ML20054K023
| ML20054K023 | |
| Person / Time | |
|---|---|
| Site: | Haddam Neck File:Connecticut Yankee Atomic Power Co icon.png |
| Issue date: | 03/31/1982 |
| From: | CONNECTICUT YANKEE ATOMIC POWER CO. |
| To: | |
| Shared Package | |
| ML20054K018 | List: |
| References | |
| NUDOCS 8206300308 | |
| Download: ML20054K023 (40) | |
Text
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x NATURAL CIRCULATION TEST AT THE CONNECTICUT YANKEE NUCLEAR POWER PLANT NORTHEAST UTILITIES SERVICE COMPANY CONNECTICUT YANKEE ATOMIC POWER COMPANY MARCH 1982 8206300308 820609 PDR ADOCK 05000213 P
r TABLE OF CONTENTS PAGE List of Tables x
ii List of Figures iii I
Abstract 1
IIA Introduction 4
IIB Plant Description 6
III Pump Trips 7
IV Natural Circulation Cooldown 9
V Cold Slug Test 14 VI Results and Discussion 17 References 20 Tables 21 Figures 22 APPENDIX:
A.
Calculation of Core Flow Using Loop Transit Time l
i
LIST OF TABLES Table No.
1 Sequence of Events for CY Tebts ii
LIST OF FIGURES Figure No.
1 Acurex Data Logger, Plant Dabe Output (Typical) 2 CY RCS Schematic Drawing 3a,b CY Reactor Vessel and Internals 4
Loop ATs - Pump Trip Test Sa,b T and T Natural Circulation Cooldown Test C
HEAD 6
Loop ATs - Natural Circulation Cooldown Test 7
Loop ATs - Cold Slug Test 8
Loop T s - Cold Slug Test C
9 Loop ATs - All Tests 10 TC (L P 3) - Strip Chart Recording During CY Tests 11 Pressurizer Pressure - Strip Chart Recording During CY Tests 12a Pressurizer Level - Strip Chart Recording During CY Tests 12b Pressurizer Level - St. Lucie Natural Circulation Cooldown iii
I.
ABSTRACT On June 11, 1980, at St. Lucie Unit 1,M loss of component cooling water flow to the seals of all four reactor coolant pumps forced these pumps to be stopped. During the subsequent plant cooldown and depressurization, under natural circulation, anomalies in pressurizer level response were observed.
It was later determined that the reactor vessel head temperature lagged behind the reactor coolant system temperature during this cooldown and that flashing had occurred, producing a steam bubble in the head. The flashing and condensing of this steam produced the observed increases and decreases in pressurizer level.
Although the formation of the steam bubble at St. Lucie didn't prevent a safe plant shutdown, it did stress the need for an improved understanding of system response and behavior during natural circulation operation.
Natural circulation tests uere performed at the Connecticut Yankee Nuclear Power Plant (CY) prior to its 1981 refueling. On September 26, 1981, approximately 10\\ hours after the plant was shutdown, the Reactor Coolant Pumps (RCPs) were stopped and natural circulation was established. While in natural cirou.ation, the Reactor Coolant System (RCS) cold leg temperature (T ) was reduced from 527 F to 480 F c
over a period of two hours. The reactor vessel head temperature (TEAD) decreased from 536*F to 497*F during the same time period.
l 1,
In going from forced flow (all four RCPs operating) to natural circulation, RCPs #1 and #3 were tripped first. Reverse flow was fully established, in Loops 1 and 3, seventy-five (75) seconds after their respective pumps were tripped. RCPs #2 and #4 were then stopped and in approximately 8 minutes natural circulation was established in all four loops.
After the two-hour cooldown period, a cold slug was intentionally produced in the RCS. By recording hot and cold leg temperatures vs.
time, as the cold slug traveled through the system, the RCS flow rate during natural circulation was calculated. This flow rate was found to be approximately 2.39 x 10s 1b/hr or 2.5% of normal flow during pumped operation.
Calculations and results of the RCP trips, natural circulation cooldown and cold slug test are discussed in this report. The data obtained during these tests:
1.
Provides factual data which can be used as training aids in i
describing system response and behavior during RCP trips and l
l natural circulation.
2.
Determines the reactor head cooldown rate vs. an RCS cooldown rate when in natural circulation.
3.
Provides benchmarking data for computer codes used by the NUSCO Reactor Engineering Branch in the prediction of CY transient behavior.
IIA. INTRODUCTION On September 25, 1981 at 2200 hours0.0255 days <br />0.611 hours <br />0.00364 weeks <br />8.371e-4 months <br />, CV began reducing generator output. By 0330 hours0.00382 days <br />0.0917 hours <br />5.456349e-4 weeks <br />1.25565e-4 months <br /> the next day the turbine was phased off of the grid. RCP vibration tests were performed between 0700 and 0900.
From 0900 to 1345 auxilliary feedwater pump tests were performed.
At 1345 hours0.0156 days <br />0.374 hours <br />0.00222 weeks <br />5.117725e-4 months <br /> the reactor was shutdown and all four RCPs were running. RCS cold leg temperature (T ) was 530 F and system C
pressure was 2007 psig.
RCPs #1 and #3 were tripped at 1400 hours0.0162 days <br />0.389 hours <br />0.00231 weeks <br />5.327e-4 months <br /> and at 1404 hours0.0163 days <br />0.39 hours <br />0.00232 weeks <br />5.34222e-4 months <br /> RCPs #2 and #4 were tripped. At approximately 1413 natural circulation flow was established. Loop ATs (T - T ) during this time ranged from H
C 10*F to 15*F.
While in natural circulation, the RCS was cooled down by dumping steam to the condenser. The cooldown continued for two hours with the RCS temperature decreasing by 47*F.
The temperature of the vessel head decreased 39*F over the same time period.
At 1615 hours0.0187 days <br />0.449 hours <br />0.00267 weeks <br />6.145075e-4 months <br />, while still in natural circulation, the flow rate of steam being dumped to the condenser was temporarily increased in order to produce a " cold slug" in the primary system. By observing T and AT instrumentation vs. time as this slug passed through the C
system, calculations were made to determine the RCS flow rate.
l l
Throughout the tests, plant data was recorded using an Acrurex-Auto Data Ten /10 Data Logger. By this means, the necessary system parameters were recorded at regular intervals. Figure 1 shows a typical data output.
At 1627 hours0.0188 days <br />0.452 hours <br />0.00269 weeks <br />6.190735e-4 months <br /> natural circulation tests were completed. The RCS temperature (T ) and pressure were 484.5*F and 2000 psia. The C
vessel head temperature was 497*F.
RCP #3 was restarted and a forced flow plant cooldown commenced.
In the following sections, details and calculations associated with the pump trips, natural circulation cooldown and cold slug test are outlined.
6-IIB. PLANT DESCRIPTION Connecticut Yankee is a Westinghouse 4* Loop Pressurized Water Reactor (PWR).
It is licensed for 1825 MWt full power operation.
As seen in Figure 2, the four loops of the RCS are nearly identical.
Instrumentation upstream and downstream of each steam generator provide a means to measure average temperature (TAVG), differential temperature (AT) and cold leg temperature (T ) f r each loop.
C Under normal conditions the plant operates at an RCS pressure of 2000 psig, average coolant temperature (TAVG) f 555 F and a loop AT of approximately 41*F.
Further information concerning plant systems and parameters can be found in References 1 and 2.
Figures 3a and 3b show the CY reactor vessel and internals. Further information concerning the reactor vesr,el is contained in Section IV, as well as in References 1 and 2.
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_ - - - _ _ = _ _. -. ~.
i III. PUMP TRIPS RCPs #1 and #3 were tripped at 1359:30' hours (this time is referenced as time = 0). The flow in Loops 1 and 3 decreased rapidly after the pumps were tripped and as a result, an increase in the ATs for these loops (ATI and AT3) was observed (see Figure 4).
At time = 48 seconds, the flow in loops I and 3 had decreased to approximately zero and reverse flow started to occur in these loops, due to the operation of RCPs #2 and #4. As reverse flow in Loops 1 and 3 increased, ATI and AT3 rapidly became negative.
Figure 4 shows that reverse flow was fully established at time = 75 seconds. At this time, the ATs in Loops 1 and 3 were approximately
-2*F.
RCPs #2 and #4 were tripped at time = 3:41 and the flow in these two loops slowly began to decrease. The ATs in Loops 2 and 4 (AT2 and AT4) increased from 0*F to about 15*F in six minutes as seen in Figure 4.
As a result of the decreasing flow in Loops 2 and 4, the reverse flow rate in Loops 1 and 3 also began to decrease. At 7:42, the flow in Loops 1 and 3 was forward again and in the following two minutes ATI and AT3 increased to approximately 13.5 F.
At 1412 hours0.0163 days <br />0.392 hours <br />0.00233 weeks <br />5.37266e-4 months <br /> (12 minutes and 45 seconds after the start of the pump trips), the AT instrumentation for each loop indicated that natural circulation conditions were established. At this time, Loop ATs
ranged between 7*F and 13 F, T was 526*F and T was 536*F. A c
g plant cooldown while in natural circulation then began.
x
IV.
NATURAL CIRCULATION C00LDOWN Heat generated in the core by the deca)-of fission products and the removal of this heat through the steam generators provides the impetus for natural circulation in the RCS. A density difference in the system results from the heat being added at a lower elevation than where it is removed. This density difference drives the natural circulation flow through the system.
As the plant cooldown rate is increased, a larger density difference in the system results, producing an increased natural circulation flow rate. Similarly, as the decay heat level in the core decrease's, the natural circulation flow rate in the RCS decreases.
During normal power operation, the reactor vessel head temperature (THEAD) is slightly less than the hot leg temperature (T ).
Typically, H
values of T and T are 570*F and 578 F, respectively. The HEAD H
difference between T and T is partly due to the cooling effect HEAD H
of bypass flow from the upper downcomer region to the vessel head (Figures 3a, b).
The bypass flow at CY is estimated to be 30 lb/sec (Reference 3).
1 Bypass flow is driven from the downcomer to the head by a pressure differential of approximately 30 psi. The flow passes through a series of small holes in the core barrel flange, upper support plate and hold down spring (Figures 3a, b).
There are 12 sets of holes, providing 12 pathways for bypass flow into the head.
Bypass flow exits the head and enters the upper plenum through two types of pathways. One path is through the 45 control rod shroud tubes and the other is through the 4 apre deluge standpipes (Figures 3a, b).
The standpipes transport most of the bypass flow since this path has a much larger area and lower flow resistance than that through the shrouds.
The four Control Rod Drive Mechanism (CRDM) fans at CY provide a second means for heat removal from the head. These fans, each rated 3
at 9,000 ft / min (Reference 4), serve primarily to cool the control rod drive components and remove heat from the reactor vessel head.
Westinghouse has estimated that these fans can remove 780 kW (2.7 x
~
106 BTU /hr) during full power operation. This translates to an upper head cooldown rate of 30 F/hr when THEAD = 572 F (Reference 5).
During natural circulation, the flow path through the head is expected to reverse. Although this condition was not detected by data taken during this test, previous studies (Reference 5) indicate its presence. Under these conditions, some of the hot, less dense water exiting the core rises into the head through the core deluge standpipes and then exits into the upper downcomer region. The higher density cold leg flow traveling down the downcomer produces a local low pressure region in the upper downcomer. This produces the necessary pressure differential which drives the flow out of the head during natural circulation.
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Additionally, the amount of bypass flow during natural circulation is less than that during pumped operation. This is because the differential pressures which exist in'the RCS during natural circula-tion are small.
Plant cooldown in natural circulation was initiated at 1412 hours0.0163 days <br />0.392 hours <br />0.00233 weeks <br />5.37266e-4 months <br /> and the cooldown period lasted approximately two hours. This cooldown was accomplished by the controlled dumping of steam to the condenser.
Figures 5a and 5b show T and T before and during the cooldown.
HEAD C
At CY, T is measured by a thermocouple located just above the HEAD upper support plate (Figure 3b). T, which was used to me.asure the C
RCS cooldown rate, was obtained by averaging the cold leg temperature readings from Loops 2, 3 and 4.
Resistance Temperature Detectors (RTDs) provide temperature readings in the hot and cold legs of the RCS.
Prior to the natural circulation tests, the Loop 1 AT indication had been observed to differ from the readings on the other three Loops.
For this reason, Loop 1 data was not used during the cooldown test.
Figure 6 shows Loop ATs during the cooldown. For the first 30 minutes of the cooldown period, these ATs varied over a wide range (6 F to 21*F) since the operators were adjusting feedwater flows to obtain the desired steam generator levels. The average AT for the remainder of the cooldown was approximately 12 F.
Individual T rea ings were H
not monitored but this AT value was used to draw a line indicating the average rate of T ecrease (Figures Sa, b).
It is seen that at H
1412 hours0.0163 days <br />0.392 hours <br />0.00233 weeks <br />5.37266e-4 months <br />, T and T were equal (~536*F). The cooldown rate H
HEAD prior to this time was negligible and this allowed these two tempera-tures to equalize.
x.
Plots of the RCS temperature vs. time show that the cooldown rate was not constant during the two-hour period.
It was originally planned to maintain a 10 F/hr cooldown for the first hour and a 25*F/hr cooldown for the second hour.
It was difficult, however, to find the proper steam dump valve position corresponding to the desired cooldown rate. After each positioning of the dump valves, the RCS cooldown rate was observed for a period of time and then the valves were repositioned.
This iterative process continued for the entire two-hour period.
Figures 5a and 5b show that the cooldown rate was largest (~45 F/hr) during the first 30 minutes of the test. The dump valves were then partially closed and the cooldown rate for the following 40-minute period was about 5*F/hr. The dump valves were repositioned two more times during the last hour of the test with resulting cooldown rates of 37*F/hr and 10 F/hr. The average RCS cooldown rate over the two-hour period was found, using a least squares fit, to be approxi-mately 23.5*F/hr.
Figure 5b also shows that the head cooldown rate lagged behind the RCS cooldown rate for the first 25 minutes of the test. This " lag time" may be associated with the time necessary for bypass flow to mix with the liquid in the head and to overcome the thermal inertia l
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effects there. It is seen, however, that during this 25-minute lag peried the RCS cooldown rate was much larger than the average rate of 23.5*F/hr. Since a decreased cooldBwn rate results in a decreased natural circulation and head bypass flow rate, the lag time would be expected to be larger than 25 minutes if a constant 23.5 F/hr cooldown rate was held for the entire test.
As the cooldown progressed, T began decreasing at a rate equal HEAD to the average RCS cooldown rate. Figure 5b shows that an average temperature difference of 9*F existed between T and TH * # "E HEAD the two-hour test. Additionally, it is seen that for periods of time where the RCS cooldown rate deviated from the two-hour average rate, the head cooldown rate stayed fairly constant. This behavior supports the fact that a lag time exists between RCS and vessel head temperature response.
Results of the natural circulation cooldown are further discussed in Section VI.
__ o V.
COLD SLUG TESTS After the two-hour natural circulationstest was completed, a cold slug test was performed. The starting time of this test was 1612 hours0.0187 days <br />0.448 hours <br />0.00267 weeks <br />6.13366e-4 months <br />.
In this test, the steam flow to the condenser was increased for a short period of time and then returned to its initial flow rate. The increased steam flow, removing additional heat from the primary system, resulted in a sudden and brief decrease in the temperature of the water leaving each steam generator. This produced a slug of cooler water in each loop of the primary system.
The transport of a cold slug in the RCS would be as follows. The slug exits the steam generator and passes through the loop's cold leg.
It is here where indication of the slug is first seen as TC decreases and AT increases. These temperature indications return to their original readings as the cold slug passes by the cold leg RTDs.
The slug then travels through the core and exits through the hot legs. The AT indication is then seen to decrease as the cold slug passes by the hot leg RTDs.
The measured transport time is the time required for the slug to travel from the cold leg RTD to the hot leg RTD. Knowing this time as well as the primary system volume displaced by the slug, the core flow rate during natural circulation may be calculated.
I l
l l
The first indication of the cold slug was at approximately 1616 as the AT in each loop increased by about 2 F (Figure 7). Figure 8 shows the corresponding decrease in T s(fr m 481.8*F to 479.8*F)
C over this time period.
Evidence of the return of AT to its previous value and the subsequent decrease in this indication, due to the cold slug passing the hot leg RTDs, was not clear cut. Figure 7 shows that no indication of the decrease in AT was seen in Loops 1, 2 and 4.
The AT for Loop 3 (AT3), however, showed the expected decrease.
In Loop 3, AT began increasing at 1615:10 hours.
It rose from 12.1*F to a maximum of 13.8 F which occurred at 1617:24. AT3 then began decreasing at a gradual rate, reaching 12.1*F again at 1620:24.
A minimum value of 10.2 F was reached at 1625:05 (Figure 7).
Using this information, the loop transport time was estimated to be 5.23 minutes. This value is believed to be an accurate interpretation of the data at hand.
It also compares well with data taken during a similar test at CY in 1967 (Reference 6).
Using this transport time, the core flow rate during natural circulation 8
was calculated to be 2.39 x 10 lb/hr (2.5% of normal flow). Using this flow rate and knowing that the Loop AT was approximately 12*F, 7
the core decay heat was calculated to be 3.247 x 10 BTU /hr. This value compares well, -12% error, with the decay heat level predicted
by ANSI /ANS (Reference 7). Assumptions and calculations used in determining the core flow during natural circulation are outlined in Appendix A.
x
VI.
RESULTS AND DISCUSSION During the RCP trip test, the system was first observed with two pumps in operation and then with all pumps stopped, as flow decayed towards natural circulation conditions.
With only RCPs #2 and #4 running, reverse flow occurred in Loops I and 3 and the AT recordings in these loops were approximately -2 F.
The magnitude of the reverse flow was estimated to be 1100 lb/sec per loop, corresponding to -15% of normal loop flow.
Eight minutes after the remaining two RCPs were stopped, natural circulation had been established in the RCS. Loop AT indications ranged between 9 F and 13 F with an average value of approximately 12*F.
The natural circulation flow rate was estimated, using Loop AT and decay heat rate, to be.63 x los Ib/hr per loop or 2.5% of normal loop flow. Figure 9 shows the loop AT indications throughout the entire natural circulation test period.
l l
While in natural circulation, the plant was cooled down over a l
two-hour period with an average cooldown rate of 23.5*F/hr. During this period, the RCS temperature was decreased 47*F (T decreased C
from 527 F to 480 ).
Figure 10 shows the plant strip chart indication of Loop 3 T during the natural circulation cooldown. Pressurizer C
pressure, as shown in Figure 11, remained relatively constant
(~1975 psig) during these tests.
After an initial lag period of about 25 minutes, the reactor vessel head temperature (Tg ) decreased at a rate equal to the average RCS cooldown rate. T decreased from 536*F to 497 F during the g
two-hour test (see Figure 5b). This lag period, as mentioned earlier, is attributable to two factors. The first is that the flow rate through the head is decreased and its path is reversed during natural circulation. For this reason, the bypass flow to cool the head is of temperature T instead of T. Sec ndly, the large metal H
C mass of the head provides thermal inertia, whereby rapid changes in T
are resisted.
gg Figure 12a shows that no rapid changes in pressurizer level occurred during the natural circulation cooldown at CY.
This fact indicates that steam flashing did not occur in the vessel head at any time during this test. Steam flashing was not expected since RCS pressure ea-maintained at approximately 2000 psia throughout the test. A strip chart of pressurizer level at St. Lucie Unit 1, where flashing did occur during a natural circulation cooldown, is shown in Figure 12b (Reference 8).
During the natural circulation cooldown, the temperature difference between the head and the hot leg (T
- T ) was 9 F (See Figure 5b).
EE H
It is likely that more rapid, prolonged RCS cooldowns would cause this temperature difference to increase. To avoid flashing in the head, the subcooled margin using T must be observed before system EE depressurization occurs.
If the head temperature is too high, a period of time at steady state conditions will allow it to decrease to T '
H
The head cooldown rate would be expected to increase for RCS cooldowns exceeding 23.5'T/hr but it is difficult to determine the extent of this increase. For larger cooldown rabes, larger density gradients will exist in the system. However, determination of the resulting increase in bypass flow to the head and improvement of the heat removal from the head is beyond the scope of this report. Also, it is expected that the head cooldown rate will reach a maximum as the RCS cooldown rate is further increased. Determination of this maximum rate would require a more detailed saalysis.
A computer analysis would predict head cooldown rates for any RCS cooldown rate by utilizing detailed heat transfer and fluid flow modeling. The results of this CY natural circulation cooldown test will provide benchmarking data for such an analysis.
Results of the cold slug test show that the core flow rate during 8
natural circulation is approximately 2.39 x 10 lb/br (2.5% of normal core flow). The decay heat rate (QDECAY) calculated using this flow rate compares well with the decay heat obtained using the ANSI /AES 5.1 method. These values, for a time approximately 14.0 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> after shutdown, agree to within -12%.
i 20 -
REFERENCES s
1.
" Facility Description and Safety Analysis," Connecticut Yankee Atomic Power Company - Haddam Neck Plant, Docket No. 50-213.
2.
Deveau, John; CY Plant Information Book - Primary Systems.
3.
" Calculation of Head Bypass Flow During Normal Operation at Connecticut Yankee," Calculation Number C2-517-304-RE.
4.
Joy Manufacturing Co., Letter to G. Bouchard, CYAPCO, Re: CRDM Replacement Fan, Dated 3/12/81.
5.
" Westinghouse Natural Circulation Cooldown Analysis," WOG-81-161, April 1981.
6.
Traggio, R. P., " Natural Circulation Test of Reactor Coolant System,"
March 1968.
7.
"American National Standard for Decay Heat Power in Light Water Reactors," ANSI /ANS-5.1-1979, August 1979.
8.
" Analysis and Evaluation of St. Lucie Unit 1 Natural Circulation Cooldown," NSAC-16/INPO-2, December 1980.
TABLE 1 SEQUENCE OF EVENTS t
TIME ON 9/26/81 HR: MIN:SEC EVENT 13:59:30 RCPs #1 and #3 tripped 14:00:18 Flow decreased to 0 in Loops 1 and 3 14:00:45 Reverse flow fully established in Loops 1 and 3 14:03:11 RCPs #2 and #4 tripped 14:07:12 Reverse flow in Loops i and 3 decreased to 0 14:12:00 Natural Circulation Conditions Present, Tg = 530*F, TC = 526*F Plant cooldown begins Loop average AT = 12*F 16:13:00 Natural Circulation Cooldown Ends, THED = 497 F TC = 484.5*F 16:13:00 Cold slug produced in RCS 16:15:10 Cold slug presence in cold leg indicated by AT instrumentation 16:20:24 Cold slug in hot leg indicated by Loop 3 AT instrumentation 16:25:00 Tests End RCP #3 Restarted Plant cooldown (forced flow) begins
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LOOPf1 TC 014 536.251 F
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"JPPORT PLATE J
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b INSTRLMENTATION THIM8LE GUIDES k.
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i Figure 3a.
CY Reactor Vessel and Internals
25 d%
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CY Reactor Vessel and Internals
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l CY NATURE CIRCUIM!ON ANEYSIS RCP TRIP TEST 20 Is l
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Figure 4.
Loop ATs - Pump Trip Test i
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NATURAL C!RCULaTION COLD 0iFN TEST 550_
A 5457 Y $40-R 530 E4%
A 5257 G $207
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i Figure Sa.
T and I - Natural Circulation Cecidown Test head c
I
28 CT NATUPA CIRCUIATION A.MYSIS 550-545--
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530-525-520-u.
n
{ 515-ag 510-o If A
c05-h, head
~
500-I l
l 495-l 490-
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h 2 hour2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> avg.
T C
400--
c 2 hour2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> avg.
(
475-l 14.0 14.5 15.0 15.5 16.0 16.5 TIME OF DAY (HR)
FIGURE Sb.
T and T - NAML CIRMATION COODOW W head' h c
29 m
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CY NAYbM CIRCULtTIGNMEYSIS COLD SLUG TEST I,
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Figure 7.
Loop ATs - Cold Slug Test
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=
I Q'NATOPE ClR:T!El0N A!'.CS!S e
i t
i COLD SLUG TEST 1
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k Figure 8.
Loop T s - Cold Slug Test c
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UXtP a T DURING ALL TESTS 2 S. 0 --
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I TIWE OF DAT ( H O U R S,a l
Figure 9.
Loop ATs - All Tests l
33
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Figure 10.
T (Loop 3) and RCS Pressure - St. ip Chart Recording During CY Tests 9
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M Figure 11.
Pressurizer Pressure - Strip Chart Recording During CY Tests
34 100' 7
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Figure 12a.
Pressurizer Level - Strip Chart Recording During CY Tests T
,100,
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8 a.m.
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(
Figure 12b.
Pressurizer Level - St. Lucie Natural Circulation Cooldown
_Aj_
APPENDIX A: CALCULATION OF CORE FLOW USING LOOP TRANSIT TItfE To calculate the mass of liquid which is displaced by the cold slug, the individual RCS volumes must be known.
x The reactor vessel and one coolant loop is shown in the figure below.
Regions through which the slug travels are labeled as are the hot and cold leg RTD locations.
e, s s p/
OV V
V 2
3 4
)
y/
Np-7 T c
8 V
1~
l,.
5-h V
g
> y
(
3 V
= 82 ft - Cold Leg RTD to RCP Inlet 3
V
= 150 ft - RCP 2
3 V
= 3g5 ft - RCP Discharge to Core Inlet Nozzle V
= 92 ft - Core Exit Nozzle to Hot Leg RTD 4
3 Vhop = 429 ft - Loop Volume Displaced By Slug 3
V
= 333 ft - Downcomer 5
3 V
= 757 ft - Lower Plenum 6
V
= 577 ft - Active Core and Core Bypass 7
3 V
= 942 ft - Upper Plenum 8
3 V,,,,y = 2609 ft y
NOTE:
The volume of the vessel head is not included since the flow through this region is small as compared to the core flow rate.
Since the slug from one loop displaces k of the vessel volume, the effective volume is:
3 Yvessel eff. = (.25) V,,,,1 = 652 ft y
and the total volume displaced by the slug is:
3
- Y1oop
- Yvessel eff. = 1081 ft I
tot Thus, the mass displaced by a slug is:
M = V,g pg = 54873 lb, where pt = 50.76 lb,/ft3 (2000 psia, 480*F) g
Th2 r:ccrd2d 1ccp tr:nsit time w:s 5.23 cin. (.0872 hr.) co the mass flew rata par lecp is:
W
= 54873 lb 6
lo0P a =.629x 10 lb /hr
.0872 br.
Total RCS flow is then:
6 W
= 2.52 x 10 lb,/hr tot During natural circulation, core bypass flow through the core baffles (1%) and through the CRC Guide Tubes (4%) is still assumed to occur.
Bypass flow through the head and from the downcomer into the outlet (T )
nozzles is neglected since system differential pressures during naturaIu circulation are small (Reference 2).
Thus, the core flow rate during natural circulation is:
6 6
W
= (.95) 2.52 x 10 = 2.39 x 10 lb,/hr c,
The decay heat rate can now be calcuated for the time when the cold slug test occured.
The test took place at 1615 hours0.0187 days <br />0.449 hours <br />0.00267 weeks <br />6.145075e-4 months <br /> on September 26, 1981, 12.25 hours2.893519e-4 days <br />0.00694 hours <br />4.133598e-5 weeks <br />9.5125e-6 months <br /> after shutdown.
The decay heat rate, Qdecay, is calculated as:
Qdecay = W C AT core p where: AT= 12*F (see Fig. 6)
Cp = 1.131 BTU /Lb,*F (2000 psia, 480*F) 6 Qdecay = (2.39 x 10 ) (1.131) (12) = 3.24 x IM BWAr The decay heat rate predicted by ANSI /ANS-5.1 (Reference 7) is:
6 BTU P
Qg gy = (1825 MWTH) (3.412 x 10 hr W P
TH o
where E =.00592 = Power Fraction
'o Qggy = 3.68 x 10 BTU /hr Thus, the error between Qdecay *" 0 is:
ANSI 3.24 68 x 100 = -11.9%
-A3-Underfulgpowerconditions,theRCSflowrateisapproximately 1.06 x 10 lb,/hr.
Assuming 9% bypass flow (Reference 2) the core flow rate is found to be:
WCORE = 1.06 x 108 (.91) = 9.65 x 107 lb,/hr.
CORE (natural circulation) = 2.39 x 10k = 0.0248 W
W (full power) 9.65 x 10' Thus, the core flow rate during natural circulation is seen to be 2.48 %
of full power flow.
. _ _ _..,.