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An on-line water contamination monitoring system
Nuclear Instruments and Methods m Physics Research A 353 (1994)
550-552
ELSEVIER
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
An on-line water contamination monitoring system K.H. Yeung, H.Y. Mok, H.K. Lam Royal Observatory, Hong Kong
Abstract A nuclear power station was recently built in the People's Republic of China, some 50 km from a reservoir which supplies fresh water to Hong Kong . An on-line water contamination monitoring system, built to the specification of the Royal Observatory, was installed at a pumping station at the border to monitor continuously the activity concentration of gamma-emitting radionuclides present in the raw water supplied to Hong Kong . The contamination monitoring system comprises a sodium iodide detector, a stainless steel water chamber, a lead shield, and a microprocessor-based spectrum analyser. Fresh water is allowed to flow through the water chamber while the activity concentrations of 4°K, 103Ru 1311 and 137CS in the water are measured. An alarm level can be set for one of the four radionuclides to alert the operator of abnormal activity concentrations. Remote display of the data and alarm status are also available. The system is calibrated one radionuclide at a time . In reality, more than one radionuclide may be present in the contaminated fresh water, and interfere with one another in the measurement . The problem of interference has been evaluated, and it was found that the presence of 1311 gave rise to a positive bias in the reported activity concentration of 137 Cs . A simple rule for correcting the bias was drawn up .
1. Introduction 70% of Hong Kong's fresh water supply comes from the bordering Guangdong Province in the People's Republic of China. Recently, a nuclear power station was built in the Guangdong Province, some 50 km from a reservoir which supplies untreated fresh water to Hong Kong . As part of the Hong Kong Government's radioactivity monitoring programme, an on-line water contamination monitoring system was installed at a pumping station at the border to monitor continuously the activity concentrations of three artificial gamma-emitting radionuclides (namely 103Ru, 1311 and 137Cs) present in untreated fresh water coming from the Guangdong Province . 2. System description The monitoring system was custom-built to meet Hong Kong's requirements [1]. It is made up of a field unit and two central processing stations . Fig. 1 shows the configuration of the system . The field unit at the pumping station comprises a 75 mm X 75 mm sodium iodide detector, a stainless steel water chamber of 22 litres in volume which encloses the detector, a 50 mm thick lead shield, and microcomputerbased spectrum analyser . During normal operation, untreated fresh water 1s allowed to flow through the water chamber at a rate of about 4 litres per minute while the 0168-9002/94/$07 .00 C 1994 Elsevier SSDI 0168-9002(94)00983-X
Science
B.V. All
radiation from gamma emitters in the water is sensed by the sodium iodide detector . The detector has a heater glued to its side and the two are enclosed by foam of 2.3 cm thick for thermal insulation . The heater-foam assembly provides thermal stability, maintaining the detector at a constant temperature of 43°C [2]. The output of the detector goes to a microcomputerbased spectrum analyser, which counts the number of gamma photons in different energy intervals. Four regions of interests are defined in the analyser for four radionuclides : 4°K, 103Ru, 1311 and 137Cs . Counting for 4° K, which is a natural radionuclide always present in the water, is used to check the system performance . The net count rate in each region of interest is converted to the activity concentration of the corresponding radionuclides based on the efficiency coefficient of the detector-analyser assembly and the volume of the water chamber. An alarm level can be set for one of the four radionuclides to alert the operator of abnormal activity concentrations . Under normal condition, the counting time is set to half an hour. Once an alarm is triggered, the counting time will be shortened to one minute for more frequent updating . The spectral and analysed data from the field unit, including the alarm status, are transmitted in real-time via leased telephone lines to two central stations, one at the water authority and another at an emergency monitoring and assessment centre, where the significance of any alarm is assessed and acted on .
rights reserved
551
KH . Yeang et al. /Nucl. Instr. and Meth. in Phys . Res. A 353 (1994) 550-552
UNTREATED SURFACE WATER IN
N,I DETECTOR
Ru respectively . Test results showed that the influ137 103 Cs and Ru on the other ROIs was negligible . The solution used in Test 4 consisted of equal amounts of 137Cs 103Ru . and Test results showed that the presence of 137Cs 103Ru and had negligible influence on the reported ence of
VENTING VALVE WATER OUT
DRAINAGE
RS 2320
SPECTRUM ANALYSER & PROCESSOR
RS 232C
MODEM
DISPLAY
activity concentration of 13 '1 .
CENTRAL PROCESSING SYSTEM
-
MODEM
TELEPHONE LINE
RS 232C DISPLAY
non-zero and practically the same . This showed that the 103 Ru increase in counts in the ROI was not correlated to 131 I. This was the increase in activity concentration of substantiated by Tests 7 to 11 . While the results of Tests 7,
CENTRAL PROCESSING SYSTEM
MODEM RS 232C
Microcomputer
Microcomputer
PRINTER
PRINTER
103Ru
Solutions used in Tests 5 and 6 did not contain 131 1. The report but significant different concentrations of 103 activity concentrations of Ru of the two solution were
R52 2C MODEM
TELEPHONE LINE
103
and
AMPLIFLIER
8 and 9 seemed to indicate a positive correlation, the
results of Tests 10 and 11 suggested a negative one. The lack of correlation is reasonable as none of the gamma 103 Ru . peaks of 131, falls within the ROI of The anomaly 103 Ru in the reported activity concentration of in Tests 5 to 103 11 was suspected to be due to plate-out residual Ru on
Fig. 1 .
3. Mixed radionuclide tests
the internal wall of the water chamber in previous mea-
The system is calibrated one radionuclide at a time for 103 Ru, 131, 137Cs and ; the most likely radionuclides to be found in fresh water contaminated by any off-site accidental release from the nuclear power station. In reality, more than one radionuclide may be present
in the contaminated fresh water, and interfere with one another in the measurement. The problem of interference has been evaluated using standard solutions of different 103 Ru, 1311 137Cs and . Table 1 activity concentrations of shows the activity concentrations of the three radionuclides of the standard solutions used for the test and the respective activity concentrations reported by the system. Solutions used in Test 2 and 3 consisted only of 137Cs
surements (i .e . in Tests 3 and 4) . This was ascertained by
repeating Tests 5 and 6 after decontamination of the water chamber, the results of which were presented as Tests 12 and 13 respectively in Table 1 .
Results of Tests 6 to 11 showed that the presence of
131,
gave rise to a positive bias in the reported activity
concentration of
137Cs,
of the order of 5% of the reported
activity concentration of
131, .
activity concentration of
This increase in the reported
137CS
could be attributed to the 637 keV gamma peak of 131, which fell almost completely within the ROI of 137Cs . Therefore, as a simple rule-ofthumb, whenever a large amount of
1311
is observed,
reported activity concentration of 137Cs should be reduced 131 by 5% of the reported activity of 1.
Table 1 137CS 1311 103Ru Reported activity concentrations of and in different solutions. (The most probable activity ratio of the three radionuclides in an accidental release has been assessed to be 1 : 15 : 1.) Test No .
1 2 3 4 5 6 7 8 9 10 11 12 13
Activity concentration (Bq/1) 137
Cs
131 1
103
Ru
reported
actual
reported
actual
reported
actual
0 530 0 484 173 27 14 82 182 564 553 180 26
0 544 0 544 152 0 0 0 109 544 544 152 0
0 0 0 0 1415 532 456 1506 1379 551 444 1472 511
0 0 0 0 1528 514 472 1516 1391 538 453 1528 514
0 0 483 445 31 33 531 194 186 67 551 0 0
0 0 502 436 0 0 498 102 100 0 470 0 0
Ratio of actual activity concentrations among 13 : 131 1 : 103 R 7CS u 0 : 0 :0 1 : 0 :0 0 : 0 :1 1 : 0 :1 1 :10 :0 0: 1 :0 0: 1 :1 1 :15 :1 1 :13 :1 1 : 1 :0 1 : 1 :1 1 :10 :0 0 : 1 :0
VIIIb. ENVIRONMENTAL APPLICATIONS
552
K.H. Yeung et al. /Nucl. Instr. and Meth. i n Phys. Res. A 353 (1994) 550-552
4. Conclusion The on-line water contamination monitoring system described in this paper uses a relatively small sodium
satisfactory output even when a mixture of the radionuclides of interest are present.
iodide detector and fairly modest processing equipment to
References
gamma emitting radionuclides present in water passing through a pumping stations . Tests with solutions of known
[1] Canberra Industries Inc., On-line Water Contamination Monitoring System, 1991 . [2] T.A . Perkins, Temperature Controller/Stabilizer/Water Vessel/Shield for Royal Observatory, 1990 .
monitor in real-time the activity concentration of a few
activity concentrations have shown that it can produce
550-552
ELSEVIER
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
An on-line water contamination monitoring system K.H. Yeung, H.Y. Mok, H.K. Lam Royal Observatory, Hong Kong
Abstract A nuclear power station was recently built in the People's Republic of China, some 50 km from a reservoir which supplies fresh water to Hong Kong . An on-line water contamination monitoring system, built to the specification of the Royal Observatory, was installed at a pumping station at the border to monitor continuously the activity concentration of gamma-emitting radionuclides present in the raw water supplied to Hong Kong . The contamination monitoring system comprises a sodium iodide detector, a stainless steel water chamber, a lead shield, and a microprocessor-based spectrum analyser. Fresh water is allowed to flow through the water chamber while the activity concentrations of 4°K, 103Ru 1311 and 137CS in the water are measured. An alarm level can be set for one of the four radionuclides to alert the operator of abnormal activity concentrations. Remote display of the data and alarm status are also available. The system is calibrated one radionuclide at a time . In reality, more than one radionuclide may be present in the contaminated fresh water, and interfere with one another in the measurement . The problem of interference has been evaluated, and it was found that the presence of 1311 gave rise to a positive bias in the reported activity concentration of 137 Cs . A simple rule for correcting the bias was drawn up .
1. Introduction 70% of Hong Kong's fresh water supply comes from the bordering Guangdong Province in the People's Republic of China. Recently, a nuclear power station was built in the Guangdong Province, some 50 km from a reservoir which supplies untreated fresh water to Hong Kong . As part of the Hong Kong Government's radioactivity monitoring programme, an on-line water contamination monitoring system was installed at a pumping station at the border to monitor continuously the activity concentrations of three artificial gamma-emitting radionuclides (namely 103Ru, 1311 and 137Cs) present in untreated fresh water coming from the Guangdong Province . 2. System description The monitoring system was custom-built to meet Hong Kong's requirements [1]. It is made up of a field unit and two central processing stations . Fig. 1 shows the configuration of the system . The field unit at the pumping station comprises a 75 mm X 75 mm sodium iodide detector, a stainless steel water chamber of 22 litres in volume which encloses the detector, a 50 mm thick lead shield, and microcomputerbased spectrum analyser . During normal operation, untreated fresh water 1s allowed to flow through the water chamber at a rate of about 4 litres per minute while the 0168-9002/94/$07 .00 C 1994 Elsevier SSDI 0168-9002(94)00983-X
Science
B.V. All
radiation from gamma emitters in the water is sensed by the sodium iodide detector . The detector has a heater glued to its side and the two are enclosed by foam of 2.3 cm thick for thermal insulation . The heater-foam assembly provides thermal stability, maintaining the detector at a constant temperature of 43°C [2]. The output of the detector goes to a microcomputerbased spectrum analyser, which counts the number of gamma photons in different energy intervals. Four regions of interests are defined in the analyser for four radionuclides : 4°K, 103Ru, 1311 and 137Cs . Counting for 4° K, which is a natural radionuclide always present in the water, is used to check the system performance . The net count rate in each region of interest is converted to the activity concentration of the corresponding radionuclides based on the efficiency coefficient of the detector-analyser assembly and the volume of the water chamber. An alarm level can be set for one of the four radionuclides to alert the operator of abnormal activity concentrations . Under normal condition, the counting time is set to half an hour. Once an alarm is triggered, the counting time will be shortened to one minute for more frequent updating . The spectral and analysed data from the field unit, including the alarm status, are transmitted in real-time via leased telephone lines to two central stations, one at the water authority and another at an emergency monitoring and assessment centre, where the significance of any alarm is assessed and acted on .
rights reserved
551
KH . Yeang et al. /Nucl. Instr. and Meth. in Phys . Res. A 353 (1994) 550-552
UNTREATED SURFACE WATER IN
N,I DETECTOR
Ru respectively . Test results showed that the influ137 103 Cs and Ru on the other ROIs was negligible . The solution used in Test 4 consisted of equal amounts of 137Cs 103Ru . and Test results showed that the presence of 137Cs 103Ru and had negligible influence on the reported ence of
VENTING VALVE WATER OUT
DRAINAGE
RS 2320
SPECTRUM ANALYSER & PROCESSOR
RS 232C
MODEM
DISPLAY
activity concentration of 13 '1 .
CENTRAL PROCESSING SYSTEM
-
MODEM
TELEPHONE LINE
RS 232C DISPLAY
non-zero and practically the same . This showed that the 103 Ru increase in counts in the ROI was not correlated to 131 I. This was the increase in activity concentration of substantiated by Tests 7 to 11 . While the results of Tests 7,
CENTRAL PROCESSING SYSTEM
MODEM RS 232C
Microcomputer
Microcomputer
PRINTER
PRINTER
103Ru
Solutions used in Tests 5 and 6 did not contain 131 1. The report but significant different concentrations of 103 activity concentrations of Ru of the two solution were
R52 2C MODEM
TELEPHONE LINE
103
and
AMPLIFLIER
8 and 9 seemed to indicate a positive correlation, the
results of Tests 10 and 11 suggested a negative one. The lack of correlation is reasonable as none of the gamma 103 Ru . peaks of 131, falls within the ROI of The anomaly 103 Ru in the reported activity concentration of in Tests 5 to 103 11 was suspected to be due to plate-out residual Ru on
Fig. 1 .
3. Mixed radionuclide tests
the internal wall of the water chamber in previous mea-
The system is calibrated one radionuclide at a time for 103 Ru, 131, 137Cs and ; the most likely radionuclides to be found in fresh water contaminated by any off-site accidental release from the nuclear power station. In reality, more than one radionuclide may be present
in the contaminated fresh water, and interfere with one another in the measurement. The problem of interference has been evaluated using standard solutions of different 103 Ru, 1311 137Cs and . Table 1 activity concentrations of shows the activity concentrations of the three radionuclides of the standard solutions used for the test and the respective activity concentrations reported by the system. Solutions used in Test 2 and 3 consisted only of 137Cs
surements (i .e . in Tests 3 and 4) . This was ascertained by
repeating Tests 5 and 6 after decontamination of the water chamber, the results of which were presented as Tests 12 and 13 respectively in Table 1 .
Results of Tests 6 to 11 showed that the presence of
131,
gave rise to a positive bias in the reported activity
concentration of
137Cs,
of the order of 5% of the reported
activity concentration of
131, .
activity concentration of
This increase in the reported
137CS
could be attributed to the 637 keV gamma peak of 131, which fell almost completely within the ROI of 137Cs . Therefore, as a simple rule-ofthumb, whenever a large amount of
1311
is observed,
reported activity concentration of 137Cs should be reduced 131 by 5% of the reported activity of 1.
Table 1 137CS 1311 103Ru Reported activity concentrations of and in different solutions. (The most probable activity ratio of the three radionuclides in an accidental release has been assessed to be 1 : 15 : 1.) Test No .
1 2 3 4 5 6 7 8 9 10 11 12 13
Activity concentration (Bq/1) 137
Cs
131 1
103
Ru
reported
actual
reported
actual
reported
actual
0 530 0 484 173 27 14 82 182 564 553 180 26
0 544 0 544 152 0 0 0 109 544 544 152 0
0 0 0 0 1415 532 456 1506 1379 551 444 1472 511
0 0 0 0 1528 514 472 1516 1391 538 453 1528 514
0 0 483 445 31 33 531 194 186 67 551 0 0
0 0 502 436 0 0 498 102 100 0 470 0 0
Ratio of actual activity concentrations among 13 : 131 1 : 103 R 7CS u 0 : 0 :0 1 : 0 :0 0 : 0 :1 1 : 0 :1 1 :10 :0 0: 1 :0 0: 1 :1 1 :15 :1 1 :13 :1 1 : 1 :0 1 : 1 :1 1 :10 :0 0 : 1 :0
VIIIb. ENVIRONMENTAL APPLICATIONS
552
K.H. Yeung et al. /Nucl. Instr. and Meth. i n Phys. Res. A 353 (1994) 550-552
4. Conclusion The on-line water contamination monitoring system described in this paper uses a relatively small sodium
satisfactory output even when a mixture of the radionuclides of interest are present.
iodide detector and fairly modest processing equipment to
References
gamma emitting radionuclides present in water passing through a pumping stations . Tests with solutions of known
[1] Canberra Industries Inc., On-line Water Contamination Monitoring System, 1991 . [2] T.A . Perkins, Temperature Controller/Stabilizer/Water Vessel/Shield for Royal Observatory, 1990 .
monitor in real-time the activity concentration of a few
activity concentrations have shown that it can produce