ML20151D345
| ML20151D345 | |
| Person / Time | |
|---|---|
| Issue date: | 03/01/1988 |
| From: | Advisory Committee on Reactor Safeguards |
| To: | Advisory Committee on Reactor Safeguards |
| References | |
| ACRS-2558, NUDOCS 8804140154 | |
| Download: ML20151D345 (33) | |
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hbkb#c?$58 NIFEDDATE ISSUED: J/1/88 l l l ADVISORY COMMITTEE ON REACTOR SAFEGUARDS COMBINED SCRAM SYSTEMS RELIABILITY AND CORE PERFORMANCE SUBCOMMITTEES MEETING FEBRUARY 19, 1988 WASHINGTON, D.C. PURPOSE: The purpose of the meeting was to review the current status of LWR plant operations (ccre reload designs, etc.) as they impact on core reactivity control operational limits (e.g., moderator temperature coefficients) in general, and ATWS analyses in particular. ATTENDEES: Principal meeting attendees included: ACRS NRC W. Kerr, Chairman W. Hodges J. Ebersole, Member D. Fieno D. Ward, Member H. Richings C. Wylie, Member P. Davis, Consultant W Owners Grou) J. Lee, Consultant R. Newton (WE)) W. Lipinski, Consultant Westinghouse CE Owners Group J. Little T. Enos* (AP&L) Combustion Engineering J. Kapinos MEETING HIGHLIGHTS. AGREEMENTS AND REQUESTS
- 1. T. Enos (AP&L - representing the B&W Owners Group) discussed the effects of plant and fuel cycle changes on the bases for the ATWS Rule. The purpose of the presentation was to: (1) review ATWS Rule requirements for B&W plants, (2) eyeluate plant and program changes which have affected the ATWS bases, (3) evaluate fuel and
- Also represented B&W Owners Group h$k41g4000301 2550 pyg
.- c . .
Combined Scram Systems Reliability
& Core Performance Meeting Minutes February 19, 1988 fuel cycle changes which have affected the ATWS bases, and (4) address the adequacy of current plant configurations relative to the ATWS bases.
Reviewing the requirements for the ATWS Rule, Mr. Enos noted that for B&W plants the limiting ATWS events were loss of feedwater (LOF)andlossofoffsitepower(LOOP). Details of the assumptions used in the generic and plant specific analyses were shown (Figures 182). Mr. Enos said the ATWS LOF plant specific peak pressure was calculated to be N3700 psig. Mr. Ebersole noted that the auxiliary feedwater (AFW) success rate may rest on the use of a steam driven turbine pump of questionable reliability. In response to Mr. Ebersole, Mr. Enos said that due to the inherent relieving capability of the RCS (head lift, etc.) actuel peak pressures for ucrse case ATWS events are limited to N 3900-4000 psig. Mr. Enos detailed the function of the B&W RPS (Figure 3). In response to Dr. Kerr, Mr. Enos said a loss of offsite power always results in a trip, but the ATWS analyses assumes no rod movement occurs. TheATWSRule-mandateddiversescramsystem(DSS)will assure complete diversity for trip by use of the silicon controlled I rectifiers (SCR) in the portion of the RPS used for control of the regulating control rods. l l l In response to Mr. Ebersole, Mr. Enos said the SCR failure rate has l been very low (1 or 2/10 years of operation). Dr. Lipinski asked if B&W had analyzed the RPS to determine the increase in scram reliability by installing the DSS. Mr. Enos said they have not, as the ATWS Rule did not require it. Dr. Kerr expressed concern
.- e . .
Combined Scram Systems Reliability
& Core Performance Meeting Minutes February 19, 1988 with this response, as he felt a Licensee would want to know what the impact of such a modification (DSS) would be on RPS reli-ability. Mr. Enos indicated that reliability analyses conducted in the early 1980's showed the DSS would increase the reliability by more than a factor of ten. I figures 4 and 5 show schematics of the DSS and AMSAC required by the ATWS Rule. In response to Dr. Kerr, Mr. Enos indicated that consideration was given in design and operation of DSS and AMSAC to minimizing the spurious scram rate.
Reviewing the elements of the ATWS Rule, Mr. Enos said the DSS was required to improve RPS reliability by a factor of ten, and AMSAC was required to assure that unacceptable consequences (core melt) would not occur for 1 of 2 ATWS events. Mr. Enos indicated that i the B&W Cwners have initiated a number of programs in the areas of I transient reduction and RPS reliability enhancement (Figure 6) to I help reduce the risk of ATWS. Results to date (Figures 7 and 8) show reductions in both spurious scrams and loss of feedwater events. Additional efforts are underway to further reduce the anticipated transient rate. Programs to address RTB failures have l resulted in no failures of RT8s to open in 1986 or 1987. Mr. Enos said current RTB unreliability is demonstrated to be *v10-4/ demand. In response to Dr. Kerr, Mr. Enos said no calculation has been done to determine the overall RPS reliability. A B&W representative l indicated there was a calculation perfomed that gave an unreli-ability value of ev10-9/ demand for the RPS. Members of the Subcomittee expressed skeptism over the validity of this calculation, given the unknowns associated with comon-mode failures. Mr. Enos reviewed the fuel design changes over the last few years. B&W plants are moving to longer fuel cycles (18 and 24 months).
l
.- e . .
, l Combined Scram Systems Reliability
& Core Perfomance Meeting Minutes February 19, 1988 This has resulted in a significant change (decrease) in the 95 percentile MTC value for the 24 months fuel cycle (to a value of
-0.43 x 10~4AK/K/ F - Figure 9).
There was discussion of the method used to detemine the MTC. Dr. Lee said the uncertainty in MTC determination at power is quite large. Dr. Lee said that at HZP + 0.4 x 10-4&K/K/*F is a reason-able value. He indicated that the uncertainty could overwhelm the MTC value given above for the 24-month cycle. Noting that the ATWS Rule allowed unacceptable consequences 50% of the time, Mr. Enos said the MTC values show the plants still maintain an acceptable margin (Figure 10). In response to Dr. Lee, B&W indicated that the soluble boron concentration used for the 24-month fuel cycle plant has been increased by 300-500 ppm, and, as expected, fuel enrichment has been increased as well. In response # 4 Dr. Kerr, Mr. Enos indicated that the trip reduction program has resulted in an overall decrease in ATWS risk that more than offsets the reduction in margin indicated on the above graph (Figure 10). Dr. Kerr said B&W has indeed decreased event frequency but at the expense of increased hazard (consequences). He asked at what point the Owners Group would "strike the balance" in judgment here. Mr. Enos indicated that one needs to examine this tradeoff in the context of overall societal risk. Dr. Kerr urged Mr. Enos to reexamine the validity of the RPS unreliability number quoted above for the B&W RPS (10-9/ demand). Concluding remarks of the B&W Owners Group were:
- Many changes have been made to reduce the probability of ATWS.
Combined Scram Systems Reliability
& Core Performance Meeting Minutes February 19, 1988
- Additional changes will be made to reduce the probability further.
- The Current design of the DSS and AMSAC exceeds the objectives of the ATWS Rule.
- No plant or fuel changes have been made or are expected which will invalidate the ATWS Rule basis.
Dr. Kerr asked if the Owners have analyzed other accidents for the impact of the change in MTC. Mr. Enos replied in the affirmative. The only accident not bounded by the FSAR events is the boron dilution event. Dr. Kerr asked if it was feasible to put excess poison in the fuel instead of the coolant. The Owners Group indicated that, while technically feasible, the excess cost may preclude such an effort.
- 2. T. Enos introduced the presentation on behalf of the CE Owners Group (CE0G). J. Kapinos (CE) provided the details of the CE0G presentation. Vey points noted by Mr. Kapinos included:
- The current trend in MTC's for CE plants was reviewed. CE showed MTC data that was a mix of experiment and analysis.
Values for "actual" MTCs and technical specification limits were shown (Figures 11-12). Also shown were graphs which trended the MTC data (Figure 13). In response to Dr. Lee, CE said they use a design criterion of zero or negative MTC at full power, and use fixed burnable poison for extended cycle design (18 to 24 months). Based on the above, CE concluded that: (1) there is no overall trend toward more positive cycle maximum MTC at full power, (2) there is no strong correbtion of changes in MTC to
r- . , Combined Scram Systems Reliability
& Core Performance Meeting Minutes February 19, 1988 cycle length, (3) their design detennines MTC at full power, and (4) concerns about positive MTC trends do not apply to CE0G utilities.
' CE evaluated the impact of the 1988 MTC trend on the 1979 ATWS analysis. CE reviewed actual plant operational data at full power initial conditions. The results (Figure 14) show for the ATWS analysis that the newer CE plant classes (3410 and 3800 MWt) have slightly less negative MTCs than the values calculated in 1979. These MTC values are considered by CE to be best estimate.
The impact of the above MTC change on ATWS peak pressure calculations (Figure 15) show slight increases for the newer plant class (3410 and 3800 MWt) and a decrease for the oldest plant class. Ors. Lee and Kerr asked what uncertainty CE would assign to the above calculations of peak pressure, given the large uncertainty in detennination of MTC. CE indicated that the uncertainty in the calculation is about the same as the pressure changes shown on Figure 15 ( ~ 200 psia). Given the requirements of the ATWS Rule (e.g., AMSAC), the overall peak pressure expected during an ATWS event actual.ly decreases. Dr. Kerr asked if it is feasible, in a practical engineering j sense, to add sufficient fixed burnable poison to assure ATWS peak pressure stays below Service Level C limits. CE indicat-ed that it is not an economically feasible option. Dr. Kerr indicated that he feels a prudent designer may want to consid-er such an approach, if it is economically feasible to do so. In response to Mr. Ebersole, CE indicated that a detailed analysis of the head lift phenomenon has not been done.
Combined Scram Systems Reliability
& Core Performance Meeting Minutes February 19, 1988
- The CE Owners Group has responded to the NRR Letter of June 12, 1987 by providing the information noted above.
Dr. Kerr asked if CE reexamined their reactivity aialyses in light of the Chernobyl accident. CE indicated that they did revisit this issue and determined that reactivity accident consequences were bounded by Chapter 15 events. As a result of further discussion, Dr. Kerr indicated that he believes the Chernobyl and TMI events demand a close reevaluation of reactivity accident assumptions, beyond the traditional analyses performed in this area.
- 3. Mr. R. Newton, representing the W Owners Group, introduced the WOG presentation. He noted that the Owners Group has begun a program to coordinate the implementation of the Rule requirements for W plants. In response to Dr. Kerr, Mr. Newton said, the+ to a great degree, concerns the NRC Staff has raised via the MTC issue had not !
been addressed in the context of implementation of the Rule requirements. Given the Staff's concerns, the W0G decided to initiate a Program to assure the Licensees will, in the future, continue to meet the intent of the ATWS Rule, for such concerns as changes in MTCs. The WOG has contracted with W for this Program ; (details noted below). V. Esposito (W) discussed the core design process used by W. Details of the design requirements and process were noted. Licen-sees are moving to longer fuel cycles (60% of reloads are now 18 month cycles). New fuel concepts have also been introduced (higher burnupfuel,lowleakagecores,etc.). The above changes have affected MTC values. Figures of average MTC values for W US plants were shown (Figures 16-18). Two-loop plants have shown an increase in MTC values, up
Combined Scram Systems Reliability
& Core Perfomance Meeting Minutes February 19, 1988 to the average of the balance of W plants (Figure 17). The overall range of MTC values has not changed appreciably over the past 8 years.
J. Little (W) discussed the ATWS Rule basis and the W program to address administration of the intent of the Rule. W reviewed the Rule requirements, noting that the Rule did address the key risks of an ATWS. Mr. Ebersole raised the issue of the recent problems with DS-416 scram breakers. Mr. Little said this problem does not change earlier W assumptions regarding RPS reliability. Given an ATWS, E plants have time to take mitigative actions (e.g., interrupt power to M/G sets, drive in control rods, etc.). Turning to the treatrent of MTC in the ATWS Rule, Mr. Little said W believes the MTC risk contributor is overstated. In response to i Dr. Kerr, Mr. Little agreed, however, that MTC is a significant risk contributor to ATWS. Mr. Little said the current fuel management programs are enveloped l by the ATVS Rule bases. However, to assure no problems arise in the future, a Program to evaluate proposed future changes to plant operational parameters is prudent. Figure 19 outlines the above WOG/W progran. The methodology for the Program will be available for review in May 1988. Details of the Program approach were described. The centerpiece of l this effort is development of a detailed ATWS risk model. Figures 20-21 provides some details of the model approach. The objective of the risk model is to define envelopes of model assumptions which are compatible with the ATWS Rule design bases. In response to Mr. Ebersole, Mr. Little said W is examining the contribution of balance of plant initiators to spurious trips.
Combined Scram Systems Reliability
& Core Performance Meeting Minutes February 19, 1988 In conclusion, W noted:
- On the average, there has been an insignificant increase in MTC for W plants.
- The current fuel management nrogram continues to be consistent with the intent of the ATWS Rule.
- WOG/ Westinghouse program will establish a systematic administration basis which will also be compatible with the severe accident policy approach.
i l
- WOG and Westinghouse believe the new program activity to be '
worthwhile to allow utilities to administratively address the basis of the ATWS Rule. l i There was discussion of the problems seen with W scram breakers. W indicated that they are aggressively addressing the problems as they arise. Dr. Kerr suggested that a reanalysis of the RTB failure probability may be in order, given the high reliability demand imposed on the RPS. Mr. Little indicated that W is fully cognizant of the ATKS risk and has had, still does have, programs underway to address the concerns here.
- 4. D. Fieno (NRC-NRR) provided NRC coments on the ATWS issue. He noted that numerous fuel cycle parameter changes led to NRR concern that PWR MTC values may now be inconsistent with the analyses preceding development of the ATUS Rule. Accordingly, NRR issued a Letter to the PWR Owners Group that requested the following infor-mation: (1) justification for the continued applicability or conservatism in ATWS-basis MTCs, (2) provide results for unique or
Combined Scram Systems Reliability
& Core Perfonnance Meeting Minutes February 19, 1988 outlier plants, (3) discuss differences in current MTCs with ATWS-basis MTCs, (4) provide plant data used. (5) provide m9th-odology used to derive ATWS MTCs, and (6) detail assumptions made.
In response to ACRS concerns with the potential reactivity opera-tional limits via potential for reactivity accidents / incidents, NRR said they are not aware of any problems in the area. However, NRC RES has initiated a program with BNL to assess the adequacy of current reactivity accident analyses. In response to Mr. Ward, Dr. Richings (NRR) said this Program is due to begin in 1-2 weeks. Mr. Fieno indicated that the uncertainty associatea with determina-tion of core reactivity parameters has been greatly reduced ovu the last few years. NRR reviewed the status of technical specification (TS) MTC values. Some plants have positive MTC TS at low powtrs. Mr. Ward asked how compliance with these TS's are assured. Dr. Fieno indicated compliance 1e .ssured by the Licensee's surveillance program. Dr. Richings indicated that assurance of acceptable MTC values is keyed to the initial core physics measurements at hot zero power. I I Based on results of recent NRR/0wners Group meetings, the ATWS MTC l 1 values for the initial rule basis and current values were shown 1 (Figure 22). NRR concluded that there is no adverse trend in MTC data. However, the B&W 24 month cycle MTC value (-0.43 x 10-5 6K/K/*F) is a potential concern. Dr. Kerr questioned the Staff l approach here. Dr. Fieno said that NRR still needs to address the ! issue of how they will treat the control of MTCs on a plant-by-plant basis, i
Combined Scram Systems Reliability
& Core Performance Meeting Minutes February 19, 1988
- 5. Dr. Kerr solicited Subcomittee coments.
J. Ebersole - Concerned with BWR plants for ATWS. Suggested modification of the ADV dump volume to assure control rods will scram. Expressed interest in the W ATWS follow-on Program. Suggested detailed study of the process associated with the "head-li?t" phenomena. D. Ward - Felt NRR needs to address the issue of MTC control vis-a-vis ATWS. He asked NRR if they have looked at the BWRs. Dr. Fieno said they have this issue under study. He said W seems to be in the best shape for the MTC issue and are addressing residual concerns here. He was surprised with CE's analysis, i.e., they seem to be in the worse shape regardd.gr peak pressures. C. Wylie - Endorses Mr. Ward's remarks. Suggested B&W perform a reliability evaluation of their RPS. He also feels that given the problems with W RTB's, the issue of trip system diversity for W plants may be worth a second look. P. Davis - Believes the BWRs should also be evaluated for their "ccmpliance" with the ATWS rule bases. He said no real cancerns with any PWR vendors' plants are evident based on the above discussions. However he doesn't have a clear understanding regarding the extent of CE's ATWS Rule compliance. J J. Lee - B&W has trended to less negative MTCs at full power. Suggest a look be given to MTC values at low (40-50%) power as it is possible the MTC could even be positive at these powers. Also suggested an analysis of limiting transients be done in lieu of the use of less negative MTCs. I
i Combined Scram Systems Reliability
& Core Performance Meeting Minutes February 19, 1988 W. Lipinski - He suggested consideration be given to the impact on plant receovery (e.g. DHR) if an RCS is subjected to peak ATWS pressures (5-6000 psia). The W RTB performance has been, and still is, poor. He wonders if the NRC plans any actions in this regard.
- 6. The meeting was adjourned at 3:23 p.m.
r NOTE: Additional meetina details can be obtained from a transcript , of this meeting available in the NRC Public Document Room. - 1717 H Street, N.W., Washington, D.C., or can be purchased 1 from ACE-Federal Reporters, 444 North Capitol Street, Suite 402, Washington, D.C. 20001.(202)347-3700. f l l
l BAW-1610 ANALYSIS OF B&W NSS RESPONSE TO ATWS EVENTS 1 JANUARY, 1980 e PERFORMED IN RESPONSE TO NUREG-0460 AND THE REQUEST OF FEBRUARY 15, 1979, FOR "EARLY VERIFICATION" REVIEW EVENTS ANALYZED: 1) LOSS OF MAIN ~FEEDWATER
~ 2) LOSS OF 0FFSITE POWER
- 3) TWO REACTOR COOLANT PUMP COASTDOWN MAJOR ASSUMPTIONS: 1) 95% MTC
- 2) NOMINAL INITIAL CONDITIONS
- 3) NO SINGLE ACTIVE FAILURES
- 4) NO CRA INSERTION, ALL OTHER SYSTEMS FUNCTION NORMALLY
- 5) TURBINE TRIP AND AUTOMATIC AFW (AMSAC) FUNCTIONED AS REQUIRED 0
PLANT SPECIFIC ATWS ANALYSIS FOR A LOSS OF MAIN FEEDWATER SEPTEMBER 1981 e PERFORMED TO DETERMINE PLANT SPECIFIC DIFFERENCES ON PEAK RCS PRESSURE EVE,NT ANALYZED: LOSS OF MAIN FEEDWATER MAJOR ASSUMPTIONS: SAME AS BAW-1610 EXCEPT: ! A) 90% MTC WITH A 1 PCM PENALTY FOR MANEUVERING . B) AFW ADDED PER SG LEVEL DEMAND e FOR COMPARATIVE PURPOSES, BAW-1610 GENERIC RESULTS CAN BE ADJUSTED BY 300 PSI (34G4 TO 3764) TO BE CONSISTENT WITH PLANT SPECIFIC ANALYSIS ASSUMPTIONS. i 4 4 I fl$ h
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.l PLANT IMPROVEMENTS WHICH l REDUCE ATWS RISK i
TRANSIENT REDUCTION PRA BASED RPS SURVEILLANCE SAFETY AND PERFORMANCE IMPROVEMENT PROGAM (SPIP) ROOT CAUSE DETERMINATION PROGRAM TRANSIENT ASSESSMENT PROGRAM TRIP RELIABILITY RTB MAINTENANCE AND SURVEILLANCE SHUNT TRIP RTB ONLINE MONITORING GENERIC RTB MAINTENANCE GENERIC RPS MAINTENANCE PROGRAM l SPIP l ROOT CAUSE DETERMINATION PROGRAM TRANSIENT ASSESSMENT PROGRAM ) 4 l
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1 l ATWS ANALYSIS MTC l 1979 VALUE 1988 VALUE l ELANT CLASS 1 2750 MWT (14X14) -0.20 -0.26 l l l 3410 MWT -0.63* -0.50 i 3800 MWT -0.68* -0.57
- ESTIMATED WITH LIMITED OPERATIONAL DATA FROM PREVIOUS DESIGNS F/d ' I
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ATWS ANALYSIS IMPACT CHANGE IN PEAK PRESSURE PLANT CENPD-263 ATWS RULE' MTC NET CLASS (1979) (1982) (1988) CHANGE (1979-1988) 2750 MWT 4220 PSIA -258 PSIA -50 PSIA ! -308 PSIA (14 X 14) l l l l 3410 MWT 4290 PSIA -347 PSIA +210 PSIA : -137 PSIA 'l I 3800 MWT 3800 PSIA -882 PSIA +200 PSIA l -682 PSIA l INCLUDES TURBINE TRIP ESTIMATED l i j l l FN. If l
e* Average Westinghouse U.S. Plants '
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T MTC: BOL, HFP, ARO, EqXe N O- 7;. . ,
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l Y0G/NESTINGHOUSE PROGRAM OVERVIET , OBJECTIVES
- DOCUMENT APPLICATION OF ATNS RULE / BASES TO TESTINGHOUSE PYR FEATURES l
- DEFINE GENERIC PROCESS FOR UCENSEE USE APPROACH !
- DEVELOP MORE DETAILED ATWS RISK MODEL - BASE UNE
- DEVELOP IMPLEMENTATION GUIDEUNES TO EVALUATE CH.GGES SCHEDULE
- PROGRAM INITIAUZATION: FEBRUARY,1988
- PROGRAM COMPLETION: MAY,1988
- Y0G/TESTINGHOUSE NRC INFORMATION MEETING: MAY,1988 l
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/ *
,' , o . RISE MODEL GUIDEINES: SEVERE ACCIDENT POIJCY; C0RE MELT FREQUENCY GUIDEINE; < 1.0 E-4 /RY SECY-83-293: ATNS CONTRIBUTION < 10 PERCENT OF TOTAL CORE MELT FREQUENCY; 1.0 E-5 / RY ATWS RISE MODEL CAN BE USED TO ETALUATE THE EFFECTS OF CORE /NSSS CHANGES ON CORE MELT FREQUENCY PROGRAM PRODUCTS e BASEllNE RISE MODEL DEFIhTr10N , 1
- RISE MODEL SENSITnTIT STUDIES i
e IMPLEMENTATION GUIDELINES 14 o
ATUS RISK MODEL / BASE IJNE MODEL CONSIDERATIONS CURRENT FREQUENCY / DISTRIBUTION OF INITIATING EVENTS
- AVAILABILITY OF REACTOR TRIP OPERATOR MANUAL SCRul/ MANUAL
-CONTROL R0D INSERTION
- AVAILABILITY OF MAIN FEEDWATER
- AVAILABILITY OF AMSAC AVAILABILITY OF AUXILIARY FEEDWATER AVAILABILITY OF LONG TERM SHUTDOWN -
AVAILABILITY OF CONTAINMENT HEAT REMOVAL AVAILABILITY OF PRESSURE RELIEF TO MAIN'I'AIN PEAK PRESSURE <3200 USING EXISTING ANALYSIS / EVALUATION BASIS CORE REACTIVITY FEEDBACK CHARACTERISTICS N N 0 0 --}}