United States Nuclear Regulatory Commission - Protecting People and the Environment

Resolution of TMI Action Item II.k.3.5, "Automatic Trip of Reactor Coolant Pumps" (Generic Letter No. 83-10d)


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

                              February 8, 1983

TO ALL LICENSEES WITH WESTINGHOUSE (W) DESIGNED NUCLEAR STEAM SUPPLY SYSTEMS  
  (NSSSs) (EXCEPT YANKEE ATOMIC ELECTRIC COMPANY) 

SUBJECT:  RESOLUTION OF TMI ACTION ITEM II.K.3.5, "AUTOMATIC TRIP OF REACTOR
          COOLANT PUMPS" (GENERIC LETTER NO. 83-10d) 


Gentlemen: 

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 Westinghouse 
Owners Group, (2) the continued acceptability of the Westinghouse 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 Westinghouse Owners Group and conclude that the evaluations 
acceptably predict the test results.  Therefore, we find the currently 
approved W 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 Westinghouse 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. 



8302090085 
.

W                                    - 2 -


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 RCP
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 RCP 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 licensee 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 licensees to comply with the Commission's rules and 
regulations. 

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 
50.12. 

.

W                                   - 3 -


For plants with low head SI pumps, we understand that RCP trip is still 
expected to occur on the low pressure trip setpoints presently proposed by W
for the design basis steam generator tube rupture.  The staff considers this
unacceptable and these licensees should identify a more discriminating 
criterion for RCP trip that would allow continued RCP operation for tube 
leaks up to the design basis steam generator tube rupture. 

For the purpose of providing uniformity of setpoints and methods and for 
minimizing potential confusion that could arise because of diverse actions 
by individual licensees, we strongly urge that licensees work collectively 
with owners of similar plants (i.e., Owners Group) and propose setpoints and
methods consistent with other licensees. 

If a licensee elects to trip RCPs, when RCP trip setpoints are developed 
which are believed to substantially meet the guidance provided in the 
enclosure, we encourage licensees to begin implementation of these new 
setpoints at operating plant(s)*.  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 licensees 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 
undesirable consequences. 

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 these inspections. 


_______________ 
*Unless implementation entails a change to technical specifications or an 
unreviewed safety question, which require NRC approval prior to 
implementation.
.

W                                     - 4 -


Alternatively, a licensee 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.  

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). 

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). 

                               Sincerely, 


                               Darrell G. Eisenhut, Director 
                               Division of Licensing 

Enclosure:
Resolution of TMI Action
  Item II.K.3.5

cc w/enclosure:
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