RADIOACTIVE MATTER IN THE ZAGREB AIR	FROM 1961 TO 1996

Radioaktivne tvari u zraku u Zagrebu od 1961. do 1996. godine.

ZDENKO FRANIĆ, DOBROSLAV CESAR, GORDANA MAROVIĆ

AND JASMINKA SENČAR

Institut za medicinska istraživanja i medicinu rada

Jedinica za zaštitu od zračenja

Ksaverska cesta 2, p.p. 291, HR-10001 Zagreb, Hrvatska

E-mail: franic@imi.hr

Abstract - This paper presents the results of long-term investigations of radioactive matter gathered from the air in the city of Zagreb. Total beta activity in the air has been measured ever since 1961. Gamma-spectrometric air analysis has been carried out continuously since 1983, and determination of 90Sr since 1987. The presented data indicate that the radioactive matter in the Zagreb air, as well as in entire Croatia from 1961 to 1996, has been manifesting a constant exponential decrease in quantity. The only exception was naturally produced 7Be, with almost unchanged specific activity concentrations throughout the measurement period. Main sources of radioactive matter in the air are still nuclear explosions conducted in the atmosphere in 1960s. The Chornobyl nuclear accident caused a significant increase in the air radioactivity only in1986. However, in the few subsequent years it decreased to pre-Chornobyl values. Despite the constant presence of radioactive matter in the Zagreb air, the activity values have never exceeded the legal limit for the last thirty-six years.

Key word index: radioactivity, air, derived activity concentrations, Chornobyl

Sažetak - U radu su opisani rezultati a dugogodišnjih istraživanja radioaktivnih tvari u zraku sakupljanom u Zagrebu. Ukupna beta aktivnost mjeri se od 1961. Gamaspektrometrijska analiza kontinuirano se provodi od 1983, a određivanje 90Sr od 1987. Prikazani podaci ukazuju na eksponencijalni pad radioaktivnih tvari u zagrebačkom zraku, kao i u cijeloj Hrvatskoj u periodu od 1961. do 1996. godine. Jedini je izuzetak prirodni 7Be, čije su specifične aktivnosti u cijelom promatranom periodu gotovo nepromijenjene.Najvažniji izvor radioaktivnih tvari u zraku još su uvijek atmosferske nuklearne eksplozije iz početka 1960-tih godina. Nuklearna nesreća u Čornobilju prouzročila je značajniji porast radioaktivnosti u zraku samo 1986. godine. No, već slijedećih godina opet je opala na pred-Čornobiljske vrijednosti. Usprkos stalnom prisustvu radioaktivnih tvari u zagrebačkom zraku, aktivnosti u proteklih trideset šest godina nikada nisu premašile zakonom dopuštene vrijednosti.

Ključne riječi: radioaktivnost, zrak, izvedene koncentracije aktivnosti, Čornobilj

INTRODUCTION

Long range atmospheric transport processes can cause widespread global distribution of radioactive matter although it sometimes, like in Chornobyl, originates in a single point. Such radioactivity eventually reaches men, either directly or indirectly through the food chain. In order to study the fate and distribution of the radionuclides introduced in the atmosphere, as well as to assess the impact on humans, it is necessary to measure the radioactivity in air (Cesar et al. 1992, Cesar and Senčar 1996).

In 1960s measurements of the total beta activity in the air has been the worldwide mostly utilized method for continuous monitoring of radioactive contamination in the air. However, in late 1970s, due to scientific advancements in the field of electronics, gammaspectrometric measurements became the basic method for qualitative and quantitative determinations of radionuclides in various samples, including the air. In addition, gammaspectrometry has numerous advantages compared to the total beta activity measurements. The latter method cannot discriminate between contribution of particular radionuclides and for more information time consuming and tedious radiochemical analysis had to be performed. Contrary, gammaspectrometrical measurement simultaneously gives information about the presence of different radionuclides. As radionuclides are mostly gamma emitters, only few important ones are left that cannot be determined gammaspectrometrically. These are 89Sr, 90Sr, 239Pu, 240Pu, 14C and 3H. Fortunately, these radionuclides are not very volatile (except 3H). Therefore, they are not expected to be released in the environment in major quantities even in the case of severe nuclear accidents.

Fission radionuclides (thirty-five) routinely gammaspectrometrically measured in the air as a part of the monitoring programme of environmental radioactive contamination in Croatia are: 24Na, 51Cr, 54Mn, 56Mn, 57Co, 58Co, 60Co, 59Fe, 65Zn, 88Rb, 95Nb, 95Zr, 99Mo, 99mTc, 103Ru,106Ru, 110mAg, 124Sb, 125Sb, 129I, 131I, 132I, 133I, 134I, 135I, 132Te, 134Cs, 136Cs, 137Cs, 138Cs, 140Ba,140La, 141Ce, 144Ce and 239Np. In addition, eleven natural radionuclides are also measured: 7Be, 40K, 208Tl, 212Bi, 214Bi, 214Pb, 226Ra, 228Ac, 234mPa, 234Th and 235U.

Investigations of the distribution and fate of natural, weapon-produced and reactor-released radionuclides in the Zagreb air have been conducted in the Unit of Radiation Protection of this Institute, as a part of an extended monitoring programme. The results have been published yearly (Popović 1962-1978, Bauman et al. 1979-1992, Kovač et al.1993-1997). Total beta activity in the air in Croatia has been measured ever since 1961, while gammaspectrometric measurements started in 1981. Investigations of 90Sr involving radiochemical methods started in 1987, a year after the Chornobyl nuclear accident. In addition to Zagreb, the radioactive contamination of air has been monitored over shorter or longer periods of time on some other locations in Croatia: Zadar (1964-1996), Pušća (1994-1996), Medvednica - Sljeme (1962-1964), Virovitica (1969-1990), Bistra (1981-1993), Stružec (1985-1987), Gospić (1986-1990) and Plomin (1987-1990).

Among anthropotropic (i.e. man-made) radioactive nuclides fission products 137Cs and 90Sr have been regarded as the fission products of a great potential hazard to living beings due to the unique combination of theirs relatively long half-lifes, and their chemical and metabolic properties resembling those of potassium and sodium respectively.

137Cs is a beta emitter with a half-life of 30 years. His radioactive daughter, 137mBa, is a gamma emitter with the half-life of 3 minutes. Caesium is alkali metal like potassium and it resembles potassium metabolically. It belongs to the category of highly radiotoxic nuclides (group II) (ICRP 1983, S.l. SFRJ 1986, S.l. SFRJ 1984, N.N. 1991).

90Sr has a radioactive half-life of 29.12 years and decays by beta emission. Its daughter 90Y is also radioactive, with a half-life of 64 hours, and decays by beta emission to the stable isotope 90Zr. Strontium is an alkaline earth element and is similar to calcium, barium and radium. Therefore, it follows calcium through the food chains from environment to man. Like calcium, strontium is retained in the body largely in bone. Therefore, 90Sr belongs to the category of highly radiotoxic nuclides (group I) (ICRP 1983, S.l. SFRJ 1986, S.l. SFRJ 1984, N.N. 1991).

7Be is a gamma emitting radionuclide with a half-life of 53.28 days. It belongs to the third group of moderately radiotoxic nuclides (group III) (ICRP 1983, S.l. SFRJ 1986, S.l. SFRJ 1984, N.N. 1991).

MATERIAL AND METHODS

Total beta activity in the air in Croatia has been measured ever since 1961. Air sampling, ranging from 100 to 200 m3 daily, was carried out 1 m above the ground. Air is continuously pumped through Schneider & Poelman filter paper (blue). The activity was measured after 120 h (in order to allow decay of short-lived beta radionuclides) by a low level anti-coincidence, shielded beta counter. For respective years were calculated mean values of a daily total beta activities.

Gamma-spectrometric air analysis has been carried out continuously since 1983. Air is pumped through an air glass-fiber filter device placed 1 m above ground, the monthly volume ranging from 10,000-30,000 m3. A gamma-ray spectrometry system based on a Ge(Li) detector (relative efficacy 15.4% with FWHM resolution of 1.87 keV at 1.33 MeV) coupled to a computerized data acquisition system (a 4096-channel pulse height analyzer and a personal computer) was used to determine radioactivity levels in the samples from their gamma-ray spectra. The detector is shielded with 10 cm thick lead that is lined with 2 mm of cadmium and 2 mm of copper. Samples were measured in cylindrical plastic containers of appropriate volume placed directly on the detector. The same samples were afterwards used for radiochemically separation (Harley 1970) of 90Sr. The radioactivity of 90Sr was determined by beta-counting its decay product (90Y), in a low-background anti-coincidence, Geiger-Müller counter.

For respective years were calculated mean values of monthly 90Sr, 137Cs and 7Be specific activities.

Counting time of all samples depended on sample activity concentration, but was never less than 60,000 s. Efficiency calibrations were carried out using sources provided by the International Atomic Energy Agency (IAEA) and World Health Organization (WHO).

RESULTS AND DISCUSSION

The moratorium on atmospheric testing of nuclear weapons in early 1960s resulted in constant decrease of radioactive matter in air, as seen from Figure 1 that shows long-term data for the total beta activity in the Zagreb air. Although some other mathematical models can lead to better fit of experimental data, the best physically acceptable fit for the experimental data is the exponential function:

where A(x) and A(0) are activity concentrations in times x and zero respectively and k is constant. Reciprocal value of constant k is the mean residence time Tm of the observed radionucide in the air, i.e.:

Transient increases in Figure 1 can be explained by smaller scale tests performed by China and France in spite of a nuclear moratorium, as well as by a variety of environmental physical factors that naturally fluctuate. For the total beta activity in the air the fit of experimental data to a function (1) was obtained by regression analysis, leading to the equation:

with P(t) < 0.001. Time is in years, the starting year x=1 being 1963. The 1986. activity peak is caused by the presence of Chornobyl radionuclides like 137Cs and 131I, as confirmed by gammaspectrometric measurements. However, a year later total beta radioactivity levels decreased to pre-Chornobyl values. Therefore, for the overall observed period, this peak did not influence on a significance of regression curve as indicated by the Fischer t-test.

FIGURE 1

HERE

Figure 2. shows the specific 137Cs activity concentrations in the Zagreb air from 1983 to 1996. The 1986. activity peak is caused by 137Cs originated in Chornobyl accident. The data for 137Cs specific activity concentrations are fitted to the exponential function:

with P(t) < 0.02. Time is in years, the starting year x=1 being 1986.

FIGURE 2

HERE

From equations (2) and (4) the mean residence time of 137Cs for the post-Chornobyl period, characterized by rapid decrease of 137Cs in the air, is 1.8 years. For comparison, mean residence time of 137Cs in fallout for the period May 1986- December 1990 is 0.71 years (Franić, 1992).

Figure 3. Shows specific 90Sr activity in Bqm-3 in the Zagreb air from 1987 to 1996. The best fit for the 90Sr specific activity concentrations is the exponential function

with P(t)<0.01. Time is in years, the starting year x=1 being 1987.

FIGURE 3

HERE

From equations (2) and (5) the mean residence time of 90Sr in the air is 2.9 years. For comparison, mean residence time of 90Sr in fallout for the period 1962 - 1993 is 5.6 years. However, the 90Sr mean residence time for the period 1962-1967, that followed the most intensive atmospheric nuclear weapon tests is only 1.27 years (Franić, 1994). Namely, like the post-
Chornobyl period for 137Cs, 1962 -1967 period is characterized by rapid decrease of 90Sr activity concentrations in the air.

The nuclear accident at Chornobyl (26 April 1986) did not cause significant increase in 90Sr activity in environmental samples in Croatia (including air), contrary to 137Cs. Unlike the debris from the atmospheric testing of nuclear weapons, the radionuclides that originated from the Chornobyl damaged reactor were not released directly into the upper atmosphere. As the result of the release mechanism (continuing releases over 10-day period, steam explosions and fire of the graphite moderator) and the prevailing meteorological conditions at the time, the less volatile components of the Chornobyl debris (e.g. 90Sr) were deposited closer to the accident location than the more volatile constituents, e.g. radiocaesium (Whitehead et al. 1988). Thus, 90Sr was not subjected to the global dispersion processes, being deposited to the Earth's surface within a period of a few days to a few weeks after the accident. Generally, changing meteorological conditions with wind of different directions at various altitudes and prolonged releases resulted in a very complex dispersion pattern over Europe. Consequently, major parts of Croatia were initially unaffected by the plumes of contaminated air (UNSCEAR 1988, UNEP 1991).

The specific activity concentrations of 7Be and in the Zagreb surface air during the study period 1983-1996 are shown in Figure 4.

FIGURE 4

HERE

Lines present mean value and corresponding standard deviations. For observed 1982-1996 period no trends in 7Be specific activity concentrations can be observed. The mean value ( ) for the 1982-1996 period is (6 ± 2)×10-3 Bqm-3 with 33% standard error (gs). Similar values have been observed elsewhere (Paatero et al. 1996). Relatively high standard error indicates pronounced fluctuations in 7Be specific activity concentrations. These fluctuations can be explained by the mechanism of production and transport of 7Be to the Earth surface. 7Be, being "cosmogenic radionuclide" is produced in the upper troposphere and stratosphere in nuclear reactions induced by cosmic rays in atmospheric oxygen and nitrogen atoms (Lujanas 1979). About 75% of 7Be is produced in the stratosphere and 25% in upper troposphere (Johnson and Viezee 1981). The descending movement of air during high-pressure situations subsequently brings 7Be downwards. Therefore, 7Be activity concentrations in near-surface air are in a very complex way dependant upon meteorological conditions.

To conveniently present the impact of the radioactive matter in the air on humans, the data for total beta activity measured and specific activities of 7Be, 137Cs and 90Sr are in Table 1 showed as the percentages of derived air concentrations (%IK), i.e. limit values determined by Croatian low. (S.l. SFRJ 1987, S.l. SFRJ 1984, N.N. 1991). It can readily be seen that in the surface air in Zagreb for the whole observed period concentrations of radioactive matter never exceeded the allowed values.

TABLE 1

HERE

CONCLUSIONS

Generally, on the basis of presented data can be concluded that for the overall 1961-1996 period the concentrations of radioactive matter in the Zagreb air exponentially decreased, the only exception being activity concentrations of naturally produced 7Be. The nuclear accident in Chornobyl caused an activity peak in 1986. However, in the subsequent years its influence on the quantities of radioactive matter in the air was negligible. Therefore, the main source of radioactive matter in the air are still the atmospheric nuclear explosions performed in 1960s.

Continuous monitoring during the observed period showed that the activity concentrations of the radioactive matter in the Zagreb air never exceeded allowed values. However, in order to protect the general public, monitoring is necessary, also as a part of general emergency preparedness in the case of a nuclear accident.

REFERENCES

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Cesar D., Kovač J., Bauman A. (1992) Radioaktivnost u zraku na području Republike Hrvatske od 1961. do 1991. godine. Zbornik radova Prvoga simpozija Hrvatskoga društva za zaštitu od zračenja, HDZZ Zagreb, 287-291.

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Franić Z. (1992) 137Cs u radioaktivnim oborinama u Zagrebu. Hrvatski meteorološki časopis, 27, 63-68

Franić Z. (1994) Distribution Analysis and Mean Residence Time of 90Sr in Wet Fallout in Zagreb. Hrvatski meteorološki časopis, 29, 76-83.

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ICRP (International Commission on Radiological Protection) (1983) Radionuclide transformations. ICRP Publication 38. Pergamon Press Oxford, New York, Toronto, Sydney, Frankfurt.

Johnson W.B. and Viezee W., 1981. Stratospheric ozone in the lower troposphere - I. Presentation and interpretation of aircraft measurements. Atmos. Env. 15(7), 1309-1323.

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N.N. (Narodne novine) 51/1991. Zakon o preuzimanju saveznih zakona iz oblasti zdravstva koji se u Republici Hrvatskoj primjenjuju kao republički zakoni. N.N. 53/1991. p. 1524.

Paatero J., Hatakka J. and Mattsson R. (1996). Measurements of Airborne Beryllium-7 in Northern Finland. In: Walderhaug T. and Einar Pall Guthlaugsson (Eds.). Proceedings of the 11th Ordinary Meeting of the Nordic Radiation Protection Society and 7th Nordic Radioecology Seminar, 26.-29. August 1996, Reykjavik, Iceland, ISBN 9979-60-351-1. p. 263-269.

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S.l. SFRJ (Službeni list) (1986). Pravilnik o stavljanju u promet i upotrebi radioaktivnih tvari iznad odre|ene granice aktivnosti, rendgenskih aparata i drugih aparata koji proizvode ionizirajuća zračenja te o zaštitnim mjerama od zračenja tih izvora S.l. SFRJ 40/1986 p. 1168.

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Figure 1: Total beta activity (b) expressed in Bqm-3 in the Zagreb air from 1961 to 1996 with the pertaining regression curve.

Slika 1. Ukupna beta aktivnost (b) u Bqm-3 u zraku grada Zagreba od 1961. do 1996. godine i pripadajuća regresijska krivulja - pravac.

Figure 2. 137Cs specific activity concentrations (aC) expressed in Bqm-3 in the Zagreb air from 1983 to 1996 with the pertaining regression curve.

Slika 2. Specifične aktivnosti 137Cs (aC) u Bqm-3 u zraku grada Zagreba od 1983. do 1996. godine i pripadajuća regresijska krivulja - pravac.

Figure 3. 90Sr specific activity concentrations (aS) expressed in Bqm-3 in the Zagreb air from 1987 to 1996 with the pertaining regression curve.

Slika 3. Specifične aktivnosti 90Sr (aS) u Bqm-3 u zraku grada Zagreba od 1987. do 1996. godine i pripadajuća regresijska krivulja - pravac.

Figure 4. 7Be specific activity concentrations (aB) expressed in Bqm-3 in the Zagreb air with the pertaining average and standard deviation.

Slika 4. Specifične koncentracije aktivnosti 7Be (aB) u Bqm-3 u zraku grada Zagreba i pripadajuća prosječna vrijednost sa standardnom devijacijom.

Table 1. Total beta activity expressed through percentage of a derived concentration for unknown mixture of radionuclides and specific 7Be, 137Cs and 90Sr activities expressed through percentage of a corresponding derived concentration in the Zagreb air from 1961 to 1996.

Tablica 1. Postoci ukupne beta aktivnosti (b) u odnosu na izvedenu koncentraciju za nepoznatu smjesu radionuklida i specifičnih aktivnosti 7Be, 137Cs i 90Sr u odnosu na odgovarajuće izvedene koncentracije u zraku grada Zagreba od 1961. do 1996. godine (%IK).

b	b	7Be	137Cs	90Sr
IK/Bqm-3	0.7	0.7	2000	10	0.4
Year	%IK	Year	%IK
1961	214	1979	0.793	-	-	-
1962	25.4	1980	1.14	-	-	-
1963	30	1981	0.886	-	-	-
1964	5.81	1982	0.129	-	-	-
1965	11.1	1983	0.129	0.0002	<1.5E-4	-
1966	0.581	1984	0.0714	0.0002	<1.5E-4	-
1967	0.423	1985	0.0886	0.0004	<1.5E-4	-
1968	0.793	1986	38.6	0.0004	1.1	-
1969	1.27	1987	0.0957	0.0006	0.0022	0.008
1970	1.49	1988	0.12	0.0002	0.0003	0.0023
1971	1.49	1989	0.127	0.0003	0.0002	0.0022
1972	0.581	1990	0.106	0.0002	0.0002	0.0045
1973	0.264	1991	0.104	0.0002	0.0002	0.0018
1974	0.896	1992	0.179	0.0003	0.0002	0
1975	0.634	1993	0.171	0.0003	<1.4E-4	0
1976	1	1994	0.29	0.0003	<1.4E-4	0
1977	2.49	1995	0.213	0.0003	<9.2E-5	0
1978	0.846	1996	0.197	0.0003	<1.0E-4	0