ML20246D413
| ML20246D413 | |
| Person / Time | |
|---|---|
| Site: | Zion File:ZionSolutions icon.png |
| Issue date: | 07/06/1989 |
| From: | COMMONWEALTH EDISON CO. |
| To: | |
| Shared Package | |
| ML20246D405 | List: |
| References | |
| NUDOCS 8907110319 | |
| Download: ML20246D413 (27) | |
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ATTACW pl_2 ZION NUCLEAR POWER STATION l
l DESCRIPT_1Q[L.Jdip REASON FOR PROPOSED MMDMERT_TD TECHNICAL SEECIf1 CAT 10EL__SEf110N 3._15 l
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Attachment.]
l Rescristinn_andLRemann_Lar Prop 9125Lhmendm_ent 1 )
This proposed Technical Specification change requests a one time relief to l perform the preventative maintenance on DG "0" while continuing operations on J Zion Unit 2. Because DG "0" is shared between the two unita and existing Technicr1 Specification 3.15.1.B requires DG "O" to be operable whenever either unit IF in operation, Zion Station is unable to perform the extended i maintenance recommended by the manufacturer every 5 years.
Under the present Technical Specification, reactor operation is allowed for only 7 days if one of the three unit DGs is found inoperable. However, this proposal requests a period of 45 days to take DG "O" out-of-service for maintenance during the Unit 1 Fall 1989 Refueling Outage.
The need for a 45 day outage period is based on the fact that in 1983, the first mejor overhaul was performed on DG "2B" by the Zion Station Mechanical Maintenance Department. This overhaul took 46 days to complete. Using this period as a reference, the station requested and received a 45 day outage period for DG "0" in 1984, (reference 1).
The overhaul of DG "0" in 1984, took 44 days to complete with some work deferred due to time restraints. DG "0" has to date hed more starts and less maintenance than the other DGs due to Technical Specification constraints.
Problems identified during the first overhaul of DG "0" indicate that continued attention will be necessary to component change-out on an overhaul time frame in order to achieve the reliability demands of Station Disckout and/or Safeguard conditions.
More recent major .erhauls on the other Zion Station DGs have been done by the engine manuf acturer in a reduced time f rame and the engine manuf acturer is scheduled to perform the overhaul on DG "0" during the Fall 1989 Unit 1 Refueling Outage. However, it must be mentioned that engine overhaul is not the only work scheduled on DG "0". Table 1 provides a list of all items to be performed on DG "0".
Although DG "0" has never been deprived of necessary maintenance, some maintenance activities have had to be deferred due to Technical Specifications time constraints. The only outage period long enough to perform all maintenance activities on DG "0" is the once every 5 year major overhaul. The benefit of this extended outage will be improved reliability of DG "0". Also, the modification to be accomplished will enhance and simplify DG "O" control and transfer capabilities when aligning to either unit ESF bus. The 5 year overhaul outage period is considered the major contributing factor toward a significant improvement in the availability of all 5 Zion Station DGs.
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AITACliMU!i'll TABLE 1 Diesel Generator "0" Unit 1 Fall 1989 Outage Overhaul It' ems
- l A. Diesel Engine Itemst
- 1) Replace 16 heads
- 2) Replace 16 fuel injectors j
- 3) Replace 16 fuel injector pumps
- 4) Replace turbocharger
- 5) Replace 4 coolers a) 2 Turbocharger coolers b) 1 Lube oil. cooler c) 1 jacket cooler
- 6) Pull, inspect and/or replace at least 2 pistons and liners
- 7) Pull, inspect and/or replace 2 shaft-driven pumps-
- 8) Pull, inspect and/or replace 2 aux pumps NOTE: Approximate time to complete this work is 28 days with 3 days to perform balancing testing including a 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> performance run.
B.. DCRDR modification to add a set of controls to the Unit 2 Main Control Board for starting DG "0" including voltage and speed control. This will require 2 days for testing.
C. Install I4od-81-09 " Synch Check" on both unit breakers for DG "0". This will require 2 days for testing.
D. Paint DG "0". This will take about 6 days.
E. There are 109 outstanding work request for Unit 1 and DG "0". 82 of these are outage related and 9 deal with modifications or modification testing.
- .The time estimates in Table 1 are based on a schedule of 3 eight hour shifts, six day per week. This is in contrast to previous outages, when the work schedule did not include around the clock coverage. !
The modification listed in items D and C of Table 1 are long standing modifications which can best be performed during an extended outage. Both modifications are scheduled so that as much work as possible will be done concurrently with the engine overhaul. The DCRDR modification is the subject of an NRC commitment pursuant to NUREG-0737 supplement 1.
l The synch-check relay installation is designed to protect the diesel generator and its loads from damage caused by the generator being synchronized to the grid out of phase with the Edison system. Similar modifications have j been performed on other diesel generators at all Edison p3 ants. !
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AIIAClEENT 3 i' ZION NUCLEAR _ POWER STATIM P.EQP_QST&_AMEEDMENT TO TECHNICAL SPECIFICATIONS, SECTION 3.15 l
l EYALIIAIJW_DF SIGNIFICATE _HAKAEDS CDUSlDIEhTICE l
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l EyJLUATION OF SIGNIFICANT HAZARDS CONSIDERATION PROPOSED E EjLTO ZION TECHNICAL SEECIElCAIIDH APPENDIX A - SECTION 3.15 AND 4.15 AUXILIAEX_ELECIELCAL POWER SYSTEM DESCRIPTION OT AMENDMEIC REQUEST An amendment to the Zion Facility Operating License As proposed to allow a 45 day outage for DG "0" during the Unit 1 Fal) 1989 Refueling Outage.
BACKGROUND l
l 10 CFR 50.92 states that a proposed amendment will involve a no significant hazards consideration if the proposed amendment does not
- 1. Involve a significant increase in the probability or consequences of an accident previously evaluated; or
- 2. Create the possibility tf a new or different kind of accident from any accident previously evaluated; or
- 3. Involve a significant reduction in a margin of safety.
The discussion below addresses each of these three criteria and demonstrates that the proposed amendment involves a no significant har,ards consideration.
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... 4 ATTACIMEtiL1 (Continued)
Basis for no significant hazards consideration:
1 The justification for no significant hazards consideration in based on the following factors:
- 1. The high reliability of the Commonwealth Edison electric system.
- 11. The small effect of the proposed outage on predicted core melt frequency, as demonstrated by a probabilistic risk assessment (PRA) based analysis.
- 111. Increased surveillance frequency and additional action statements which will ensure that all ESF components (other than those powered by bus 247) are operable with both normal and alternate AC power at I all times during the O diesel generator outage.
iv. Consideration in the Zion Emergency Operating Procedures (EOP's) of proper operator action in the unlikely event that only one of three 4160 volt ESF busses is available during an accident.
Attachment 4 summarizes the conclusion of the Zion Probabilistic Safety Study regarding Loss of Offsite Power (LOOP). Due to the reliable design of the switchyard ring bus (featuring six incoming lines) and the historical stability of the Edison system, the probability of total loss of offsite power at Zion Station is considerably less than the industry average.
l Table 1 to this Attachment provides a list of ESF equipment powered from I each ESF bus and Table 2 identifies the minimun number of components required to meet the accident analyses. Table 3 provides references identifying the bases for minimum component requirements.
The Commonwealth Edison PRA group has analyzed the probability and consequences of a LOCA coincident with total' loss of offsite power, for both the normal case and the case of a 45 day 0 D/G outage. The resu3ts are presented in Attachment D. The largest contributor to risk was found to be a hypothetical event involving the small break LOCA with simultaneous loss of all offsite power and failure of ESF bus 248 or 249 to energize. The analysis shows that a 45 day diesel outage does incresse the probability that charging will not be available during a LOCA. However, the PRA has used deterministic methods (also discussed in Attachment 5) to ane. lyze the probability of core melt with one of four charging or SI pwnps available. The analysis shows that the risk of core melt is not significantly increased by the 45 day outage, because coolant is supplied by the available SI pump. This conclusion is consistent with the Zion Probabilistic Safety Study (ZPSS), which requires only one of four charging or SI pumps for safe shutdown from a LOCA. A description of the methods and computer codes used for this analysis is also incorporated into Attachment 5.
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_ _ _ _ _ _ _ _ _ __________________________-____________a
ATTACHMENT _3 (Continued)
In response to NRC concerns, the Commonwealth Edison Nuclear Fuel Services department has analyzed the effects of the 45 day O diesel generator outage on l containment pressure reduction capability during a postulated accident. The l accident most sensitive to containment pressure reduction concerns is the large break LOCA. Using NRC approved methodology, a containment integrity analysis was run assuming a large break LOCA with only one containment spray pump and two Reactor Containment Fan Cooler's (RCFC's) operable. The analysis showed that peak containment pressure was still within design pressure under this scenario. Additionally, the PRA analysis presented in Attachment D shows
! that the large break LOCA with only one of the three ESF busses available is not a significant contributor to risk, Page 255b of the proposed Technical Specification shows the additional surveillance and action statements which will apply during the proposed 45 day outage. The action statements proposed are more limiting than Zion's existing Technical Specifications, and include additional actions which were not in effect during the similar 45 day O diesel outage of 1984.
Specifically, the Unit 2 diesels 2A and 2B will be tested and proven operable every 14 days. Failure of one of the unit 2 diesels would require shutdown within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />, per Zion's existing Technical Specifications.
Failure of 1A or 2A diesel would require immediate and daily testing of all co.cponent cooling (CC) and service water (SW) pumps which are powered by the three remaining operating diesels. This will ensure that the required CC and SW pumps (and their normal and alternate AC power supplied) are operable at all times for Unit 2. Operability of the remaining ECCS components on Unit 2 busses 248 and 249 is already required by existing Zion Technical Specifications, particularly Technical Specification 3.0.5.
This schedule of action statements does not reduce the margin of safety because it ensures the operability of all components required for Unit 2 operation. In the event of a IA or IB diesel failure, the only effect on Unit 2 would be the at allability of SW and CC pumps. Three SW and three CC pumps would still be guaranteed Operable with emergency AC power supplied by diesels 2A and 2B and the operable Unit I diesel.
Th9 Zion EOP's have been reviewed for applicability during a LOCA with only one ESF bus available. The review showed that the possibility of having either one available charging pump or one available SI pump is accounted for in the EOP's, and that the procedure allows for unit shutdown following a LOCA using either a single charging pump or a single SI pump.
The diesel generator test circuitry is designed so that EST component actuation is not interfered with in the event of a safeguards actuation during a diesel test. This is confirmed every outage during performance of procedure T.S.S. 35, which simulates loss of offsite power and Safeguards Actuation and verifies proper operation of the circuits which sequence EST loads onto the diesels.
Based on these analyses, Commonwealth Edison has concluded that the probability and consequences of accidents previously evaluated are not significantly increased, the possibility of a new accident is not created, and the margin of safety is not significantly reduced. We believe this request meets the criteria for no significant hazards consideration.
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l AIIACIRiENI__4 ZION NUCLEAR POWER STATION l
EXCERPT FROM ZION PROBABILJSIlf EAEETY STUDY, VOLUME 3 I
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- s ATTACHMENT 4 Excerpt from Zion Probabilistic Safety Study. Vol. 3 It is assumed in this study that all initiating events ultimately result in a trip of the main generator on the affected unit. Since each of the Zion units is rated at 1085 MW(e), the instantaneous loss of this input could have a'significant effect upon the stability of the offsite power supply network l: due to reduced transmission voltages, frequency fluctuations, or power flow
'- imbalances as the grid recovers from the transient. The Commonwealth Edison transmission network has been designed to provide a stable power supply grid uader conditions of multiple, large generating unit and major transmission line outages. Utility interconnections to the north, west, south, and southeast. provide excellent geographical stability. Two of these tie lines (to the Wisconsin Electric Power Company) directly supply the Zion 345 kV switchyard, providing increased diversity in the power sources for this specific site. Detailed guidelines have been established for the entire Commonwealth Edison power supply network which define the basis for system operation under a wide variety of steady-state and transient conditions. A prime consideration in the establishment of these guidelines is the requirement that no single loss of a generating unit or transmission facility should result in an unacceptable condition of degraded system operation.
System operating contingencies are defined by these guidelines and specify the need to provide additional generating capacity from Commonwealth Edison's own facilities or to provide power from network inter ties long before critical operating stability limits are approached. A detailed voltage reduction and selective load shedding program is also specified in order to maintain grid stability with adequate margins under the most severe conditions. Detailed system stability studies have been performed to verify the efficacy of these operating guidelines under a wide range of scenarios. The combined effects of these guidelines and the overall design stability of the Commonwealth Edison transmission network are demonstrated by the fact that there has never been a major grid f ailure or a local f ailure of the offsite power supply to Zion as a result of the loss of a single generating unit. (In fact, there has never been a loss of all offsite power at Zion from any cause.)
The assignment of'a distribution for the probability of losing all of f-site power to the Zion switchyard as a result of the trip of one of the Zion units is an extremely difficult task. Factors affecting this condition are total system load, available spinning reserve capacity, the fraction of the load being supplied from the Zion units, the status of neighboring utilities' networks, scheduled and unscheduled outages of specific generating units and transmission lines, etc. The analysis of this problem presented in WASH-1400 applies a median value of 10-3 for the conditional loss of offsite power as a result of a unit trip.*
- WASH-1400, Appendix II, page 34.
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j ATTACFMEliT 4 (Continued) l Several factors limit the applicability of this value to the specific problem faced in this study. The WASH-1400 distribution was developed from a review of Federal Power Commission studies of power supply networks in a wide variety of locations east of the Rocky Mountains. While it may be applicable I to the composite site studied in NASH-1400, it is certainly not directly applicable to the Zion site. It must also be recognized that the reference study is now several years outdated and that significant advances in the design, operation and overall stability of virtually all transmission networks in the United States have been made during the intervening years.
Unfortunately, no specific studies have been conducted to address the i precise problem posed at Zion. However, a review of the available l Commonwealth Edison system design criteria and operating guidelines, several l grid stability studies focusing specifically upon the Zion site, and conversations with Commonwealth Edison engineers have provided a significant amount of applicable information. As a result of these inputs, we have assigned the following distribution for the probability of losing offsite power to the Zion switchyard as a result of a trip of either of the Zion units:
Median: 2 x 10-6 failure / unit trip Sth Percentile: 4x 10-7 failure / unit trip 95th Percentile: 1 x 10-3 failure / unit trip The mean and variance of this assumed log normal distribution are Meant 3.38 x 10-4 failure / unit trip variance: 3.26 x 10-5, A few observations must be made in order to place this distribution in a proper perspective. It is our best estimate of a conservative distribution to -
be applied to this analysis only. Although it is broadly baced upon the results of Commonwealth Edison grid stability studies developed for the Zion site, none of the studies reviewed was directly applicable to the problem at hand. Most of the studies adoress a condition less severe than the total loss of offsite power, and those reports developed for the loss of power do not address it from the cause-effect relationship presented in this analysis. For ;
these reasons, we feel that the median value of our distribution is a very conservative estimate for the frequency of this event. However, we also feel that the assigned broad distribution adequately accounts for our uncertainty ,
in this value. The given distribution is thus considered to represent a conservatively bounding estimate for the conditional failure of offsite power, which is as specialized to the Zion site as is possible with the existing information base.
The mean frequency for the loss of offsite power at a generic plant site from the given population, excluding the Zion evidence, was determined to be 0.194 events per site sAlend u yS n. The updated mean frequency for the loss of offsite power at the Zion sits is 0.068 events per site nalendar year-
/sc1:0114T:26 1 i
9 ATTACliMI;NI._4 (Continued)
Excerpt from " Response to the BNL Peer Review of the Zion Probabilistic Study," F. G. Lentine to II. R. Denton, dated 9/6/82:
l A two-stage Bayesian update was performed using the data from Table 1. As suggested by the BNL review comments, the Zion data was excluded from the plant population for the first step of the analysis. The plant population data was applied using the total number of offsite power failure events for each site and the total number of site years listed in Table 1 (i.e., not l
accounting for the effects of unit availability). The generic data was updated using the Zion site specific evidence of no failures in 9 site years.
The resulting distribution provides the specialized calendar ygar frequency of loss of offsite power events at the Zion site, regardless of unit operating conditions. This distribution was then multiplied by the average Zion unit I availability (0.71) to obtain the frequency of losses of offsite power to a Zion unit during_ppyer opeJ.atinn. The parameters of this updated and scaled dist ribution are 5th Percentile: 1.04 x 10-2 failure / unit operating year Median 3.63 x 10-2 failure / unit operating year 95th Percentile: 1.27 x 10-1 failure / unit operating year Mean: 4.85 x 10-2 failure / unit operating year The mean frequency for the loss of offsite power at a generic plant site from the given population, excluding the Zion evidence, was determined to be 0.194 events per site salmndar year. The updated mean frequency for the loss of of fsite power at the Zion site is 0.068 events per site salandAI yEBI.
l
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ATTACHMENT 5 ZION NQCLEAR POWER STATION PRA EVALUATION OF A 15-DAY OUTAGE OF O" DIESEL GENERATOR l
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/sc1:0114T 28
. _ - _ _ _ _ _ _ __ ____________________________________________________0
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ATIACliMENT 5 PRA Evaluation of 45 day "O" D/G Outage with I unit in cold shutdown and 1 unit operating.
We have exanined a number of cases to establish the potential effects of extending the allowed outage time for the diesel. First of all, we consider the.large LOCA's as initial events. In terms of the FSAR analyses, the "O" diesel is immaterial since the FSAR requires one out of two (1/2) RIIR trains and 3/4 accumulators for large LOCA's. The RHR trains operate from emergency buses served by the'non-shared diesel. We have used the MAAP computer code to examine the response of the ECCS to large LOCA events on a realistic basis and find that, for breaks greater than .06 square feet and up to 2.0 square feet, any one of the six ECCS pumps will prevent core melt. Our study did not go beyond breaks of that size since it is judged that the likelihood of even that sized break is extremely small. (Please note that the MAAP study discussed herein covers a wide verlety of cases and is attached as .%ppendix A for your information.) Given the fact that, on a realistic basis, our success criteria is viewed as 1/6 pumps, we could argue that increasing the "O" diesel outage time increases the probability of core melt. We now have to determine if the increase is,.In any way, significant.
We center our evaluation around the probabilities associated with large LOCA and no AC power. Our base point for these derivations is the Zion Probabilistic Safety Study (ZPSS). For the base case evaluation, we look at a LOCA frequency of 9.4E-4 and a total loss of AC to the emergency buses occurring with a conditional frequency of 1.85E-7 (table 1.3.4.1-1). The associated base case core melt probability is the product of these values or 1.74E-10. Now, .how much does this increase if we extend the allowed outage on the "O" diesel to 45 days't By using the same table in ZPSS, we find that the frequency of losing buses.146 and 149 is 2.28E-6. We can also establish a new frequency for the loss of bus 147, given the extended outage, of 2.3E-1. The new associated core melt frequency is determined to be 4.93E-10. The extended outage does have an effect on the core melt frequency but, given the overall large LOCA core melt frequency, from all causes, of about 9.4E-4 times 6.6E-3, or about 6.4E-5, the extended outage has an insignificant effect on large LOCA core melt sequence probability The next step is to bound the issue at the lower end by examining small LOCA events. We take a similar approach to that described above but note that, for small LOCA's the FSAR success criteria does involve the 147 bus since, for the smallest breaks considered, 1/2 charging pumps are required.
The situatit for FSAR success criteria 1st Base case LOCA 4 LOOP + loss of 247 & 248 or LOCA + LOOP 4 loss of 2 chg trains = 7416E-6 45 day OOS case: similarly = 7.1E-5
/sc1:0114T:29 l
4 ATTACIRIEH'L5 (Continued)
We note that this case does result in a significant increase in the estimated core melt frequency, roughly a factor of ten. We cannot judge the effect of this on overall core melt frequency since these values reflect conservative FSAR success criteria. To put the situation in perspective, we have to consider t'ae impact of the 45 day extension on the plant using realistic success criteria such as those in the ZPSS. The ZPSS requires 1/4 charging or hi-head SI pumps be available to prevent core melt. (Our MAAP study confirms this anac moreover, shows that 1/6 of ECCS pumps will prevent core melt for all break siras between .06 square feet and 2.0 square feet.)
Performing.the evaluatitn in a manner similar to that discussed above, we note that the conditional frequency of core melt from all small LOCA events is about 6.33E-4. The frequency of the small LOCA is 3.54E-2. This gives us a total probability of core melt from small LOCA events of 2.2E-5. Now evaluating the effect of a 45 day extension on the "O" diesel, we find a new core melt frequency of 2.2E-5. In short, the extension has no effect given realistic success criteria.
The conclusion we draw from this evaluation is that a 45 day outage for the "O" diesel will not have any adverse impact on core melt frequency associated with LOCA sequences, if realistic success criteria are employed.
G. Klopp PRA Group Commonwealth Edison 1
/sc1:0114T:30
ATTACHMT;;lII_h (Continued)
I DISCUSSION OF THE "MAAP" COMPUTER CODE METHODOLOGY USED TO PERFORM DETERMINISTIC ASSESSMENT OF EFFECT OF 45 DAY O D/G OUTAGE ON PLANT RISK ASSUMING LOCA WITHOUT OFFSITE POWER The following is a discussion of the methods used by the Commonwealth y Edison PRA/IPE group in assessing Zion's capability to shutdown safety l following a postulated small break LOCA coincident with Loss of All Offsite Power (LOOP) during the proposed 45 day O diesel generator outage. It was assumed that one other diesel also falls during the event, requiring unit recovery from the accident with no charging pump available. The small break LOCA was identified as the accident nnost sensitive to charging pump inoperability, because for large breaks the RCS pressure decreases quickly to below the shutoff head of the other ECCS pumps.
The basic analysis tool used for this evaluatlon was the MAAP computer code and its associated Zion parameter file. The code was run on a VAX computer owned and operated by Edison. The results of the MAAP runs were consolidated and plotted using the MATCHAD code (2.0) on an IBM AT.
The MAAP code, while not an NRC licensed code for accident analysis, was l developed by the industry as part of the Industry Degraded Core Program to I
provide a realistic basis for transient evaluation in the area of severe accidents. The code has been benchmarked against established codes and l
experimental data. EPRI reports documenting the benchmarking of this code are listed as references.
The LOCA events postulated were all hot leg breaks. No major impact, if any, would result from using cold leg breaks. Zion recognized that the cold leg small break LOCA is normally the limiting case in FSAR Chapter 14 analysis. This is largely because the codes employed are focusing on Peak Clad Temperature (PCT) as a key variable, and because of the inherent conservatism in normal FSAR analysis methods required by the Appendix K criteria. Using "best estimate" methods and focusing on fuel melt temperature rather than PCT as a key variable, it was found that the difference between l hot leg and cold leg break is not as significant. The time to reach fuel melt l
temperature was found to be much less sensitive to hot leg versus cold leg breaks when using a "best estimate" thermal-hydraulic methodology. This has been confirmed by running the code both ways in previous analyses. Break sizes ranged from 0.005 square feet up to 2.0 square feet. The upper limit of 2.0 square feet was selected subjectively. Clearly, for risk assessments, the design basis break has a vanishingly low importance.
i When engineered safeguards were operational, only the minimum number of trains were allowed to run. Por example, only one RHR train out of two would run. Oals applied to containment safeguards as well.
/sc1 0114T 31
- E 1
(Continued)
Study Methodology The conditions to be analyzed were established in the form of a matrix with the columns consisting of break sizes and the rows consisting of cases" showing various combinations of ECCS availability. A total of nine break sizes was employed and six cases were developed. This led to 54 runs for the MAAP code. Each run was allowed to proceed through 50 hours5.787037e-4 days <br />0.0139 hours <br />8.267196e-5 weeks <br />1.9025e-5 months <br /> of accident time on the VAX. In each case, the operation of ECCS in the recirculation mode was not permitted. A total of six key variables was plotted for each run as a function of time. In addition, the time to empty the RWST (eg. the termination of ECCS operation) was extracted from the data file for each run.
The break cizes evaluated were:
0.005 sq. ft.
0.01 sq. ft.
0.025 sq. ft.
0.05 sq. ft.
0.075 sq. ft.
0.1 sq. ft.
0.5 sq. ft.
l 1.0 sq. ft.
2.0 sq. ft.
The ECCS system combinations employed were:
Case 1, RHR (LPI) only Case 2, RHR and SI only Case 3, RHR and Charging /SI only Case 4, SI only Case 5, SI and Charging /SI only Case 6, Charging /SI only The six data plots made from each of the resulting 54 MAAP runs were time 1: dependent plots of; MCR, the mass of intact core material PPS, the pressure in the primary system MH2CRT, the mass of hydrogen produced in the core PB, the containment pressure MCMC, the mass of molten core material in the vessel cavity MCMB, the mass of molten core material on the containment basemat The variable of major import is the very first one, MCR. It allows one to establish the onset of ccre melt in an unambiguous manrat. Also, for analysts familiar with severe accident work, it allows one to establish the onset of metal water reactions. The other five variabler were selected to provide clarifications, confirmations, and perspectives on the accident sequences being considered.
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/sc1:0114T:32
r ATIACIMEIE_li (Continued)
Once the 324 plots were in hand and had been reviewed for consistency, tabulations were made for each of the six cases of the time to onset of core melt as a function of break size and of the time to empty the RWST as function of break size. It quickly became apparent that only three cases (1, 4 & 6) were of interest. These were plotted using MATHCAD. Another case, case 3, was also tabulated and plotted to verify the consistency of the results.
Et2Jd16 The results for each of the four cases are depicted in figures 1 through 4.
Figure 1 shows the study result for case 1, operating with only the RHR system. Clearly, the RHR system protects the core for the larger breaks (down to about 0.06 sq. ft.) as expected. For the smaller breaks, the primary system remains pressurized above the level at which the RHR pumps can deliver enough water to cool the core. In these circumstances, core melt occurs before the RWST is depleted.
Figures 2 and 3 show the results for cases 4 and 6 respectively. In both cases, the core is protected throughout the injection phase of ECCS operation. This indicates that both the charging /SI system and the SI system will provide adequate core cooling with one pump from either system for the spectrum of breaks considered, Herein lies the merit of this analysis approach. Even the most optimistic subjective evaluations to date assumed that 2 out of the 4 pumps in these systems would be required. By showing that only one pump is required, we have substantially reduced the success criteria
. burden for risk assessment work. We have also provided a fresh perspective on this matter for other purposes.
Figure 4 shows the results for case 3 and simply reinforces the previous results. The figures provide a great deal of other information relative to the timing of events. For exemple, by looking at the figures, it becomes clear that the shortest time to a core melt occurs for case 1 and is about 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> after the start of the accident. More typically, the earliest times are on the order of 4 to 5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> and range up to a full day. These types of perspectives may well prove useful for a variety of purposes.
EDTIRDtlCEB
/sc1:0114T:33
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