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Gaseous standards preparation with the radionuclide Ar-41 for stack monitors calibration and verification in nuclear facilities
ARTICLE IN PRESS
Applied Radiation and Isotopes 66 (2008) 796–798 www.elsevier.com/locate/apradiso
Gaseous standards preparation with the radionuclide Ar-41 for stack monitors calibration and verification in nuclear facilities Petr Kovar, Pavel Dryak Czech Metrology Institute, Inspectorate for Ionizing Radiation (CMI), Radiova 1, CZ-102 00 Prague 10, Czech Republic
Abstract The Czech Metrology Institute performs calibration and verification of noble gases stack monitors in nuclear power plants and nuclear research facilities. Together with Kr-85 and Xe-133, the radionuclide Ar-41 is measured using HPGe detectors and its activity is determined using a gamma-ray peak at 1293 keV. The counting efficiency used in these measurements was calculated by the Monte Carlo method using the MCNP code. Radioactive gas standard is prepared by irradiation of argon in a high-pressure vessel by a Cf-252 neutron generator. The inner shape and thickness of the cylinder walls were determined by radiography. The argon volume under normal conditions is determined from the high-pressure vessel volume and by a precise gas pressure measurement. As a result, the activity concentration of Ar-41 at normal conditions is certified. r 2008 Elsevier Ltd. All rights reserved. Keywords: Ar-41; Gaseous standards; Nuclear facilities; Monte Carlo
1. Introduction Argon-41 is an important gamma-ray emitter, which has to be monitored in gaseous effluents from nuclear facilities. Regarding the relatively high energy of gamma-ray photons, 1293 keV, its presence influences also the measurement of the other radionuclides with lower energies. Because of its short half-life of 1.827 h the standards for calibration and verification of the noble gases monitors cannot be purchased from foreign suppliers. Therefore the Czech Metrology Institute started its own production of these standards. The following requirements have to be fulfilled during the standardization:
maximum relative combined standard uncertainty of Ar-41 volumic activity: 2%; minimum Ar-41 volumic activity for verification or calibration: 100 Bq L1; Corresponding author. Tel.: +420 266020407, +420 266020497;
fax: +420 266020466. E-mail addresses: [email protected] (P. Kovar), [email protected] (P. Dryak). 0969-8043/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2008.02.069
minimum volume of prepared gas: 150 L at normal conditions.
The high-pressure vessel with a nominal volume of 0.8 L is filled with non-active argon at the pressure of 20 MPa with special filling equipment. The gas amount in the vessel is determined by the overpressure, which is measured by a manometer. Then the high-pressure vessel is inserted into a container with water, serving as neutron moderator, located near the Cf-252 source and irradiated for 15 h. After the activation, the high-pressure vessel with the activated argon is measured on the germanium spectrometer, the total activity of Ar-41 is determined and the volumic activity of Ar-41 in the vessel is calculated. The high-pressure vessel with the known argon volumic activity can be directly used for calibration and verification of the noble gas monitors in the nuclear facilities. 2. Method 2.1. Set-up of the detector and measuring geometry model The gas is irradiated and measured in standard steel high-pressure vessel type CSN EN 10083 of volume
ARTICLE IN PRESS P. Kovar, P. Dryak / Applied Radiation and Isotopes 66 (2008) 796–798
Vs ¼ (0.859570.0050) L. The volume Vs was determined gravimetrically using water. The vessel is filled up to a pressure of 25 MPa with special filling device consisting of a storage vessel with non-active argon, precise manometer KELLER, MANO GAUGE IM/300bar/81060.C and filling pipes. The activity of Ar-41 is measured by a coaxial HPGe detector GC4018 with relative efficiency 40% and resolution 1.8 keV for Co-60 (1332 keV). During the measurement the steel vessel is located at the constant distance of about 20 cm from the detector. The vessel and the detector are located within a lead shield, the axis of the bottle coinciding with the detector axis. The GenieTM 2000 spectrometric software is used for spectra evaluation. The spectra are collected during the acquisition live time 1000 s, using 8 k channels for the 5–2000 keV energy range. The peak area is computed using the summation method and the continuum is subtracted using step function. The net area is corrected for the decay during the measurement. The software algorithms are described in Genie 2000 Customization Tools Manual (2002). For detection efficiency calculation by the MCNP-4A (1993) code, the model of the HPGe detector, previously described by Dryak and Kovar (2006), was used. The characterization of the measurement geometry was based on the description of the vessel shape, its wall thickness and composition. Besides the internal and external dimensions, which were obtained from the manufacturer’s data, the data of the bottom thickness, obtained by radiography (Fig. 1), were taken into account. The parameters of the vessel used for MCNP model are shown in Table 1. In spite of the high energy of gamma rays from Ar-41, their attenuation in the vessel wall is about 15–20%, which was estimated from the bottom thickness of the vessel. Since the vessel is filled to pressures up to 25 MPa, the correction for the attenuation of gamma rays in the argon gas must be taken into account. The detection efficiency was calculated for various gas pressures in the bottle and the relation of the detection efficiency versus the gas pressure in the bottle was determined (Fig. 2). The validation of the data set used for calculation of the counting efficiency was performed by calculating the efficiency at the energy of 514 keV. A vessel filled with certified activity of Kr-85 was measured and an agreement of 0.6% between the measured and certified activities was achieved. 2.2. Volumic activity of Ar-41 determination The activity of radionuclide Ar-41 in the pressure vessel is determined by high-resolution gamma-ray spectrometry. The activity A of the gas in the vessel is calculated from the net area S of the photon peak at the energy of 1293 keV. The activity of Ar-41 at the counting time is calculated using following formula: A ¼ S=ðEta Y tÞ,
(1)
where Eta denotes the full-energy peak efficiency of photons at 1293 keV, Y the emission probability of photons
797
Fig. 1. Radiography of high-pressure vessel bottom. Table 1 Parameters of the high-pressure vessel Chemical composition Fe C Si Mn Cr Mo P S
97.31% 0.33% 0.30% 0.75% 1.05% 0.22% 0.02% 0.02%
Dimensions Inner diameter Side walls thickness Bottom outer radius Bottom inner disc spheroid axes a, b Bottom inner disc spheroid axis c
83 mm 2.5 mm 41.5 mm 39.0 mm 36.2 mm
at 1293 keV (Y ¼ (0.991670.0010), Browne and Firestone, 1986) and t the live time of the measurement. The volumic activity of the argon gas is determined by a ¼ A=V ,
(2)
where V denotes the volume at normal conditions (T0 ¼ 0 1C and P0 ¼ 0.10132 MPa) of the argon gas in the vessel. The temperature is measured on the vessel surface using a precise thermometer with an accuracy 0.1 1C. The volume is calculated from the overpressure Pp (MPa) in the vessel by V ¼ ðV s ðPp þ 0:10132Þ=0:10132Þð273:15=ð273:15 þ TÞÞ. (3)
ARTICLE IN PRESS P. Kovar, P. Dryak / Applied Radiation and Isotopes 66 (2008) 796–798
798
2,700E-04 2,650E-04 2,600E-04 2,550E-04
E ta
2,500E-04 2,450E-04 2,400E-04 2,350E-04 2,300E-04 2,250E-04 2,200E-04 2,150E-04
0
5
10
15 P, MPa
20
25
30
Fig. 2. Detection efficiency as a function of gas pressure.
as a function of the pressure in the vessel as
Table 2 Relative standard uncertainties
Etað1293 keVÞ ¼ 0:0000017445ð50ÞP ðMPaÞ Net peak area (1293 keV), Us Efficiency uncertainty (1293 keV), UEta Emission probability (1293 keV), UY Activity of Ar-41, UA Pressure vessel volume, UVs Overpressure in the vessel, UPp Gas volume, UV Volumic activity of Ar-41, Ua
0.5% 0.8% 0.1% 1.1% 0.6% 0.5% 1.0% 1.5%
Here T (1C) denotes the gas temperature in the vessel and Vs the vessel volume. The combined relative standard uncertainty Ua of the volumic activity Ar-41 is calculated by U a ¼ ðU 2A þ U 2V Þ1=2 ,
(5)
where US denotes the relative uncertainty of the net peak area at 1293 keV, UEta the relative uncertainty of the efficiency at 1293 keV and UY the relative uncertainty of the 1293 keV photon emission probability. UV denotes the relative uncertainty of the gas volume at normal conditions. It can be calculated as U V ¼ ðU 2Vs þ U 2Pp Þ1=2 .
where P (MPa) denotes the absolute pressure in the vessel at the temperature of 22 1C. At the described conditions the combined relative standard uncertainty of Ar-41 activity in the pressure vessel is 1.1% and the combined relative standard uncertainty of radioactive gas volume at atmospheric pressure is 1.0%. It follows that the combined relative standard uncertainty of Ar-41 volumic activity in gas at atmospheric pressure is 1.5%.
(4)
where UA denotes the relative uncertainty of the Ar41activity. It is given by U A ¼ ðU 2S þ U 2Eta þ U 2Y Þ1=2 ,
þ 0:00026413ð180Þ,
4. Comment and conclusion The method for standardization of the Ar-41 activity in gas was developed. The gas is contained in a pressure vessel of the volume of 0.8 L at the pressure of about 25 MPa, which corresponds to the volume of about 200 L at atmospheric pressure. The standard uncertainty of its activity is 1.5%. The standard can be used immediately after preparation, which is important in order to minimize the losses due to radioactive decay.
(6)
Here UVs denotes the relative uncertainty of the pressure vessel volume and UPp the relative uncertainty of the overpressure in the vessel. The uncertainty budget is given in Table 2. 3. Results The full-energy peak efficiency at 1293 keV was determined using the Monte Carlo method and MCNP program
References Browne, E., Firestone, R.B., 1986. Table of Radioactive Isotopes. Wiley, New York. Canberra Industries Inc., 2002. Genie 2000 Customization Tools Manual. Dryak, P., Kovar, P., 2006. Experimental and MC determination of HPGe detector efficiency in the 40–2754 keV energy range for measuring point source geometry with the source-to-detector distance of 25 cm. Appl. Radiat. Isot. 64, 1346–1349. MCNP 4A, 1993. Monte Carlo N-Particle Transport Code System. Los Alamos National Laboratory, LA-12-625-M.
Applied Radiation and Isotopes 66 (2008) 796–798 www.elsevier.com/locate/apradiso
Gaseous standards preparation with the radionuclide Ar-41 for stack monitors calibration and verification in nuclear facilities Petr Kovar, Pavel Dryak Czech Metrology Institute, Inspectorate for Ionizing Radiation (CMI), Radiova 1, CZ-102 00 Prague 10, Czech Republic
Abstract The Czech Metrology Institute performs calibration and verification of noble gases stack monitors in nuclear power plants and nuclear research facilities. Together with Kr-85 and Xe-133, the radionuclide Ar-41 is measured using HPGe detectors and its activity is determined using a gamma-ray peak at 1293 keV. The counting efficiency used in these measurements was calculated by the Monte Carlo method using the MCNP code. Radioactive gas standard is prepared by irradiation of argon in a high-pressure vessel by a Cf-252 neutron generator. The inner shape and thickness of the cylinder walls were determined by radiography. The argon volume under normal conditions is determined from the high-pressure vessel volume and by a precise gas pressure measurement. As a result, the activity concentration of Ar-41 at normal conditions is certified. r 2008 Elsevier Ltd. All rights reserved. Keywords: Ar-41; Gaseous standards; Nuclear facilities; Monte Carlo
1. Introduction Argon-41 is an important gamma-ray emitter, which has to be monitored in gaseous effluents from nuclear facilities. Regarding the relatively high energy of gamma-ray photons, 1293 keV, its presence influences also the measurement of the other radionuclides with lower energies. Because of its short half-life of 1.827 h the standards for calibration and verification of the noble gases monitors cannot be purchased from foreign suppliers. Therefore the Czech Metrology Institute started its own production of these standards. The following requirements have to be fulfilled during the standardization:
maximum relative combined standard uncertainty of Ar-41 volumic activity: 2%; minimum Ar-41 volumic activity for verification or calibration: 100 Bq L1; Corresponding author. Tel.: +420 266020407, +420 266020497;
fax: +420 266020466. E-mail addresses: [email protected] (P. Kovar), [email protected] (P. Dryak). 0969-8043/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2008.02.069
minimum volume of prepared gas: 150 L at normal conditions.
The high-pressure vessel with a nominal volume of 0.8 L is filled with non-active argon at the pressure of 20 MPa with special filling equipment. The gas amount in the vessel is determined by the overpressure, which is measured by a manometer. Then the high-pressure vessel is inserted into a container with water, serving as neutron moderator, located near the Cf-252 source and irradiated for 15 h. After the activation, the high-pressure vessel with the activated argon is measured on the germanium spectrometer, the total activity of Ar-41 is determined and the volumic activity of Ar-41 in the vessel is calculated. The high-pressure vessel with the known argon volumic activity can be directly used for calibration and verification of the noble gas monitors in the nuclear facilities. 2. Method 2.1. Set-up of the detector and measuring geometry model The gas is irradiated and measured in standard steel high-pressure vessel type CSN EN 10083 of volume
ARTICLE IN PRESS P. Kovar, P. Dryak / Applied Radiation and Isotopes 66 (2008) 796–798
Vs ¼ (0.859570.0050) L. The volume Vs was determined gravimetrically using water. The vessel is filled up to a pressure of 25 MPa with special filling device consisting of a storage vessel with non-active argon, precise manometer KELLER, MANO GAUGE IM/300bar/81060.C and filling pipes. The activity of Ar-41 is measured by a coaxial HPGe detector GC4018 with relative efficiency 40% and resolution 1.8 keV for Co-60 (1332 keV). During the measurement the steel vessel is located at the constant distance of about 20 cm from the detector. The vessel and the detector are located within a lead shield, the axis of the bottle coinciding with the detector axis. The GenieTM 2000 spectrometric software is used for spectra evaluation. The spectra are collected during the acquisition live time 1000 s, using 8 k channels for the 5–2000 keV energy range. The peak area is computed using the summation method and the continuum is subtracted using step function. The net area is corrected for the decay during the measurement. The software algorithms are described in Genie 2000 Customization Tools Manual (2002). For detection efficiency calculation by the MCNP-4A (1993) code, the model of the HPGe detector, previously described by Dryak and Kovar (2006), was used. The characterization of the measurement geometry was based on the description of the vessel shape, its wall thickness and composition. Besides the internal and external dimensions, which were obtained from the manufacturer’s data, the data of the bottom thickness, obtained by radiography (Fig. 1), were taken into account. The parameters of the vessel used for MCNP model are shown in Table 1. In spite of the high energy of gamma rays from Ar-41, their attenuation in the vessel wall is about 15–20%, which was estimated from the bottom thickness of the vessel. Since the vessel is filled to pressures up to 25 MPa, the correction for the attenuation of gamma rays in the argon gas must be taken into account. The detection efficiency was calculated for various gas pressures in the bottle and the relation of the detection efficiency versus the gas pressure in the bottle was determined (Fig. 2). The validation of the data set used for calculation of the counting efficiency was performed by calculating the efficiency at the energy of 514 keV. A vessel filled with certified activity of Kr-85 was measured and an agreement of 0.6% between the measured and certified activities was achieved. 2.2. Volumic activity of Ar-41 determination The activity of radionuclide Ar-41 in the pressure vessel is determined by high-resolution gamma-ray spectrometry. The activity A of the gas in the vessel is calculated from the net area S of the photon peak at the energy of 1293 keV. The activity of Ar-41 at the counting time is calculated using following formula: A ¼ S=ðEta Y tÞ,
(1)
where Eta denotes the full-energy peak efficiency of photons at 1293 keV, Y the emission probability of photons
797
Fig. 1. Radiography of high-pressure vessel bottom. Table 1 Parameters of the high-pressure vessel Chemical composition Fe C Si Mn Cr Mo P S
97.31% 0.33% 0.30% 0.75% 1.05% 0.22% 0.02% 0.02%
Dimensions Inner diameter Side walls thickness Bottom outer radius Bottom inner disc spheroid axes a, b Bottom inner disc spheroid axis c
83 mm 2.5 mm 41.5 mm 39.0 mm 36.2 mm
at 1293 keV (Y ¼ (0.991670.0010), Browne and Firestone, 1986) and t the live time of the measurement. The volumic activity of the argon gas is determined by a ¼ A=V ,
(2)
where V denotes the volume at normal conditions (T0 ¼ 0 1C and P0 ¼ 0.10132 MPa) of the argon gas in the vessel. The temperature is measured on the vessel surface using a precise thermometer with an accuracy 0.1 1C. The volume is calculated from the overpressure Pp (MPa) in the vessel by V ¼ ðV s ðPp þ 0:10132Þ=0:10132Þð273:15=ð273:15 þ TÞÞ. (3)
ARTICLE IN PRESS P. Kovar, P. Dryak / Applied Radiation and Isotopes 66 (2008) 796–798
798
2,700E-04 2,650E-04 2,600E-04 2,550E-04
E ta
2,500E-04 2,450E-04 2,400E-04 2,350E-04 2,300E-04 2,250E-04 2,200E-04 2,150E-04
0
5
10
15 P, MPa
20
25
30
Fig. 2. Detection efficiency as a function of gas pressure.
as a function of the pressure in the vessel as
Table 2 Relative standard uncertainties
Etað1293 keVÞ ¼ 0:0000017445ð50ÞP ðMPaÞ Net peak area (1293 keV), Us Efficiency uncertainty (1293 keV), UEta Emission probability (1293 keV), UY Activity of Ar-41, UA Pressure vessel volume, UVs Overpressure in the vessel, UPp Gas volume, UV Volumic activity of Ar-41, Ua
0.5% 0.8% 0.1% 1.1% 0.6% 0.5% 1.0% 1.5%
Here T (1C) denotes the gas temperature in the vessel and Vs the vessel volume. The combined relative standard uncertainty Ua of the volumic activity Ar-41 is calculated by U a ¼ ðU 2A þ U 2V Þ1=2 ,
(5)
where US denotes the relative uncertainty of the net peak area at 1293 keV, UEta the relative uncertainty of the efficiency at 1293 keV and UY the relative uncertainty of the 1293 keV photon emission probability. UV denotes the relative uncertainty of the gas volume at normal conditions. It can be calculated as U V ¼ ðU 2Vs þ U 2Pp Þ1=2 .
where P (MPa) denotes the absolute pressure in the vessel at the temperature of 22 1C. At the described conditions the combined relative standard uncertainty of Ar-41 activity in the pressure vessel is 1.1% and the combined relative standard uncertainty of radioactive gas volume at atmospheric pressure is 1.0%. It follows that the combined relative standard uncertainty of Ar-41 volumic activity in gas at atmospheric pressure is 1.5%.
(4)
where UA denotes the relative uncertainty of the Ar41activity. It is given by U A ¼ ðU 2S þ U 2Eta þ U 2Y Þ1=2 ,
þ 0:00026413ð180Þ,
4. Comment and conclusion The method for standardization of the Ar-41 activity in gas was developed. The gas is contained in a pressure vessel of the volume of 0.8 L at the pressure of about 25 MPa, which corresponds to the volume of about 200 L at atmospheric pressure. The standard uncertainty of its activity is 1.5%. The standard can be used immediately after preparation, which is important in order to minimize the losses due to radioactive decay.
(6)
Here UVs denotes the relative uncertainty of the pressure vessel volume and UPp the relative uncertainty of the overpressure in the vessel. The uncertainty budget is given in Table 2. 3. Results The full-energy peak efficiency at 1293 keV was determined using the Monte Carlo method and MCNP program
References Browne, E., Firestone, R.B., 1986. Table of Radioactive Isotopes. Wiley, New York. Canberra Industries Inc., 2002. Genie 2000 Customization Tools Manual. Dryak, P., Kovar, P., 2006. Experimental and MC determination of HPGe detector efficiency in the 40–2754 keV energy range for measuring point source geometry with the source-to-detector distance of 25 cm. Appl. Radiat. Isot. 64, 1346–1349. MCNP 4A, 1993. Monte Carlo N-Particle Transport Code System. Los Alamos National Laboratory, LA-12-625-M.