ML19224B733
| ML19224B733 | |
| Person / Time | |
|---|---|
| Site: | Crane  |
| Issue date: | 06/01/1979 |
| From: | Holahan G Office of Nuclear Reactor Regulation |
| To: | Rosztoczy Z Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 7906220157 | |
| Download: ML19224B733 (5) | |
Text
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pa nic u UNITED STATES
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NUCLEAR REGULATORY COMMISSION
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JUN 0 1 ;379 MEMORANDUM FOR:
Zoltan R. Rosztoczy, Chief, Analysis Branch, DSS FROM:
G.M. Holahan, Analysis Branch, DSS THRU:
L.E. Phillips, Section Leader, Analysis Branch, DSS
SUBJECT:
RECOMMENDATIONS RELATIVE TO REESTABLISHING NATURAL CIRCULATION AT TMI-2 IN THE EVENT A LOSS OF NATURAL CIRCULATION The purpose of this memorandum is to document the results of calculations performed to determine the most efficient method of reestablishing natural circulation in the Three Mile Island Unit 2 reactor in the event of loss of natural circulation from the present mode of operation (Steam Generator A steaming).
Guidance relative to the expected system response to various possible actions is included as well as recomendations relative to actions to be taken in the event of loss of natural circulation.
The natural circulation calculations presented here were performed with a comput' program written especially for this purpose.
The results of these calculations have been compared to available B&W and INEL calcalations to assure consistency. The program used for these calculation consists of a reactor coolant system representation with 97 equal volume nodes. At each time step the temperature and density in each node are calculated using the loop flow rates from the previous time step. The density and elevation of each node are then used to calculate new loop flow rates. Heat Transfer is modelled in the core and steam generator nodes. Boundary conditions are established on the shell side of the steam generators by specifying fluid temperature and heat transfer Jefficients for each steam generator node as a function of time. The various modes of operation of the steam generators are therefore modelled by charging the boundary conditions on the steam generator shell side.
For the cases included in this study, tile temperatures and heat transfer coefficients for the various modes of operation were calculated by hand.
In all of these calculations a core resistance of 4000 (i.e., 425 times _
normal) was used.
The model was used to predict the loop flow rate with the present conditions, that is, with OTSG A steaming. The calculated results show loop A flow of 143 lb/sec., loop B flow of -4.5 lb/sec., and core flow of 138 lb/sec.
This core flow is in good agreement with the best estimate core flow of 136 lb/sec.
whicn has been obtained usina the cold leg anc hot leg RTD measurements from TMI-2.
The actual loop B flow rate may be zero or slightly positive.
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7906220/5 7
Zoltan R. Rosztoczy A summary of the results of the cases studied follows:
Assumed Initial Conditions Loop A Flow 143 lb/sec.
Loop B Flow
-4.5 lb/sec.
Cases 1 through 5 assume the water in the lower elevations of Orig B is initially at 139.5"F.
Cases 6 through 8 assume the water in the lower elevation of OTSG B is initially at 160*F.
_ Case 1:
Doenina of the Bypass Valve on OTSB B (Valve area < Valve area of OTSB A) 1.
Loop "B" flow increase until the cold slug of water (initially in OTSB B and cold leg) reaches the core; 2.
Loop "B" flow stagnates then oscillates positive and negative (period --1000 seconds.'
3.
Loop "A" flow oscillates between +70 lb/sec. and +140 lb/sec.
(period 1000 seconds).
Case 2: Opening of Bypass Valve on OTSG B (Valve area ~ Valve area on OTSG A) 1.
Loop A and Loop B flows increase until the cold slug of water (initially in OTSG B and colo leg B) reaches the core.
2.
Loop A and Loop B flows decrease to l/2 the initial values as the cold water passes thrcugh the core.
3.
Loop A and Loop B flows increase and stabilize at approximately equal values with a total core flow of 170 lb/sec.
4.
Both hot leg temperatures decrease continuously during the first hour and stabilize.
Case 3: Ooening of Bypass Valve on OTSG B (Full Coening of Valve; Area >
Area on OTSG A Loop B ficw increases rapidly to 143 lb/sec. while Loop A flow decreases 1.
from 143 lb/sec. to 104 lb/sec.
2.
Both Loop B and Loop A flows decrease and reach minimum values at -- 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />; Loop A flow equals 12 lb/sec. and Loop B equals 110 lb/sec. 253 183
Zoltan R. Rosztoczy 3.
Loop A flow increases and stabilizes at 65 lb/sec. at 2.2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />.
Loop B flow continues to decrease until it stabilizes at 102 lb/sec. at 2.2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />.
4.
Total core flow at 2.2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> is 167 lb/sec.
5.
Both hot leg temperatures decrease continuously during the first 2.2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> and then stabilize.
Case 4: Auxiliary Feedwater (500 GPM) is Turned On In OTSG B for ~ 5 Minutes, Byoass Valve on JTSG B is also Opened at Time Zero.
1.
Loop B flow increases steadily from -4.5 lb/sec. to 131 lb/sec. in 20 minutes, during tnis same period Loop A flow drops rapidly to zero.
2.
Loop A flow increase slightly to 4 lb/sec. at ^ i hour then returns to zero.
3.
Loop B flow stabilizes at 131 lb/sec. (this value may be less than the initial Loop A flow rate because of the reduction in T associated cold with the use of auxiliary feedwaterl.
4.
Both hot leg temperatures decrease continuously during the first hour.
Case 5: Auxiliary Feedwater (500 GPM) into An Initially Empty OTSG B 1.
Loop B flow increases steadily from -4.5 lb/sec. to +149 lb/sec. during the first 20 minutes while Loop A flow decrease to 7 lb/sec.
2.
Loop A flow continues to decrease until it reaches zero at-- 40 minutes.
At this time the Loop B flow has stabilized at 148 lb/sec.
3.
Both hot leg temperatures decrease contir.uously during the transient.
Note that auxiliary feedwater flow must be stopped at-- 15 minutes if norma'. water level is to be maintained in OTSG B.
This would result in a decrease in Loop B flow to a- '" lb/sec.
If auxiliary feedwater flow is not stopped, OTSG B will fill at -- 30 minutes.
At this point flow should be transferred to the main feedwater system at a flow rate of 1000 GPM or greater. The Loop B flow rate will remair at 148 lb/sec. or greater depending on the feedwate, flow rate.
It should be noted that the B loop inlet temperature will drop durina water solid operation from 150 F to,110 F to 140 F again depending on the feedwater flow rate.
Case 5:
Ooenino of Byoass Valve on OTSG B ( Area > Area OTSG A) 1.
Loop B flow oscillates between +100 lb/sec. and -80 lb/sec.
2.
Loop A flow oscillates between +100 lb/sec. and + 183 lb/sec.
3.
Loop flows do not stabilize within 3.6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and do not appear to be stabilizinc.
~
253 i8o 4
The hot lea tcmoaratures remain aooroximatelv constant.
+2 F
Zoltan R. Rosztnczy Case 7: Openina of Bypass Valve on OTSG B ( Area ~~ Area of OTSG A Bypass Valve) 1.
Loop B flow increases slowly and continuously to 81 lb/sec. at ~ 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br />.
2.
Loop A flow decreases slowly and continuously to 122 lb/sec. at~3 hours.
3.
Loop flows will stabilize at approximately equal values (
100 lb/sec. each) wichin 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />.
4.
Both loop hot leg * - ceratures decrease continuously during the transient.
Case 8: Openino of Byoc Valve on OTSG B (Full Osening: Area > Area of OTSG A) 1.
Loop B flow increase slowly to 100 lb/sec. at ~ 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />.
2.
Loop A flow decreases slowly to 113 lb/sec. at~2 hours.
3.
Both hot leg temperatures decrease continuously during the transient.
Note that in this case water level in OTSG B was held constant at the initial value which is somewhat lower than the level in OTSG A.
If the level in OTSG B is brought up to the 100% level the loop B will increase and be slightly greater than the flow in loop A.
Conclusions and Recommendations 1.
Cold water in the lower elevation of OTSG B aid in the B loop cold leg has an adverse effect on the start up of flow in the 8 loop. The transition to flow in the B loop is much smoother and more predictable if the inlet temperature in the B loop is close to the inlet temperature in the A loop. Therefore, we recomend that, if possible, the Loop B cold leg temperature be increased to approximately the' temperature of the Loop A cold leg.
2.
The cases snow that reduced flow conditions are associated with either low water level in one of the two OTSG's or with too little heat transfer in thec steam region of OYSG (associated with too small an opening of the bypass valve). These conditions should be avoided. This concern has been properly reflected in the TMI2 emergency procedure for loss of natural circulation (EP-34).
3.
There are several modes of operation which provide adequate core flow during natural circulation.
The following list presents the possible modes of operation in order cf decreasing core flow rate:
Mode 1.
Water solid operation of both OTSG's.
Mode 2.
500 GPM auxiliary feedwater to each OTSG to raise the level to 5 ft. above 100%; folhwed by steaming of both OTSG's.
Mode 3.
Wates solid operation of one OTSG.
9-
Zoltan R. Rosztoczy Mode 4.
Steaming of boch OTSG's.
Mcde 5.
500 GPM auxiliary feedwater to one OTSG to raise the level from 100% to 5 ft. above 1005; followed by steaming of one OTSG.
Mode 6 500 GPM auxiliary feedwater to one initially empty OTSG, followed by steaming of one OTSG.
Mode 7.
Steaming of one OTSG.
Since OTSG A is operating in a steady-state steaming condition, the first mode ;; not readily achievable. The maximum ccre flows possible are there-fore acnieved by using auxiliary feeddater on both OTSG's (allowing the levels to increase to 5 feet above the 100% level), and by continuing to steam on both OTSG's after auxiliary feedwater flow is stopped (Mode 2).
In order to establish approximately equal steaming on both OTSG's auxiliary feedwater flow should be supplied in equal quantities to ea:h OTSG. This implies that approximately equal levels should be maintained in the two OTSG's during steady-state operation of OTSG A.
The use of auxiliary feedwater flow into an initially empty 0TSG B (Mode 5) is a viable citernative. However, it is unlikely that loop A flow could be maintained during the period of time of the auxiliary feedwater flow into OTSG B.
This would probably result in only flow in the B loop and at a lower flow rate than achievable from the steaming of both OTSG's.
The present emergency procedure EP-34 calls for the use of auxiliary feedwater in one OTSG after steaming on both OTSG's has been attempted. We recommend that the emergency procedure be modified to call for ecual auxiliary feedwater flow (500 GPM) in each of the two OTSG's (Mode 2), followed by operation of both OTSG's in the steaming model holding *.he water levels at 5 feet above the 100% operating level.
gA, I%
G.M. h]lahan Analys"s Branch Division of Systems Safety cc:
R. Mattson R. Tedesco V. Stello R. Vollmer AB Personnel T. Novak G. Mazetis S. _Is rael
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