NRC Generic Letter 85-12, Implementation of TMI Action Item II.K.3.5, "Automatic Trip of Reactor Coolant Pumps
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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 Letters83-10c 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 Letters83-10c 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 Letters83-10c 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
8507010252requirement but only identifies information specified in Generic Letters83-10c 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 Letters83-10c 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 Letters83-10a, 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).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 summarizesSection 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. 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 2200<deg>F. The highest PCT calculated with most probable best estimate assumptions was 1255<deg>F.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 Letters83-10c 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.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.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 Letters83-10c 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.In order to complete the response to Generic Letters83-10c 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.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:
- (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