ML20205J300
| ML20205J300 | |
| Person / Time | |
|---|---|
| Site: | Maine Yankee |
| Issue date: | 01/31/1986 |
| From: | Maine Yankee |
| To: | |
| Shared Package | |
| ML20205J290 | List: |
| References | |
| NUDOCS 8601300150 | |
| Download: ML20205J300 (45) | |
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I o I MAINE YANMEE ATOMIC POWER COMPANY A
1 PRIMARY INVENTORY TREND SYSTEM MAIE YAtKEE ATOMIC POWER STATION TABLE OF CONTENTS J
Page LI ST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
' LIST ~0F FIGURES................................................... iv LI ST OF REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v 1.0 EXECUT IVE SUhNARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2.0 PITS SYSTEM DESCRIPTION........................................... 3 2.1 General Description......................................... 3 2.2 Sy stem Ca libra tio n. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.3 Calibration Test............................................ 6 2.4 PIT System Caution Band..................................... 7 2.5 Interpreta tion of PITS Readings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.0 SYSTEM ASSESSENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.1 Control Variable Calcula tion Summary . . . . . . . . . . . . . . . . . . . . . . . . 11 3.2 Stuck Open PORV Transient and Results. . . . . . . . . . . . . . . . . . . . . . . 12 3.3 Cold Leg Small Break Transient and Results.................. 14 4.0 C ONCLU SI ONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
s MAINE hANKEE ATOMIC POWER COMPANY ~
A 4
LIST OF TAELES Nursber Title Sequence of Events for Stuck Open PORV Transient .
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3.2
.[ 3.3' Sequence of Events for Three-Inch Cold Leg Break Transient -
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MAIN 2 YANKEE ATOMIC POWER COMPANY
- A LIST OF FIGURES Nurtber Title-2.1.1 PITS Layout Description 2.2.1 Calibration of the PITS 2.2.2 Control Room Display 4
2.2.3 PITS Display Indication for Solid, Hot R'S With RCPs off 3.1 RCS Elevation Diagram 3.2 MY RELAP5YA Model for Stuck Open PORV Transient 3.3 System Pressure Versus Time, Stuck Open PORV Transient 3.4 Breakflow Versus Time, Stuck _Open PORV Transient 3.5 PITS Indication, Stuck Open PORV Transient 3.6 PT-3001 Mass Inventory and C611apsed Liquid Mass 3.7 Liquid Volume Fraction in Top Three Nodes of Pressurizer 3.8 PT-3002 Mass Inventory and Collapsed Liquid Mass 3.9 PT-3003 Mass Inventory and Collapsed Liquid Mass 3.10 MY RELAP5YA Model 'for Cold Leg Break Transient 3.11 System Pressure Versus Time, Cold Leg Break Transient 3.12 Break Flow Versus Time, Cold Leg Break Transient
'3.13 PITS Indication, Cold Leg Break Transient 3.14 PT-3001, Mass . Inventory and Collapsed Liquid Mass 3.15 Mass Inventory in Primary System and Vessel 3.16 PT-3002 Mass Inventory and Collapsed Liquid Mass 3.17- PT-3003 Mass Inventory and Collapsed Liquid Mass f
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4 M AINE YANKEE ATOMIC POWEO COMPANY 4
LIST OF REFERENCES '
(1) . Maine Yankee Letter to USNRC, dated April'9, 1984, MN-85-56, Justification of Reactor Coolant Pump Operation During Small Break LOCA-Transients, YAEC Report.1423.
(2) Maine Yankee' Letter to.US E , dated March 8, 1985, m-85-47, Inadequate Core Cooling Instrumentation System.
-(3) Maine Yankee Letter.to US E , dated September 6, 1985, MN-85-159, Answers to Questions Concerning Inadequate Core Cooling Instrumentation. -
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M AINE Y ANKEE ATOMIC POWER COMPANV .
1.0 EXECUTIVE SLM4ARY The purpose of this document is to provide'the. design / operational analysis necessary to demonstrate the adequacy of the Maine Yankee Primary Inventory Trend System (PITS). Analysis of the PITS instrumentation system is' required by NUREG-0737, Item II.F.2. The results of this- analysis predict that the PITS inferred readings are representative of the actual coolant inventory in the primary system.
The Primary Inventory Trend System consists of three differential pressure (dp) transmitters connected across three locations within the Reactor Coolant System (RCS). The dp measurements can be used to assess trends in the mass inventory within the primary system during a small breek aEc!Jent such as experienced at TMI, Unit 2. The mass inventory trend system, the saturation monitors, and core exit thermocouples, provide the operator information concerning the progress of an accident and recovery.
The objective of this' analysis was to demonstrate that each of the PITS-
- transmitters will adequately trend the change of mass inventory within their respective regions of the primary system. This was done 'for two different break scenarios; a cold leg break and a stuck open PORV.' These scenarios were chosen so that the PITS response could be assessed for breaks which are characteristically different. The response of the PITS transmitters was predicted by calculating a time history of varying pressure differences across each of.the transmitters for the two scenarios. The output of each transmitter was converted to an inferred mass. inventory within the region of
- measurement. The validity of the PITS response was assessed by comparing the trend of the PITS inferred mass inventory against the calculated inventory ~ in the same region of the primary system. For a small break TMI-type scenario, this analysis showed that with the Reactor Coolant Pumps (RCPs) off, each PITS transmitter provides a representative trend in mass inventory within its region of measurement. Fractional pressure-drops which occur during the'small-break transients have negligible effect on.the dp measurements.
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MQiNE YANKEE ATOMIC POWER COMPANY Corrections ~ for system errors caused by radiation, density variations,'
voiding in sensing lines due to flashing of dissolved gases were not factored into the analytical output. These uncertainties however, are accounted.for by use of a caution band on the PITS Control Room display. The caution band-provides a conservative means of warning the operator of an approach to possible core uncovery. The trend information and other primary instrumentation assist the operator'in assessing the status of core cooling and accident recovery.
Section 2.0 of this report provides a general description of the PIT System and design objectives. System calibration, caution band and interpretation of PITS readings are also included in this section.
The analytical assessment of PITS is presented in Section 3.0. This section includes: (1) a summary of the control variable calculations used to model the PITS in RELAP5YA, (2) a description of the transients and (3) the results of assessment.
Section 4.0 provides concluding statements and observations relative to the acceptability of the PITS-design.
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MAINE YONMEE ATOMIC POWEO COMPANV 4
2.0 PRIMARY INVENTORY TREND SYSTEM DESCRIPTION 2.1 General Description The Primary Inventory Trend System (PITS) consists of three differential pressure transmitters connec.ted to the reactor coolant system as illustrated in Figure 2.1.1. The three dp measurements or channels ~are arranged to indicate trends in the reactor vessel and primary system mass inventories during a small break event.
Transmitter PT-3001 (Channel IZ) trends mass inventory across the primary system. Its reference line is connected at the top of the pressurizer and the variable sensing line is connected to the bottom of the reactor vessel via an incore instrument tube. ' Transmitter PT-3002 (Channel IY) trends mass ,
inventory within the reactor vessel. Its reference line is connected to the reactor vessel head east instrument flange and the variable sensing line is connected to the same incore instrument tube as. Transmitter PT-3001.
Transmitter PT-3003 (Channel IX) trends mass inventory in the reactor vessel above the bottom of the reactor coolant loop piping. Its reference line is connected to the reactor vessel head west instrument flange and the variable sensing line is connected to the bottom of the Loop 2 hot leg. Each dp channel drives an associated pen on a strip chart recorder located on the-Main Control Board (M'B).
The PITS is designed for use after the reactor coolant pumps are shut off. By procedure, the operators trip the RCPs when margin to saturation ~in the primary system is reduced to less than 25 UF. At this point, the effects of pressure drop due to forced circulation in the primary system diminish, and the dp transmitters measure the static pressure differential between the.
varying primary system and respective reference legs.
The dp measurements in the primary system are used to detect changes in system mass inventory. It is not intended as a measurement of liquid level.
The readings provide an indication of. the effectiveness of any actions taken
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to maintain or recover reactor vessel inventory during accident mitigation.
The inventory information in addition to the saturation monitor, and core exit 6932L-FWS 3
M AINE Y ANKEE ATOMIC POWER COMPANV s
thermocouples provide the operator with a status of accident core cooling conditions. The PITS information is not_ required to directly support any automatic or manual operator actions.
2.2 System Calibration Refer to Figure 2.2.1 for a description of the PIT System calibration.
The vertical ~ distance between the bottom of the core and the top of the pressurizer is appro'ximately 768 inches. The top of the reactor head is located at approximately half this distance or 384 inches from the bottom of the core. The vertical distance from the bottom of the loop piping to the top of the' reactor head is' approximately one fourth the total distance or 192 inches. Thus, the elevation of the primary system can be divided into four equal _ portions relative to the bottom of the core.
All three PITS transmitters are cold calibrated to a range of 0 to 768 inches of water column (density of water approximately 62.34 lbs/ft3 ). The corresponding cold calibrated output of each transmitter is 20 mA at 0 inches dp and 4 mA at 768 inches dp. Consequently, the output of each transmitter will change by 1 mA for every 48 inches dp. The PITS transmitters and recorder are calibrated each refueling outage to assure proper calibration of system components.
t Figure 2.2.2 illustrates the PITS recorder display on the M'B. Each channel is easily identified by appropriate labeling and three different colored ink pens, 1
PT-3001, Channel IZ - blue pen l t
i PT-3002, Channel IY - green pen l PT-3003, Channel IX - red pen The output of each channel is indicated on the vertilcal scales located
- on the right side of the display. Recording of the channel outputs is I
provided on the strip chart which travels from right to left at' a speed of l
6932L-FWS. 4 j
MAINE YANKEE ATOMIC POWER COMPQNV 3/4" per hour. The strip chart has a "0" to "100" scale corresponding to cold
~ calibration. Urrbr hot' conditions, the actual inventory will be conservatively higher Ulan that indicated. During an accident the trend of inventory indicates the progress of accident recovery.
2.2.1 PITS Readings - RCS Cold, No Reactor Coolant Pumps The PITS recorder channels are individually calibrated to represent associated regions of the reactor and primary system under cold conditions.
With the RCS cold and the RCPs turned off, the PITS channels will indicate the ,
following for each transmitter:
PT-3001 (Channel IZ) l Transmitter PT-3001 measures dp across the primary system. With the primary system full to the top of the pressurizer, Channel IZ (blue pen) will '
indicate "100." With the water inventury at the top;of the reactor head, the channel will indicate "48-50", and water. inventory at the bottom of the core will indicate 90."
PT-3002 (Channel IY)
Transmitter PT-3002 measures dp across the reactor vessel. With the water inventory at the top of the reactor head or higher,. Channel IY (green pen) will indicate half scale, "48-50." With inventory at the bottom of.the core the channel will indicate "0."
i PT-3003 (Channel IX)
Transmitter PT-3003 measures dp across the upper reactor vessel and <-
head region. With water inventory. at the top of the reactor head or higher, Channel IX (red pen) will indicate half . scale, "48-50." With inventory at. the i bottom of the loop piping or lower the channel will indicate "25."
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MAINE YANKEE ATOMIC POWER COMPANY 2.2.2 PITS Readings - RCS Hot, No Reactor Coolant Pumps Since the transmitters are cold calibrated and there is no correction for RCS density variations, heating of the RCS will cause the PITS channels to indicate a lower inventory than the actual. For example, with the RCS at an 0
average temperature 'of'532 F and with the reactor coolant- pumps turned off,
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the PITS recorder would indicate as follows for each transmitter. These readings are illustrated in Figure 2.2.3.
With a water solid pressurizer and vessel, PT-3001 will indicate at approximately "76" on the display. PT-3002 will indicate at approximately "38" and PT-3003 will indicate at approximately "44".
The percent density shift for each channel is the ratio of the hot density to the density corresponding to cold calibration. For the above illustration, the density. shift for PT-3002 (from "50" to "38") is twice the density shif t for PT-3003. (from "50" to "44"). This is because PT-3002 is measuring twice the height of liquid in the reactor vessel as PT-3003 ("384" versus "192") and the offset is r; eater.
As the inventory in the crimary system empties due ta e po;tolated break, the actual density shift for each channel becomes less since the height of measured inventory is less.
2.3 Calibration Test The PITS response has been measured against a known vessel level to verify the correct cold calibration of.the inventory channels. This was done during the Cycle 8/9 refueling outage with the reactor coolant system stagnant.
and vented. With the current calibration, all three PITS channels will indicate at approximately the same point on the chart recorder for a given water level ~anywhere between the top of the head (50% scale) and bottom of the loop piping (25% scale). The response of the PITS was observed with the vessel water level approximately 8 inches below the vessel flange (El.
approximately 19' 3-1/2"). At this mark, ~ the channels properly indicated.the 6932L-FWS 6
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MAINE YANKEE ATOMIC POWER COMPANV s
inventory at 36-37% of full scale. Other functional tests which were satisfactorily performed included a system. leak check and an electrical loop.
check to verify the absence of cross-talk among the inventory channels.
2.4- PIT System Caution Band Since the PITS transmitters are located inside containment, the system output may be subject to inaccuracies caused by the post-LOCA environment.
Approximated errors resulting from the harsh conditions were submitted to NRC in References 2 and 3. In summary, the accuracy of the system can be affected by Die following:
- 1. Transmitter inaccuracy due to increase in ambient temperature and radiation fields.
- 2. Variation of fluid dens'ity in the reference lines as a result of increased containment temperature.
- 3. Postulated voiding in the sensing lines caused by flashing of dissolved gases in the sensing lines during RCS depressurization.
- 4. Recorder inaccuracy.
As a result of these effects, the small break LOCA total system errors in percent ~of indicated span.are:
PT-3001 -4 ~ 0 to +10.0 PT-3002 -4,0 to +7.0 PT-3003 -3.0 to +5.0 The errors in the negative direction produce a conservative output
. indicating slightly lower than the actual mass inventory. Errors in' the positive direction, however, produce a nonconservative output so the indicated inventory would be slightly higher than the actual. To account for the
'6932L-FWS 7
MAINE YANKEE ATOMIC POWED COMPANY nonconservative error, a nominal caution band (shaded area) is indicated on the PITS display refer to Figure 2.2.2. The caution bcad _begins at 32% of inventory scale which would be equivalent to a collapsed liquid inventory at or above the bottom of the loop piping. Use of the conservative caution band is as follows:
o Any decreasing reading epproaching the top of the band is interpreted as inventory decreasing to the bottom of the loop piping and the potential loss of the steam generator as an efficient. heat sink.
o A further decreasing trend within the band is interpreted as inventory approaching the top of the' core. (The operator wot.1d verify that ECCS injection was properly initiated and would also monitor other primary variables such as core exit thermocouples.)
o Readings within the caution band indicates a potential for inadequate core cooling dJe'to voiding in the reactor vessel. The PITS information in combination with other indications of core cooling, such as core exit thermocouple and subcooled margin-readings, would be assessed before any reduction in safety
- injection flow would be made.
i 2.5 Interpretation' of PITS Readings for Accident Response During a small break accident situation, similar to TMI, the plant operator would monitor the decreasing margin to saturation in the primary system. Before margin to saturation is completely lost, the operator would trip the RCPs and the PITS inventory channels would begin to indicate within their respective operating ranges. The operators are trained to use the PITS information only after the RCPs are off. The operator would assess inventory
.and its trend as' inferred by the chart recorder (increasing, decreasing or -
steady).
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f M AINE YANKEE ATOMIC POWER COMPANY o
The movement of the individual channels on the PITS display indicates the inventory distribution in the system. For example, with the formation of a steam void in the upper reactor vessel and head, both vessel channels (PT-3002 and PT-3003) respond with a downward trend. As the void continues to grow, both channels continue trending downward until the loop piping is uncovered. At this point, PT-3003 will bottom out (approximately 25% of scale) and PT-3002 continues to follow the decreasing inventory into the core region. Conversely, during recovery phase, PT-3002 will first show an increase in vessel inventory. If the vessel inventory is restored above the loop piping (for a given accident situation), then PT-3003 will show some increase.
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The inventory of PT-3001 can be different from that of the . vessel
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channels. This is because PT-3001 is affected by the pressurizer volume.
However, this channel will trend the mass changes within its own region of measurement. An increase in pressurizer inventory can cause PT-3001 to trend an increase in system inventory at the same time both reactor vessel channels indicate the presence of a growing steam void in the vessel. When the loop pipe is uncovered and the pressurizer has drained, PT-3001 will trend the vessel inventory similar to PT-3002. Monitoring of the PITS and other primary variables, such as pressurizer level channels, core exit thermocouples, and subcooled margin monitor provides the operator with information relative to inadequate core cooling.
With any readings approaching or within the 32% caution band, the operator conservatively assumes that loop piping could begin to empty, resulting in loss of the steam generators as an efficient heat sink. (Reflux cooling is possible within the steam generator primary tubes). The operator verifies that the ECCS injection systems have automatically initiated and are injecting water into the vessel. PITS readings within the caution band indicate a potential for inadequate core cooling due to voiding in the reactor vessel. The adequacy of core cooling is monitored throughout the accident and recovery phases by use of the PITS information, the saturation monitors and the core exit thermocouples. Assessment of all the primary system indications is made before safety injection flow is reduced.
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M AINE Y ANKEE ATOMIC POWER COMPANY 3.0 PITS ASSESSMENT The intent of this analysis is to demonstrate that each of- the PITS transmitters can adequately measure in mass inventory for typical small break accident' conditions. Two representative small break scenarios, a stuck open PORV and a cold leg break, were chosen for this assessment. The break characteristics of these scenarios are significantly different and therefore provide a variety of thermal-hydraulic conditions for which the PITS can be ,
assessed.
The validity of the PITS indications is determined by comparing the simulated response of each pressure transmitter to the calculated collapsed liquid mass in the same region of measurement. The results of this assessment establish that, during a small break transient, the PITS instrumentation can representatively trend primary system mass inventory after the RCPs are shut off. The effects of dynamic head (due to RCS small break blowdown),
frictional pressure drop and localized boiling are negligible contributors to the dp measurements. These effects do not mask the trend of mass inventory even during the RCS depressurization. This assessment deals with the transmitter response to the thermal-hydraulic conditions occurring during the assumed accident scenario. The PIT. System inaccuracies discussed in Section 2.4 are not part of the analytical assessment. Refer to Section 2.4 to see how Ulese uncertainties are factored into the actual PIT System.
The Primary Inventory Trend System was represented by control . variables '
in RELAP5YA. For each transient these control variables and corresponding actual collapsed liquid mass were calculated as a function of time using previously calculated thermal-hydraulic conditions. A comparison of. simulated PITS response to actual collapsed liquid mass provides a valid assessment of the PITS. The pertinent control variables used for modeling PITS are summarized in the next section and the results of the assessment are given in Sections 3.2 and 3.3.
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MAINE YANKEE ATOMIC POWER COMPANY
' 3.'1 Control Variable Calculation Summary The control variables used for modeling the PITS are based on the following assumptions:
Calculational Assumptions
- 1. The density in the sensing line between points 1 and 2 of. Figure
' 3.1 is equal to the density in the lower plenum. This is an ,
acceptable assumption since the sensing line offers very little thermal resistance.
- 2. Density in the sensing line between points 3 and 4 is equal to the e density in the vessel at point 4. This assumption is expected to
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}- simulate the drainlag of the sensing line when the mass in the vessel drops below point 4 in Figure 3.1.
- 3. Density in the sensing lines everywhere else remains at 62.34 3
lb/ft throughout the simulated transients.
- 4. The PITS transmitters were calibrated under cold conditions (density in primary system = 62.34 lb/ft 3).
Using these assumptions the PITS transmitter outputs and actual equivalent collapsed liquid mass in the associated region of measurement were calculated.
For each small break case, a time history of the pre'ssure differences I (dp) across each of the PITS transmitters was calculated using RELAP5YA.
Using the calibration conditions discussed in' Section 2.2,' the dp data was
. converted to a percent output of equivalent liquid mass as would be indicated on 'the recorder / display.
A time history of the actual equivalent collapsed liquid mass in eact.
region of measurement was also calculated by the RELAP5YA code. The equivalent collapsed liquid mass (in feet) was also converted to an equivalent.
percent collapsed liquid mass using the same calibration conditions.
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MAINE YDNKEE ATOMIC POWER COMPANY The actual equivalent collapsed liquid mass.ls plotted against the PITS output. This is done to compare what would be seen by the plant operator on the PITS recorder to diat is seen by the RELAP5YA code. For comparison purposes, the plot of PITS output represents a recorder chart speed which is faster than the actual speed.
The thermal-hydraulic variables used . . calculating the PITS output are pressure and density, whereas the actual collapsed liquid mass is calculated independently from void fraction. Therefore, a comparison of these two parameters is a v~ alid assessment of the PITS. The results of the PITS assessment for the two small break transients are given in Sections 3.2.1 and 3.3.1. The transients are described in Sections 3.2 and 3.3.
3.2 Stuck Doen FORV Transient The " stuck open PORV transient" was simulated in RELAP5YA using the model shown in Figure 3.2. Initially the reactor was assumed to be operating at 102 percent power and normal operating conditions. The transient was
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initiated by assuming that the PORV failed open. The FPSI System was intentionally left secured so that the PITS response could be monitored over an extereded transient recovery time. The only heat removal mechanisms were PORV flow and steam generator safety relief valve flow. Emergency feedwater flow was assumed to be unavailable for the first 900 seconds. At this time, the emergency feedwater pump flow was assumed to be 500 gpm. The transient was continued under these conditions up to 3600 seconds. At 3600 seconds the PORV was closed and FPSI flow was started. The transient was terminated at 5000 seconds. The transient can be characterized by the pressure history plotted in Figure 3.3 and break flow rate plotted in Figure .3.4. A list of significant events that occurred during this transient is given in Table 3.2, and the results of the assessment are discussed in the following section.
3.2.1 Results of Assessment: Stuck Open PORV The PITS output is plotted versus time in Figure 3.5. The indications of mass inventory by.each of the three PITS transmitters are compared to corresponding actual collapsed 11guld mass in Figures 3.6, 3.8 and 3.9.
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MAINE YANKEE ATOMIC FOWER COMPANY
- The indication of mass inventory by the Transmitter PT-3001 is compared to the actual collapsed liquid mass in Figure 3.6. During steady-state plant operation (time = 0.0 sec.), the PITS indication is overranged high caused by the high velocity head resulting from reactor coolant pump operation.
Following the initiation of the transient at 50 seconds, the increase in density caused by the reactor trip, and decrease in the pressure difference caused by the RCP trip reduces the equivalent collapsed liquid mass to 65 percent and PITS indication to 42 percent. Since PITS is cold calibrated (density of water approximately 62.34 lb/ft ), the PITS mass inventory shown on the plot is less than the indicated actual inventory at hot conditions.
The objective of tris assessment is to demonstrate that PITS correctly trends the mass inventory ct.snges in the system and not the magnitudes, therefore, no attempt has been made 1.7 compare the exact magnitudes.
The actual equivalent collapsed liquid mass increases from 65 percent at 100 seconds to 95 percent at 620 seconds and indicated mass increases from 42 percent to 64 percent during the same time period. This increase is caused by the accumulation of mass in the pressurizer vapor region as evident from Figure 3.7, where the mass in Nodes 730-1, 730-2 and 730-3 are plotted versus time. The mass accumulated in the' pressurizer is transported from~ the vessel upper head by the pressure gradient resulting from the opening of PORV.
Also indicated on Figure 3.6, from 620 seconds to approximately 1000
. seconds the mass accumulated in the upper section of the pressurizer exits the system through the PORV (see Figure 3.4) causing a rapid decrease in mass inventory. After 1000 seconds the PORV flow is predominantly- steam, hence, the collapsed liquid mass and the indicated liquid mass decreases s. lowly until 3600 seconds. - At -3600 seconds of Figure 3.6, the PORV was closed and MSI-flow was started, causing the mass inventory to increase.
Throughout the transient the mass indicated by PT-3001 is -
representative of the actual equivalent liquid mass. Hence, this transmitter is expected to adequately trend the mass changes during a hot leg break.
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MAINE YANKEE ATOMIC POWER COMPANY In Figure 3.8, the indication of mass inventory by Transmitter PT-3002 is compared to the actual equivalent collapsed 11guld mass. At steady-state operation,-the transmitter output is overranged high. This is caused by the core dp when the RCPs are operating. After the.RCPs are tripped (50.0 seconds), however, and throughout the' transient, the indicated mass inventory follows the trends of the actual equivalent collapsed liquid mass very closely. In summary, the mass inventory trend indicated by the pressure Transmitter PT-3002 is representative of the actual mass.
In Figure 3.9, the response of the Pressure Transmitter PT-3003 is compared to the actual equivalent collapsed liquid mass. At steady-state plant operation (0.0 seconds), the transmitter output is overranged low. This is because the dynamic head in the hot leg resulting from RCPs operating causes a reversal of indication on Transmitter PT-3003. After the pumps are tripped (approximately 50 seconds), the pen returns to approximately 37-38 percent and the indicated mass follows the trend of collapsed liquid mass very closely.
In summary, the mass trends indicated by the PITS transmitters during a
" stuck open PORV" transient are a good indication of the actual liquid mass trend. These results are extrapolated to conclude that the PITS will correctly indicate the changing mass inventory during hot leg breaks.
3.3 Cold Leg Small Break Transient The plant response to a 3-inch (0.05 ft )2 break in the pump discharge side of a cold leg was simulated in RELAP5YA. The system was modeled as shown in Figure 3.10 and " evaluation model" assumptions were used in the analysis.
Details of this transient are given in Reference 1; however, the sequence of events given in Table 3.3, the system pressure history plotted ' in Figure 3.11 and break flow rate plotted in' Figure 3.12 adequately describe the transient.
The results of PITS assessment during tnis transient are given in the following section.
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MAINE YANKEE ATOMIC POWER COMPANY 3 . 3.'l Results of Assessment: 3-Inch Cold Leg Break Transient The PITS output is plotted versus time in Figure 3.13.
Figure 3.14 compares the mass inventory indicated by Transmitter PT-3001 to the actual equivalent collapsed liquid mass in the region of measurement. During the first 200 seconds of the transient the RCPs were operating; therefore, the PT-3001 pen is overranged high due' to the effects of forced flow. Therefore, during this period, the mass trend indicated by PT-3001 is not pertinent. At 200 seconds, the RCPs are tripped and the mass inventory decreases due to break flow. At 670 seconds, the mass from the .
steam generators drains back into the vessel and causes the vessel mass inventory to increase abruptly. This phenomenon is also evident in Figure 3.15 at the same time period. On Figure 3.15, the total mass inventory in the primary system is shown to be decreasing during this period, while the mass inventory in the core is shown to be increasing. This suggests that at 670 seconds the abrupt increase in the vessel mass inventory is caused by a redistribution of. the mass in the primary system.
After 800 seconds, in Figure 3.14, the break flow is predominantly steam, hence, the mass in the system decreases at a slower rate. This trend continues until 2000 seconds when the FPSI flow exceeds the break flow,.
causing the mass -inventory in the system to increase. In conclusion, after the RCP trip, the mass inventory indicated by Transmitter PT-3001 correctly trends the actual equivalent collapsed liquid mass.
In Figure 3.16, the mass indicated by Transmitter PT-3002 is compared to the actual equivalent collapsed liquid mass. The mass indicated by Transmitter PT-3002 follows trends of the collapsed liquid mass after the RCP trip (200 seconds). The behavior of the mass inventory in the vessel is a result of the phenomenon and events discussed in the previous paragraphs pertaining to Figure 3.14.
In Figure 3.17, the output of Transmitter PT-3003, which indicates mass in the upper head, is compared to the actual equivalent collapsed liquid mass. Initially, the transmitter ~ is overranged low due to RCP operation.
6932L-FWS 15
MAINE YANKEE 5 ATOMIC POWER COMPANY e
Following the RCP trip, the upper head becomes completely voided as indicated by Transmitter PT-3003 and the actual equivalent collapsed liquid mass.
In conclusion, the catput of PITS transmitters provide a representative ,
indication and trend of mass inventory in the -system.
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l
- 6932L-EWS. 16 .
MAINE YANKEE ATOMIC POWER COMPANY
4.0 CONCLUSION
S
- 1. With the reactor coolant pumps tripped, the output of the~ PITS transmitters provides a representative of mass inventory within the associated regions of measurement for a TMI-type small break scenario. , Frictional pressure drops as a result of postulated small break transients have negligible impact-on the dp
' measurements.-
- 2. The trend of the mass inventory'provides operators with additional information relative to_ the progress of a small break accident and the effectiveness of any mitigation or recovery actions taken.
- 3. The inventory information in addition to other primary system variables such as the saturation monitor, and core exit thermocouples provide the operator with a status of core cooling and accident recovery. The caution band indicated on the PITS display, provides a conservative means of warning the operator of an approach to or potential for inadequate core cooling.
6932L-FWS 17
MAINE YANKEE ATOMIC POWER COMPANY -
. TABLE 3.2 Sequence of Events for Stuck Open PORV Transient Time 2
(Sec) Events 0.0 o Normal Steady-State Operation 50.0 o PORV Opens
'o Reactor Scram.
o Loss of Main Feed and Steam Flow o RCPs Trip 900.0 o Auxiliary Feed Flow Starts (500 GPM) o Main Steam Flow Starts (4.25 lbm/sec) 3600.0 o PORV Closed o HPSI' Flow Starts 5000.0 o Transient Terminated e
e 6932L-FNS '18
MAINE YANKEE ATOMIC POWER COMPANY TABLE' 3.3 Sequence of Events for 'Three-Inch Cold Leg Break Transient Time (Sec) Events 0.0 0 Normal Steady-State Operation 50.0 0 Break Opens 69.0 o Reactor' Scram on Low Pressure
~
o Loss of Main Feed Flow o- Loss of. Main Steam Flow 81.0 o High Pressure Safety Injection Actuation 86.0 o Auxiliary Feed Flow Starts and Stays on Throughout the Transient 201.0 0 Reactor. Coolant Pumps Tripped 1610.0 o Steam Generator Pressure Exceeds Primary Pressure 3500.0 o Transient Terminated 6932L-FWS 19
FIGURE 2.1.1 MAINE YANKEE - - - 100_
PRIMARY INVENTORY TREND SYSTEM SYSTEM ILLUSTRATION w
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V V TRANSMITTER AND RECORDER CALIBRATION MAINE YANKEE PRIMARY INVENTORY TREND SYSTEM i
PITS INDICATION SCALES (Percent of Scale)
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CONTROL ROOM DISPLAY MAINE YANKEE PRIMARY INVENTORY TREND SYSTEM l
l
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MAINE YANKEE ATOMIC POWER COMPANY TABLE 3.2-Sequence of Events for Stuck Open PORV Transient
. Time (Sec) Events 0.0 0 Normal Steady-State Operation-50.0' o PORV Opens o Reactor Scram o Loss of Main Feed and Steam Flow o RCPs Trip 900.0 o Auxiliary Feed Flow Starts '(500 GPM) o Main Steam Flow Starts-(4.25 lbm/sec) 3600.0 o PORY Closed o HPSI Flow Starts 5000.0 o Transient Terminated 18 '3
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MAINE YANKEE ATOMIC POWER COMPANY TABLE'3.3 Sequence of Events for Three-Inch Cold Leg Break Transient Time (Sec) Events 0.0 o Normal Steady-State Operation 50.0 o Break Opens 69.0 o Reactor' Scram on Low Pressure 2 o Loss-of Main Feed Flow o Loss of Main Steam Flow 81.0 o High Pressure Safety Injection Actuation 86.0 o Auxiliary Feed Flow Starts and Stays on -Throughout the Transient 1
201.0 o Reactor Coolant Pumps Tripped 1610.0 o Steam Generator Pressure Exceeds Primary Pressure 3500.0 o Transient Terminated 19
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Figure 3.11 SYSTEM PRESSURE VERSUS TIME
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