ML20041D335
| ML20041D335 | |
| Person / Time | |
|---|---|
| Site: | Midland |
| Issue date: | 03/01/1982 |
| From: | Shiek K BABCOCK & WILCOX CO. |
| To: | |
| Shared Package | |
| ML20041D330 | List: |
| References | |
| 51-1131546, 51-1131546-00, BWNP-20440, NUDOCS 8203050239 | |
| Download: ML20041D335 (13) | |
Text
--
BWNP-20440(4-80)
BABC0CK & WILCOX - NPGD Enclosure I to serial 16o08 ENGINEERING IflFORMATION RECORD I
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PDR ADOCK 05000329 h
E111131546 00 Loss of Feedwater (LOFW) Accident I.
INTRODUCTION This report provides a sumary of the results, assumptions, and methods used to evaluate the response of the Consumer's Power Company Miifland plant to a LOFW event.- The LOFW was considered total, main and auxiliary, until operator action was taken to resolve the incident. Two actions were considered:
(1) operator activation of one high pressure injection (HPI) train, or (2) operator activation of auxiliary feedwater (550 gpm),
II.
SUMMARY
AND CONCLUSIONS
~ Based on the results of this analyses the following conclusions are made:
1.
Consistent with the operating procedures, it was assumed that the opera-tor would initiate one HPI (Intnl by 201ninutes. The results show, that, for
~
1.0SiSDecayHeat,oneHPIaloneprovidessufficientmakeuptoprevent core uncovery. The core decay heat is removed via feed and bleed pro-cess through the pressurizer safety valves and power operated relief valve (PORV).
2.
The activation of ARI at 25 minutes will successfully mitigate the ac-cident, assuming 1.0 ANS Decay Heat and no HPI. The plant would con-tinue to expel fluid through the pressurizer, PORV only, unt:' approxi-mately 73 minutes and thereafter return to natural circulation with a gradual cooldown and depressurization.
In sumary, a safe shutdown condition can be maintained for the itidiand plant following a LOFW accident if one HPI or ARI is actuated within 20 minutes.
.g.
- ~ ~ -
5111131546 00 III. itETHODS A!!D ASSUftPTIOf!S The analysis in this report was performed using a six-node CRAFT r;:odel to '
evaluate the transient behavior of the primary system hydrodynamics.
Figure 1 shows a schematic diagram of the model. Node 1 comprises the cold leg pump discharge piping, the reactor vessel downcomer, and the lower ple-num. Mode 2 represents the primary side of the steam generators, and node 3 represents the core, upper plenum, and the hot leg piping. Nodes 4', 5 and 6 represents the pressurizer, containment and the secondary side of the steam generators, respectively. The assumptions used in the analysis
~
are listed below:
1.
The reactor is operating at 102% of the steady-state power level of 2772 HHt.
2.
Loss of main feedwater flow to the steam generator occurs at time zero.
For the activation of HPI (500 gpm, 600 psi) at 20 minutes, no AFW was available. For the activation of AFW at 25 minutes no activation of HPI was considered.
3.
Offsite power is not available.
4.
The reactor trips on high pressure at 2300 psig.
5.
No credit is taken for operation of the PORV. (for one HPI case only) 6.
The pressurizer safety valves start to open at the set pressure of 2500 psig. They are assumed to be full open at 103% of the set pres-sure.
7.
The discharge rate through the code safety valves is calculated using
' the Bernoulli equation, for subcooled fluid discharging through the valve. The flow area utilized for the. safety valves was chosen such that the Moody calculated discharge rate, for steam flow through the valve at the valve rated pressure, is equivalent to the design capa-city of the valve.
.3
511113154G 00 8.
Actuation of one HPI train, via operato action at 20 minutes, was assumed for the appropriate case.
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t 9.
In order to simulate the realistic decay heat curve,1.0 times the 1971 ANS Standard was utilized.
A CRAFT run was made for the LOFW accident with the actuation of one HPI at 20 minutes. Then, a hand calculation was performed based on the data from this run at 20 minutes to evaluate the liquid inventory for the case with the actuation of AFW at 25 minutes and no HPI flow. A constant AFW flow rate of 550 GPM with the inlet enthalpy of 58 btu /lbm is used. The AFW is assumed to be heated to a saturation temperature corresponding to the secondary s'ide pressure by absorbing energy from the primary side.
The volume balance in the primary side is perfomed to detemine the mass i
flow rate through the PORV.
IV. RESULTS Activation of HPI at 20 flinutes Figures 2 through 4 show the transient system responses predicted by the CRAFT model assuming actuation of one HPI at 20 minutes. The following '
table presents the sequence of events for this analysis:
Sequence of Events Time, s l.
1.
Loss of main feedwater, turbine trip, loss of O.0 offsite power 2.
Reactor trip on high pressure 8.0
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~
3.
SG inventory boiled-off
~
100.0 4.
Pressurizer goes solid (indicated) 350.0 5.
Pressurizer code safeties open 400.0 6.
Long term cooling established - one HPI 8900.0
-+-
w1
=
5111131546 00 Following the simultaneous LORI and offsite power, the RCS pressure in-creased and reached the high pressure trip setpoint, thus causing the The RCS unde'went a brief depressurization reactor to scram at 8 seconds.
r foll,owing the scram and repressurized rapidly when heat sink in the steam generators was lost. The RCS pressure, as shown in Figure 2, exceeded the PZRsafetyvalvesetpressure(2500psig). The primary system discharged fluid through the safety valves to relieve portions of decay energy as shown in Figure 3.
At 20 minutes, one HPI was actuated and tb system re-mained in a feed and bleed mode of cooling until the long term cooling was established at 8900 seconds. Figure 4 shows that the core remained in a safe condition throughout the transient.
Activation of AFW at 25 Minutes A conservative hand calculation was performed for the case with no HPI and activation of the AFW at 25 minutes. The transient up to 20 minutes is identical to the HPI case reported above. A slight additional mass loss occurs between 20 and 25 minutes at a rate equivalent to that occurring at 20 minutes. Upon activation of AFW (25 minutes), the mass loss is severely reduced and the system placed in natural circulation. The effect o'f the AFW was treated very conservatively in that none of the AFW was allowed to boil. Thus the ARI could not fully remove decay heat until 4400 seconds and some alternative system energy loss path was required.
That path would be through the PORV. After 4400 seconds the AFW will re-move all decay heat, the system will slowly ddpressurize and gradually re-turn to solid subcooled natural circulation.
Normal makeup flow would supply water to replace system volume contraction during the cooldown.
Tha following table presents the sequence of events.
-m e
51 1131546. 00
- ' Seouence of Events Time, s 0.0 1.
Loss of main feedwater, turbine trio, loss of offsite power 8.0 2.
Reactor trip on high oressure 100.0 3.
S.G. inventory boiled-off 350.0
- 4. 'Pressuri:er goes solid '(indicated) 400.0 5.
Pressurizer code safeties'open 1500.0 6.
AFW actuated 4400.0 7.
Pressurizer PORY closes 4400.0 8.
Long term cooling established Figure 5 shows that liquid inventory is sufficient to maintain the core covered throughout the transient. Figure 6 shows the system pressure transient. As the AFH was initiated at 25 minutes, the system pressure dropped to the PORV setpoint (2300 psig) and renained at this pressure until 4400 seconds when the AFW matched the decay heat.
V.
CONSERVATISils Because the transients were evaluated on a generic basis there are several.
conservatisms relative to the expected behavior of the Midland plant.
These are listed below along with their impact:
Core Power - The power level assumed in the analysis was 2772 f1Ht.
1.
3 e
,1 The flidland power level is 2452 11Wt. This is a 13% difference and For flidland the generally has a speeding up effect on the transient.
transients would develop more slowing and cooling mechanisms would j
be more effective. Use of the code safeties would be reduced.
Reactor Trip Time - The high pressure trip function was used in the 2.
l tiidland has an anticipatory reactor trip on loss of feed-l analysis.
It is anticipated that this trip would have occurred at about water.
1 i.
.==w
5111131546.00 5.0 seconds. Three full power seconds extra heat is significant to
- these evaluations. The effect would be similar to the power level in that there would be a general slowing down of the transient and the time during which the code safeties were employed would be shortened.
3.
PORV Usage - For the analysis no use of the PORV prior to 25 minutes was made. Midland has a safety grade PORY which could have been em-
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ployed and would have actuated at approximately 2250 psig. The com-bined effect of the PORV, the lower power, and the earlier trip time will reduce the time the code safety is challenged prior to 20 or 25 minutes allowing operator to actuate the AFW. The effect on the HPI activation case would not be significant and the safeties would still be challenged for that case.
4.
AFW Cooling Rate - In the AFW activation case, the AFW was only heated
(
to saturation.
It was not allowed to boil.
In reality a great deal of boilding would occur while the RCS was at high temperature. Boil-ing would be more limited after the RCS cooled. The effect would be -
to almost double the heat removal capability of the AFW and, therefore, bring about long term cooling and an end to system mass loss instan-taneously with the activation of AFW.
5.
AFW Flow Rate - The analysis employed only 550 GPM. The Midland AFW system provides a minimum of 885 GPM. This would again increase the f
effectiveness of the AFW in performing system energy removal.
_1
51
- 1131546, 00 Mj. /
CRAFT 2NODlnGDIAGRAMFORSMLLBREAKS (6 NODE MODEL)
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