MONITORING OF KRYPTON-85 ACTIVITY IN THE ATMOSPHERE AROUND PRAGUE L U D M I L A WILHELMOV,/~, M I L A N T O M A S E K and K A R E L S T U K H E I L Institute of Radiation Dosimetry, Czech Academy of Science, Prague, Czech Republic
Abstract. The results of measurements of Krypton-85 (85Kr) concentrations in the ground-level air of Prague between 1983 and 1992 are presented and time-related changes analysed. The long-term trend in activity level of SSKr has been steadily increasing with a growth rate of 0.04 Bq.m -3 (STP) per year. Some peaks of 85Kr activity were observed due to the influence of undispersed radioactive plumes coming from distant sources. Short-term variations within a typical range of concentrations from 0.61 to 1.25 Bq.m -3 (STP) were found to be seasonally dependent, with the maximum occurring in spring.
1. Introduction
Pollution of the atmosphere by long-lived radionuclide 85Kr (T1/2 = 10.76 y) results from human activity in the exploitation of nuclear energy for peaceful and military purposes. Man-made 85Kr originates by fission in nuclear reactors and is among the waste products routinely released from the nuclear facilities into the atmosphere. Emissions from nuclear reactors during normal operation are negligible. The vast majority of actual 85Kr environmental supplies comes'from the gaseous effluents of spent fuel reprocessing plants and military plants producing plutonium (Weiss et al., 1989). As 85Kr is an inert gas, it is removed from the atmosphere mainly by radioactive decay. Due to high mobility this radionuclide became widely dispersed in the Earth's atmosphere at present and acts as a long-term low-level source of exposure concerning the world's population. Radiological assessment of global effects necessitates analytical methods of detection with sensitivities well below those required for radiation protection. The basic radiological information on 85Kr in the atmosphere is provided by measuring its activity in ground-level air. The available time-series of 85Kr data are related mostly to its source region in the middle latitudes of the Northern Hemisphere, where the maximum SSKr concentration is also observed (Weiss et al., 1989; Csongor, 1980; Wilhelmovti et al., 1990; Styro and Butkus, 1988; Cimbfik and Povinec, 1985; Tertyshnik et al., 1982). The regular monitoring of 85Kr in the atmosphere in the Institute of Radiation Dosimetry, Prague, began in 1983. The aim of this work was to establish the base-line level of this radionuclide for emergency situations and to investigate the potential influence of reprocessing plants operating in Europe. Moreover, the results obtained can help to understand the behaviour of 85Kr in the atmosphere and may appear to be of interest to the study of the mesoscale atmospheric transport and dispersion. Environmental Monitoring and Assessment 34: 145-149, 1995. @ 1995 Kluwer Academic Publishers. Printed in the Netherlands.
146
LUDMILA WILHELMOV,~ ET AL.
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,
*-~.
~_ ~.~_
d"
rn
;0
0,5 __
I
,
198Z,
I
I
I
1986
I
1988
1
199'0
I
Years
1992
Fig. 1. Measurementsof 85Kr activity in the surface air of Prague during 1983-1992. Full line denotesthe linear fit of long-termtrend.
2. Experimental The 85Kr air activity present cannot be quantified by direct measurement. Therefore the samples of natural atmospheric krypton obtained by laboratory separation techniques are used for this purpose. The method consists of a sampling and purification set-up and counting procedure. The technique used in our Institute for the preparation of krypton samples is based on cryogenic selective adsorption of air components on charcoal beds combined with standard chromatographic analysis of isolated krypton volume. Since the atmospheric content of natural krypton is constant (1.14 cm 3.m -3 STP) the last step allows its recovery to be determined from the treated air in each experiment. The analysis of 85Kr activity in the isolated gaseous sample is performed using a counting chamber equipped with a CaF2 (Eu) scintillation detector. With conventional 85Kr activity measurements (efficiency 13%, measuring time 5 x 103 s, both in the case of background 0.3 cps and 85Kr sample activity 5-10 Bq) the relative error of less than (3%)0.95 is achieved. The apparatus and other details of experimental procedure were previously described in Wilhelmov~i et al. (1985), Wilhelmov~i et al. (1986). In routine experiments the air sample of about 10 m 3 is treated. Sampling is performed in the locality of the Institute in Prague (50°04 N; 14°26 E). The results obtained over the period 1983-1988 corresponded to the air samples collected regularly one day each month. Since 1989 they represented the 85Kr activity of
MONITORING OF KRYPTON-85
147
Fig. 2. Backwardtrajectoriesfor the outlyingobservation: 1 - 84/7/11, 2 - 85/2/13, 3 - 86/6/8, 487/11/11, 5 - 88/6/8. Locationsof nuclear fuel reprocessing plants: A - Sellafield,B - La Hague, C - Marcoule, D - KarlsnLhe,E - middleUral.
air samples averaged continuously at monthly intervals. The compatibility of these two kinds of data was checked by experiments carried our simultaneously during the year 1989. The statistical treatment of these measurements revealed the rather large scatter of spot (one-day) observation, but the yearly trend as well as its mean value proved to be identical. The reason for the change in methodology of 85Kr measurements was the radiological relevance of the acquired data.
3. Results and Discussion The concentrations of SSKr measured in the ground-level air of Prague are shown in Figure 1. The extreme values (exceeding 1.3 Bq.m -3 STP) registered as being exceptional in the spot-series of data were identified as outlying observations. Their occurrence may have been connected with the passage of undispersed air masses from distant sources. Supporting information can provide the trajectories of contaminated air constructed at 850 hPa within the interval of 96 h before the time of sampling, as shown in Figure 2. It follows from this picture that elevated 85Kr
148
LUDMILAWILI-IELMOV,~ET AL.
!'
I
I
I
I
"l
I
1.2 o_ 1.0 ti3
o
o%~ooo o ~ 8 S~oo 04, ¸
~o
m
0.2o
o88
0
f
1960
I
1
1
1970
I,,
1980
I
1
1990
YeQrs Fig. 3. Evolution of mean 85Kr activity levels in the surface air of middle latitudes of the Northern Hemisphere. o - estimate from literary data; • - this work.
levels in Prague can be attributed to the plumes originating in the source regions of western Europe as well as in Russia. Further variations of 85Kr air activity with an amplitude of about 12% of mean annual value have been recorded throughout the whole period of observation. As can be seen from Figure 1, this variability is seasonally dependent, with the maximum occurring mostly in the early spring. The reason for this 85Kr behaviour is not fully clear. It may be caused by discontinuity in operation of reprocessing plants (Weiss et al., 1989); however, the influence of natural factors cannot be excluded so far and require further investigation. As apparent from Figure 1, the long-term trend of 85Kr air activity has been steadily increasing over the period examined (19 83-1992). It can be approximated by linear form that provides the growth rate of about 0.04 Bq/m 3 (STP) per year. The mean annual values of 85Kr concentrations in the air around Prague are plotted in Figure 3 together with the estimate of those derived by statistical treatment (Dvo~Lk and Wilhelmov~i, 1984) from all available measurements in the middle latitudes of the Northern Hemisphere. The pattern in Figure 3 may be considered to be characteristic for the evolution of environmental 85Kr contamination in given geographical zones. Our observations concur with this tendency and indicate that the 85Kr air content over the Czech Republic generally reflects the changes of zonally dispersed levels.
MONITORING OF KRYPTON-85
149
The mean value of 85Kr concentration, amounting to 1.15 Bq.m -3 (STP) at present is about seven orders of magnitude higher than its natural background estimated before the nuclear age. Despite this difference it does not pose any serious radiation risk to the population. The corresponding individual doses (4.7 × 10 -7 Sv.y -1 for the skin and 3.5 × 10 -9 Sv.y -I for the whole body) represent only 0.01% of the values given by standards of hygiene. However, the growth of the nuclear industry might change this situation in the future, unless steps are taken to collect and store 85Kr during fuel reprocessing. References Cimb~k, S. and Povinec, P.: 1985, Environment International 11, 69. Csongor, E.: 1982, Proc. 2nd lnt. Conf. on Low Level Counting, High Tatras 1980, Povinec, E (Ed.), VEDA, Bratislava, p. 357. Dvof'~k, Z. and Wilhelmov~, L.: 1984, Jad. energie 30, 23. Styro, B. and Butkus, D.: 1988, Geophysical Problems of Krypton-85 in the Atmosphere, Mosklas, Vilnius. Tertyshnik, E.G., Severin, A.A., Baranov, V.G. and Vakulovskij, S.M.: 1982, At. Energia 53, 114. Weiss, W., Sartorius, H. and Stockburger, H.: 1989, 'Isotopes of Noble Gases as Tracers in Environmental Studies', Proc. Consultants Meeting, IAEA, Vienna, 29 May-2 June, p. 29. Wilhelmov~, L., Torn~ek, ,M. and Dvorak, Z.: 1985, J. Radioanal. Nucl. Chem. Letters 95, 45. Wilhelmov~, L., Dvo~ftk, Z., Tom~ek, M. and Stukheil, K.: 1986, Appl. Rad. Isotopes, Part A 37, 247. Wilhelmov~, L., Torn~ek, M. and Stukheil, K.: 1990, J. Radioanal. Nucl. Chem. Letters 144, 125.
Abstract. The results of measurements of Krypton-85 (85Kr) concentrations in the ground-level air of Prague between 1983 and 1992 are presented and time-related changes analysed. The long-term trend in activity level of SSKr has been steadily increasing with a growth rate of 0.04 Bq.m -3 (STP) per year. Some peaks of 85Kr activity were observed due to the influence of undispersed radioactive plumes coming from distant sources. Short-term variations within a typical range of concentrations from 0.61 to 1.25 Bq.m -3 (STP) were found to be seasonally dependent, with the maximum occurring in spring.
1. Introduction
Pollution of the atmosphere by long-lived radionuclide 85Kr (T1/2 = 10.76 y) results from human activity in the exploitation of nuclear energy for peaceful and military purposes. Man-made 85Kr originates by fission in nuclear reactors and is among the waste products routinely released from the nuclear facilities into the atmosphere. Emissions from nuclear reactors during normal operation are negligible. The vast majority of actual 85Kr environmental supplies comes'from the gaseous effluents of spent fuel reprocessing plants and military plants producing plutonium (Weiss et al., 1989). As 85Kr is an inert gas, it is removed from the atmosphere mainly by radioactive decay. Due to high mobility this radionuclide became widely dispersed in the Earth's atmosphere at present and acts as a long-term low-level source of exposure concerning the world's population. Radiological assessment of global effects necessitates analytical methods of detection with sensitivities well below those required for radiation protection. The basic radiological information on 85Kr in the atmosphere is provided by measuring its activity in ground-level air. The available time-series of 85Kr data are related mostly to its source region in the middle latitudes of the Northern Hemisphere, where the maximum SSKr concentration is also observed (Weiss et al., 1989; Csongor, 1980; Wilhelmovti et al., 1990; Styro and Butkus, 1988; Cimbfik and Povinec, 1985; Tertyshnik et al., 1982). The regular monitoring of 85Kr in the atmosphere in the Institute of Radiation Dosimetry, Prague, began in 1983. The aim of this work was to establish the base-line level of this radionuclide for emergency situations and to investigate the potential influence of reprocessing plants operating in Europe. Moreover, the results obtained can help to understand the behaviour of 85Kr in the atmosphere and may appear to be of interest to the study of the mesoscale atmospheric transport and dispersion. Environmental Monitoring and Assessment 34: 145-149, 1995. @ 1995 Kluwer Academic Publishers. Printed in the Netherlands.
146
LUDMILA WILHELMOV,~ ET AL.
"~ 1-5 I,.-t./)
,
*-~.
~_ ~.~_
d"
rn
;0
0,5 __
I
,
198Z,
I
I
I
1986
I
1988
1
199'0
I
Years
1992
Fig. 1. Measurementsof 85Kr activity in the surface air of Prague during 1983-1992. Full line denotesthe linear fit of long-termtrend.
2. Experimental The 85Kr air activity present cannot be quantified by direct measurement. Therefore the samples of natural atmospheric krypton obtained by laboratory separation techniques are used for this purpose. The method consists of a sampling and purification set-up and counting procedure. The technique used in our Institute for the preparation of krypton samples is based on cryogenic selective adsorption of air components on charcoal beds combined with standard chromatographic analysis of isolated krypton volume. Since the atmospheric content of natural krypton is constant (1.14 cm 3.m -3 STP) the last step allows its recovery to be determined from the treated air in each experiment. The analysis of 85Kr activity in the isolated gaseous sample is performed using a counting chamber equipped with a CaF2 (Eu) scintillation detector. With conventional 85Kr activity measurements (efficiency 13%, measuring time 5 x 103 s, both in the case of background 0.3 cps and 85Kr sample activity 5-10 Bq) the relative error of less than (3%)0.95 is achieved. The apparatus and other details of experimental procedure were previously described in Wilhelmov~i et al. (1985), Wilhelmov~i et al. (1986). In routine experiments the air sample of about 10 m 3 is treated. Sampling is performed in the locality of the Institute in Prague (50°04 N; 14°26 E). The results obtained over the period 1983-1988 corresponded to the air samples collected regularly one day each month. Since 1989 they represented the 85Kr activity of
MONITORING OF KRYPTON-85
147
Fig. 2. Backwardtrajectoriesfor the outlyingobservation: 1 - 84/7/11, 2 - 85/2/13, 3 - 86/6/8, 487/11/11, 5 - 88/6/8. Locationsof nuclear fuel reprocessing plants: A - Sellafield,B - La Hague, C - Marcoule, D - KarlsnLhe,E - middleUral.
air samples averaged continuously at monthly intervals. The compatibility of these two kinds of data was checked by experiments carried our simultaneously during the year 1989. The statistical treatment of these measurements revealed the rather large scatter of spot (one-day) observation, but the yearly trend as well as its mean value proved to be identical. The reason for the change in methodology of 85Kr measurements was the radiological relevance of the acquired data.
3. Results and Discussion The concentrations of SSKr measured in the ground-level air of Prague are shown in Figure 1. The extreme values (exceeding 1.3 Bq.m -3 STP) registered as being exceptional in the spot-series of data were identified as outlying observations. Their occurrence may have been connected with the passage of undispersed air masses from distant sources. Supporting information can provide the trajectories of contaminated air constructed at 850 hPa within the interval of 96 h before the time of sampling, as shown in Figure 2. It follows from this picture that elevated 85Kr
148
LUDMILAWILI-IELMOV,~ET AL.
!'
I
I
I
I
"l
I
1.2 o_ 1.0 ti3
o
o%~ooo o ~ 8 S~oo 04, ¸
~o
m
0.2o
o88
0
f
1960
I
1
1
1970
I,,
1980
I
1
1990
YeQrs Fig. 3. Evolution of mean 85Kr activity levels in the surface air of middle latitudes of the Northern Hemisphere. o - estimate from literary data; • - this work.
levels in Prague can be attributed to the plumes originating in the source regions of western Europe as well as in Russia. Further variations of 85Kr air activity with an amplitude of about 12% of mean annual value have been recorded throughout the whole period of observation. As can be seen from Figure 1, this variability is seasonally dependent, with the maximum occurring mostly in the early spring. The reason for this 85Kr behaviour is not fully clear. It may be caused by discontinuity in operation of reprocessing plants (Weiss et al., 1989); however, the influence of natural factors cannot be excluded so far and require further investigation. As apparent from Figure 1, the long-term trend of 85Kr air activity has been steadily increasing over the period examined (19 83-1992). It can be approximated by linear form that provides the growth rate of about 0.04 Bq/m 3 (STP) per year. The mean annual values of 85Kr concentrations in the air around Prague are plotted in Figure 3 together with the estimate of those derived by statistical treatment (Dvo~Lk and Wilhelmov~i, 1984) from all available measurements in the middle latitudes of the Northern Hemisphere. The pattern in Figure 3 may be considered to be characteristic for the evolution of environmental 85Kr contamination in given geographical zones. Our observations concur with this tendency and indicate that the 85Kr air content over the Czech Republic generally reflects the changes of zonally dispersed levels.
MONITORING OF KRYPTON-85
149
The mean value of 85Kr concentration, amounting to 1.15 Bq.m -3 (STP) at present is about seven orders of magnitude higher than its natural background estimated before the nuclear age. Despite this difference it does not pose any serious radiation risk to the population. The corresponding individual doses (4.7 × 10 -7 Sv.y -1 for the skin and 3.5 × 10 -9 Sv.y -I for the whole body) represent only 0.01% of the values given by standards of hygiene. However, the growth of the nuclear industry might change this situation in the future, unless steps are taken to collect and store 85Kr during fuel reprocessing. References Cimb~k, S. and Povinec, P.: 1985, Environment International 11, 69. Csongor, E.: 1982, Proc. 2nd lnt. Conf. on Low Level Counting, High Tatras 1980, Povinec, E (Ed.), VEDA, Bratislava, p. 357. Dvof'~k, Z. and Wilhelmov~, L.: 1984, Jad. energie 30, 23. Styro, B. and Butkus, D.: 1988, Geophysical Problems of Krypton-85 in the Atmosphere, Mosklas, Vilnius. Tertyshnik, E.G., Severin, A.A., Baranov, V.G. and Vakulovskij, S.M.: 1982, At. Energia 53, 114. Weiss, W., Sartorius, H. and Stockburger, H.: 1989, 'Isotopes of Noble Gases as Tracers in Environmental Studies', Proc. Consultants Meeting, IAEA, Vienna, 29 May-2 June, p. 29. Wilhelmov~, L., Torn~ek, ,M. and Dvorak, Z.: 1985, J. Radioanal. Nucl. Chem. Letters 95, 45. Wilhelmov~, L., Dvo~ftk, Z., Tom~ek, M. and Stukheil, K.: 1986, Appl. Rad. Isotopes, Part A 37, 247. Wilhelmov~, L., Torn~ek, M. and Stukheil, K.: 1990, J. Radioanal. Nucl. Chem. Letters 144, 125.
