Resolution of TMI Action Item II.k.3.5, "Automatic Trip of Reactor Coolant Pumps" (Generic Letter No. 83-10a)
NUCLEAR REGULATORY COMMISSION
WASHINGTON, D.C. 20555
February 8, 1983
TO ALL APPLICANTS WITH COMBUSTION ENGINEERING (CE) DESIGNED NUCLEAR STEAM
SUPPLY SYSTEMS (NSSSs)
SUBJECT: RESOLUTION OF TMI ACTION ITEM II.K.3.5, "AUTOMATIC TRIP OF
REACTOR COOLANT PUMPS" (GENERIC LETTER NO. 83-10a)
The purpose of this letter is to inform you of (1) the staff's conclusions
regarding the analysis of LOFT Test L3-6 submitted by the CE Owners Group,
(2) the continued acceptability of the CE ECCS evaluation model for
predicting small break LOCAs with Reactor Coolant Pump (RCP) operation and
(3) criteria for resolution of TMI Action Item II.K.3.5, "Automatic Trip of
Reactor Coolant Pumps."
We have completed our evaluation of the analyses of LOFT Test L3-6 performed
by the CE Owners Group and conclude that the evaluations acceptably predict
the test results. Therefore, we find the currently approved CE evaluation
model for small break LOCAs in continued conformance with Appendix K to 10
CFR 50 for the case of limited RCP operation after reactor trip and for the
range of licensed CE reactor designs.
We have reviewed industry analyses and performed our own analyses to
determine whether RCP trip is necessary during LOCAs, and evaluated the
desirability of continued RCP operation during non-LOCA transients and
accidents, including steam generator tube ruptures. We have concluded that
there is a wide range of transients and LOCAs where it is beneficial for the
operators to maintain forced circulation cooling and mixing through
operation of the RCPs. However, some of the calculations show that for
certain small break LOCAs, primarily those with only one of the two High
Pressure Safety Injection (HPSI) Pumps assumed available, continued
operation of the RCPs or continued operation of the RCPs followed by delayed
RCP trip could lead to core damage.
Some uncertainty in these conclusions remains. Specifically, there is a
complex interrelationship among break size, break location, RCP trip delay
time, available safety systems, and peak cladding temperature (PCT) for each
type of NSSS design. Moreover, although the staff's and each vendor's
calculational models adequately predicted LOFT test L3-6, there appear to be
subtle differences embedded in the computer models which, when applied to
large, commercial, PWR designs, yield differing results regarding the
necessity for RCP trip during small LOCAs.
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Because of this, we place substantial weight on the views of the reactor
designers and the utilities which are almost unanimous in asserting that for
some small LOCAs with less than the maximum available HPSI flow, delayed RCS
trip could lead to core damage. Some utilities indicated their preference
to keep the RCPs running for all events; however, this view appeared to be
based solely on the desire to maintain forced circulation and did not
consider the consequences of delayed RCS trip.
While acknowledging the industry's general conclusion that the RCPs should
be tripped for small LOCAs, both the staff and the industry recognized that
there are other accident sequences of much higher probability than the small
LOCA where the absence of forced circulation makes the operator's job more
difficult and can increase the likelihood of operator errors. For this
reason, we believe that a balance should be struck between the competing
risks associated with tripping the RCPs early and leaving them running
following transient and accident events.
Based on our discussions with both utilities and the reactor manufacturers,
and our internal evaluations, we believe that appropriate pump trip
setpoints can be developed by the industry that would not require RCP trip
for those transients and accidents where forced 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.
In summary, we have concluded that the need for RCP trip following a
transient or accident should be determined by each applicant on a
case-by-case basis, considering the Owners Group input. However, the staff
must ensure that whatever decision is made regarding pump operation, it will
result in safe, reliable operation of reactors and will not adversely affect
the ability of applicants to comply with the Commission's rules and
The enclosure to this letter provides guidance for the development of either
(1) satisfactory setpoints for RCP trip or (2) the technical bases for
allowing continued RCP operation in the event of a small LOCA at a
licensee's facility. As stated in the enclosure, manual tripping of the
RCPs for a LOCA can be allowed under certain conditions.
We recognize that possible differences exist between the requirements of 10
CFR 50.46, which assure ample core cooling capacity, and the approaches
described in the enclosure which are based upon assuring proper operator/
system response under conditions that may be faced during accidents and
transients. Accordingly, in such cases, we will consider a request for
exemption from specific requirements of 10 CFR 50.46 pursuant to 10 CFR
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For the purpose of providing uniformity of setpoints and methods and for
minimizing potential confusion that could arise because of diverse actions
by individual applicants, we strongly urge that applicants work collectively
with owners of similar plants (i.e., Owners Group) and propose setpoints and
methods consistent with other applicants.
We caution that careful judgment should be used when developing proposed
methods and setpoints in accordance with the guidance in the enclosure. If
RCPs are to be tripped, we recommend that the applicants utilize event trees
to systematically evaluate RCP trip setpoints to minimize the potential for
undesirable consequences due to a misdiagnosed event.
Specifically, we recommend the setpoints be evaluated for events where the
RCPs could be tripped when it is preferable they remain operational. We
further recommend the setpoints also be evaluated for the case when the RCPs
are not tripped early in the event and for which a delayed trip may lead to
For applicants who have already received an NRC Operating License Safety
Evaluation Report (OLSER), we are not requiring a formal submittal of the
analyses which support either RCP trip setpoints or the decision to leave
the RCPs operational for all events. However, once the technical bases for
the decision are established, we intend to conduct inspections of individual
applicants led by Regional personnel. During these inspections, we will
examine the translation of the 10 CFR 50, Appendix K, and RCP operation mode
evaluations into plant procedures. We would expect the evaluations to
include consideration of the guidance contained in the enclosure to this
letter. Copies of these evaluations should be made available to the staff at
Alternatively, an applicant may choose to make either an individual
submittal or reference a generic (i.e., Owners Group) submittal which
provides the technical justification for treatment of RCPs during transients
and accidents. In that case, an inspection would not be necessary.
Applicants who have not received an OLSER should plan to make such a
The requirements set forth in this letter supersede the actions required in
IE Bulletins 79-05C and 79-06C.
Accordingly, within 60 days following receipt of this letter, please provide
your plans and schedules for resolution of this issue for your facility.
You should also indicate whether you desire to make a submittal concerning
this issue. If you cannot respond within 60 days, you should indicate
within 30 days when your schedule will be submitted. The information
requested should be sent to Mr. D. G. Eisenhut, Director, Division of
Licensing, Washington, D.C. 20555, pursuant to 10 CFR 50.54(f).
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This request for information was approved by the Office of Management and
Budget under clearance number 3150-0065 which expires May 31, 1983.
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 Dr. B. Sheron (301-492-7460).
Darrell G. Eisenhut, Director
Division of Licensing
Resolution of TMI Action
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