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08.O.05 Determination of the activity-size distribution of radioactive aerosols released from the planned waste repository konrad
J. Aerosol Sci., Vol. 25, Suppl. I, pp. $75-$76. 1994
Pergamon
08
O
05
Copyright(~)1994 Elsevier Science Lid Printed in Great Britain. All right. . . . . ed 0021-8502/94 $7.00 + 0.00
DETERMINATION OF THE ACTIVITY-SIZE DISTRIBUTION OF RADIOACTIVE AEROSOLS RELEASED FROM THE PLANNED WASTE REPOSITORY KONRAD
H.P. B e r g 3), H. J. Fett ~), F. L a n g e ~), R. M a r t e n s ~), J. PorstendOffer 2), A. Reineking 2) 1)Gesellschaft fiir Anlagen- und Reaktorsicherheit (GRS)mbH, Cologne, Germany, 2) Isotopenlabor fiir biol. u. med. Forschung der Universit/lt G6ttingen, Germany, 3)Bundesamt fiir Strahlenschutz, Salzgitter, Germany
KEYWORDS Acitivity-size distribution; radon and thoron daughter products; background aerosol; iron ore mine
The former iron ore mine Konrad situated within an industrial area of the city of Salzgitter in Germany is intended to be used as a final repository for low- to medium-level radioactive wastes. In the frame of the licensing procedure detailed safety analyses have been made which concem the operating and postoperating phase of the repository. Airborne radionuclide releases from the repository to the environment can result from several sources: •
Leakage of radionuclides from waste packages (diffusion type processes) stored in deepunderground emplacement rooms (operational releases).
•
Radionuclides released by incidents such as drop of a package during handling operations or tire of a diesel-driven transport vehicle in underground galleries affecting transported waste packages.
•
Emanation of radon (Rn 222) and thoron (Rn 220) from natural radioactivity in the host rock of the repository and associated build-up of radioactive daughter products.
In most cases the radioactivity will be attached to aerosol particles and will be released from the repository to the environment after having been transported with the ventilation air for rather long distances (order of kin) through underground galleries and upwards through the exhaust air shaft (ca. 80Ore). In this context the question was raised whether the rather dusty atmosphere of the former iron ore mine (1-2 mg/m 3) could have a noticeable influence on the activity-size distribution of particulate matter released with the exhaust air to the environment. Could agglomeration processes result in a shift of activity-size distributions to larger aerodynamic diameters and consequently to increased deposition velocities after release to the environment via the 45 m high diffusor? An experimental program has been conducted to investigate this question. Mass-size distributions of the background aerosol and activity-size distributions of radon daughter products have been measured at four different locations of the mine: in an underground gallery far from the exhaust air shaft, about 3.2 km further downstream closer to the exhaust air shaft, in a lower section of the exhaust air shaft (-660m) and close to the upper exit. $75
$76
H . P . BERG et al.
Mass-size distributions and activity-size distributions of aesosol-attached short-lived radon and thoron decay products were measured by a low-pressure cascade impactor (type BERNER, 8 stages, 1.8 m3/h) with experimentally determined/REI 84/50% cutoffs between 80 and 8000 nm. Measurements at the four locations in the mine were always during typical working conditions with collection times of 3 to 7 hours. During all impactor measurements the particle-number concentrations in the size range N 5 nm 1000 nm were measured with a calibrated condensation particle counter/SCH 86/. Aerosol particles were deposited on the different stages of the impactor on aluminum or plastic foils which were later weighed to determine mass-size distributions and inserted into a NaJ/TI or into a Germanium gamma spectrometer to determine the size-fractionated activities. The experimental data were fitted by taking account of the known deposition characteristics of the impactor with a nonlinear optimization procedure (Simplex method) to produce activity size distributions which are represented by the sum of logarithmic normal distributions/REI 86/. Free parameters in this numerical minimization algorithm are AMAD (Activity Median Aerodynamic Diameter) or MMAD (Mass Median Aerodynamic Diameter), respectively, og (geometric standard deviation) and f (relative weight of the mode). The mass-size distributions of the background aerosol at all four measuring locations are very well represented by the sum of two logarithmic normal distributions with MMAD in the range between 150300 nm (accumulation-mode, og3 = 2.5 - 3.5) and 3600 - 7000 nm (coarse-mode, og4= 1.4 - 2.0). The activity-size distributions of radon and thoron daughter products attached to aerosol particles are on the other hand all unimodal with an AMAD 3 between 150 - 200 nm (og3= 2.0 - 2.5). This was to be expected because the attachment of initially unattached daughter products is dominated by the very large number of background aerosol particles in the accumulation mode compared to the relatively small number of particles in the course mode which dominates the mass-size distribution. The measured particle number densities at the four locations were in the range 3 • 105 to 8 ° 105 per cm 3, typically an order of magnitude above outdoor conditions. Because of the d to d 2 dependence (d = aerodynamic diameter) of the attachment processes the activitysize distributions will be shifted to larger aerodynamic diameters compared to the number-size distributions. This is supported by comparing particle number distributions derived analytically from the measured mass-size distributions and particle number distributions weighed with d and d 2 with the measured activity-size distributions. From the close similarity of the measured activity-size distributions at the different locations in the mine it can be concluded that during typical traveling times of around 10 min. agglomeration processes do not lead to a shift to larger particle diameters. The deposition velocity of 1.5 • 10.3 m/s applied for the assessment of potential radiation exposures from routine releases and of radon and thoron daughter products is therefore justified.
REFERENCES /REI 84/
Reineking, A., Scheibel, H.G., Hussin, A., Becker, K.H., and Porstend0rfer, J.: Measurement of Stage Efficiency Functions including Interstage Losses for a Sierra and a Bemer lmpactor and Evaluation of Data by a Modified Simplex Method, J. Aerosol Sci., 15 (1984) 376.
/SCH 86/
Scheibel, H.G. and PorstendOrfer, J.: Counting Efficiency and Detection Limit of Condensation Nuclei Counters for Submicrometer Aerosols. II. Measurements with Monodisperse Hydrophobic Ag and Hygroscopic NaC1 Aerosols with Particle Diameters beween 2 and 100 rim, J. of Colloid and Interface Sci., 109 (1986) 275.
Pergamon
08
O
05
Copyright(~)1994 Elsevier Science Lid Printed in Great Britain. All right. . . . . ed 0021-8502/94 $7.00 + 0.00
DETERMINATION OF THE ACTIVITY-SIZE DISTRIBUTION OF RADIOACTIVE AEROSOLS RELEASED FROM THE PLANNED WASTE REPOSITORY KONRAD
H.P. B e r g 3), H. J. Fett ~), F. L a n g e ~), R. M a r t e n s ~), J. PorstendOffer 2), A. Reineking 2) 1)Gesellschaft fiir Anlagen- und Reaktorsicherheit (GRS)mbH, Cologne, Germany, 2) Isotopenlabor fiir biol. u. med. Forschung der Universit/lt G6ttingen, Germany, 3)Bundesamt fiir Strahlenschutz, Salzgitter, Germany
KEYWORDS Acitivity-size distribution; radon and thoron daughter products; background aerosol; iron ore mine
The former iron ore mine Konrad situated within an industrial area of the city of Salzgitter in Germany is intended to be used as a final repository for low- to medium-level radioactive wastes. In the frame of the licensing procedure detailed safety analyses have been made which concem the operating and postoperating phase of the repository. Airborne radionuclide releases from the repository to the environment can result from several sources: •
Leakage of radionuclides from waste packages (diffusion type processes) stored in deepunderground emplacement rooms (operational releases).
•
Radionuclides released by incidents such as drop of a package during handling operations or tire of a diesel-driven transport vehicle in underground galleries affecting transported waste packages.
•
Emanation of radon (Rn 222) and thoron (Rn 220) from natural radioactivity in the host rock of the repository and associated build-up of radioactive daughter products.
In most cases the radioactivity will be attached to aerosol particles and will be released from the repository to the environment after having been transported with the ventilation air for rather long distances (order of kin) through underground galleries and upwards through the exhaust air shaft (ca. 80Ore). In this context the question was raised whether the rather dusty atmosphere of the former iron ore mine (1-2 mg/m 3) could have a noticeable influence on the activity-size distribution of particulate matter released with the exhaust air to the environment. Could agglomeration processes result in a shift of activity-size distributions to larger aerodynamic diameters and consequently to increased deposition velocities after release to the environment via the 45 m high diffusor? An experimental program has been conducted to investigate this question. Mass-size distributions of the background aerosol and activity-size distributions of radon daughter products have been measured at four different locations of the mine: in an underground gallery far from the exhaust air shaft, about 3.2 km further downstream closer to the exhaust air shaft, in a lower section of the exhaust air shaft (-660m) and close to the upper exit. $75
$76
H . P . BERG et al.
Mass-size distributions and activity-size distributions of aesosol-attached short-lived radon and thoron decay products were measured by a low-pressure cascade impactor (type BERNER, 8 stages, 1.8 m3/h) with experimentally determined/REI 84/50% cutoffs between 80 and 8000 nm. Measurements at the four locations in the mine were always during typical working conditions with collection times of 3 to 7 hours. During all impactor measurements the particle-number concentrations in the size range N 5 nm 1000 nm were measured with a calibrated condensation particle counter/SCH 86/. Aerosol particles were deposited on the different stages of the impactor on aluminum or plastic foils which were later weighed to determine mass-size distributions and inserted into a NaJ/TI or into a Germanium gamma spectrometer to determine the size-fractionated activities. The experimental data were fitted by taking account of the known deposition characteristics of the impactor with a nonlinear optimization procedure (Simplex method) to produce activity size distributions which are represented by the sum of logarithmic normal distributions/REI 86/. Free parameters in this numerical minimization algorithm are AMAD (Activity Median Aerodynamic Diameter) or MMAD (Mass Median Aerodynamic Diameter), respectively, og (geometric standard deviation) and f (relative weight of the mode). The mass-size distributions of the background aerosol at all four measuring locations are very well represented by the sum of two logarithmic normal distributions with MMAD in the range between 150300 nm (accumulation-mode, og3 = 2.5 - 3.5) and 3600 - 7000 nm (coarse-mode, og4= 1.4 - 2.0). The activity-size distributions of radon and thoron daughter products attached to aerosol particles are on the other hand all unimodal with an AMAD 3 between 150 - 200 nm (og3= 2.0 - 2.5). This was to be expected because the attachment of initially unattached daughter products is dominated by the very large number of background aerosol particles in the accumulation mode compared to the relatively small number of particles in the course mode which dominates the mass-size distribution. The measured particle number densities at the four locations were in the range 3 • 105 to 8 ° 105 per cm 3, typically an order of magnitude above outdoor conditions. Because of the d to d 2 dependence (d = aerodynamic diameter) of the attachment processes the activitysize distributions will be shifted to larger aerodynamic diameters compared to the number-size distributions. This is supported by comparing particle number distributions derived analytically from the measured mass-size distributions and particle number distributions weighed with d and d 2 with the measured activity-size distributions. From the close similarity of the measured activity-size distributions at the different locations in the mine it can be concluded that during typical traveling times of around 10 min. agglomeration processes do not lead to a shift to larger particle diameters. The deposition velocity of 1.5 • 10.3 m/s applied for the assessment of potential radiation exposures from routine releases and of radon and thoron daughter products is therefore justified.
REFERENCES /REI 84/
Reineking, A., Scheibel, H.G., Hussin, A., Becker, K.H., and Porstend0rfer, J.: Measurement of Stage Efficiency Functions including Interstage Losses for a Sierra and a Bemer lmpactor and Evaluation of Data by a Modified Simplex Method, J. Aerosol Sci., 15 (1984) 376.
/SCH 86/
Scheibel, H.G. and PorstendOrfer, J.: Counting Efficiency and Detection Limit of Condensation Nuclei Counters for Submicrometer Aerosols. II. Measurements with Monodisperse Hydrophobic Ag and Hygroscopic NaC1 Aerosols with Particle Diameters beween 2 and 100 rim, J. of Colloid and Interface Sci., 109 (1986) 275.