United States Nuclear Regulatory Commission - Protecting People and the Environment

Summary of Meeting Held on October 12, 1979 to Discuss Responses to IE Bulletins 79-05C and 79-06C and HPI Termination Criteria (Generic Letter 79-55)



GL79055 

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

TO ALL PRESSURIZED WATER REACTOR LICENSEES 

SUBJECT:  SUMMARY OF MEETING HELD ON OCTOBER 12, 1979 TO DISCUSS RESPONSES 
          TO IE BULLETINS 79-05C AND 79-06C AND HIGH PRESSURE INJECTION 
          (HPI) TERMINATION CRITERIA 

On October 12, 1979, members of the NRC staff (the staff) met with the three
pressurized water reactor (PWR) vendors and representatives of the 
corresponding Owners' Groups in Bethesda, Maryland. The purpose of the 
meeting was to discuss the variations in analyses and operator guidelines 
submitted in response to IE Bulletins 79-05C and 79-06C and to discuss 
differences in proposed HPI termination criteria. A list of attendees is 
provided as Enclosure 1. 

BACKGROUND 

The initial review of the preliminary chronology of events which occurred at 
the Three Mile Island Nuclear Station, Unit 2 (TMI-2) led to the 
identification of six potential human, design, and mechanical failures which 
appeared to have led to core damage in the reactor. One of the identified 
items was the tripping of the reactor coolant pumps (RCPs) during the course 
of the accident. Once forced flow was terminated by securing the RCPs, core 
damage occurred since the steam and water separated in the primary system 
and caused the core to become uncovered. Instructions were issued to the 
holders of operating licenses, in the form of IE bulletins, which required 
operating procedures to specify that in the event of HPI initiation with 
RCPs operating, forced flow would be maintained in the RCS. 

Subsequent analyses by each of the Nuclear Steam Supply System (NSSS) 
vendors indicated that for a certain spectrum of small breaks in the RCS, 
continued operation of the RCPs would increase the mass lost through the 
break and prolong or aggravate the uncovering of the reactor core if the 
RCPs were subsequently tripped at certain times into the accident. While the 
size and location of such a break and the time of tripping the RCPs, which 
would produce unacceptable results, varied for each analysis, all three 
vendors agreed that an acceptable action under loss-of-coolant-accident 
(LOCA) symptoms would be to trip all operating RCPs immediately, before 
significant voiding in the RCS occurred. As a result of this information, IE 
Bulletins 79-05C and 79-06C were issued which required immediate tripping of 
all operating RCPs upon reactor trip and initiation of HPI caused by system 
low pressure. These bulletins also required the licensees to perform more 
detailed analyses on the effect of RCP trip for both LOCA and non-LOCA 
transients. 

The conclusions reached by the PWR vendors in these analyses vary to a 
considerable degree as summarized in Enclosure 2. It is the NCR staff's 
present judgment that the major differences in results are attributable to 
model differences, which are highlighted in Enclosure 3. The various RCP 
trip criteria that have been proposed by the vendors are shown in Enclosure 
4. 
.

                                    - 2 -

An additional concern relates to the criteria for HPI termination. Enclosure 
5 lists the criteria proposed by the various vendors. The staff can see no 
reason why the same set of safety considerations would lead to more than one 
uniform criterion for termination of HPI. 

This meeting was requested by the staff to discuss, in an administrative 
sense, how the PWR regulated industry can reach agreement on these two 
issues. 

DISCUSSION 

The NRC staff opened the meeting by stating that its goal for the 
discussions was to attempt to reach agreement among the staff, the Owners' 
Groups, and the NSSS vendors on several issues involving small break LOCA 
analyses and RCP trip criteria, as well as the criteria which should be used 
for termination of HPI. The staff pointed out that NUREG-0578 ("TMI-2 
Lessons Learned Task Force Status Report and Short-Term Recommendations" - 
July 1979) requires that the implementation of small break LOCA emergency 
procedures and the associated operator training be completed by December 31, 
1979. Therefore, the staff considers timely resolution of these issues to be 
imperative. 

1.   SMALL BREAK LOCA AND RCP TRIP REQUIREMENTS 

     There was general agreement among the participants that: (1) there are 
     significant differences in the small break LOCA models used by the 
     vendors; (2) there are differences of opinion on what model features 
     are conservative; and (3) none of the models have been demonstrated to 
     be overall conservative by integral test comparison. The staff said it 
     has done some calculations with RELAP-4/Mod 7, as summarized in 
     Enclosure 6; however, these calculations are also not definitive enough 
     to enable the staff to determine which set of model assumptions is 
     suitably conservative. Based on the above conditions, the staff pointed 
     out that it is not surprising that the conclusions reached by each 
     vendor in their small break LOCA calculations vary with those reached 
     by the other vendors. The staff stated, it believes that the different 
     results obtained by the vendors cannot be attributed solely to physical 
     plant differences. The other participants, however, were not able to 
     reach the same conclusion. 
     
     All participants were able to agree that resolution of differences is 
     needed in order to proceed with the task of writing/rewriting the small 
     break LOCA operator guidelines. The staff pointed out that it is very 
     important for the licensed operators to be convinced that tripping of 
     RCPs is the proper course of action for a small break LOCA, and in 
     order to achieve this, there should be general agreement among the 
     vendors, supported by analyses, on the criteria to be used to formulate 
     the licensees' emergency procedures. 

     Westinghouse (W) stated that the resolution of this issue was on its 
     critical path for completing the requirements of NUREG-0578 (Section 
     2.1.9) on schedule. Combustion Engineering (CE) stated that it was not 
     a critical issue at this time; however, continued inability to reach a 
     common agreement would impact its schedule. Babcock & Wilcox (B&W) has 
     completed its small break LOCA guidelines, detailed emergency 
     procedures, and operator training. B&W's concern was centered around 
     not confusing the operator. B&W does not want to modify 
.

                                    - 3 -

     its guidelines and subsequent licensees' emergency procedures and 
     training based upon another interim position. B&W would like to resolve 
     the issue in a timely fashion, and if changes are needed in the 
     guidelines, procedures, and training, the changes should not be made 
     until the issue is resolved. 
     
     The next item discussed was the need for an automatic RCP trip for 
     small break LOCAs. B&W desires that an automatic trip design be 
     approved by the staff based on its recent proposed design submittal. 
     The B&W design uses coincident signals from: (a) safety injection 
     initiation (1600 psig) and (b) RCP current. W desired to have a 
     detailed technical meeting in the near future to discuss its proposal 
     of using coincident signals from: (a) safety injection initiation and 
     (b) low primary system pressure of 1250 psig. CE has not decided about 
     the need for an automatic trip. CE's analysis shows that in most cases 
     it would require only two out of four RCPs to be tripped. In addition, 
     the CE analysis shows that no RCP trip would be required provided at 
     least two HPI pumps were used during the accident. 
     
     The NRC staff questioned whether the requirements of IE Bulletins 
     79-05C and 79-06C to "immediately" trip all operating RCPs upon a 
     reactor trip and HPI initiation caused by low pressure is too 
     restrictive. Agreement was reached by the participants that the 
     requirement to immediately trip the RCPs should be modified such that 
     additional time is allowed for the operator to determine if the 
     transient which causes the low pressure is a LOCA or non-LOCA event. It 
     was generally agreed that the reduced pressure for the RCP trip should 
     fall between the setpoint of the steam generator relief valves and the 
     safety injection actuation setpoint. B&W stated that while they agreed 
     that a lower pressure than the 1600 psig used for safety injection 
     actuation could be acceptable, the reasons it had chosen its coincident 
     signals was because the B&W plants do not have a safety-grade pressure 
     transmitter that would assure a reliable signal, at a lower pressure, 
     from which to trip the RCPs. CE modified its response by stating that 
     it also agreed with the concept of using a lower pressure; however, it 
     still felt that a certain time delay (after reaching the specified 
     pressure limit) should be afforded the operator prior to tripping the 
     RCPs to allow for operator actions to recover from the transient, if 
     appropriate. 
     
     This topic was closed out by a restatement by the staff that it was 
     critical for the industry to reach agreement on the subject to assure 
     that the licensed operators understood that tripping of the RCPs and 
     the criteria for doing so do not represent an arbitrary NRC decision, 
     but in fact are based on the recommendation of the NSSS vendors and 
     have the endorsement of the licensees. 
     
2.   HIGH PRESSURE INJECTION TERMINATION CRITERIA 

     The basic thrust of this portion of the meeting was to come to 
     agreement on the necessity to pursue a common set of criteria for the 
     manual termination of HPI following automatic initiation caused by 
     reactor coolant system (RCS) low pressure. 
.

                                    - 4 -

     As a result of the investigation of the TMI-2 accident, one of the 
     prime contributors to the damage which occurred to the core was 
     attributed to the premature termination of HPI. Shortly after the 
     accident, IE Bulletins 79-05A, 79-06A, and 79-06B (applicable to B&W, 
     W, and CE operating plants respectively) were issued which directed 
     licensees to take certain actions which would minimize the chances of 
     such an accident. One of the action items of these bulletins dealt with 
     HPI termination criteria. In general, the bulletins required, as an 
     interim measure, that if HPI automatically initiated because of a low 
     pressure condition in the RCS, it must remain operating until one of 
     two criteria is met. These criteria are: 
     
     a.   Both low pressure injection (LPI) pumps are in operation and 
          flowing for 20 minutes or longer at a flow rate which would assure 
          stable plant behavior or 
          
     b.   The HPI system has been in operation for 20 minutes, and all hot 
          leg and cold leg temperatures are at least 50F below the 
          saturation temperature for the existing RCS pressure. If the 
          50F subcooling cannot be maintained after HPI cutoff, HPI 
          shall be reactivated. The degree of subcooling and the length of 
          time HPI is in operation shall be limited by the 
          pressure/temperature considerations for vessel integrity. 
          
     Subsequent to these bulletins and based on vendor analyses, vendor 
     guidelines were developed independently for HPI termination criteria. 
     The various criteria are summarized in Enclosure 5. The staff stated 
     its position that there should only be one set of criteria for HPI 
     termination and that it should be equally applicable to all PWR 
     facilities. All participants agreed on this position. The staff's goal 
     in this area is to get one set of reliable criteria which can be easily 
     monitored and will not cause operator confusion. 

     The mechanics of getting agreement on the criteria to be used was then 
     discussed. It was agreed that the simplest set of criteria which would 
     still satisfy all concerns would be the logical choice. The criteria 
     selected would have to hold for all cases; i.e., could not lead the 
     operator to erroneous conclusions concerning the status of the RCS 
     inventory and degree of subcooling. The criteria would also have to 
     assure that the influence, if any, on pressure vessel integrity would 
     be considered. All participants, including the staff, agreed that these 
     criteria do not need to include a requirement that the HPI system be 
     operated for any arbitrary length of time, such as the 20-minute 
     requirement specified in the IE bulletins. 
     
CONCLUSIONS 

All participants agreed that, based on the information presently available 
to the staff, no clear solution to establishing a unified set of criteria 
for RCP trip during small break LOCAs and HPI termination is readily 
apparent. An indepth technical meeting with the W Owners' Group to discuss 
its analysis and criteria associated with these issues will be scheduled 
shortly. A similar meeting will be arranged with the CE Owners' Group if it 
is necessary. A meeting with B&W does not appear necessary at this time. 
Following the technical meeting(s), it may be 
.

                                    - 5 -

necessary to have another administrative meeting with the Chairmen of the 
Owners' Groups and the NSSS vendors to obtain a decision on criteria 
selection. 

The criteria selected by the staff should have full vendor and licensee 
endorsement. These criteria will most likely be transmitted to the Chairmen 
of the Owners' Groups for comment followed by individual letters being sent 
to the licensees. 



                                        R. A. Capra, B&W Project Manager 
                                        Project Management Group 
                                        Bulletins & Orders Task Force 

Enclosures:
1.   List of Attendees 
2.   Summary of Conclusions on IE Bulletins 79-05C and 79-06C 
3.   Model Differences for SBLOCA Analyses 
4.   RCP Trip Criteria 
5.   HPI Termination Criteria 
6.   Summary of Staff Calculations

Page Last Reviewed/Updated Wednesday, May 22, 2013