actions for resolution of that
issue in Generic Letter 86-02, "Long-Term Solutions to Thermal-Hydraulic
Instabilities in Boiling Water Reactors," which requested BWR licensees to
examine each core reload and to impose operating limitations, as appropriate,
to ensure compliance with GDC 10 and 12. GDC 10 requires that the reactor
core be designed with appropriate margin to assure that specified acceptable
fuel design limits will not be exceeded during any condition of normal
operation, including the effects of anticipated operational occurrences.
GDC 12 requires assurance that power oscillations which can result in
conditions exceeding specified acceptable fuel design limits are either not
possible or can be reliably and readily detected and suppressed.
On March 9, 1988, LaSalle Unit 2 experienced an instability event. The work
by both the staff and industry organizations following the event has provided
additional insight into thermal-hydraulic instabilities in BWR cores. The
LaSalle event is described in NRC Information Notice 88-39, "LaSalle Unit 2
Loss of Recirculation Pumps With Power Oscillation Event," dated June 15,
1988. NRC Bulletin 88-07, also dated June 15, 1988, highlighted the generic
concerns identified in light of the LaSalle event and requested all BWR
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July 11, 1994
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licensees, regardless of BWR type or analytical core stability margin, to
review the adequacy of procedures and instrumentation to respond to power
oscillations, and requested review of operator training programs with regard
to power oscillations. In response to these concerns, the BWR Owners' Group
(BWROG) initiated a project to investigate actions that should be taken to
resolve the BWR stability issue.
On October 28, 1988, the General Electric Company (GE) notified the NRC under
10 CFR Part 21 that thermal margins might not be sufficient to prevent
violation of the minimum critical power ratio safety limit for some BWR plants
if a 10-percent average power range monitor (APRM) oscillation was used as a
procedural action point for manual scram of the plant. Based on this
possibility, GE recommended stability "interim corrective actions" in a
November 1988 letter to BWR utilities. On December 30, 1988, the NRC issued
Bulletin 88-07, Supplement 1, approving the proposed BWROG/GE interim
operating recommendations and stating additional conditions. One of these
conditions addressed the applicability of the experience-based stability
exclusion boundaries defined in the interim operating recommendations, and
noted the need to reevaluate and justify these boundaries for cores that
include new fuel designs. This bulletin also discussed long-term corrective
actions. Such corrective actions might include hardware modifications or
additions to facilitate manual or automatic protective response to avoid
neutron flux oscillations or to suppress oscillations should they occur.
Since it is possible for some oscillations to grow to levels exceeding NRC
safety limits in the order of a minute, automatic protection action is
generally indicated. The detailed design specifications for the automatic
protection are being defined by an expanded post-LaSalle BWROG study to
develop a generic resolution to the stability issue.
In June 1991, the BWROG issued NEDO-31960 (Ref. 1) which documented proposed
long-term solutions to the stability issue as well as methodologies that have
been developed to support the design of these long-term solutions. Supple-
ment 1 to NEDO-31960 (Ref. 2) was issued in March 1992 and contained final
methodology details and additional information requested by the NRC. By a
July 1993 letter from A. C. Thadani (NRC) to L. A. England (BWROG), the NRC
transmitted its safety evaluation report (Ref. 3) on NEDO-31960 and its
Supplement 1 to the BWROG. Reference 3 describes the results of the staff
review of the proposed solution concepts and their associated methodologies.
This generic letter requests information about licensee plans for implementa-
tion of the approved solution concepts and actions taken in response to later
BWROG guidance and stability experience until the long-term solution is imple-
mented. This resolution assumes the protection system will function when
required and does not consider the combination of instability and anticipated
transients without scram (ATWS). That subject is being addressed by other NRC
and BWROG activities.
Need for Enhancement of Interim Operating Recommendations
In early 1992, the BWROG, citing its continuing studies, provided its members
additional guidance on implementation of the interim recommendations for
stability actions attached to NRC Bulletin 88-07, Supplement 1. In the 1992
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guidance, the BWROG emphasized the need for caution when operating near the
exclusion regions and recommended reexamination of procedures and training to
ensure that uncertainties in the definition of exclusion region boundaries
were appropriately reflected. The NRC staff considered this guidance in
conjunction with an Augmented Inspection Team (AIT) review of a Washington
Nuclear Power Unit 2 (WNP-2) 1992 instability event. The AIT report (Ref. 4)
discusses that review and the BWROG guidance.
On August 15, 1992, Washington Nuclear Power Unit 2 (WNP-2) experienced power
oscillations during startup. The WNP-2 operators recognized the oscillations
and responded promptly, consistent with their procedures and training, to
initiate a manual scram. The NRC evaluated this event, concluding that
the primary cause of the oscillations was very skewed radial and bottom peaked
axial power distributions due to insufficient procedural control of control
rod removal patterns during power ascension. It was concluded from
discussions with other licensees that similar procedural practices were not
unusual for some other BWRs. The skewed power distributions make the core
tend towards the "harder to detect" out-of-phase oscillation mode. The
WNP-2 power distribution was inconsistent with the more normal operating
conditions that have been associated with the experience-based stability
exclusion boundaries, and was also inconsistent with the power distribution
assumptions employed in the methodology for development of long-term solution
exclusion region boundaries based only on power and flow parameters. The
WNP-2 core design, consisting of a mixture of 9x9 and 8x8 fuel types which
caused unbalanced flow and pressure drop characteristics, was also a
contributor to uncertainty in its stability exclusion boundary. The WNP-2
event is described in NRC Information Notice 92-74, "Power Oscillations at
Washington Nuclear Power Unit 2," dated November 10, 1992.
Most of the BWROG long-term solutions proposed in NEDO-31960 (Ref. 1) involve
substantial modifications to the plant protection system hardware; these
modifications are not expected to be ready for implementation until 1995-1996.
The staff review of analytical studies in support of these solutions, the
additional guidance provided by the BWROG, and the circumstances leading to
the WNP-2 event, have identified the following items which are appropriate to
enhance the effectiveness of the interim procedures and training, implemented
in response to Bulletin 88-07, Supplement 1, until implementation of the
long-term solutions can be accomplished:
(1) Bulletin 88-07, Supplement 1, requested licensees of BWRs to take
actions including a procedural requirement for a manual scram under all
circumstances in which there are no recirculation pumps operating with the
reactor in the RUN mode. This action was not applicable to plants with
effective automatic scram protection against out-of-phase regional
oscillations. Bulletin 88-07, Supplement 1 indicated that a flow-biased APRM
scram circuit without a simulated thermal power monitor (filtered APRM
signals) would provide such protection. However, more recent analyses by the
BWROG show that the flow-biased APRM scram does not provide sufficient
protection for the out-of-phase mode of instability, which can produce very
large asymmetric oscillations before exceeding the average power scram
setpoint. The need for protection against .GL 94-02
July 11, 1994
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out-of-phase oscillations remains under review for a few small core
plants with small inlet orifices. Likewise, NRC plant-specific reviews
are incomplete for the BWR2 plants, Oyster Creek and Nine Mile Point 1,
to ensure the effectiveness of the quadrant based APRM scram protection
provided for these plants. Therefore, prior to completion of reviews of
the long-term solutions for individual plants, all plants are assumed to
have ineffective scram protection against out-of-phase oscillations and
should comply with the Bulletin 88-07, Supplement 1 requested procedural
requirement for manual scram.
(2)Bulletin 88-07, Supplement 1, endorsed the experience-based power/flow
boundaries of the interim corrective actions based on the assumption
that other factors important to the core stability characteristics
(e.g., radial and axial peaking, feedwater temperature, and thermal-hydraulic
compatibility of mixed fuel types) were consistent with previous experience
and the bounding values expected during normal operation. The BWROG studies
and the precautions recommended in the early 1992 letter to its members
indicated that uncertainties existed in the definition of these boundaries
(Ref. 4). The WNP-2 instability event and subsequent NRC evaluation
determined that some licensees may have given inadequate attention to the
impact on core stability of the reload core design and operating procedures
for changing reactor power. The WNP-2 experience also highlighted the value
of using on-line stability monitors as an operational aid to avoid unstable
operation; the capability for on-line stability monitoring does not exist
currently for most BWRs.
Requested Actions
1.All licensees of BWRs, except for Big Rock Point which does not have the
capability for operation under variable flow conditions, are requested
to review their current procedures and training programs and modify them
as appropriate to strengthen the administrative provisions intended to
avoid power oscillations or to detect and suppress them if they occur
prior to implementation of the long-term solutions. The experience
gained at WNP-2 should be a primary guide in this review. In doing
this, each licensee of a BWR (except for Big Rock Point) should:
a.Ensure that procedural requirements exist for initiation of a manual
scram under all operating conditions when all recirculation pumps
trip (or there are no pumps operating) with the reactor in the RUN
mode, and ensure that operators are aware of the potential for very
large power oscillations and the potential for exceeding core
thermal safety limits before automatic protection systems function
following the trip of all recirculation pumps (the procedural manual
scram is not necessary after long-term solutions are approved and
implemented for individual plants); and
b.Ensure that factors important to core stability characteristics
(e.g., radial and axial peaking, feedwater temperature, and thermal
hydraulic compatibility of mixed fuel types) are controlled within
appropriate limits consistent with the core design, power/flow .GL 94-02
July 11, 1994
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exclusion boundaries, and core monitoring capabilities of the
reactor in question, and that these factors are controlled through
procedures governing changes in reactor power, including startup and
shutdown, particularly at low-flow operating conditions. Each
licensee should review its procedures and determine if instability
can be avoided by these procedures and if the procedures can be
carried out using existing instrument information. If it is
concluded that a near-term upgrade of core monitoring capability is
called for to ease the burden on operators, determine the need to
incorporate on-line stability monitoring or monitors for stability
sensitive parameters and inform the NRC of the schedule and
technical evaluation for such upgrades found to be necessary.
(These procedural operation controls will no longer be necessary for
licensees which implement fully automatic long-term solutions, such
as Options III or IIIa of Reference 2. Licensees should propose for
plant-specific review the administrative controls to be retained in
conjunction with other long-term solutions.)
2.All licensees of BWRs, except for Big Rock Point, are requested to
develop and submit to the NRC a plan for long-term stability corrective
actions, including design specifications for any hardware modifications
or additions to facilitate manual or automatic protective response
needed to ensure that the plant is in compliance with General Design
Criteria 10 and 12. An acceptable plan could provide for implementing
one of the long-term stability solution options proposed by the BWROG
and approved by the NRC in Reference 3 or in subsequent documentation.
The plan should include a description of the action proposed and a
schedule of any submittal requiring plant-specific design review and
approval by the NRC and an installation schedule (if applicable). The
plan should also address the need for near-term and long-term technical
specification modifications. Generic BWROG documents or planned
submittal may be referenced in the plan.
Reporting Requirements
Pursuant to Section 182a of the Atomic Energy Act of 1954, as amended, and
10 CFR 50.54(f), each holder of an operating license for a BWR, except for Big
Rock Point, shall:
1.Within sixty (60) days of the date of this letter:
a.Inform the NRC, in writing and under oath or affirmation, of the
licensee's plans and status with respect to the actions requested in
this letter; and
b.If the licensee does not plan to take an action requested in this
letter, the reasons for not taking the action, a description of the
nature of any substitute action, and a schedule for completing or
implementing the substitute action;
2.If the licensee plans to take an action requested, or a substitute
action, within thirty (30) days of the completion of the action, inform.GL
94-02 July 11, 1994
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the NRC, in writing and under oath or affirmation, of the action taken
and verify its completion or implementation.
Each submittal shall be addressed to the U.S. Nuclear Regulatory Commission,
ATTN: Document Control Desk, Washington, DC 20555. A copy shall also be
submitted to the appropriate Regional Administrator.
This generic letter requires submittal of information that will enable the NRC
to verify that the licensee is complying with its current licensing basis
regarding GDC 10 and 12. Accordingly, an evaluation justifying this
information request is not necessary under 10 CFR 50.54(f).
References
1.NEDO-31960, "BWR Owners' Group Long-Term Stability Solutions Licensing
Methodology," June 1991.
2.NEDO-31960, Supplement 1, "BWR Owners' Group Long-Term Stability
Solutions Licensing Methodology," March 1992.
3.Letter from A. Thadani, NRC, to L. A. England, Chairman, BWR Owners'
Group, Acceptance for Referencing of Topical Reports NEDO-31960 and
NEDO-31960, Supplement 1, "BWR Owners' Group Long-Term Stability
Solutions Licensing Methodology," dated July 1993.
4.Letter from J. B. Martin, NRC, to A. L. Oxsen, Washington Public Power
Supply System, "NRC Augmented Inspection of Washington Nuclear Project,
Unit 2", September 29, 1992.
Paperwork Reduction Act Statement
The information collections contained in this request are covered by the
Office of Management and Budget clearance number 3150-0011, which expires
September 30, 1994. The public reporting burden for this collection of
information is estimated to average 300 hours per response, including the time
for reviewing instructions, searching existing data sources, gathering and
maintaining the data needed, and completing and reviewing the collection of
information. Send comments regarding this burden estimate or any other aspect
of this collection of information, including suggestions for reducing this
burden, to the Information and Records Management Branch, (T-6 F33),
U.S. Nuclear Regulatory Commission, Washington, D.C., 20555, and to the Desk
Officer, Office of Information and Regulatory Affairs, NEOB-10202,
(3150-0011), Office of Management and Budget, Washington D.C. 20503.
Compliance with the following request for information is voluntary. The
information would assist the NRC in evaluating the cost of complying with this
generic letter.
(1)the licensee staff time and costs to perform requested procedure
reviews and implementation of changes;
.GL 94-02
July 11, 1994
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(2)the licensee staff time and costs to prepare the requested reports and
documentation;
(3)the additional short-term costs incurred to address the changes, such as
the costs of the corrective actions or the costs of down time; and
(4)an estimate of the additional long-term costs that will be incurred as a
result of implementation commitments.
Backfit Discussion
This generic letter defines the requested actions and reporting requirements
for all holders of operating licenses for BWRs, except for Big Rock Point, in
order to enhance the current interim operating recommendations and to provide
a long-term solution to the issue of thermal-hydraulic instabilities in BWRs.
The staff has concluded that these requested actions are a backfit that is
necessary to ensure compliance with GDC 10 and 12. The basis for the
determination is stated in the preceding discussion of the generic letter
(Background section). Accordingly, pursuant to 10 CFR 50.109(a)(4)(i), a
backfit analysis is not required.
If you have any questions about this matter, please contact the technical
contact listed below or the appropriate NRR project manager.
/S/'D BY RP ZIMMERMAN
Roy P. Zimmerman
Associate Director for Projects
Office of Nuclear Reactor Regulation
Technical contact: Larry Phillips, NRR
(301) 504-3232
Attachment:
List of Recently Issued NRC Generic Letters
