The Radioactive Aerosol Particle Size Distribution in the Air Effluents from Nuclear Power Plants obtained by the Use of Cascade Impactor

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1 The Radioactive Aerosol Particle Size Distribution in the Air Effluents from Nuclear Power Plants obtained by the Use of Cascade Impactor P. Rulík National Radiation Protection Institute, Srobarova 48, Prague, , Czech Republic Abstract. The aerosol particle size distribution in the effluents from the new nuclear power plant (NPP) Temelín, the Czech Republic was determined. The 6-stages cascade impactor was placed in the ventilation stack of NPP. The results of gamma spectrometry analysis of the individual collection substrates from the stages were used to evaluate the particle size distribution for individual radionuclides in terms of the activity median aerodynamic diameter (AMAD) and the geometric standard deviation (GSD). The values determined in the samples from NPP Temelin effluents were compared to those determined in the samples from NPP Dukovany (Czech Republic), NPP Jaslovske Bohunice (Slovak Republic) and from the environment in 1986 after the Chernobyl accident. The values were compared to the values of the natural radionuclides, too. Over 50% of the activity of the radionuclides originated in NPP Temelin is attached to the particles larger than 2 µm with the mean AMAD of all the evaluated radionuclides about 2.2 µm and GSD about 2.3 and size distribution does not depend on the kind of a radionuclide; such magnitude corresponds to common industrial aerosol. It originates from mechanical disintegration of coarser material. All the size distributions are slightly bimodal with the boundary under 0.4 µm. The mean values of AMADs for both NPP Dukovany and NPP Jaslovske Bohunice were about 3.8 µm and 3.3 µm respectively. Most of 7 Be and radon daughter products activities are connected with particles of aerodynamic diameter smaller than 0.4 µm. This magnitude of the particles corresponds to aerosol originated in the environment by coagulation and condensation. In the case of a nuclear accident as the Chernobyl accident the aerodynamic diameter of the aerosols depends on the conditions of the aerosol origin - combustion of the fuel, explosion of the fuel, etc. - thus AMADs are rather higher for refractory radionuclides and lower for the volatile ones. 1. Introduction The radiological effect of radioactive aerosol particles depends not only on the kind of radionuclide and its chemical form, but also on the aerodynamic properties of the particles described usually by their aerodynamic diameter or, more exactly, by aerosol particle size distribution. An aerodynamic diameter is a key parameter to understanding of filtration and deposition of particles. It is an important parameter in the models of aerosol transport in the atmosphere; it is also essential for calculation of the dose from an inhalation intake. In many cases the particle size distribution of radioactive aerosols can be described by log-normal distribution and characterized in terms of the activity median aerodynamic diameter (AMAD) and the geometric standard deviation (GSD). Many measurements of the AMADs of the radioactive aerosols in different working environments (nuclear power industry, uranium mills, fuel handling facilities) have been performed [1]. The AMAD varied in the large range from about 0.1 µm to over 25 µm with the median value 4.4 µm. Both the nuclear power and nuclear fuel handling industries gave the median value of about 4 µm. Although these results come from working places and the aerosol distributions in the effluents can be shifted or distorted in comparison with them due to filtration of the main part of the exhausted air, this distortion needn t be large. Our aim was to measure the intrinsic aerosol size distribution from the new nuclear power plant (NPP) Temelin, the Czech Republic in dependence on the kind of a radionuclide and compare it to the size distribution of other nuclear power plants, namely NPP Dukovany (the Czech Republic) and NPP Jaslovske Bohunice (the Slovak Republic), and to the size distribution of the aerosol containing artificial radionuclides collected after the Chernobyl accident in the environment on the Czech Republic territory, and to the aerosol size distribution of natural radionuclides in the environment. 1

2 2. Materials and Methods In all the samplings the 5-stages (in NPP Jaslovske Bohunice, after the Chernobyl accident in the environment) or 6-stages (in NPP Jaslovske Bohunice, NPP Dukovany, NPP Temelin) cascade impactors were used The basic concepts Cascade impactor is an equipment, which subsequently sorts aerosol particles according to their aerodynamic diameter by means of increasing linear velocity in the slots of individual impactor stages leading to the separation of larger particles by inertia and to their settling by impaction and to the deflection of smaller particles sidelong to the next stage. Each stage is characterised by a so called cutoff parameter. The cut-off parameter of a stage is the aerodynamic diameter of the particle, which has 50% probability that it will be caught on the given stage. If a particle has an aerodynamic diameter only a bit larger the probability increases quickly to 100%, if a particle has an aerodynamic diameter only a bit smaller the probability decreases quickly to zero. It can be said that the given stage will hold particles with aerodynamic diameter approximately in the range from the cut-off of the given stage to the cut-off of the previous stage. The back-up filter, which collects the rest of the particles, is placed behind the last stage. Aerodynamic diameter is the diameter of sphere with the density of 1 g/cm 3 that has the same settling velocity as the particle in question. It means that the aerodynamic diameter standardises particles with respect to their shape and mass density. A particle of a specific aerodynamic diameter is aerodynamically indistinguishable from other particles of different sizes, shapes, and mass densities having the same aerodynamic diameter [2]. AMAD is defined so that 50% of an aerosol activity is associated with particles of aerodynamic diameter larger than AMAD and 50% with particles of aerodynamic diameter lesser than AMAD Sampling in the ventilation stack of NPP Temelin The pressurised water reactor (PWR Novovoronezh type VVER-1000/320, 1000 MW E ) with the capacity of 1000 MW near the village of Temelin was put into operation in the year This NNP is unique, because the main part of the production block comes from the former Soviet Union and next engineering parts were produced in the Czech Republic; the fuel (UO 2 with the average enrichment 3.5% of the 235 U), the control and direction system are from Westinghouse, USA. The active zone contents 163 fuel sets, each one with 312 rods combined - 92 tons of the uranium fuel. Under normal operation of the reactor the air flows through the ventilation filters of NPP to the internal ventilation stack with the average flow rate of m 3 /h; during outage periods (in the process of depressurization and shut-down for refuelling and/or maintenance purposes) the air flows also to the external ventilation stacks (surrounds the internal ventilation stack) with the average flow rate of m 3 /h. A system of the pipelines by which a sample of the exhausted air with aerosols is led to different analyses begins at the height of 50 m above the bottom of a ventilation stack. The system of pipelines ensures the isokinetic sampling. The six stages cascade impactor, (Model Sierra Andersen SA-236 see Fig.1) attached to the air sampler with the flow control system was connected to one of the sampling lines of the pipelines system. The collection substrate (slotted glass fibre filter) was placed on each stage. The substrates were soaked with 1% solution silicone oil in hexane in order to reduce rebounds of the incident particles and to reduce removal of the already caught particles by the air stream. Under the last stage the back-up filter was placed. After each sampling the alcoholic wipe-test samples from the stages, from the area around them and from the area of back-up filter were taken for determination of interstage losses. 2

3 All the sampling intervals were 14 days with the flow rate of m 3 /min. An aerodynamic diameter cut-off values of individual cascade impactor stages for the used flow rate given by the manufacturer are presented in the Tab.1. Total volume of the air throughput for one sample was about m 3. FIG. 1. Six stages cascade impactor Sierra Andersen, Model SA-236 Table 1. Aerodynamic diameter cut-off for the cascade impactor SA 236 for the flow rate m 3 /min. Impactor stage Cut-off [ µm ] Evaluation procedure Gamma-spectrometric analysis with HPGe detectors to determine the activity of radionuclides on each collection substrate from the individual stages and on the back-up filter was performed after the sampling. If the activity of an evaluated radionuclide on any stage lay under the minimum significant activity (MSA), half-value of the MSA was used as the best estimate of the activity. The MSA for individual radionuclides was in the range of Bq/m 3 depending on the gamma ray intensity and energy, when approx. counting time of 80 hours and detectors having a % relative efficiency including well-type detector were used. The results of the gamma-spectrometry analysis of the collection substrates from the individual stages and the back-up filter (under the assumption of logarithmic-normal distribution of the aerodynamic diameters) were used to define the particle size distribution for individual radionuclides in exhausted air in terms of the activity median aerodynamic diameter (AMAD) and geometric standard deviation (GSD). The AMAD and GSD for individual radionuclides were evaluated only for those sample sets, where the activities lay above the MSA on 4 stages at least (including the back-up filter). 3

4 3. Results 3.1. Results from NPP Temelin From the October 2001 to the December 2003 fourteen sample sets have been collected during normal operation and during outage periods. AMAD and GSD were determined for 11 different artificial radionuclides. The samplings were performed not continuously because of very low activities in the air effluents. By the April 2002 (6 sample sets) no artificial radionuclides (radionuclides for which AMAD and GSD were possible to calculate) were detected. Only in 6 sample sets some artificial radionuclides were detected. The basic characterization of the samplings is presented in the table 2. Table 2. The time schedule of the samplings in which some artificial radionuclides were detected Sample No. Date of sampling Mode of operation of the reactor Ventilation stack Analysed radionuclides Outage Internal Outage Internal Outage External Normal operation Internal Outage Internal Outage External 60 Co, 54 Mn, 124 Sb, 110m Ag, 58 Co, 60 Co, 51 Cr, 137 Cs, 59 Fe, 54 Mn, 124 Sb, 125 Sb, 113 Sn, 95 Zr 110m Ag, 60 Co, 137 Cs, 124 Sb 110m Ag 110m Ag, 54 Mn 110m Ag, 54 Mn Notices: Normal operation - normal power operation of the reactor Outage - the reactor in an outage for refuelling and/or maintenance Analysed radionuclides - radionuclides for which AMAD and GSD were possible to calculate The AMAD and GSD of the natural radionuclide 7 Be, originated in higher layers of the atmosphere, were determined in 5 sets, too. The total activity concentrations (sum of the activity concentrations on the 6 stages and on the back-up filter) in individual sampling sets for radionuclides for which the AMADs were calculated are presented in Fig.2. The normalized fraction of the activity concentrations related to the logarithmic width of the size interval for individual radionuclides for the sample set No.2 is shown in the Fig.3. Radionuclides in the other sample sets have similar distributions. A significant maximum is usually in the third stage. The analysis of the data shows that results acquired without taking into account the back-up filter can be best fit to log-normal distribution (the highest correlation coefficient) for all radionuclides originated in NPP. It corresponds to the findings of other authors [1] that the distribution is bimodal with the boundary around 0.5 µm. As an example the cumulative distribution (in percentages) of the activity connected to the particles with aerodynamic diameter lesser than given cut-off for the individual radionuclides and corresponding values of AMAD and GSD for the sampling No.2 are shown in the table 3. 4

5 FIG. 2. The total activity concentrations in the individual sampling sets 1-6 (sum of the activity concentrations on the 6 stages and back-up filter) FIG. 3. The normalized fraction of the activity concentrations related to the logarithmic width of the size interval for individual radionuclides for the sample set No.2 Notices: Normalized da/d(ln x) is the ratio of the activity A i on the given stage and logarithmic distance between the maximum and the minimum size of the particles on the given stage related to the sum of the da/d(ln x) from all stages. The smallest size interval is taken from 0.01 µm to 0.39 µm and the largest interval from 10.2 µm to 100 µm because particles smaller than 0.01 µm and larger than 100 µm are usually in ventilation stacks only with very low probability. 5

6 Table 3. The cumulative distribution (in percentages) of the activity connected to the particles with aerodynamic diameter lesser than given cut-off for the individual radionuclides and corresponding values of AMAD and GSD for the sampling No.2 Particle aerodynamic diameter [µm] < 0.39 < 0.69 < 1.3 < 2.1 < 4.2 < 10.2 Cumulative distribution of the activity [ % ] AMAD [µm ] 110m Ag Co Co Cs Mn Sb Zr Fe Sb Sn Cr Arithmetic mean Be GSD Notice: AMADs and GSDs were determined without taking into account the back-up filter with exception of 7 Be; it was calculated with back-up filter but without the 1 st stage. The distribution of all the values of AMAD in probability graph is given in Fig. 4. The data very well correspond to normal distribution (they are aligned). The arithmetic mean of the AMADs is 2.22 µm and of the GSDs 2.32 and does not depend on the kind of the radionuclide; such magnitude corresponds to common industrial aerosol called coarse mode and originates from mechanical disintegration of coarser material. The uncertainty of the parameters of the distribution The combined uncertainty consists of the following errors and uncertainties: an uncertainty of the flow rate and its stability in the ventilation stack and in the sampler, an uncertainty given by interstages losses on the walls of the stages, an uncertainty given by rebounds of the dropped particles and by removal to the air stream of the already caught particles from the higher stages to the lower ones and an uncertainty of the gamma-spectrometric analysis. The losses given by settling of the aerosol on the impactor walls were estimated by means of the wipetest samples taken from walls and the surface around the back-up filter after each sampling and by their measurements. Mean losses were estimated to be 15%; the same value is brought out by [3]. The total uncertainty of the results was estimated to about 30%. 6

7 FIG. 4. The distribution of all values of the AMAD of all radionuclides in a probability graph 3 Quantil of normal distribution AMAD [ µm ] Notice: For orientation the relation between quantil of normal distribution and cumulative fraction is in the Tab.4. Table 4. The relation between quantil of normal distribution and cumulative fraction Quantil of normal distribution Cumulative fraction [ % ] The comparison of the aerosol particle size distribution in the ventilation stacks of the NPPs Temelin, Dukovany and Jaslovske Bohunice In the ventilation stack V-1 of the NPP Jaslovské Bohunice which is common for 2 pressurised water reactors (PWR Novovoronezh type 230, 440 MW E ) first five-, and then six-stages cascade impactor were placed. The impactors were placed directly in the ventilation stack, 2 meters above the bottom of the stack. About 80 samplings during the years from 1985 to 1993 [4] were performed. A ventilation stack of the NPP Dukovany is common for 2 pressurised water reactors (PWR Novovoronezh type 213, 440 MW E ). A system of the sampling of the aerosol was the same as in NPP Temelin. 28 samplings during the years from 1999 to 2000 [4] were performed. 7

8 All the distributions are bimodal with the boundary under 0.5 µm, thus AMADs and GSDs were determined without taking into account the back-up filter. The arithmetic means of AMAD and GSD determined for three NPPs are given in the table 5. The arithmetic mean of AMAD of aerosols from NPP Temelin is significantly lower (on the 95% level) than from those of both NPP Dukovany and NPP Jaslovske Bohunice. The smaller AMAD values can be caused by production of lesser amount of large particles due to short time of the NPP operation. All results are in the range of AMADs determined by other authors in the NPP working environment [1]. Table 5. The comparison of the AMAD and GSD determined from the data from the NPP Dukovany and the NPP Jaslovské Bohunice Arithmetic mean AMAD GSD NPP Temelin NPP Dukovany NPP Jaslovské Bohunice The comparison of the aerosol particle size distribution in the ventilation stacks of the NPPs and after the Chernobyl accident in the environment After the Chernobyl accident 9 sampling sets of aerosol were collected in the Czech Republic in the period from to by 5 stage cascade impactor. The particle size distributions of 12 radionuclides were determined. The radionuclides could be divided into two groups: the volatile 131 I, 132 Te, 134 Cs, 137 Cs, 103 Ru, 106 Ru belong to the first group with lower AMAD values, while the refractory 140 La, 140 Ba, 141 Ce, 144 Ce, 95 Zr, 95 Nb belong to the second one, with higher AMAD values. In some cases the distributions were bimodal. The existence of the bimodal distribution and mainly the existence of these two groups of radionuclides is an evidence for supposed two distinct ways of origin of the aerosol particles: the condensation of volatile radionuclides and the dispersion of the nuclear fuel by explosion. In general, the AMADs of the aerosol particles in the environment after the Chernobyl accident are smaller than AMADs from NPP, with an important part of the activity on particles smaller than 0.5 µm (often over 40%) [5] The comparison of the aerosol particle size distribution in the ventilation stacks of the NPPs and in the environment Our results [6] show that in the open atmosphere the main part of radon daughters ( 210 Pb and shortlived radon progeny) and of 7 Be are attached to the fine aerosol with aerodynamic diameter under 0.4 µm. In most cases the AMADs were in the range from 0.05 to 0.35 µm. These results agree well with [1, 7, 8]. This size of the particles corresponds to aerosol originated in the environment by coagulation of smaller aerosol particles and by condensation of the vapour on a finer aerosol so called accumulation mode of the particle distribution. 4. Conclusions The NPP Temelin is a new and modern nuclear facility; the radioactive aerosol effluents are very low. Unique information about AMAD and GSD of individual radionuclides in aerosol effluents from three NPPs was obtained. During the long-time observation, AMAD and GSD of all radionuclides varied very little (NPP Dukovany and Jaslovske Bohunice), so it is possible to use these values for characterisation of air effluents and use them as an input in models of atmospheric dispersion. For NPP Temelin it will have to be verified by a longer time survey. 8

9 AMAD of all the evaluated radionuclides originated in the NPP Temelin is about 2,2 µm with GSD about 2.3 and does not depend on the kind of the radionuclide; such magnitude corresponds to common industrial aerosol called coarse mode and originates from mechanical disintegration of coarser material. All the size distributions are slightly bimodal with the boundary under 0.4 µm. Over 75% activity of the radionuclides originated in NPP Temelin is attached to the particles larger than 1.3 µm and over 50% to the particles larger than 2 µm. The smaller AMAD values (in comparison to NPPs Dukovany and Jaslovske Bohunice) can be caused by production of lesser amount of large particles due to short time of the NPP operation. All results are in the range of AMADs determined by other authors in the NPP working environment [1]. So far it has not yet been possible to study differences between the particle size distributions during outages and during normal operation periods due to the lack of experimental data mainly during normal operation. (The differences were not significant from the practical point of view for the NPP Dukovany and Jaslovske Bohunice.) The particle size distribution of the 7 Be differs from the distribution of the radionuclides originated in NPP; the most of 7 Be activity is connected with particles with aerodynamic diameter smaller than 0.4 µm. This size of the particles - so called accumulation mode - corresponds to aerosol originated in the environment by coagulation and by condensation. Beryllium 7 joins this aerosol in the environment (in the higher atmosphere layers). This aerosol is not attached to the industrial aerosol subsequently. Comparison of results from NPP to results of measurements of aerosol in air masses coming after the Chernobyl accident and to naturally occurring radionuclides documents the capacity of this method to trace the origin of radionuclides. It follows from the obtained results that a cascade impactor is a useful tool for better characterization of aerosol effluents from NPP and for investigations in an abnormal situation, too. 5. Acknowledgements The help of Mr. J. Pospichal and Mr. Y. Glaway from NPP Temelin is greatly appreciated. 6. References 1 Dorrian, M.D., Bailey, M.R, Particle Size Distribution of Radioactive Aerosols Measured in Workplaces, Radiation Protection Dosimetry, Vol. 60, No. 2, pp , (1995). 2 Hinds, W.C., Aerosol Technology, A Wiley-Interscience Publication, John Wiley and Sons, (1982), ISBN Reineking, A., Porstendorfer, J., Becker, K.H., Comparison Measurements of Radon Daughter Activity Size Distribution with Cascade Impactors and High-volume Screen Diffusion Batteries, Aerosols: Formation and Reactivity 2 nd Int. Conf., Berlin, (1986), Pergamon Journals Ltd. Printed in Great Britain, pp Malatova, I., Rulik, P., Bucina, I.: Study of the radioactive aerosol particle size distribution in the effluent air from NPP with the use of cascade impactor, Bezpecnost jaderne energie 1(39), (1993). 5 Rulik, P., Bucina, I., Malatova, I., Aerosol particle size distribution in dependence on the type of radionuclide after the Chernobyl accident and in the NPP effluents, pp in The Radioecology of Natural and Artificial Radionuclides, Proc. 15 th Regional Congress of IRPA, Visby, Gotland, Sweden, (1989). 6 Malatova, I., Rulik, P., Drabova, D., Cespirova, I., Bucina, I., The particle size distribution of 210 Pb in the air, part of International Atomic Energy Agency Co-ordinated Programme Radon in the human environment, Research Contract No 7716/RB, Final Report, (1994). 7 Dorrian, M.D., Bailey, M.R, Particle Size Distribution of Radioactive Aerosols in the Environment, Radiation Protection Dosimetry, Vol. 69, No. 2, pp , (1997). 8 Yamasaki, K., Suzuki, K., Activity Size Distribution of Radon Progeny in Indoor Air: Comparison of Model to Data, Health Physics, Volume 63, No. 4, (October 1992). 9