United States Nuclear Regulatory Commission - Protecting People and the Environment

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
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