Implementation of TMI Action ITEM II.K.3.5, "Automatic Trip of Reactor Coolant Pumps" (Generic Letter No. 85-12)
UNITED STATES
NUCLEAR REGULATORY COMMISSION
WASHINGTON, D. C. 20555
June 28, 1985
TO ALL APPLICANTS AND LICENSEES WITH WESTINGHOUSE (W) DESIGNED NUCLEAR STEAM
SUPPLY SYSTEMS (NSSSs)
SUBJECT: IMPLEMENTATION OF TMI ACTION ITEM II.K.3.5, "AUTOMATIC TRIP OF
REACTOR COOLANT PUMPS" (GENERIC LETTER NO. 85-12 )
Gentlemen:
The purpose of this letter is to inform you of (1) the staff's conclusions
regarding the Westinghouse Owners Group (WOG) submittals on reactor coolant
pump trip in response to Generic Letters 83-10c and d, and (2) provide
guidance concerning implementation of the reactor coolant pump trip
criteria. Our Safety Evaluation (SE) on this subject is enclosed for your
use.
With regard to the WOG submittals referenced in Section V of the enclosed
SE, we conclude that the methods employed by the WOG to justify manual
reactor coolant pump (RCP) trip are consistent with the guidelines and
criteria provided in Generic Letters 83-10c and d. The approved Westinghouse
Small Break LOCA Evaluation Model was used to demonstrate compliance with 10
CFR 50.46 and Appendix K to 10 CFR Part 50.
We have determined that the information provided by the WOG in support, of
the alternative RGP trip criteria is acceptable on a generic basis. A
suitable reactor coolant pump trip criterion can be selected by each
licensee to minimize reactor coolant pump trip during steam generator tube
ruptures and non-LOCA events, while still providing for RCP trip for small
break LOCAs.
With regard to implementation, we note that the WOG RCP trip methodology
allows applicants/licensees to select among three alternate RCP trip
criteria. The selection is based upon obtaining maximum discrimination
between a small break LOCA (which requires RCP trip) and a steam generator
tube rupture (which does not require RCP trip). Tn reviewing the WOG RCP
trip criteria, we note that the process of criterion selection involves a
number of considerations which were assigned plant-specific status by the
WOG during the process of the trip criteria review.
Accordingly, we request that operating reactor licensees select and
implement an appropriate RCP trip criterion based upon the WOG methodology.
Schedules for submittal of information requested in Section IV of the SE
(refer to Appendix A for considerations associated with Generic Letters
83-10c and d) should be developed with your individual project managers
within 45 days from receipt of this letter. The requested information does
not constitute a new
CONTACT: D. Jaffe X28140
8507010252
- 2 -
requirement but only identifies information specified in Generic Letters
83-10c and d which has not been provided under the WOG generic program. In
the event that licensees decide not to trip the RCP (an option provided for
in Generic Letters 83-10c and d), they should respond to the questions in
Section IV of the SE and refer to Appendix B of the SE. Applicants should
provide the appropriate response to the extent that this information is
known at this time.
Those applicants and licensees who choose not to endorse the WOG methodology
should submit a schedule for submittal of plant specific RCP trip criteria
or Justification for non-trip of RCPs within 45 days of receipt of this
letter.
This request for information was approved by the Office of Management and
Budget under clearance number 3150-0065 which expires September 30, 1985.
Comments on burden and duplication may be directed to the Office of
Management and Budget, Reports Management, Room 3208, New Executive Office
Building, Washington, D.C. 20503.
If you believe further clarification regarding this issue is necessary or
desirable, please contact Mr. D. Jaffe 301 492-8140).
Sincerely,
Hugh L. Thompson, Jr., Director
Division of Licensing
Enclosure:
Safety Evaluation
cc w/enclosure:
Service Lists
UNITED STATES
NUCLEAR REGULATORY COMMISSION
WASHINGTON, D. C. 20556
SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION
FOR THE
WESTINGHOUSE OWNERS GROUP
REACTOR COOLANT PUMP TRIP
I. INTRODUCTION
TMI Action Plan Item II.K.3.5 of NUREG-0737 required all licensees to
consider other solutions to the small-break loss-of-coolant-accident
(LOCA) problems because tripping the reactor coolant pumps (RCPs) was
not considered the ideal solution. Automatic trip of the RCPs in the
case of a small-break LOCA was recommended until a better solution was
found. A summary of both the industry programs and the NRC programs
concerning RCP trip is provided in Generic Letters 83-10a, b, c, d, and
e, which are included in the NRC report, SECY-82-475, from W. J. Dircks
to the NRC Commissioners, "Staff Resolution of the Reactor Coolant Trip
Issue" (November 30, 1982). SECY-82-475 also provided the NRC
guidelines and criteria for the resolution of TMI Action Item II.K.3.5,
"Automatic Trip of Reactor Coolant Pumps."
In SECY-82-475 the NRC concluded: "...that appropriate pump trip
setpoints can be developed by the industry that would not require RCP
trip for those transients and accidents where forced convection
circulation and pressurizer pressure control is a major aid to the
operators, yet would alert the operators to trip the RCPs for those
small LOCAs where continued operation or delayed trip might result in
core damage."
SECY-82-475 also stated: "The resolution provided in the enclosures
[Generic Letters 83-10] is intended to ensure that for whatever mode of
pump operation a licensee elects, a) a sound technical basis for that
decision exists, b) the plant continues to meet the Commission's rules
and regulations, and c) as a minimum, the pumps will remain running for
those non-LOCA transients and accidents where forced convection cooling
and pressurizer pressure control would enhance plant control. This
would include steam generator tube ruptures (SGT up to approximately
the design basis event (one tube).
The Westinghouse Owners Group (WOG) submitted two reports to the NRC in
response to the Westinghouse specific Generic Letters, 83-10c and d.
The first report provided an "Evaluation of Alternate RCP Trip
Criteria" (Reference I). The second report provided the "Justification
of Manual RCP Trip for Small Break LOCA Events" (Reference 2). The WOG
also provided additional information (Reference 3) in response to our
request for this information, based on the review of the WOG
submittals. We have also performed analyses of selected events to
support our review (Reference 4).
- 2 -
Appendix A to this report summarizes Section l of the enclosure to
Generic Letter 83-10 for "Pump-Operation Criteria that Can Result in
RCP Trip During Transients and Accident," and Appendix B summarizes
Section II, "Pump-Operation Criteria That Will Not Result in RCP Trip
During Transients and Accident."
II. SUMMARY
The WOG has developed a set of three alternative reactor coolant pump
(RCP) trip criteria - each one being reported to be equally suitable in
meeting the intent of Generic Letter 83-10. The revised criteria
replace the current RCP trip criterion of low reactor coolant system
(RCS) pressure, which could result in RCP trip for SGTR and non-LOCA
events.
The objective of the WOG study was to evaluate alternative RCP trip
criteria to determine if a criterion could be established to reduce the
probability of RCP trip for SGTRs and non-LOCA events, while still
providing for RCP trip for small break LOCAs.
The parameters which were considered for the evaluation of alternative
RCP trip criteria included RCS pressure, RCS subcooling, and secondary
pressure dependent RCS pressure (RCS/secondary pressure differential).
Because SGTRs and most non-LOCA events will not,result in adverse
containment conditions, the no mal instrument uncertainties associated
with the measurement of these parameters can be utilized in evaluating
the effectiveness of these alternative criteria in preventing pump trip
for most SGTRs and non-LOCAs.
The alternative RCP trip criteria which were evaluated are:
1. RCS pressure with normal instrument uncertainties.
This criterion would be established in the same manner as the
current RCS pressure criterion, with the exception that the normal
instrument uncertainties would be utilized in determining the RCP
trip setpoint for normal containment conditions. The instrument
uncertainties associated with post-accident containment conditions
would continue to be used to determine the RCP trip setpoint for
adverse containment conditions.
2. Reactor coolant subcooling.
This method would provide a direct indication of the need for pump
trip, since pump trip is not required as long as the reactor
coolant remains subcooled. The RCP trip setpoint would be
established as zero degrees subcooling in the RCS hot legs, plus
the uncertainty in the subcooling monitor to assure that the pumps
are tripped before subcooling is actually lost. The normal
instrument uncertainties would be used for normal containment
conditions, whereas the instrument uncertainties associated with
post-accident containment conditions would be used for adverse
containment conditions.
- 3 -
3. Secondary pressure dependent RCS pressure.
With the current method of using RCS pressure, the trip criteria
is conservatively derived assuming that the secondary pressure is
at the lowest secondary safety valve set pressure. However, the
secondary pressure may actually be significantly less than this
value, particularly if the condenser steam dump system is in
operation. With this method, the RCS pressure setpoint for pump
trip would be continuously evaluated based on the actual secondary
pressure. The alternate RCP trip criterion can also be expressed
as the RCS/secondary pressure differential. The combined
instrument uncertainties for the RCS and secondary pressure
measurements would be included in determining the RCP trip
setpoint. The normal instrument uncertainties would be used for
normal containment conditions, whereas the instrument
uncertainties associated with post-accident containment conditions
would be used for adverse containment conditions.
The results of the small break LOCA analysis demonstrate that each of
the alternative criteria is adequate in providing an indication for the
operator to trip the RCPs for small break LOCAs. Thus, the selection of
the RCP trip criterion can be based on the capability to preclude a
pump trip for SGTRs and non-LOCAs. The minimum values of each of the
parameters used to evaluate the alternative criteria were also
determined for SGTRs and non-LOCA events for each category of plants in
the study. A methodology was provided to determine the RCP trip
setpoints for each of the three alternative criteria. Using the results
of the evaluation and the setpoints calculated for each of the
alternative criteria, each utility can determine which of the criteria
will prevent pump trip for SGTRs and non-LOCA events for their
respective plants. The criterion that is considered most appropriate in
providing pump trip discrimination between LOCAs and SGTR or non-LOCA
events can then be selected by the utility for each plant.
Based on these studies, the WOG concluded that the RCP trip criterion
can generally be implemented using existing qualified instrumentation
already available in the plants, and additional instrumentation is not
required.
The WOG followed the guidelines provided in Generic Letter 83-10c and
d to justify manual RCP trip for small-break LOCAs. (See Appendix A,
Section D.) The WOG studies concluded that:
1. Every Westinghouse plant's FSAR ECCS analysis demonstrates
compliance with 10 CFR 50.46 if operator action to trip the RCPs
is taken within two minutes after the RCP trip criterion is
reached.
2. Most probable best estimate analyses indicate that in all
Westinghouse plants the RCPs may be tripped at any time during a
small break LOCA event without reaching clad temperatures of
2200F. The highest PCT calculated with most probable best
estimate assumptions was 1255F.
- 4 -
The WOG concluded that automatic reactor coolant pump trip is not
required since adequate time for manually tripping the RCPs is
demonstrated using 10 CFR Part 50, Appendix K assumptions as well as
most probable best estimate analysis results. It was also concluded
that the most probable best estimate analysis results demonstrate that
the RCPs can be tripped at any time during the LOCA (if the operator
should fail to trip the pumps when the trip criterion is reached)
without incurring unacceptable clad temperature results. Therefore, the
WOG concludes that the existing guidelines in Revision 1 of the
Emergency Response Guidelines (ERGs are sufficient and complete with
respect to RCP status for all accident situations, and no additional
"missed RCP trip setpoint" steps are required.
The methods (References 5 and 6) employed by the WOG to,justify manual
RCP trip are consistent with the guidelines and criteria provided in
Generic Letters 83-10c and d.
We have reviewed the assumptions and models employed by the WOG to
study steam generator tube rupture (SGTR) and non-LOCA events. The
LOFTRAN computer program (Reference 7) was used to study these events.
Best estimate assumptions and models were used. However, the SGTR break
flow model incorporated into LOFTRAN does result in higher than
expected mass flow rates for a given break size. (This model was
approved for SAR SGTR analyses, where the high flow rates result in a
conservative evaluation of offsite dose.) The WOG position with respect
to the use of this conservative model is that the analysis results are
bounding for the design basis SGTR event of a single tube.
The WOG considered all other FSAR Chapter 15 non-LOCA events for
evaluation against the alternate RCP trip criteria. It was concluded
that the feedline and steamline breaks needed to be considered because
their transient characteristics would be the most limiting with respect
to the three criteria. For the steamline break accident, a "credible"
(4.5 inch diameter) break size was considered - essentially equivalent
in size to a single steam generator PORV failing open. For the feedline
break, a full double-ended rupture of the main feedwater line was
considered.
We have reviewed the assumptions, models and plant groupings used to
perform the SGTR and non-LOCA studies and have determined that the
information provided is acceptable. Table 1 provides a summary of the
WOG studies. We believe that the three RCP trip criteria may be
marginal for some plants for the SGTR event. We base our conclusion on,
the following:
1. The SGTR event gives the minimal values for all three alternative
RCP trip criteria for all but a few plants, and
2. The uncertainty analysis of instrument error provided by the WOG
for use to evaluate the trip set points for each alternative
criteria (for both normal and adverse containment conditions) may
not be bounding for all plants.
- 5 -
In particular, the reactor coolant system pressure set point RCP trip
criterion appears to offer the least in reducing unnecessary RCP trip.
This confirms our position as discussed in SECY-82-475 and Generic
Letter 83-10.
The WOG objective for the SGTR and non-LOCA analyses was to consider
design basis accidents with more realistic assumptions, to enable the
development of a RCP trip criterion which would provide reasonable
assurance of continued pump operation for these accidents. While it is
possible that other accident conditions could result in more limiting
parameter values, the design basis accidents which were defined for the
analyses combined with the conservatisms which are incorporated in the
analytical model provide assurance that the analysis results will be
bounding for most SGTR and non-LOCA events. The WOG does not consider
it to be practical or necessary to develop a RCP trip criterion which
will provide for continued pump operation for all possible SGTR and
non-LOCA events. It would not be a safety problem if RCP trip should
occur for a SGTR or non-LOCA event, since the plant safety systems are
designed to handle those accidents with a loss of offsite power and,
therefore, with RCP trip. The objective was to demonstrate that the
RCPs will remain on for most of the expected cases of these accidents,
so that the operator can retain normal pressurizer pressure control and
will not be required to open the pressurizer PORVs. In addition,
maintaining forced reactor coolant system flow will reduce the
likelihood of generating voids in the reactor vessel upper head region.
The WOG response to our concern that none of the three alternative
criteria would prevent RCP trip for a SGTR or a non-LOCA event (on a
plant specific bases) is the recommendation to use the criterion which
demonstrates the greatest discrimination capability.
In doing so, the WOG expects that a large range of SGTRs and non-LOCA
events still would not require RCP trip. In the event of RCP trip
occurring for SGTRs and non-LOCAs, the WOG position is that the
Emergency Response Guidelines (ERGs) provide specific contingency
actions to recover the plant even though RCP operation is not
available. Also, specific RCP restart steps are built into the ERGs
where deemed beneficial although they are not required for safe plant
shutdown. The WOG expects, however, that at least one of the
alternative criteria will be successful in preventing pump trip for
SGTRs and non-LOCA events for each of the plants.
The studies performed by the WOG to determine the transient
characteristics for the SGTR and non-LOCA events were based on best
estimate input assumptions and models (to the extent practical with the
computer programs used). Based on our experiences, with other
thermal-hydraulic programs used to perform similar types of analyses,
we believe there are uncertainties associated with the numerical
results of any calculated system transient. Each licensee must consider
these uncertainties when selecting the criterion which demonstrates the
greatest discrimination capability, and be prepared to explain how they
were considered during future inspections.
- 6 -
The generic nature of the WOG submittals concerning RCP trip by nature
do not include any plant specific information, other than that needed
to determine plant groupings for analysis. We have therefore included
a section (Implementation) in this report which describes those plant
specific items we require each licensee to address when incorporating
the RCP trip criteria into the plant procedures.
III CONCLUSIONS
We have determined that the information provided by the WOG for the
justification of manual reactor coolant pump trip is acceptable. The
methods employed by the WOG to justify manual reactor coolant pump trip
are consistent with the guidelines and criteria provided in Generic
Letters 83-10c and d. The approved Westinghouse Small Break LOCA
Evaluation Model was used to demonstrate compliance with 10 CFR 50.46
and Appendix K to 10 CFR Part 50.
We have determined that the information provided by the WOG in support
of the alternative reactor coolant pump trip criteria is acceptable. A
suitable reactor coolant pump trip criterion can be selected by each
licensee to minimize reactor coolant pump trip during steam generator
tube ruptures and non-LOCA events, while still providing for RCP trip
for small break LOCAs.
The results presented by the WOG, for the plant groups studied, -imply
that one of the alternative RCP criteria would prevent RCP trip for the
design basis SGTR and for design basis non-LOCA events. This would be
a true statement if the numerical results from the calculation
performed were error free and if each plant responds exactly as the
simulation model predicts. Also, the uncertainty analysis for
instrument error would have to be bounding for each plant, with normal
containment conditions. Adverse containment conditions are not expected
for design basis SGTRs or non-LOCA events.
We believe the analysis tools employed by the WOG are capable of
qualitatively providing the appropriate information to evaluate the
alternate RCP criteria. It should be obvious however that the
quantitative values provided cannot be considered absolute. In our
judgement, the alternate RCP trip criteria, as defined, may provide
only marginal assurance of preventing RCP trip for the design base SGTR
event for some Westinghouse plants.
We have concluded that the WOG has developed acceptable criteria for
tripping the reactor coolant pumps during small-break LOCAs and to .
minimize reactor coolant pump trip for SGTR and non-LOCA events.
IV IMPLEMENTATION
The generic information presented by the WOG does not address plant
specific concerns about instrumentation uncertainties, potential
reactor coolant pump problems and operator training and procedures as
requested in Generic Letter 83-10. Appendix A contains a summary
related to these issues and may be used as a guideline to assure that
these issues are adequately addressed.
- 7 -
In order to complete the response to Generic Letters 83-10c and d, each W
licensee is required to submit the following information to the NRC Tor
plant specific reviews:
A. Determination of RCP Trip Criteria
1. Identify the instrumentation to be used to determine the RCP trip
set point, including the degree of redundancy of each parameter
signal needed for the criterion chosen.
2. Identify the instrumentation uncertainties for both normal and
adverse containment conditions. Describe the basis for the
selection of the adverse containment parameters. Address, as
appropriate, local conditions such as fluid jets or pipe whip
which might influence the instrumentation reliability.
3. In addressing the selection of the criterion, consideration to
uncertainties associated with the WOG supplied analyses values
must be provided. These uncertainties include both uncertainties
in the computer program results and uncertainties resulting from
plant specific features not representative of the generic data
group.
If a licensee determines that the WOG alternative criteria are
marginal for preventing unneeded RCP trip, it is recommended that
a more discriminating plant-specific procedure be developed. For
example, use of the NRC-required inadequate-core-cooling
instrumentation may be useful to indicate the need for RCP trip.
Licensees should take credit for all equipment (instrumentation)
available to the operators for which the licensee has sufficient
confidence that it will be operable during the expected
conditions.
B. Potential Reactor Coolant Pump Problems
1. Assure that containment isolation, including inadvertent
isolation, will not cause problems if it occurs for non-LOCA
transients and accidents.
a. Demonstrate that, if water services needed for RCP operations
are terminated, they can be restored fast enough once a
non-LOCA situation is confirmed to prevent seal damage or
failure.
b. Confirm that containment isolation with continued pump
operation will not lead to seal or pump damage or failure.
2. Identify the,components required to trip the RCPs, including
relays, power supplies and breakers. Assure that RCP trip, when
determined to be necessary, will occur. If necessary, as a result
of the location of any critical component, include the effects of
adverse containment conditions on RCP trip reliability. Describe
the basis for the adverse containment parameters selected.
- 8 -
C. Operator Training and Procedures (RCP Trip)
1. Describe the operator training program for RCP trip. Include the
general philosophy regarding the need to trip pumps versus the
desire to keep pumps running.
2. Identify those procedures which include RCP trip related
operations:
(a) RCP trip using WOG alternate criteria
(b) RCP restart
(c) Decay heat removal by natural circulation
(d) Primary system void removal
(e) Use of steam generators with and without RCPs operating
(f) RCP trip for other reasons
Page Last Reviewed/Updated Tuesday, March 09, 2021