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Reactor coolant pressure boundary leak detection systems in JapanesePWR plants
Nuclear Engineering and Design 128 (1991) 35-42 North-Holland
35
Reactor coolant pressure boundary leak detection systems in JapanesePWR plants Kazuo Aoki
System Engineering Department, Mitsubishi Atomic Power Industries, Inc., 2-4-1, Shibakouen, Minato-ku, Tokyo 105, Japan Received 15 May 1990
The current Reactor Coolant Pressure Boundary (RCPB) leak detection systems in Japanese PWR plants consist of the air cooler condensate "measuring system, the containment sump level monitoring system, the containment air particulate monitoring system and the containment radioactive gas monitoring system. These systems were reassessed in the course of the study on an application of the LBB concept and mainly the leak detection capabilities were discussed in this reassessment. The leak detection capabilities of these systems are described and, through comparison of these systems, appropriate methods to detect small leakage, more than 3.8 kg/min (1 gpm), are shown.
1. Introduction T h e current R C P B leak detection systems in J a p a n e s e p l a n t s were reassessed in the course of the study o n the application of the LBB concept. In the reassessment it was concluded that the air cooler c o n d e n s a t e measuring system and the c o n t a i n m e n t s u m p level m o n i t o r i n g system were sensitive e n o u g h to detect small leakage, more than 3.8 k g / m i n , in a short time. T h e concept a n d the sensitivity of the current leak
LETDOWN ORIFICE
-~
detection systems in J a p a n e s e P W R p l a n t s are described in the following, a n d the conclusion applicable to the LBB concept is introduced.
2. Behavior of leakage and normal operating systems If a leak in R C P B would occur, the leakage would partially evaporate a n d the v a p o r p o r t i o n could be t r a n s p o r t e d to the c o n t a i n m e n t vessel a t m o s p h e r e a n d
INSIDE OUTSIDE CONTAIi'~MI~'4T CONTA II'~IENT VESSEL VESSEL COOLING WATER
-~. SUBSYST~
LETDOWN COOLER
STEAM GENERATOR ~- - ~ . . . . . . . . . . .ER--~ .........
(~ ~R~ss~R,~ (~T;~TO R ', ' ~ REACTOR COOLANT PUMP
REACTOR VESSEL
REACTOR COOLANT pUMP
~
"
REACTOR MAKEUp WATER SYSTE2~ ~
~
--
CHARGING PUMP
Fig. 1. Conceptual flow diagram of the volume control system. 0029-5493/91/$03.50
© 1991 - E l s e v i e r S c i e n c e P u b l i s h e r s B.V. ( N o r t h - H o l l a n d )
~RI C ACID SUPPLY UBSYSTI~d
K. A oki / Reactor coolant pressure boundary leak detection ,~,sterns
36
Table 1 Behavior of normal operating systems and leakage detectable methods in case of normal letdown flow rate of 17 ton/hr Leakage rate
System behavior deviated from normal operating condition
Leakage detectable methods except for RCPB leak detection systems
less than 3,8 kg/min (0.23 ton/hr)
- The charging flow rate slightly increases. However, operators might not notice a difference from normal flow rate. - The amount of the unscheduled reactor makeup water usage increases.
- To be calculated from the reactor coolant inventory changes and the makeup water usage but to be done for more than several hours.
3.8-200 kg/min (0.23-12 ton/hr)
- The leakage could be compensated by the makeup water without the letdown line isolation. - The charging flow rate increases. Operators could notice a difference from normal flow rate. - Frequency of the reactor coolant makeup increases.
Calculation of the coolant inventry changes and the makeup water usage. Frequency of the coolant makeup. Increased unbalance between letdown and charging flow rate.
200-480 kg/min (12-29 ton/hr)
- The leakage could be compensated by the makeup water with the letdown line isolation. - The pressurizer level decreases and dispatches the normal level deviation alarm, and then automatically isolates the letdown line. - The auto-makeup water line is continuously operated and, in worse case, the volume control tank level decreases and reaches the low level alarm set point.
- Continuous auto-makeup water line operation. - Charging flow rate high alarm. - Pressurizer normal level deviation alarm. - Volume control tank level low alarm.
more than 480 kg/min (29 ton/hr)
-
Impossible to compensate the leakage by normal operating systems.
mixed by the c o n t a i n m e n t vessel air recirculation system, and the liquid portion could be routed to the c o n t a i n m e n t sump, Then, leak detection systems in both the c o n t a i n m e n t a t m o s p h e r e and the sump could detect the leakage. In normal operation of the volume control system, since the total flow leaving the reactor coolant system via the normal letdown line remains constant, the total charging flow via the charging p u m p s increases to make up for leakage. Thus, in case of a large a m o u n t of leakage, the normal operating system such as the volume control system could also detect the leakage. Figure 1 shows the conceptual flow diagram of the volume control system. Table 1 shows the behavior of the normal operating systems under the condition of various amount of leakage and the leakage detection methods. Discussed
Ditto. - Pressurizer level low alarm. -
hereafter are the leak detection systems for small leakage, in the order of 3.8 k g / m i n .
3. Leak
detection
systems
3.1. Design criterion and requirements " G u i d e l i n e for Safety Design of Light-Water Reactor Power Plant" [1] was p r o m u l g a t e d in 1977 by the former Japanese Atomic Energy Commission, and its Criterion 36 " t h e leak detection from Reactor Coolant Pressure Boundary" requires: the R C P B shah have de-
sign features that makes it possible to surely and promptly detect the leakage of the coolant.
37
K. A oki / Reactor coolant pressure boundary leak detection systems
In order to meet this requirement, the Japanese PWR plants are designed to be provided with the following standardized systems. (1) Air cooler condensate measuring system (2) Sump level monitoring system (3) Containment air particulate monitoring system (4) Containment radioactive gas monitoring system, The above four systems can provide the following two functions: (1) To determine the leak source category, such as high or low temperature line and radioactive or nonradioactive line. (2) To detect leaks and determine the leakage rate for leaks of 3.8 k g / m i n or greater within one hour, in accordance with U.S. Regulatory Guide 1.45 [2].
~ \ 1.5 .
o,..1 " ~<
1.0
z O
/
I 10
0
I 20
I 30
I 40
I 50
I 60
TIME AFTER CONDI~SATIONOCCURRED (MINUTES) Fig. 3. Time dependent condensate flow rate increase in case of 3.8 kg/min reactor coolant leakage.
3.2. System description and leak detectability 3.2.1. Air cooler condensate measuring system Figure 2 shows the concept of the air cooler condensate measuring system and the containment sump level monitoring system. This system collects the liquid runoff from the containment air recirculation cooling units and the CRDM air cooling unit, and measures the amount of the liquid. It consists of a containment cooler drain collection header, a vertical standpipe, valving, and standpipe level instrumentation. The condensate from the air cooler units flows via the drain collection header to the vertical standpipe. A flow resistant device, like a needle valve, is installed downstream of the standpipe. A differential pressure transmitter provides a standpipe level
! AIR COOLING
LEVEL IbSD ICATOR ] RECORDER ALARM - -
signal and this level signal can be converted into a condensate flow rate. Assuming leakage of the reactor coolant and isenthalpic change of the leakage, approximately 40% of the leakage mass is discharged as steam into the containment atmosphere and the remaining 60% is dropped in the form of water. The leaked steam raises the humidity of the containment air recirculation cooling units and the resulting increase in the condensate flow rate is given by the equation
(',
C~IR4DENSATE FROMCONTAIb~4ENT ] R RECIRCULATION
~ST
INSIDE C O N T A INWIENT VESSEL
OUTSIDE COTA I VESSEL
&\DP I PE
]I
] IB
CONTAINWlI~xvrSUI~~ LEVEL MONITORINGSYSTEM
LEVEL ALARM RATE R ~DRAIN FLOW'~ E C O R D E R A L A R M MAKEUP
~//////
s~ .
.
.
~Ps / / / / / ~ / .
.
.
.
.
CONTAI b~SENT SUNW
Fig. 2. Air cooler condensate measuring system and containment sump level monitoring system.
38
K. Aoki / Reactor coolant pressure boundary leak detection ,!vstems I
4oo
/
B P PE
LEVEL 30C
---
WH~
I"LOWINE-
PE
200
100
0
. . . . . . . . . 2 4 B
8
I . . . . . 10
CONDI~'SATE
FLOW
i
i
RATE
i
i
I 20
i
~ i
i
I
i
( LITER]MLNUTE
i
i
i
I 30
)
Fig. 4. Standpipe level dependency on condensate flow rate.
Figure 3 shows the condensate flow rate after the condensation has occured in a typical P W R plant. In normal operation, air in the containment vessel is well mixed by the containment air recirculation fan and, therefore, condensate can be generated in a short time after leak occurrence, and then condensate flow can reach the steady state approximately half an hour later as shown in fig. 3. The steady-state standpipe level signal determines the condensate flow rate. Figure 4 shows the relation between the standpipe level and condensate flow rate.
delay in leak detection. However, most RCPB pipes in Japanese P W R plants are insulated by metallic insulation and the insulation is divided into small pieces in order to be easily handled for the inspection of the pipes. The leakage, partially vaporizing and partially remaining water, can leak out through the gaps between the small pieces. The calculation result showed that the leakage can leak out in about ten minutes, even when using conservative assumptions. The leaked steam is absorbed by the environment and then condensed by the air coolers as previously described. The leaked water, if it is still water, may drop to the containment floor and flow to the floor drain lines in a short time since the floor has a slope of 1 / 1 0 0 to the drain inlets. The drained water can flow to the containment sump within five minutes even in the case of a longest drain line because each drain line also has a slope of 1 / 1 0 0 to the sump. Therefore, the drained water can be detected in a short time. The flow rate in the drain line is obtained from following formulas [3]. Ch~zy's formula: (2)
v = C~ii,
and Ganguillet and Kutter's formula: C =
23 + 1 / n + 0.00155/i I + n(23 + O . O 0 1 5 5 / i ) / f m
.
(3)
The containment sump level monitoring system can detect the leakage rate more precisely after the condensate flow reaches steady state since the condensate from the air condensate measuring system flows to the containment sump.
3.2.2. Containment sump level monitoring system
The concept of the containment sump level monitoring system is shown in fig. 2. Since any leak in the containment vessel would result in the flow of leaked fluid into the containment sump, leakag e would be indicated by a level increase in the sump. Indication of increasing sump level is transmitted from the sump to the control room and then converted to the leakage flow rate by means of a level-increase-toflow-rate converter. In order to detect the level change in the sump in a short time, the size of the sump is limited. The level change rate in the sump of a typical Japanese plant is about 6% of the level indicator span per quarter hour in case of 3.8 k g / m i n leakage. The delay time of the leaked fluid from the leak point to the sump may be of concern. The leakage may be absorbed by the thermal insulation, if the insulator would be made of water absorbable material like rockwool, and the absorption may result in a significant
3.2.3. Containment air particulate monitoring system
The concept of the containment air particulate monitoring system and the containment radioactive gas monitoring system is shown in fig. 5.
INSIDE C(~TAINMENT VESSEL
OUTSIDE CONTAINMI~qT V~SSEL
' PARTICULATE MONI TOR
i
CONTROL
ROOM
MONITOR
INDICATOR
i HIGH
ALARM
MONITOR I N D I C A T O R
Fig. 5. Containment air particulate momtoring system and containment radioactive gas monitoring system.
39
K. Aoki / Reactor coolant pressure boundary leak detection systems
ASSUMPTION
: 1 CONTALNMENrPVESSEL FREE VOLUME ;73.700m a 2 PATICULATE ACTIVITY CONCFI%TRATION IN REACTOR COOLANT ; 1,5X103Bq/~-wfi 3. PARTITION" FACTOR
; 0.1
4I---
3 8kq/min
LEAKAGE
F41 q
~5 0 38kq/min LEAKAGE
g
4E--
-
4E-7
-
~
.............
III:CI'IIL~LL/MI i . . . . . . . . . .
t
L
i
i
i
10
20
30
.io
50
TIME
AFTER
LEAK
OCCURRED
60
7O
(MINUTES1
Fig. 6. Time dependent particulate activity increases. The containment air particulate monitoring system is one of the process monitoring systems which monitors the radioactivity of the particulate by continuously obtaining containment air samples. An air sample is drawn outside the containment into a closed system by a sample pump and is then passed through a particulate filter with a scintillation detector, and a gaseous monitor chamber with a detector, and is then returned to the containment atmosphere. The minimum detectable concentration of the particulate monitor is around 4 × 1 0 .6 B q / c m 3 and the output signal of the monitor is recorded in the control room.
The particulate activity concentration in the containment atmosphere is determined from the containment free volume, the particulate activity in the coolant, the release rate of the activity and the purification rate of the containment atmosphere. The release rate of the activity depends on the leakage rate and the partition factor of particulates. Figure 6 shows that particulate activity increases in the containment atmosphere after leak occurrence. The assumptions are shown in this figure. The containment air particulate monitoring system has the best sensitivity to detect leak of the reactor
40
K. Aoki / Reactor coolant pressure boundao' leak detectton systems
coolant compared with other systems, but cannot determine the leakage rate since the particulate activity concentration is determined by unsteady conditions such as background level, particulate activity concentration in the coolant and the partition factor of particulates.
signal of the monitor is recorded in the control room. The gaseous radioactivity is determined from the containment free volume and the gaseous activity concentration of the reactor coolant. The leakage flow rate can be determined from the count rate when the specific background radioactivity is known. The background activity is dependent on the power level and the percentage of fuel defects. However, since the percentage of fuel defects is kept quite low in most plants and the background activity from Ar-41 always exists during power operation, the containment radioactive gas monitoring system is less sensitive than the particulate monitoring system. Figure 7 shows gaseous radioactivity increases in the containment atmosphere after leak occurrence. The assumptions are shown in this figure.
3.2.4. Containment radioactive gas monitoring system
The containment radioactive gas monitoring system is also one of the process monitoring systems and determines the gaseous radioactivity of the air sample after passing through the particulate filter in the containment air particulate monitoring system. Each sample is continuously mixed in a fixed and shielded volume where its activity is monitored by a scintilation detector. The minimum detectable concentration of the gaseous monitor is around 2 × 10 -2 Bq/cm 3 and the output
Ablb:I:MPTION
I X~' I: 3 A C T I V I T Y PE:\(T()R
IN
C(X)I.A2VI
; 4,
CON~I'A I N M I ~ 1 " V F S : S E L FRFTK \ ' O I Y M F ~
104 I$q
cm 3
; 7 3,7 0 0 In3
c-
152kq mir~
c 7-
V Ill i rl
v-
\
BACKGROI;ND \ FP,D M A r 11
ACTIVITY
0.01 21) TIME
10
3O
AFTER
LEAK
50
6O
OCCURPd';D M I N I J T E S
Fig. 7. Time dependent gaseous activity increases.
70
A0
K. Aoki / Reactor coolantpressure boundary leak detection systems
4. Evaluation of leak detection systems
atmosphere. The operator can estimate the leakage rate from the level signal of the standpipe as shown in fig. 4. Accurate measurements can be made when the steady state of the level signal is reached. As shown in fig. 3, steady state can be reached after approximately thirty minutes from the beginning of the condensation. Therefore, this system can detect a leakage in the order of 3.8 k g / m i n within one hour. The containment sump level monitoring system measures the level change rate in the sump and converts it to the leakage rate. Since any leak in the containment vessel results in a flow into the sump, this system can most precisely determine the leakage rate. The size of the sump is limited in order to detect the level change in the sump within about thirty minutes in case of 3.8 k g / m i n leakage. The delay time of the leaked fluid from the leak point to the sump is approximately fifteen minutes. Therefore, this system can detect a leakage in the order of 3.8 k g / m i n within our hour. The containment air particulate monitoring system has the best sensitivity to detect a leak of the reactor coolant. This system could detect a leakage of 3.8 k g / m i n within ten minutes if the particulate activity concentration in the coolant would be 1.5 × 10 3 B q / c m 3 as shown in fig. 6. However, this system cannot determine the precise leakage rate since the particulate activity concentration in the containment atmosphere depends on many factors. The major limiting factors are: (a) the background activity level, (b) the particulate activity concentration in the coolant, (c) the coolant temperature and pressure which affect the flashing fraction and the partition factor of particulates, (d) the purification rate of the containment atmosphere. The containment radioactive gas monitoring system is less sensitive than the particulate monitoring system,
4.1. Identification of the source of leakage The air cooler condensate measuring system collects and measures the moisture condensed from the containment atmosphere onto the cooling coils of the cooling units. This system determines leakage from the hightemperature water or steam system within the containment vessel. The containment sump level monitoring system collects and measures the flow of the leaked fluid into the sump. Since any leak in the containment vessel results in a flow into the sump, this system cannot directly determine the source of the leakage. The containment air particulate monitoring system monitors the radioactivity of the particulate in the containment atmosphere. This system determines leakage of the reactor coolant which contains radioactive particulate. The containment radioactive gas monitoring system determines leakage of the reactor coolant which contains radioactive gas. This system does not work if the reactor fuel cladding has no defect. Figure 8 shows the relation between each leak detection system and the sources of leakage to be detected. As shown in fig. 8, the source category of leakage can be identified by the information from these leak detection systems, Since most lines in the RCPB would contain high-temperature and radioactive fluid, all of these detection systems could work in case of leak occurring in RCPB.
4.2. Early detection of leakage The air cooler condensate measuring system measures the moisture condensed from the containment
LEAK DETECTION SYSTEM (1) AIR COOLER CONDENSATE MEASURING SYSTEM
SOURCE OF LEAKAGE '~_
~.2' HIGH TEMPERATUREAND RADIOACTIVE FLUID
(2) C O N T A I ~ SUMP LEVEL MONITORING SYSTEM (3) C O N T A I ~ T
AIR PARTICULATE
41
HIGH TEMPERATUREAND NON-RADIOACTIVE FLUID /
~ ....
LOW TEMPERATURE AND RADIOACTIVE FLUID LOW TEMPERATUREAND NON-RADIOACTIVE FLUID
(4) C O N T A I ~ RADIOACTIVE GAS MONITORING SYSTI~¢I
.....
In case of' fuel defect
Fig. 8. Relation between leak detection system and source of leakage.
42
K. Aoki / Reactor coolant pressure boundary, leak detection systems
Table 2 Capabilities of leak detection systems Leak detection system
Minimum detectable leak rate within one hour
Response time in case of 3.8 kg/min leakage
Source of leakage to be detected
Air cooler condensate Measuring System
less than 3.8 kg/min
within 1 h
high-temperature liquid or steam
Containment sump level monitoring system
less than 3.8 kg/min
approximately 1 h
all
Containment air particulate monitoring system
approximately 0.4 kg/min assuming no fuel defect and 1.5 × 103 Bq/cm 3 corrosion product concentration in coolant
within 1 h
high-temperature liquid containing radioactive particulate (reactor coolant)
Containment radioactive gas monitoring system
approximately 8 kg/min assuming 4× 104 Bq/cm 3 Xe 133 concentration in coolant
within 2 h
liquid or steam containing radioactive gas (reactor coolant with fuel defect)
and would function in the event that significant reactor coolant gaseous activity exists due to fuel cladding defects. Assuming a gaseous activity of 4 x 104 B q / c m 3 as Xe-133 in the reactor coolant, leakage of 7.6 k g / m i n would be detected within one hour. Table 2 shows a summary of the capability of each leak detection system. To detect leakage in a short time, the containment air particulate monitoring system and the air cooler condensate measuring system are effective. Since the detectability of the containment air particulate monitoring system and the radioactive gas monitoring system is strongly dependent on the radioactivity concentrations in the reactor coolant, these systems are more effective to identify the source category of leakage rather than to determine the leakage rate. The containment sump level monitoring system could provide relatively precise measurement of the leakage rate. 5. Conclusions
(1) The current RCPB leak detection systems can provide early detection of small leakage in the order of 3.8 k g / m i n and identify the source category of the leakage. (2) The radiation monitors would be available if the leakage would contain enough amount of radioactive materials. (3) The air cooler condensate measuring system can most quickly work in any case of leak occurrence in high-energy lines. (4) The containment sump level monitoring system is available to determine the leakage rate.
(5) Since so many non- or low-radioactive high-energy lines exist in the containment vessel, the air cooler condensate measuring system and the containment sump level monitoring system are appropriate methods applicable to any leakage from any high-energy line and quantitatively detectable.
Nomenclature
C D L Q V i m n t u
coefficient given by eq. (3), condensate flow rate ( k g / m i n ) , evaporated leakage (kg/min), containment air recirculation cooling rate (m3/min), containment free volume (m3), gradient (-), average water depth (m), roughness factor, 0,01 for stainless steel, time after condensation occurred (min), average flow velocity ( N m / s ) .
References
[1] Japanese Atomic Energy Commission, Guideline for safety Design of Light-Water Reactor Power Plant (1977). [2] U.S. Nuclear Regulatory Commission~ Regulatory Guide 1.45 (1973). [3] JSME, Mechanical Engineering Handbook (JSME, Tokyo, 1968) pp. 8-21-8-22.
35
Reactor coolant pressure boundary leak detection systems in JapanesePWR plants Kazuo Aoki
System Engineering Department, Mitsubishi Atomic Power Industries, Inc., 2-4-1, Shibakouen, Minato-ku, Tokyo 105, Japan Received 15 May 1990
The current Reactor Coolant Pressure Boundary (RCPB) leak detection systems in Japanese PWR plants consist of the air cooler condensate "measuring system, the containment sump level monitoring system, the containment air particulate monitoring system and the containment radioactive gas monitoring system. These systems were reassessed in the course of the study on an application of the LBB concept and mainly the leak detection capabilities were discussed in this reassessment. The leak detection capabilities of these systems are described and, through comparison of these systems, appropriate methods to detect small leakage, more than 3.8 kg/min (1 gpm), are shown.
1. Introduction T h e current R C P B leak detection systems in J a p a n e s e p l a n t s were reassessed in the course of the study o n the application of the LBB concept. In the reassessment it was concluded that the air cooler c o n d e n s a t e measuring system and the c o n t a i n m e n t s u m p level m o n i t o r i n g system were sensitive e n o u g h to detect small leakage, more than 3.8 k g / m i n , in a short time. T h e concept a n d the sensitivity of the current leak
LETDOWN ORIFICE
-~
detection systems in J a p a n e s e P W R p l a n t s are described in the following, a n d the conclusion applicable to the LBB concept is introduced.
2. Behavior of leakage and normal operating systems If a leak in R C P B would occur, the leakage would partially evaporate a n d the v a p o r p o r t i o n could be t r a n s p o r t e d to the c o n t a i n m e n t vessel a t m o s p h e r e a n d
INSIDE OUTSIDE CONTAIi'~MI~'4T CONTA II'~IENT VESSEL VESSEL COOLING WATER
-~. SUBSYST~
LETDOWN COOLER
STEAM GENERATOR ~- - ~ . . . . . . . . . . .ER--~ .........
(~ ~R~ss~R,~ (~T;~TO R ', ' ~ REACTOR COOLANT PUMP
REACTOR VESSEL
REACTOR COOLANT pUMP
~
"
REACTOR MAKEUp WATER SYSTE2~ ~
~
--
CHARGING PUMP
Fig. 1. Conceptual flow diagram of the volume control system. 0029-5493/91/$03.50
© 1991 - E l s e v i e r S c i e n c e P u b l i s h e r s B.V. ( N o r t h - H o l l a n d )
~RI C ACID SUPPLY UBSYSTI~d
K. A oki / Reactor coolant pressure boundary leak detection ,~,sterns
36
Table 1 Behavior of normal operating systems and leakage detectable methods in case of normal letdown flow rate of 17 ton/hr Leakage rate
System behavior deviated from normal operating condition
Leakage detectable methods except for RCPB leak detection systems
less than 3,8 kg/min (0.23 ton/hr)
- The charging flow rate slightly increases. However, operators might not notice a difference from normal flow rate. - The amount of the unscheduled reactor makeup water usage increases.
- To be calculated from the reactor coolant inventory changes and the makeup water usage but to be done for more than several hours.
3.8-200 kg/min (0.23-12 ton/hr)
- The leakage could be compensated by the makeup water without the letdown line isolation. - The charging flow rate increases. Operators could notice a difference from normal flow rate. - Frequency of the reactor coolant makeup increases.
Calculation of the coolant inventry changes and the makeup water usage. Frequency of the coolant makeup. Increased unbalance between letdown and charging flow rate.
200-480 kg/min (12-29 ton/hr)
- The leakage could be compensated by the makeup water with the letdown line isolation. - The pressurizer level decreases and dispatches the normal level deviation alarm, and then automatically isolates the letdown line. - The auto-makeup water line is continuously operated and, in worse case, the volume control tank level decreases and reaches the low level alarm set point.
- Continuous auto-makeup water line operation. - Charging flow rate high alarm. - Pressurizer normal level deviation alarm. - Volume control tank level low alarm.
more than 480 kg/min (29 ton/hr)
-
Impossible to compensate the leakage by normal operating systems.
mixed by the c o n t a i n m e n t vessel air recirculation system, and the liquid portion could be routed to the c o n t a i n m e n t sump, Then, leak detection systems in both the c o n t a i n m e n t a t m o s p h e r e and the sump could detect the leakage. In normal operation of the volume control system, since the total flow leaving the reactor coolant system via the normal letdown line remains constant, the total charging flow via the charging p u m p s increases to make up for leakage. Thus, in case of a large a m o u n t of leakage, the normal operating system such as the volume control system could also detect the leakage. Figure 1 shows the conceptual flow diagram of the volume control system. Table 1 shows the behavior of the normal operating systems under the condition of various amount of leakage and the leakage detection methods. Discussed
Ditto. - Pressurizer level low alarm. -
hereafter are the leak detection systems for small leakage, in the order of 3.8 k g / m i n .
3. Leak
detection
systems
3.1. Design criterion and requirements " G u i d e l i n e for Safety Design of Light-Water Reactor Power Plant" [1] was p r o m u l g a t e d in 1977 by the former Japanese Atomic Energy Commission, and its Criterion 36 " t h e leak detection from Reactor Coolant Pressure Boundary" requires: the R C P B shah have de-
sign features that makes it possible to surely and promptly detect the leakage of the coolant.
37
K. A oki / Reactor coolant pressure boundary leak detection systems
In order to meet this requirement, the Japanese PWR plants are designed to be provided with the following standardized systems. (1) Air cooler condensate measuring system (2) Sump level monitoring system (3) Containment air particulate monitoring system (4) Containment radioactive gas monitoring system, The above four systems can provide the following two functions: (1) To determine the leak source category, such as high or low temperature line and radioactive or nonradioactive line. (2) To detect leaks and determine the leakage rate for leaks of 3.8 k g / m i n or greater within one hour, in accordance with U.S. Regulatory Guide 1.45 [2].
~ \ 1.5 .
o,..1 " ~<
1.0
z O
/
I 10
0
I 20
I 30
I 40
I 50
I 60
TIME AFTER CONDI~SATIONOCCURRED (MINUTES) Fig. 3. Time dependent condensate flow rate increase in case of 3.8 kg/min reactor coolant leakage.
3.2. System description and leak detectability 3.2.1. Air cooler condensate measuring system Figure 2 shows the concept of the air cooler condensate measuring system and the containment sump level monitoring system. This system collects the liquid runoff from the containment air recirculation cooling units and the CRDM air cooling unit, and measures the amount of the liquid. It consists of a containment cooler drain collection header, a vertical standpipe, valving, and standpipe level instrumentation. The condensate from the air cooler units flows via the drain collection header to the vertical standpipe. A flow resistant device, like a needle valve, is installed downstream of the standpipe. A differential pressure transmitter provides a standpipe level
! AIR COOLING
LEVEL IbSD ICATOR ] RECORDER ALARM - -
signal and this level signal can be converted into a condensate flow rate. Assuming leakage of the reactor coolant and isenthalpic change of the leakage, approximately 40% of the leakage mass is discharged as steam into the containment atmosphere and the remaining 60% is dropped in the form of water. The leaked steam raises the humidity of the containment air recirculation cooling units and the resulting increase in the condensate flow rate is given by the equation
(',
C~IR4DENSATE FROMCONTAIb~4ENT ] R RECIRCULATION
~ST
INSIDE C O N T A INWIENT VESSEL
OUTSIDE COTA I VESSEL
&\DP I PE
]I
] IB
CONTAINWlI~xvrSUI~~ LEVEL MONITORINGSYSTEM
LEVEL ALARM RATE R ~DRAIN FLOW'~ E C O R D E R A L A R M MAKEUP
~//////
s~ .
.
.
~Ps / / / / / ~ / .
.
.
.
.
CONTAI b~SENT SUNW
Fig. 2. Air cooler condensate measuring system and containment sump level monitoring system.
38
K. Aoki / Reactor coolant pressure boundary leak detection ,!vstems I
4oo
/
B P PE
LEVEL 30C
---
WH~
I"LOWINE-
PE
200
100
0
. . . . . . . . . 2 4 B
8
I . . . . . 10
CONDI~'SATE
FLOW
i
i
RATE
i
i
I 20
i
~ i
i
I
i
( LITER]MLNUTE
i
i
i
I 30
)
Fig. 4. Standpipe level dependency on condensate flow rate.
Figure 3 shows the condensate flow rate after the condensation has occured in a typical P W R plant. In normal operation, air in the containment vessel is well mixed by the containment air recirculation fan and, therefore, condensate can be generated in a short time after leak occurrence, and then condensate flow can reach the steady state approximately half an hour later as shown in fig. 3. The steady-state standpipe level signal determines the condensate flow rate. Figure 4 shows the relation between the standpipe level and condensate flow rate.
delay in leak detection. However, most RCPB pipes in Japanese P W R plants are insulated by metallic insulation and the insulation is divided into small pieces in order to be easily handled for the inspection of the pipes. The leakage, partially vaporizing and partially remaining water, can leak out through the gaps between the small pieces. The calculation result showed that the leakage can leak out in about ten minutes, even when using conservative assumptions. The leaked steam is absorbed by the environment and then condensed by the air coolers as previously described. The leaked water, if it is still water, may drop to the containment floor and flow to the floor drain lines in a short time since the floor has a slope of 1 / 1 0 0 to the drain inlets. The drained water can flow to the containment sump within five minutes even in the case of a longest drain line because each drain line also has a slope of 1 / 1 0 0 to the sump. Therefore, the drained water can be detected in a short time. The flow rate in the drain line is obtained from following formulas [3]. Ch~zy's formula: (2)
v = C~ii,
and Ganguillet and Kutter's formula: C =
23 + 1 / n + 0.00155/i I + n(23 + O . O 0 1 5 5 / i ) / f m
.
(3)
The containment sump level monitoring system can detect the leakage rate more precisely after the condensate flow reaches steady state since the condensate from the air condensate measuring system flows to the containment sump.
3.2.2. Containment sump level monitoring system
The concept of the containment sump level monitoring system is shown in fig. 2. Since any leak in the containment vessel would result in the flow of leaked fluid into the containment sump, leakag e would be indicated by a level increase in the sump. Indication of increasing sump level is transmitted from the sump to the control room and then converted to the leakage flow rate by means of a level-increase-toflow-rate converter. In order to detect the level change in the sump in a short time, the size of the sump is limited. The level change rate in the sump of a typical Japanese plant is about 6% of the level indicator span per quarter hour in case of 3.8 k g / m i n leakage. The delay time of the leaked fluid from the leak point to the sump may be of concern. The leakage may be absorbed by the thermal insulation, if the insulator would be made of water absorbable material like rockwool, and the absorption may result in a significant
3.2.3. Containment air particulate monitoring system
The concept of the containment air particulate monitoring system and the containment radioactive gas monitoring system is shown in fig. 5.
INSIDE C(~TAINMENT VESSEL
OUTSIDE CONTAINMI~qT V~SSEL
' PARTICULATE MONI TOR
i
CONTROL
ROOM
MONITOR
INDICATOR
i HIGH
ALARM
MONITOR I N D I C A T O R
Fig. 5. Containment air particulate momtoring system and containment radioactive gas monitoring system.
39
K. Aoki / Reactor coolant pressure boundary leak detection systems
ASSUMPTION
: 1 CONTALNMENrPVESSEL FREE VOLUME ;73.700m a 2 PATICULATE ACTIVITY CONCFI%TRATION IN REACTOR COOLANT ; 1,5X103Bq/~-wfi 3. PARTITION" FACTOR
; 0.1
4I---
3 8kq/min
LEAKAGE
F41 q
~5 0 38kq/min LEAKAGE
g
4E--
-
4E-7
-
~
.............
III:CI'IIL~LL/MI i . . . . . . . . . .
t
L
i
i
i
10
20
30
.io
50
TIME
AFTER
LEAK
OCCURRED
60
7O
(MINUTES1
Fig. 6. Time dependent particulate activity increases. The containment air particulate monitoring system is one of the process monitoring systems which monitors the radioactivity of the particulate by continuously obtaining containment air samples. An air sample is drawn outside the containment into a closed system by a sample pump and is then passed through a particulate filter with a scintillation detector, and a gaseous monitor chamber with a detector, and is then returned to the containment atmosphere. The minimum detectable concentration of the particulate monitor is around 4 × 1 0 .6 B q / c m 3 and the output signal of the monitor is recorded in the control room.
The particulate activity concentration in the containment atmosphere is determined from the containment free volume, the particulate activity in the coolant, the release rate of the activity and the purification rate of the containment atmosphere. The release rate of the activity depends on the leakage rate and the partition factor of particulates. Figure 6 shows that particulate activity increases in the containment atmosphere after leak occurrence. The assumptions are shown in this figure. The containment air particulate monitoring system has the best sensitivity to detect leak of the reactor
40
K. Aoki / Reactor coolant pressure boundao' leak detectton systems
coolant compared with other systems, but cannot determine the leakage rate since the particulate activity concentration is determined by unsteady conditions such as background level, particulate activity concentration in the coolant and the partition factor of particulates.
signal of the monitor is recorded in the control room. The gaseous radioactivity is determined from the containment free volume and the gaseous activity concentration of the reactor coolant. The leakage flow rate can be determined from the count rate when the specific background radioactivity is known. The background activity is dependent on the power level and the percentage of fuel defects. However, since the percentage of fuel defects is kept quite low in most plants and the background activity from Ar-41 always exists during power operation, the containment radioactive gas monitoring system is less sensitive than the particulate monitoring system. Figure 7 shows gaseous radioactivity increases in the containment atmosphere after leak occurrence. The assumptions are shown in this figure.
3.2.4. Containment radioactive gas monitoring system
The containment radioactive gas monitoring system is also one of the process monitoring systems and determines the gaseous radioactivity of the air sample after passing through the particulate filter in the containment air particulate monitoring system. Each sample is continuously mixed in a fixed and shielded volume where its activity is monitored by a scintilation detector. The minimum detectable concentration of the gaseous monitor is around 2 × 10 -2 Bq/cm 3 and the output
Ablb:I:MPTION
I X~' I: 3 A C T I V I T Y PE:\(T()R
IN
C(X)I.A2VI
; 4,
CON~I'A I N M I ~ 1 " V F S : S E L FRFTK \ ' O I Y M F ~
104 I$q
cm 3
; 7 3,7 0 0 In3
c-
152kq mir~
c 7-
V Ill i rl
v-
\
BACKGROI;ND \ FP,D M A r 11
ACTIVITY
0.01 21) TIME
10
3O
AFTER
LEAK
50
6O
OCCURPd';D M I N I J T E S
Fig. 7. Time dependent gaseous activity increases.
70
A0
K. Aoki / Reactor coolantpressure boundary leak detection systems
4. Evaluation of leak detection systems
atmosphere. The operator can estimate the leakage rate from the level signal of the standpipe as shown in fig. 4. Accurate measurements can be made when the steady state of the level signal is reached. As shown in fig. 3, steady state can be reached after approximately thirty minutes from the beginning of the condensation. Therefore, this system can detect a leakage in the order of 3.8 k g / m i n within one hour. The containment sump level monitoring system measures the level change rate in the sump and converts it to the leakage rate. Since any leak in the containment vessel results in a flow into the sump, this system can most precisely determine the leakage rate. The size of the sump is limited in order to detect the level change in the sump within about thirty minutes in case of 3.8 k g / m i n leakage. The delay time of the leaked fluid from the leak point to the sump is approximately fifteen minutes. Therefore, this system can detect a leakage in the order of 3.8 k g / m i n within our hour. The containment air particulate monitoring system has the best sensitivity to detect a leak of the reactor coolant. This system could detect a leakage of 3.8 k g / m i n within ten minutes if the particulate activity concentration in the coolant would be 1.5 × 10 3 B q / c m 3 as shown in fig. 6. However, this system cannot determine the precise leakage rate since the particulate activity concentration in the containment atmosphere depends on many factors. The major limiting factors are: (a) the background activity level, (b) the particulate activity concentration in the coolant, (c) the coolant temperature and pressure which affect the flashing fraction and the partition factor of particulates, (d) the purification rate of the containment atmosphere. The containment radioactive gas monitoring system is less sensitive than the particulate monitoring system,
4.1. Identification of the source of leakage The air cooler condensate measuring system collects and measures the moisture condensed from the containment atmosphere onto the cooling coils of the cooling units. This system determines leakage from the hightemperature water or steam system within the containment vessel. The containment sump level monitoring system collects and measures the flow of the leaked fluid into the sump. Since any leak in the containment vessel results in a flow into the sump, this system cannot directly determine the source of the leakage. The containment air particulate monitoring system monitors the radioactivity of the particulate in the containment atmosphere. This system determines leakage of the reactor coolant which contains radioactive particulate. The containment radioactive gas monitoring system determines leakage of the reactor coolant which contains radioactive gas. This system does not work if the reactor fuel cladding has no defect. Figure 8 shows the relation between each leak detection system and the sources of leakage to be detected. As shown in fig. 8, the source category of leakage can be identified by the information from these leak detection systems, Since most lines in the RCPB would contain high-temperature and radioactive fluid, all of these detection systems could work in case of leak occurring in RCPB.
4.2. Early detection of leakage The air cooler condensate measuring system measures the moisture condensed from the containment
LEAK DETECTION SYSTEM (1) AIR COOLER CONDENSATE MEASURING SYSTEM
SOURCE OF LEAKAGE '~_
~.2' HIGH TEMPERATUREAND RADIOACTIVE FLUID
(2) C O N T A I ~ SUMP LEVEL MONITORING SYSTEM (3) C O N T A I ~ T
AIR PARTICULATE
41
HIGH TEMPERATUREAND NON-RADIOACTIVE FLUID /
~ ....
LOW TEMPERATURE AND RADIOACTIVE FLUID LOW TEMPERATUREAND NON-RADIOACTIVE FLUID
(4) C O N T A I ~ RADIOACTIVE GAS MONITORING SYSTI~¢I
.....
In case of' fuel defect
Fig. 8. Relation between leak detection system and source of leakage.
42
K. Aoki / Reactor coolant pressure boundary, leak detection systems
Table 2 Capabilities of leak detection systems Leak detection system
Minimum detectable leak rate within one hour
Response time in case of 3.8 kg/min leakage
Source of leakage to be detected
Air cooler condensate Measuring System
less than 3.8 kg/min
within 1 h
high-temperature liquid or steam
Containment sump level monitoring system
less than 3.8 kg/min
approximately 1 h
all
Containment air particulate monitoring system
approximately 0.4 kg/min assuming no fuel defect and 1.5 × 103 Bq/cm 3 corrosion product concentration in coolant
within 1 h
high-temperature liquid containing radioactive particulate (reactor coolant)
Containment radioactive gas monitoring system
approximately 8 kg/min assuming 4× 104 Bq/cm 3 Xe 133 concentration in coolant
within 2 h
liquid or steam containing radioactive gas (reactor coolant with fuel defect)
and would function in the event that significant reactor coolant gaseous activity exists due to fuel cladding defects. Assuming a gaseous activity of 4 x 104 B q / c m 3 as Xe-133 in the reactor coolant, leakage of 7.6 k g / m i n would be detected within one hour. Table 2 shows a summary of the capability of each leak detection system. To detect leakage in a short time, the containment air particulate monitoring system and the air cooler condensate measuring system are effective. Since the detectability of the containment air particulate monitoring system and the radioactive gas monitoring system is strongly dependent on the radioactivity concentrations in the reactor coolant, these systems are more effective to identify the source category of leakage rather than to determine the leakage rate. The containment sump level monitoring system could provide relatively precise measurement of the leakage rate. 5. Conclusions
(1) The current RCPB leak detection systems can provide early detection of small leakage in the order of 3.8 k g / m i n and identify the source category of the leakage. (2) The radiation monitors would be available if the leakage would contain enough amount of radioactive materials. (3) The air cooler condensate measuring system can most quickly work in any case of leak occurrence in high-energy lines. (4) The containment sump level monitoring system is available to determine the leakage rate.
(5) Since so many non- or low-radioactive high-energy lines exist in the containment vessel, the air cooler condensate measuring system and the containment sump level monitoring system are appropriate methods applicable to any leakage from any high-energy line and quantitatively detectable.
Nomenclature
C D L Q V i m n t u
coefficient given by eq. (3), condensate flow rate ( k g / m i n ) , evaporated leakage (kg/min), containment air recirculation cooling rate (m3/min), containment free volume (m3), gradient (-), average water depth (m), roughness factor, 0,01 for stainless steel, time after condensation occurred (min), average flow velocity ( N m / s ) .
References
[1] Japanese Atomic Energy Commission, Guideline for safety Design of Light-Water Reactor Power Plant (1977). [2] U.S. Nuclear Regulatory Commission~ Regulatory Guide 1.45 (1973). [3] JSME, Mechanical Engineering Handbook (JSME, Tokyo, 1968) pp. 8-21-8-22.