The Radiological Impact of Hot Beta-Particles from the Chernobyl Fallout: Risk Assessment" September 1993; Tjujgaria

INTERNATIONAL ATOMIC ENERGY AGENCY RCM "The Radiological Impact of Hot Beta-Particles from the Chernobyl Fallout: Risk Assessment" September 1993; TjuJgaria

INVESTIGATION ON THE DISTRIBUTION AND THE EVOLUTION OF HOT PARTICLES IN BULGARIA AFTER THE CHERNOBYL ACCIDENT S. Bogoeva, Ts. Bonchev, E. Dinolova*, K. Dinolova, F. Helal, T. Rida, T. Semova, V. Vinchev, S. Vladimirov Faculty of Physics, Sofia University, 1126 Sofia, Bulgaria * Institute of Applied Mineralogy, Sofia

This paper, printed in brief in the Kiev meeting proceedings^.concerns an attempt to find the distribution of the hot particles activity in air for the days 1-7 May 1986 when their concentration was the highest one. With that end in view autoradiographs of air filters are used. In principle it would be expected that the hot particles formed at the time of the Chernobyl accident will follow the logarithmic-normal distribution law in radiuses concerning particles obtained by grinding of solid substances. This is confirmed by the extensive studies of Georgi et allJ2! from many samples of various regions in Europe. The probability density of that distribution is given by:

III where: r - the radius, and the main parameters are: lnr median - /;„ = e

mean value - r - r ,e* Hm As the activity of one particle, A, is proportional to its volume it follows that for the activity distribution in /I/ A> would be replaced by r. In fact Kritidistin 2\ Mandjukov et a\l\Q and our results presented later set up that in the probability density HI must be put A rather than A' . At present we can not give the explanation of that fact which most probably is due to the hot particles separation in activity in the process of their deposition. According to the plan of investigation.of the problem "hot particles" by autoradiography the following results were obtained up to'now: The development of a method for processing of autoradiographs of air filters with digital image processor is completed. The end result of such a processing is the distribution of the number of hot particle images versus the diameter of the images or their area. The digital image processor makes possible the separation of several hundred hot particle images per 1 cm2. An example of the output information from the digital analyser is presented in Fig.l.

The development of a method which makes possible the determination of the beta-activity of hot particles according to the diameter of their images obtained with the digital image processor is completed. The calibration is performed for roentgen films and also for conventional black and white films which have certain advantages for such type of autoradiographs. An example of such a calibration is presented in Fig.2. It has been proved after a great number of experimental evidences that the activity distribution of the hot particles follows a logarithmic-normal distribution.

The above mentioned methods were deployed for the determination of the activity distribution of hot particles in Bulgarian cities in which since the early 60-ties air samples are collected at the stations of the Chief Department of "Hydrology and Meteorology" at the Bulgarian Academy of Science, namely: Pleven, Sofia, Plovdiv, Burgas and Varna. The autoradiographs of the air filters for the time 1-7 May 1986 when there was a significant number of hot particles in the. air were processed. Autoradiographs of those filters obtained on the corresponding dates were processed as follows:

- 29 May 1986, exposition time 24 hours; - end of December 1987, exposition time 7 days;

- July-August 1990, exposition time 45 days.

The consecutive autoradiographs made possible the tracking of the hot particle beta-activity in time. The results of those processing are presented in the following Table. There are no figures for the town of Varna since the air filters from that town contain negligible number of hot particles.

Data given in the Table shows that between the expositions on the same date there are not significant differences concerning the values of \nA as well those of A where the latter depend also on the disperse a. That is why because of the lack of better possibilities for the exposition on 29 May 1986 we take the average values of In/I and a for all measurements. This averaging was done also reading the "relative weights" of separate measurements concerning the filter total beta-activity, Ap , which does not change in particular the values of In A and a obtained by simple averaging and finally we accepted the values In7l = 5.516 and CT = 0.576. However the digital image analyser discards the spots with the smallest diameters (i.e., the particles with the least activity). This was experimentally proved by a visual processing of the results for the filter on 03.05.1986 in Sofia. The results of this processing are shown in Fig.3 with a solid line. In Fig.3 the probability density is marked by N following the equation HI concerning the particle number per unity range of the activity scale, AN N = . The big spot at the curve lower end corresponds to the result AA obtained for particles with an average activity about 200 Bq collected from various outside surfaces (road pavements, flat roofs). In the same Fig.3 the logarithmic-normal distribution is given by circles obtained for the above mentioned filters with averaged parameters. The sharp drop of the probability density, N, for values of A<0.1 Bq is due to the above mentioned disadvantage of the digital analyser. Ultimately for the "averaged citizen" of the towns of Pleven, Sofia, Plovdiv and Burgas we propose the equation including the parameters of the logarithmic-normal distribution for the solid line: 1.6 (lnA,-5. where: ANy is the hot particles number in 1 m3 of air in the activity range Aj + Aj ; average activity Ajj= (Aj + Aj)/2 which a grown-up individual has breatned-in (assuming 20 m^ per 24 hours) for a time T hours outdoors in the period 1-7 May 1986.

Of course we recognise the very approximate approach of this evaluation but because of the lack of in-time representative samples this is the best one. Data given in the Table for .02.1987 expositions clearly show the "evolution" of the hot particles concerning their activity. The last of these exposition prolonged 45 days shows also that the hot particles concentration with low activities in air through the first days of May 1986 had been very high in fact. At that time these particles could not be visualised as a very long exposition would lead to fully blacken image. Data of the three expositions could be analysed thoroughly but we have not done this up to now.

In our opinion using the sedimentation method of hot particle collection in combination with air filters usage, their autoradiography and processing by the above mentioned method would lead to the correct data of hot particle concentration in air and consequently to the risk assessment. • HISTOGRAM OF OCIRCLE fUtlUtHCY rouHTt JJJ • t e •) END nov • ovccnou 7 ciotttt >• noDui i .mi L IOUHC • .lie U BOVXD 21 it niKinun J.52J ! '.. ' . • " • V .«'

*<-i 1DCN1.NO «.J NCW P0R1 itnin CLBSSlfICOT10K LIST OT PCIRCLE PIX LOST COUNTEl 33? MIHIHU1I i UNDCRFLOU 0 OVERFLOW 0 KODUl.1 10. 000 HflllMtjni MCANI 3.079 EDI • »O9I Cnss FROM HE JIOMCLOSSI 1 TO OD5 HEL 1 0.000 10.000 331 IOC . CX< %

a) b)

Fig.l. a) Autoradiograph of an air filter from the afternoon on 3 of May 1986 from Pleven, through which 20 m3 of air have been filtered. The exposition time in July-August 1990 was 45 days. The background is removed and the sharpness is increased by a specially developed photographic processing procedure;

b) Results from the processing of the radiograph with the Digital Image Processor KONTRON. The following values are printed in the listing: - number of processed images of hot particles (332); - minimum and maximum values of the processed image diameters, the average value and its dispersion; - logarithms of the above quantities (for the logarithmic-normal distribution); A histogram of the diameter distribution (in pixels), the solid curve is the fitted logarithmic-normal distribution. 6 tn(M) 5

1 - /

0 . •. .i i ii • ' • i—i—i— • 1 -2 -1 inO

Fig.2. An emperical dependence of the image diameter of a hot pai'ticle (mm) and its measured beta-activity (Bq) for an exposition time of t (days).

0.2 0.1 0.6 1 i—m

0.1 10 100 1000 A [ Bcj ]

AiV Fig.3. A^ = , probability density measured in particles/AA = l mBq in aA 1 m3 of air; NQ -the hot particle average concentration in 1 m3 of air. The particle diameter is given on the upper scale, corresponding to A, provided they are fuel particles with a specific activity of 11 3 Ao = 3.10 Bq/kg and specific density of p = 10 g/cm . TABLE OF HOT PARTICLES BASIC PARAMETERS IN SOME TOWNS Results from the image processing of air filters from the towns of Pleven, Sofia, Plovdiv and Burgas for the period L-7 May 1986, the exposition is on 20 May 1986, December 1987 and July-August 1990. V - filtered air, m3; Ap - total beta-activity measured with low background facility (anticoincidence);

u, a2 - parameters of the logarithmic-normal distribution;

A - average activity of a hot particle;

At - total activity of the hot particles.

3 All values for Ap , n and AL are normalised for 1 m of air. The activities are in mBq.

Certain data of some of the filters are missing due to the fact that in 1987 and especially in 1986 no autoradiographs were obtained. The same holds for the total beta measurements in 1987.

In some cases At > Ap which is not possible but is a result of measurement errors. d d d PLEVEN 29.05.1986 z. texp = l .12.1987 2. texp = 7 .07.1990 z. texp = 45

V n n n Date Ap A At A At A At

3 .mBq .mBq mBq .mBq. mBq May r i 2 o im ] I—r-3 J L—d a [mBq] a- [mBq] 1 3 3 3 [mBq] 1 1 1986 2. m m3 m3 m m m m3 m 1 01. 21 433 75 0.21 9.4 1.578 5.40 51 0.209 2 02. 23.6 6800 0.93 5.785 404 376 1.2 3.795 53 62.4 488 0.21 50 1.984 8.50 420 0.430 0.334 0.305 3 03.1 20 4500 1.1 5.830 474 522 3.5 3.700 57 200 440 0.29 78 1.963 8.20 640 0.664 0.670 0.281 4 O3.n 20 2800 2.5 5.563 320 790 4.0 3.733 51 200 450 0.15 16.6 2.251 11.80 196 0.417 0.390 0.430 03. - 3650 1.8 5.670 375 670 3.7 3.718 54 200 440 0.22 47 2.030 8.93 433 0.516 0.530 0.318 5 04.06 n. 10 2600

6 04.14 H. 10 700 3.6 5.500 280 1010 7.4 3.706 47 350 140 0.44 29 1.798 7.20 208 0.270 0.302 0.245 7 04.18 *. 10 1600 3.1 3.785 44 135 236 0.44 37 1.806 7.30 270 0.177 0.353 8 04.22 H. 10 . 630 6.1 3.927 58 355 86 - 24 1.900 7.80 186 0.284 0.314 04. 1380 3.6 5.500 280 1010 5.5 3.812 52 240 154 0.44 30 1.784 7.00 210 0.270 0.274 0.321 9 05.02 H. 10 9.8 3.792 53 520 120 18 1.848 7.70 140 0.360 0.390 10 05.06 n. 10 1100 7 5.500 290 2010 8.4 3.661 48 400 242 12 1.879 7.50 90 0.310 0.400 0.264 d PLEVEN 29.05.1986 z. 1t = l .12.1987 e. 7d .07.1990 z. 45d exp texP =

Date V Ap n V- A At n I At Ap n A At mBq mBq .mBq. .mBq, mBq May ,mBq. r i 2 2 r l 2 N. 3 L—d a [mBq] 1 3 J a [mBq] 1 3 J 3 1 L—d a [mBq] 1 3 i 1986 z. m m3 m m3 m m m m3 m 11 05.10 M. 10 6300 17 5.434 260 4360 640 0.05 40 1.522 5.30 208 0.238 0.211 1411 12 05. - 10 2100 2.9 5.311 245 710 446 17.5 1.690 6.00 105 0.382 0.209 13 05.18 *• 10 4.7 3.670 45 210 810 - 94 1.732 6.30 590 0.269 0.220 14 05.22 n. 10 2.5 3.863 53 133 1100 0.02 92 1.856 7.20 664 0.258 0.236 05. 3170 9.0 5.441 264 2380 6.4 3.738 50 320 560 0.03 55 1.856 6.60 360 0.272 0.346 0.236 15 06.02^- 20 2900 3.5 5.258 320 1120 0.482 16 06.06 u. 10 6000 9.8 5.309 230 2270 1.4 3.551 41 57 730 - 84 1.810 7.00 590 0.277 0.298 0.275 17 06.10 H. 10 1300 12.2 5.311 228 2780 1.1 3.614 45 50 798 0.237 0.391 4 Xi !l8 06.1 - 10 990 5.0 3.640 45 226 220 45 1.929 7.70 350 0.342 0.215 19 06.18 H. 10 3600 10 5.324 243 2426 2.8 3.855 57 160 416 26 1.727 6.50 170 0.334 0.388 0.275 06. 2900 8.9 5.363 250 2210 2.6 3.700 48 126 450 42 1.825 7.00 290 0.307 0.357 0.253

20 07.02 M. 10 157 0.15 28 1.617 5.60 154 0.210 d 7d d PLEVEN 29.05.1986 2. texp = l .12.1987 2. .07.1990 z. t = 45 texp = exP n Date V H n A* n V- A At Ap V- A At

May 3 ^mBq^ 2 a ratiq rmBq. ,mBq, 2 mBq [m ] z a [ 3 ] [mBq] 3 a [mBq] 1 3 J m [ 3] m 3 m m m 2 tn 1986 z. /7Z OT m

21 07.06 H. 10 8.2 3.572 39 320 - 0.35 120 1.907 5.60 910 0.201 0.231 22 07.14*- 291 0.12 3.7 1.261 4.80 18 0.611 23 07.20 «• 380 0.067 36 1.429 4.70 170 0.235 24 07.22 q. 10 8.9 3.800 52 460 0.304

07. 8.6 3.706 43 390 243 0.17 47 1.873 7.40 350 0.217 0.234 i d d SOFIA 29.05.1986 2. 1'exp x .12.1987 2. texp = 7 .07.1990 2. •"exp ~ 45

Date V Ap n V- A At n A At Ap n A At

mBq 2 mBq 2 mBq mBq 2 N May L , J [mBq] 1 z [mBq] 1 3 J I—j-J [mBq] 3 J 4 3 a m 3 a m 3 3 a m 1986 2. m m m m m 25 01. 105 730 96 0.14 3.20 2.040 9.00 29 0.310 26 02. 105 8700 1.10 5.924 484 530 0.96 3.977 78 75 940 0.19 1.07 1.554 5.50 5.8 0.516 0.762 0.283 27 03. 107 4000 1.33 5.510 290 380 1.06 3.519 45 47 392 0.12 4.12 2.033 8.90 36.5 0.332 0.566 0.297 2* 04. 105 1000 0.59 5.414 260 153 0.78 3.263 29 23 66 0.07 4.70 2.188 10.60 50 0.291 0.210 0.293 29 05. 104 1400 0.16 6.168 714 117 0.60 3.567 46 27 122 0.06 4.40 2.076 9.20 40 0.770 0.514 0.293 30 06. 112 2300 0.34 5.885 411 140 0.54 3.730 52 28 213 0.10 0.47 1.840 7.60 3.6 0.267 0.427 0.388 31 07. 109 78 0.083 1.27 1.997 8.90 11.2 0.372 _ iid 7d d PLOVDIV 29.05.1986 2. 1'exp .12.1987 2. 'exp .07.1990 2. fcexp = 45

Date V Ap n A At n A At Ap A. n P- ~A At

3 mBq mBq mBq mBq 2 2 .mBq, tmBq, r i 2 N, May [m ] 1 3 J [mBq] 1 3 i [mBq] 1 3 CT [mBq] m { 3 a m 3 a m' m I j J 3 mr 1986 2. m ^ m m m 32 01. 27.4 84

33 02. 25.4 1400 3.07 5.487 274 840 2.07 3.546 39 78 206 0 12 1.227 3.67 44 0.252 0.218 0.150 34 03. 25.9 2300 4.10 5.492 274 1125 2.10 3.486 36 77 300 0 52 2.036 8.80 46 0.248 0.202 0.270 35 04. 33.0 320 25 0

36 05. 30.5 2300 2.70 5.634 320 860 1.80 3.169 26 48 154 0.09 16 1.374 4.33 6S 0.270 0.212 0.187

37 06. 32.1 4100 1.10 5.620 310 350 2.30 3.659 45 104 260 0.17 37 1.929 7.80 290 0.225 0.281 0.260 38 07. 28.7 3200 1.50 3.682 45 69 100 0.23 7.8 1.253 3.70 29 0.256 0.150 39 08. 26.3 230 13 - 3.5 1.159 3.40 12 0.122 d d d BURGAS 29.05.1986 2. texp = l .12.1987 2. texp = 7 .07.1990 2. texp = 45

V n n Date n A A At A At

3 -tnBcf. rmBq mBq N. May 2 2 2 lm ] a [mBq] 1 3 J a [mBq] 1 3 J z 1 a [mBq] 1 3 J 1986 z. m m3 m m3 m m m [m3] m 40 01. 35.6 450 49 10.3 1.229 3.70 3S 0.131 41 02. 28.3 12200 3.8 5.521 294 1120 3.7 3.640 43 160 1790 0 13.6 1.303 4.00 55 0.323 0.256 0.164 42 03. 31.3 680 61

43 04. 50.0 909 2.6 5.154 190 500 2.0 3.365 32 64 73 0.09 13 1.507 5.00 63 0.309 0.228 0.189 44 05. 42.0 5000 1.9 5.728 420 790 1.3 3.921 64 84 390 0.014 36 1.544 5.20 186 0.621 0.485 0.214 45 06. 58.0 90

46 07. 56.0 680 1.1 5.240 230 250 0.86 3.642 45 39 40 0.011 3.9 1.426 4.70 18 0.405 0.323 0.230