- Email: [email protected]
Activity of 239+240Pu and 238Pu in atmospheric deposits
Applied Radiation and Isotopes 55 (2001) 97–102
Activity of
239+240
Pu and
238
Pu in atmospheric deposits
M.P. Rubio Montero, A. Martı´ n Sa´nchez* Departamento de Fı´sica, Universidad de Extremadura, 06071 Badajoz, Spain Received 17 June 2000; received in revised form 21 October 2000; accepted 23 October 2000
Abstract Radioactive concentrations of 239+240Pu and 238Pu were determined in rainwater samples collected at Badajoz (in a semi-arid zone in the south-west of Spain) from 1992 to 1996. Total plutonium deposition (including dry and wet deposition) was investigated during dry and wet periods. Anomalously high depositions of 239+240Pu ((6.8 0.4) mBq/ m2) and 238Pu ((2.2 0.2) mBq/m2) were found in the second-half of 1995. The 238Pu/239+240Pu activity ratio (0.32 0.04) obtained for that sample was higher than in global fallout. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Plutonium; Man-made radioactivity; Alpha spectrometry
1. Introduction As a result of the activities of the nuclear industry, there have been discharges of actinides (mainly plutonium) into the environment, either premeditated (as in nuclear weapons testing) or accidental (as in the nuclear accident of Chernobyl in 1986). Atmospheric monitoring is a useful tool to study the decade-scale dispersion process of the radioactive plumes from those events. Thus, in studying the geochemical behaviour of transuranic elements in the Mediterranean Sea it became necessary to estimate the deposition of 238Pu and 239+240 Pu onto the sea surface, where rain carries most of the total fallout delivery (Thein et al., 1980). Later, widespread deposition of plutonium and other products was observed over large areas of the former USSR and Europe as a consequence of the accident in 1986 in Chernobyl. Information about the concentration of plutonium in atmospheric samples in Europe was reported by Holm et al. (1992). After the Chernobyl accident and in the last decade, the redistribution of plutonium has been explained by resuspension of airborne particles (Rosner et al., 1997). *Corresponding author. Tel.: +34-924-289-526; +34-924-289-651. E-mail address: [email protected] (A. Martı´ n Sa´nchez).
fax:
Our lab has been monitoring aerosols and rainwater (including dry deposition) by activity measurements since 1992 in a general campaign involving about 25 sampling sites in Spain. We here present the results of determining alpha-activity concentrations of some plutonium isotopes (238Pu, 239+240Pu) in the atmospheric depositions collected at Badajoz (in a semi-arid zone located in the south-west of Spain) from 1992 to 1996. The period from 1992 to the first-half of 1995 corresponded to a prolonged drought, while the secondhalf of 1995 and all 1996 corresponded to a wet period. Significant differences were found for the concentrations of plutonium in these two periods.
2. Analytical procedure 2.1. Sampling The sampling period corresponded to natural months from April 1992 to December 1996. Rainwater samples (including dry deposition, as explained below) were collected monthly using a 1 m2 stainless-steel collector (IAEA, 1989) placed in the sports area at the University of Extremadura, Badajoz, Spain (388510 N, 78W). The tray collector had a hole at the bottom to feed the water through a silicone tube to a polyethylene container (25 l
0969-8043/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 4 3 ( 0 0 ) 0 0 3 6 5 - 1
M.P. Rubio Montero, A. Martı´n Sa´nchez / Applied Radiation and Isotopes 55 (2001) 97–102
98
Residues were stored in new containers. Independent chemical pretreatments of both liquid and solid phases were made to achieve the maximum recovery of the plutonium content for each sample. After adding known amounts of 242Pu tracer and Fe carrier (2.5–3 mg of Fe3+ per litre) to the aqueous phase, the sample was stirred and homogenized for 2 or 3 days. Iron and actinides were coprecipitated as hydroxides by adding ammonium hydroxide until pH 9 was reached. The supernatant was discarded and the iron extracted from the precipitate with diisopropylether (DIE) in 8 N HCl using a conventional decantation funnel. Residues on filters were weighed and then digested with H2O2 in 8 N HNO3 in a covered beaker by heating on a hot plate, after the addition of 242Pu tracer. The residue was centrifuged, and fresh acid was added. The process was repeated three times. The total volume of acid used was evaporated down to a few millilitres and dissolved with deionized water. Iron and actinides were coprecipitated as for the aqueous phase, and iron was eliminated by extraction with DIE. Finally, the two resulting aliquots in 8 N HCl (from the aqueous and filtered phases) were mixed and evaporated to dryness.
volume). Precipitation was measured each month using a pluviometer next door the collector. The annual precipitation amount was less than 400 mm during the years 1992–1994. At the end of each sampling period (one month) or when the container was full, the collected water was acidified without filtration with 3 ml of 60% HNO3 per litre of sample and stored in the containers for subsequent chemical treatment. Also at the end of each sampling period the volume of the collected rainwater was determined and the collector was washed with less than 1 l of fresh deionized water to collect dry deposition, and the washing water was added to each sample. When there had been no rainfall, dry deposition was collected by washing with 3 l of fresh deionized water. The results therefore correspond to total deposition including wet and dry depositions, although the activity concentration will be quoted without taking into consideration the volume corresponding to the washing water. Composed samples for different periods (each including waters from several months) were obtained mixing and homogenizing the collected monthly samples, due to the expected very low concentration of plutonium in rainwater. An aliquot of about 100 l from each composed sample was taken for analysis, as quoted in Table 1, together with other quantities of interest in this work and explained in the following sections.
2.3. Anion exchange purification The residue from the pretreatment step was dissolved in the minimum volume needed (less than 10 ml) of 1N HCl, and a few milligrams of hydroxylamine hydrochloride (NH2OHHCl) were added. The mixture was evaporated to dryness in a covered beaker by heating on a hot plate. The resulting residue was dissolved in
2.2. Pretreatment of the samples Acidified samples were filtered through a Millipore filter with a pore size of 0.45 mm to separate suspended matter that had fallen together with the aqueous phase.
Table 1 Set of composed samples considered in this work. The volume, the suspended matter mass (residue), and precipitation in the period analyzed are indicated for each sample. Results for the activity concentrations (mBq/L) and depositions (mBq/m2) of 238Pu and 239+240 Pu are given in the four last columns. Uncertainties of the mean values are 1s for all cases Year
Months
1992
Apr/Dec (92)
1993
[239+240Pu] (mBq/l)
[238Pu] (mBq/l)
[239+240Pu] (mBq/m2)
6.2 0.7a
5 1.4
2.0 0.2
5 0.4
10.5 1.6 12.7 1.3
5 3.6 5 6.4
2.2 0.3 2.3 0.3
5 0.8 5 1.2
225 153
5.6 0.7 14.0 1.1
5 1.6 5 2.2
1.3 0.2 2.1 0.2
5 0.4 5 0.3
34.6 36.9
91 367
15.1 0.8 18.6 1.1
5 1.2 5.9 0.7
1.37 0.07 6.8 0.4
5 0.1 2.2 0.2
8.9 40.2 6.6
283 259 222
5 1.9 4.5 0.8 4.2 0.3
5 2.0 5 1.3 5 1.6
5 0.5 1.2 0.2 0.92 0.07
5 0.6 5 0.3 5 0.4
Residue (mg/l)
Precipitation (mm)
94
50.5
321
Jan/Jun (93A) July/Dec (93B)
90 66
19.7 21.8
212 185
1994
Jan/Jun (94A) July/Dec (94B)
71 57
15.5 45.8
1995
Jan/Jun (95A) July/Dec (95B)
68 90
1996
January (96A) Feb/Oct (96B) Nov/Dec (96C)
92 127 140
a
Volume analyzed (l)
Activity concentration determined only for the aqueous phase.
[238Pu] (mBq/m2)
M.P. Rubio Montero, A. Martı´n Sa´nchez / Applied Radiation and Isotopes 55 (2001) 97–102
5–10 ml 8 M HNO3, and a few milligrams of NaNO2 were added to the solution. Once the salt had dissolved, the sample was passed through an anion exchange column containing the resin Dowex 1 8, where Pu(IV) was retained. Adsorbed plutonium was purified from interfering elements by washing the column with 200– 250 ml 8 N HNO3, and 200 ml 10 N HCl, and then eluted with 100–150 ml of a mixture of 9 N HCl+0.1N HI (Yamato, 1982; Gasco´n et al., 1994). The solution containing the plutonium was electrodeposited onto a stainless–steel disc planchet from a sulphuric acid electrolyte at pH 2.1–2.4 (Hallstadius, 1984) for 50 min (Vera Tome´ and Martı´ n Sa´nchez, 1991; Vera Tome´ et al., 1994). Chemical recoveries were 60–91% for volumes of sample that ranged between 70 and 150 l. 2.4. Counting details Alpha-particle spectra were obtained by measuring the plutonium planchets with passivated ion-implanted silicon detectors of 450 mm2 active area. Sources were situated as near to the detector as possible. The counting efficiency was 33% measured with a 241Am standard sample of 164.5 Bq in the same geometrical arrangement as for the unknown samples. The minimum detectable activity was lower than 0.2 mBq for a typical collecting time of 1.2 106 s (about 14 d).
99
(except for sample 96A, which was below the limit of detection as will be described in the following section).
3. Results and discussion The activity concentrations of 238Pu and 239+240Pu obtained are summarized in Table 1. The values of the concentration of 239+240Pu and the deposition in the studied period are plotted in Fig. 1, and compared with the rainfall. Only in the second semester of 1995 was the 238 Pu concentration significantly higher than the minimum detectable activity. However, a systematic increase of activity was observed in the second semesters of 1993– 1995. In order to better understand the situation, the average monthly precipitations for the period 1993–1996 are plotted in Fig. 2, showing the increase of precipitation from September to December. The scarcity of rain in spring at Badajoz could well be the cause of the lag in
2.5. Quality control in chemical treatment and measure Since the total activities of the samples were very low, extreme precaution was required to prevent crosscontamination between the samples during the chemical treatment and measure. New plastic and glass containers, and fresh ion exchange resins were used for each sample. In order to reach high recoveries in the whole chemical treatment, large volume standard samples of deionized water traced with low activities of 242Pu were prepared before proceeding with rainwater samples. These were also used to check the chemical method. All the reagents were of analytical grade. No other different samples were prepared in the lab at the same time and the site was thoroughly cleaned after each preparation. New detectors were used in the measurements of the samples. Detector backgrounds were determined before and after measuring each source. Counting periods were prolonged to achieve a net area clearly greater than that corresponding to the background counts. To detect any possible systematic misfunctioning of the measuring device, the overall time of counting was divided into smaller intervals and each spectrum was analyzed as a check of the final results. Background spectra were also analyzed in order to detect any contamination in the detector. All the activities reported in this work are more than 2.5–3 standard deviations above the minimum detectable activity obtained for each measurement
Fig. 1. Activity concentration of 239+240Pu and deposition found in atmospheric samples at Badajoz compared with precipitation in the period from 1992 to 1996. Quoted error bars are 1 s.
100
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Fig. 2. Average monthly precipitation expressed as a percentage of the mean annual precipitation in the period 1993–1996 (497 l/m2).
the maximum plutonium deposition compared with wetter zones, such as Monaco (438 450 N, 78 250 E). Higher activity concentrations were found in the drought years (1992–1995) compared with those for the wet year (1996). Depositions measured are plotted in Fig. 3 versus the rainfall (Fig. 3a), and versus the concentration of suspended matter in rainwater (Fig. 3b). Based on these factors, three stages can be distinguished: 1992 } first-half of 1995, second half of 1995, and the year 1996. During the drought stage (1992 } first-half of 1995), although a dilution of the activity concentration was observed when the volume of precipitation increased, the activity deposition was approximately constant, ranging between 1.3 0.2 and 2.2 0.3 mBq/m2. This effect of constant deposition could be caused by the contributions of the dry and wet deposition jointly, which would be in agreement with the results giving equal contributions of wet and dry fallout in Monaco found in rainwater samples by Ballestra et al. (1991). In the second stage (second-half of 1995) an increase of activity was observed (sample 95B). This is the only case in which the concentration of 238Pu was higher than the minimum detectable activity with our experimental procedure. Deposition of 239+240Pu in this period ranged between three and five times higher than in previous samples ((6.8 0.4) mBq/m2). In the third stage, the wet period (1996), three samples were analyzed, corresponding to January, February to October, and November to December. The activity
concentrations and the depositions of 239+240Pu measured during this year were at lower levels than in the preceding samples. No significant differences between the concentrations of 239+240Pu were found between the three samples. For the sake of completeness we shall compare some values taken from the literature with those presented in this work. Total deposition of 239+240Pu by rainwater for the 5-year period studied (1992–1996) in Badajoz was 21 2 mBq/m2. At Monaco, for the 12-month period (April 1978–April 1979) after the Chinese tests, this value was estimated to be 296 4 mBq/m2 (Ballestra et al., 1988); the total deposition of 239+240Pu from April 26 to the end of May 1986 (after the Chernobyl accident) was estimated to be 10 1 mBq/m2 (of which 20% was in the form of dry deposition); the total deposition of 239+240 Pu (including dry deposition) was estimated to be 19 2 mBq/m2 for the period 26 April–31 December 1986, of which 8.825 mBq/m2 corresponded to the rainfall deposition (Ballestra et al., 1991). High values of the activity concentration and deposition for 239+240Pu, and the presence of 238Pu, could be signs of an anomalous contribution of activity in the second-half of 1995. The 238Pu/239+240Pu activity ratio may characterize the source of the contamination. Some results can be found for this activity ratio in the literature. In rainwater samples collected at Monaco (1978–1979), Thein et al. (1980) observed ratios that ranged between 0.012 and 0.033, the annual average ratio being 0.022 0.001.This value is similar to the
M.P. Rubio Montero, A. Martı´n Sa´nchez / Applied Radiation and Isotopes 55 (2001) 97–102
101
Fig. 3. (a) Variation of deposition of 239+240Pu with rainfall. (b) Variation of deposition of 239+240Pu with the mean concentration of suspended matter (mg/L of residue) for each sample (sample 92 is not plotted in this case because the plutonium measured in this sample corresponded only to the aqueous phase). Dashed lines are the means of the values for each variable, and dotted lines are the standard deviation from the mean (1s). Sample 95B is not included in either graph for the reasons explained in the text.
typical value found in nuclear weapons test debris (around 0.03). The values of this activity ratio found in releases from nuclear fuel reprocessing plants, and weapons grade plutonium are about 0.25, and 0.014, respectively (Holm et al., 1992). This ratio was incidentally higher due to the burn-up of the power source of the SNAP satellite in the atmosphere that occurred in 1964. By 1980, more than 95% of this contamination had already been deposited on the Earth’s surface (Thein et al.,1980). However, the Chernobyl source term was in general characterized by the relatively high activity ratio of 238Pu/239+240Pu and the presence of isotopes of curium, due to the high amount of these and other isotopes in the nuclear fuel that burned. Thus, the values found in atmospheric samples during the Chernobyl period were 0.57 0.07 measured in south Sweden, 0.58 0.05 in Denmark, 0.47 0.03 at Monaco, and 0.64 0.09 in south Finland (Holm et al., 1992). These values would reflect an increased activity ratio due to the decay of 242Cm (T1/2=162 d) present in the Chernobyl fallout. In our work, the 238Pu/239+240Pu ratio in the second half of 1995 (sample 95B) was 0.32 0.04. That is one order of magnitude higher than the characteristic ratio for
nuclear weapons tests. It is, therefore, possible to conclude that the plutonium contamination found in sample 95B is not only due to global fallout from nuclear testing. Moreover, in this same type of study, other anomalously high activity concentrations of 238Pu in atmospheric samples (aerosol particulates) have been found, as in the surface air of Prague in 1993–1995 (Ho¨lgye and Filgas 1996), without there as yet being any clear explanation.
4. Conclusions An increase of the activity concentration of 239+240Pu in atmospheric deposits was observed systematically during the second part of each year in the period 1992– 1996 at Badajoz. Lower activity concentrations were found in the wet year (1996) than in the relatively dry years of the study (1992–1995). However, the total deposition seems to be approximately constant in both cases. The anomalously high values of the activity concentrations and deposition of plutonium isotopes (239+240Pu and 238Pu) found in the second-half of 1995 seem to indicate that there had been some extra
102
M.P. Rubio Montero, A. Martı´n Sa´nchez / Applied Radiation and Isotopes 55 (2001) 97–102
contribution of plutonium. The high value found for the ratio 238Pu/239+240Pu is evidence that this contribution cannot be exclusively due to weapon fallout debris.
Acknowledgements Thanks are due to Drs Gasco´n Murillo and Crespo Va´zquez from CIEMAT for their advice about the chemical procedure used for the rainwater samples, and to Ms Blanco Rodrı´ guez for her help in the treatment of the rainwater samples in the laboratory. Financial support from the Junta of Extremadura (Consejerı´ a de Educacio´n y Juventud, Project no. IPR98C021) and DGICYT (Project no. PB95-1139A) is acknowledged.
References Ballestra, S., Garci, G., Holm, E., Gastaud, J., Fukai, R., 1988. Plutonium-241 as an age indicator for radioactive atmospheric fallout. Lanthanide and Actinide Res. 2, 201–209. Ballestra, S., Gastaud, J., Lopez, J.J., 1991. Radiochemical procedures used at IAEA-ILMR Monaco for measuring artificial radionuclides resulting from the Chernobyl accident. In: Garcı´ a-Leo´n, M., Madurga, G. (Eds.), Low-level Measurements of Man-made Radionuclides in the Environment. World Scientific, Singapore. Gasco´n, J.L., Acen˜a, M.L., Sua´rez, J.A., Rodrı´ guez, M., 1994. Radiochemical methods for the determination of plutonium,
americium and curium in typical waste streams. J. Alloys Compounds 213/214, 557–559. Hallstadius, L., 1984. A method for electrodeposition of actinides. Nucl. Instrum. Meth. Phys. Res. 223, 266–267. Ho¨lgye, Z., Filgas, R., 1996. Concentrations of 239,240Pu and 238 Pu in the surface air of Prague in 1993, 1994 and 1995. J. Radioanal. Nucl. Chem. Letters. 214, 303-308. Holm, E., Rioseco, J., Petterson, H., 1992. Fallout of transuranium elements following the Chernobyl accident. J. Radioanal. Nucl. Chem. Articles 156, 183–200. IAEA, 1989. Measurement of radionuclides in food and the environment. IAEA Technical Reports Series 295, IAEA, Vienna. Rosner, G., Ho¨tzl, H., Winkler, R., 1997. Long-term behaviour of plutonium in air and deposition and the role of resuspension in a semi-rural environment in Germany. Sci. Total Environ. 196, 255–261. Thein, M., Ballestra, S., Yamato, A., Fukai, R., 1980. Delivery of transuranic elements by rain to the Mediterranean Sea. Geochim. et Cosmochim. Acta 44, 1091–1097. VeraTome´, F., Martı´ n Sa´nchez, A., 1991. Optimizing the parameters affecting the yield and energy resolution in the electrodeposition of uranium. Appl. Radiat. Isot. 42, 135–140. VeraTome´, F., JuradoVargas, M., Martı´ n Sa´nchez, A., 1994. SIMPLEX method for optimization of experiments. Application to electrodeposition in alpha spectrometry. Nucl. Instr. Meth. Phys. Res. A 348, 183–187. Yamato, A., 1982. An anion exchange method for the determination of 241Am and plutonium in environmental and biological samples. J. Radioanal. Nucl. Chem. 75, 265–273.
Activity of
239+240
Pu and
238
Pu in atmospheric deposits
M.P. Rubio Montero, A. Martı´ n Sa´nchez* Departamento de Fı´sica, Universidad de Extremadura, 06071 Badajoz, Spain Received 17 June 2000; received in revised form 21 October 2000; accepted 23 October 2000
Abstract Radioactive concentrations of 239+240Pu and 238Pu were determined in rainwater samples collected at Badajoz (in a semi-arid zone in the south-west of Spain) from 1992 to 1996. Total plutonium deposition (including dry and wet deposition) was investigated during dry and wet periods. Anomalously high depositions of 239+240Pu ((6.8 0.4) mBq/ m2) and 238Pu ((2.2 0.2) mBq/m2) were found in the second-half of 1995. The 238Pu/239+240Pu activity ratio (0.32 0.04) obtained for that sample was higher than in global fallout. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Plutonium; Man-made radioactivity; Alpha spectrometry
1. Introduction As a result of the activities of the nuclear industry, there have been discharges of actinides (mainly plutonium) into the environment, either premeditated (as in nuclear weapons testing) or accidental (as in the nuclear accident of Chernobyl in 1986). Atmospheric monitoring is a useful tool to study the decade-scale dispersion process of the radioactive plumes from those events. Thus, in studying the geochemical behaviour of transuranic elements in the Mediterranean Sea it became necessary to estimate the deposition of 238Pu and 239+240 Pu onto the sea surface, where rain carries most of the total fallout delivery (Thein et al., 1980). Later, widespread deposition of plutonium and other products was observed over large areas of the former USSR and Europe as a consequence of the accident in 1986 in Chernobyl. Information about the concentration of plutonium in atmospheric samples in Europe was reported by Holm et al. (1992). After the Chernobyl accident and in the last decade, the redistribution of plutonium has been explained by resuspension of airborne particles (Rosner et al., 1997). *Corresponding author. Tel.: +34-924-289-526; +34-924-289-651. E-mail address: [email protected] (A. Martı´ n Sa´nchez).
fax:
Our lab has been monitoring aerosols and rainwater (including dry deposition) by activity measurements since 1992 in a general campaign involving about 25 sampling sites in Spain. We here present the results of determining alpha-activity concentrations of some plutonium isotopes (238Pu, 239+240Pu) in the atmospheric depositions collected at Badajoz (in a semi-arid zone located in the south-west of Spain) from 1992 to 1996. The period from 1992 to the first-half of 1995 corresponded to a prolonged drought, while the secondhalf of 1995 and all 1996 corresponded to a wet period. Significant differences were found for the concentrations of plutonium in these two periods.
2. Analytical procedure 2.1. Sampling The sampling period corresponded to natural months from April 1992 to December 1996. Rainwater samples (including dry deposition, as explained below) were collected monthly using a 1 m2 stainless-steel collector (IAEA, 1989) placed in the sports area at the University of Extremadura, Badajoz, Spain (388510 N, 78W). The tray collector had a hole at the bottom to feed the water through a silicone tube to a polyethylene container (25 l
0969-8043/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 4 3 ( 0 0 ) 0 0 3 6 5 - 1
M.P. Rubio Montero, A. Martı´n Sa´nchez / Applied Radiation and Isotopes 55 (2001) 97–102
98
Residues were stored in new containers. Independent chemical pretreatments of both liquid and solid phases were made to achieve the maximum recovery of the plutonium content for each sample. After adding known amounts of 242Pu tracer and Fe carrier (2.5–3 mg of Fe3+ per litre) to the aqueous phase, the sample was stirred and homogenized for 2 or 3 days. Iron and actinides were coprecipitated as hydroxides by adding ammonium hydroxide until pH 9 was reached. The supernatant was discarded and the iron extracted from the precipitate with diisopropylether (DIE) in 8 N HCl using a conventional decantation funnel. Residues on filters were weighed and then digested with H2O2 in 8 N HNO3 in a covered beaker by heating on a hot plate, after the addition of 242Pu tracer. The residue was centrifuged, and fresh acid was added. The process was repeated three times. The total volume of acid used was evaporated down to a few millilitres and dissolved with deionized water. Iron and actinides were coprecipitated as for the aqueous phase, and iron was eliminated by extraction with DIE. Finally, the two resulting aliquots in 8 N HCl (from the aqueous and filtered phases) were mixed and evaporated to dryness.
volume). Precipitation was measured each month using a pluviometer next door the collector. The annual precipitation amount was less than 400 mm during the years 1992–1994. At the end of each sampling period (one month) or when the container was full, the collected water was acidified without filtration with 3 ml of 60% HNO3 per litre of sample and stored in the containers for subsequent chemical treatment. Also at the end of each sampling period the volume of the collected rainwater was determined and the collector was washed with less than 1 l of fresh deionized water to collect dry deposition, and the washing water was added to each sample. When there had been no rainfall, dry deposition was collected by washing with 3 l of fresh deionized water. The results therefore correspond to total deposition including wet and dry depositions, although the activity concentration will be quoted without taking into consideration the volume corresponding to the washing water. Composed samples for different periods (each including waters from several months) were obtained mixing and homogenizing the collected monthly samples, due to the expected very low concentration of plutonium in rainwater. An aliquot of about 100 l from each composed sample was taken for analysis, as quoted in Table 1, together with other quantities of interest in this work and explained in the following sections.
2.3. Anion exchange purification The residue from the pretreatment step was dissolved in the minimum volume needed (less than 10 ml) of 1N HCl, and a few milligrams of hydroxylamine hydrochloride (NH2OHHCl) were added. The mixture was evaporated to dryness in a covered beaker by heating on a hot plate. The resulting residue was dissolved in
2.2. Pretreatment of the samples Acidified samples were filtered through a Millipore filter with a pore size of 0.45 mm to separate suspended matter that had fallen together with the aqueous phase.
Table 1 Set of composed samples considered in this work. The volume, the suspended matter mass (residue), and precipitation in the period analyzed are indicated for each sample. Results for the activity concentrations (mBq/L) and depositions (mBq/m2) of 238Pu and 239+240 Pu are given in the four last columns. Uncertainties of the mean values are 1s for all cases Year
Months
1992
Apr/Dec (92)
1993
[239+240Pu] (mBq/l)
[238Pu] (mBq/l)
[239+240Pu] (mBq/m2)
6.2 0.7a
5 1.4
2.0 0.2
5 0.4
10.5 1.6 12.7 1.3
5 3.6 5 6.4
2.2 0.3 2.3 0.3
5 0.8 5 1.2
225 153
5.6 0.7 14.0 1.1
5 1.6 5 2.2
1.3 0.2 2.1 0.2
5 0.4 5 0.3
34.6 36.9
91 367
15.1 0.8 18.6 1.1
5 1.2 5.9 0.7
1.37 0.07 6.8 0.4
5 0.1 2.2 0.2
8.9 40.2 6.6
283 259 222
5 1.9 4.5 0.8 4.2 0.3
5 2.0 5 1.3 5 1.6
5 0.5 1.2 0.2 0.92 0.07
5 0.6 5 0.3 5 0.4
Residue (mg/l)
Precipitation (mm)
94
50.5
321
Jan/Jun (93A) July/Dec (93B)
90 66
19.7 21.8
212 185
1994
Jan/Jun (94A) July/Dec (94B)
71 57
15.5 45.8
1995
Jan/Jun (95A) July/Dec (95B)
68 90
1996
January (96A) Feb/Oct (96B) Nov/Dec (96C)
92 127 140
a
Volume analyzed (l)
Activity concentration determined only for the aqueous phase.
[238Pu] (mBq/m2)
M.P. Rubio Montero, A. Martı´n Sa´nchez / Applied Radiation and Isotopes 55 (2001) 97–102
5–10 ml 8 M HNO3, and a few milligrams of NaNO2 were added to the solution. Once the salt had dissolved, the sample was passed through an anion exchange column containing the resin Dowex 1 8, where Pu(IV) was retained. Adsorbed plutonium was purified from interfering elements by washing the column with 200– 250 ml 8 N HNO3, and 200 ml 10 N HCl, and then eluted with 100–150 ml of a mixture of 9 N HCl+0.1N HI (Yamato, 1982; Gasco´n et al., 1994). The solution containing the plutonium was electrodeposited onto a stainless–steel disc planchet from a sulphuric acid electrolyte at pH 2.1–2.4 (Hallstadius, 1984) for 50 min (Vera Tome´ and Martı´ n Sa´nchez, 1991; Vera Tome´ et al., 1994). Chemical recoveries were 60–91% for volumes of sample that ranged between 70 and 150 l. 2.4. Counting details Alpha-particle spectra were obtained by measuring the plutonium planchets with passivated ion-implanted silicon detectors of 450 mm2 active area. Sources were situated as near to the detector as possible. The counting efficiency was 33% measured with a 241Am standard sample of 164.5 Bq in the same geometrical arrangement as for the unknown samples. The minimum detectable activity was lower than 0.2 mBq for a typical collecting time of 1.2 106 s (about 14 d).
99
(except for sample 96A, which was below the limit of detection as will be described in the following section).
3. Results and discussion The activity concentrations of 238Pu and 239+240Pu obtained are summarized in Table 1. The values of the concentration of 239+240Pu and the deposition in the studied period are plotted in Fig. 1, and compared with the rainfall. Only in the second semester of 1995 was the 238 Pu concentration significantly higher than the minimum detectable activity. However, a systematic increase of activity was observed in the second semesters of 1993– 1995. In order to better understand the situation, the average monthly precipitations for the period 1993–1996 are plotted in Fig. 2, showing the increase of precipitation from September to December. The scarcity of rain in spring at Badajoz could well be the cause of the lag in
2.5. Quality control in chemical treatment and measure Since the total activities of the samples were very low, extreme precaution was required to prevent crosscontamination between the samples during the chemical treatment and measure. New plastic and glass containers, and fresh ion exchange resins were used for each sample. In order to reach high recoveries in the whole chemical treatment, large volume standard samples of deionized water traced with low activities of 242Pu were prepared before proceeding with rainwater samples. These were also used to check the chemical method. All the reagents were of analytical grade. No other different samples were prepared in the lab at the same time and the site was thoroughly cleaned after each preparation. New detectors were used in the measurements of the samples. Detector backgrounds were determined before and after measuring each source. Counting periods were prolonged to achieve a net area clearly greater than that corresponding to the background counts. To detect any possible systematic misfunctioning of the measuring device, the overall time of counting was divided into smaller intervals and each spectrum was analyzed as a check of the final results. Background spectra were also analyzed in order to detect any contamination in the detector. All the activities reported in this work are more than 2.5–3 standard deviations above the minimum detectable activity obtained for each measurement
Fig. 1. Activity concentration of 239+240Pu and deposition found in atmospheric samples at Badajoz compared with precipitation in the period from 1992 to 1996. Quoted error bars are 1 s.
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Fig. 2. Average monthly precipitation expressed as a percentage of the mean annual precipitation in the period 1993–1996 (497 l/m2).
the maximum plutonium deposition compared with wetter zones, such as Monaco (438 450 N, 78 250 E). Higher activity concentrations were found in the drought years (1992–1995) compared with those for the wet year (1996). Depositions measured are plotted in Fig. 3 versus the rainfall (Fig. 3a), and versus the concentration of suspended matter in rainwater (Fig. 3b). Based on these factors, three stages can be distinguished: 1992 } first-half of 1995, second half of 1995, and the year 1996. During the drought stage (1992 } first-half of 1995), although a dilution of the activity concentration was observed when the volume of precipitation increased, the activity deposition was approximately constant, ranging between 1.3 0.2 and 2.2 0.3 mBq/m2. This effect of constant deposition could be caused by the contributions of the dry and wet deposition jointly, which would be in agreement with the results giving equal contributions of wet and dry fallout in Monaco found in rainwater samples by Ballestra et al. (1991). In the second stage (second-half of 1995) an increase of activity was observed (sample 95B). This is the only case in which the concentration of 238Pu was higher than the minimum detectable activity with our experimental procedure. Deposition of 239+240Pu in this period ranged between three and five times higher than in previous samples ((6.8 0.4) mBq/m2). In the third stage, the wet period (1996), three samples were analyzed, corresponding to January, February to October, and November to December. The activity
concentrations and the depositions of 239+240Pu measured during this year were at lower levels than in the preceding samples. No significant differences between the concentrations of 239+240Pu were found between the three samples. For the sake of completeness we shall compare some values taken from the literature with those presented in this work. Total deposition of 239+240Pu by rainwater for the 5-year period studied (1992–1996) in Badajoz was 21 2 mBq/m2. At Monaco, for the 12-month period (April 1978–April 1979) after the Chinese tests, this value was estimated to be 296 4 mBq/m2 (Ballestra et al., 1988); the total deposition of 239+240Pu from April 26 to the end of May 1986 (after the Chernobyl accident) was estimated to be 10 1 mBq/m2 (of which 20% was in the form of dry deposition); the total deposition of 239+240 Pu (including dry deposition) was estimated to be 19 2 mBq/m2 for the period 26 April–31 December 1986, of which 8.825 mBq/m2 corresponded to the rainfall deposition (Ballestra et al., 1991). High values of the activity concentration and deposition for 239+240Pu, and the presence of 238Pu, could be signs of an anomalous contribution of activity in the second-half of 1995. The 238Pu/239+240Pu activity ratio may characterize the source of the contamination. Some results can be found for this activity ratio in the literature. In rainwater samples collected at Monaco (1978–1979), Thein et al. (1980) observed ratios that ranged between 0.012 and 0.033, the annual average ratio being 0.022 0.001.This value is similar to the
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Fig. 3. (a) Variation of deposition of 239+240Pu with rainfall. (b) Variation of deposition of 239+240Pu with the mean concentration of suspended matter (mg/L of residue) for each sample (sample 92 is not plotted in this case because the plutonium measured in this sample corresponded only to the aqueous phase). Dashed lines are the means of the values for each variable, and dotted lines are the standard deviation from the mean (1s). Sample 95B is not included in either graph for the reasons explained in the text.
typical value found in nuclear weapons test debris (around 0.03). The values of this activity ratio found in releases from nuclear fuel reprocessing plants, and weapons grade plutonium are about 0.25, and 0.014, respectively (Holm et al., 1992). This ratio was incidentally higher due to the burn-up of the power source of the SNAP satellite in the atmosphere that occurred in 1964. By 1980, more than 95% of this contamination had already been deposited on the Earth’s surface (Thein et al.,1980). However, the Chernobyl source term was in general characterized by the relatively high activity ratio of 238Pu/239+240Pu and the presence of isotopes of curium, due to the high amount of these and other isotopes in the nuclear fuel that burned. Thus, the values found in atmospheric samples during the Chernobyl period were 0.57 0.07 measured in south Sweden, 0.58 0.05 in Denmark, 0.47 0.03 at Monaco, and 0.64 0.09 in south Finland (Holm et al., 1992). These values would reflect an increased activity ratio due to the decay of 242Cm (T1/2=162 d) present in the Chernobyl fallout. In our work, the 238Pu/239+240Pu ratio in the second half of 1995 (sample 95B) was 0.32 0.04. That is one order of magnitude higher than the characteristic ratio for
nuclear weapons tests. It is, therefore, possible to conclude that the plutonium contamination found in sample 95B is not only due to global fallout from nuclear testing. Moreover, in this same type of study, other anomalously high activity concentrations of 238Pu in atmospheric samples (aerosol particulates) have been found, as in the surface air of Prague in 1993–1995 (Ho¨lgye and Filgas 1996), without there as yet being any clear explanation.
4. Conclusions An increase of the activity concentration of 239+240Pu in atmospheric deposits was observed systematically during the second part of each year in the period 1992– 1996 at Badajoz. Lower activity concentrations were found in the wet year (1996) than in the relatively dry years of the study (1992–1995). However, the total deposition seems to be approximately constant in both cases. The anomalously high values of the activity concentrations and deposition of plutonium isotopes (239+240Pu and 238Pu) found in the second-half of 1995 seem to indicate that there had been some extra
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contribution of plutonium. The high value found for the ratio 238Pu/239+240Pu is evidence that this contribution cannot be exclusively due to weapon fallout debris.
Acknowledgements Thanks are due to Drs Gasco´n Murillo and Crespo Va´zquez from CIEMAT for their advice about the chemical procedure used for the rainwater samples, and to Ms Blanco Rodrı´ guez for her help in the treatment of the rainwater samples in the laboratory. Financial support from the Junta of Extremadura (Consejerı´ a de Educacio´n y Juventud, Project no. IPR98C021) and DGICYT (Project no. PB95-1139A) is acknowledged.
References Ballestra, S., Garci, G., Holm, E., Gastaud, J., Fukai, R., 1988. Plutonium-241 as an age indicator for radioactive atmospheric fallout. Lanthanide and Actinide Res. 2, 201–209. Ballestra, S., Gastaud, J., Lopez, J.J., 1991. Radiochemical procedures used at IAEA-ILMR Monaco for measuring artificial radionuclides resulting from the Chernobyl accident. In: Garcı´ a-Leo´n, M., Madurga, G. (Eds.), Low-level Measurements of Man-made Radionuclides in the Environment. World Scientific, Singapore. Gasco´n, J.L., Acen˜a, M.L., Sua´rez, J.A., Rodrı´ guez, M., 1994. Radiochemical methods for the determination of plutonium,
americium and curium in typical waste streams. J. Alloys Compounds 213/214, 557–559. Hallstadius, L., 1984. A method for electrodeposition of actinides. Nucl. Instrum. Meth. Phys. Res. 223, 266–267. Ho¨lgye, Z., Filgas, R., 1996. Concentrations of 239,240Pu and 238 Pu in the surface air of Prague in 1993, 1994 and 1995. J. Radioanal. Nucl. Chem. Letters. 214, 303-308. Holm, E., Rioseco, J., Petterson, H., 1992. Fallout of transuranium elements following the Chernobyl accident. J. Radioanal. Nucl. Chem. Articles 156, 183–200. IAEA, 1989. Measurement of radionuclides in food and the environment. IAEA Technical Reports Series 295, IAEA, Vienna. Rosner, G., Ho¨tzl, H., Winkler, R., 1997. Long-term behaviour of plutonium in air and deposition and the role of resuspension in a semi-rural environment in Germany. Sci. Total Environ. 196, 255–261. Thein, M., Ballestra, S., Yamato, A., Fukai, R., 1980. Delivery of transuranic elements by rain to the Mediterranean Sea. Geochim. et Cosmochim. Acta 44, 1091–1097. VeraTome´, F., Martı´ n Sa´nchez, A., 1991. Optimizing the parameters affecting the yield and energy resolution in the electrodeposition of uranium. Appl. Radiat. Isot. 42, 135–140. VeraTome´, F., JuradoVargas, M., Martı´ n Sa´nchez, A., 1994. SIMPLEX method for optimization of experiments. Application to electrodeposition in alpha spectrometry. Nucl. Instr. Meth. Phys. Res. A 348, 183–187. Yamato, A., 1982. An anion exchange method for the determination of 241Am and plutonium in environmental and biological samples. J. Radioanal. Nucl. Chem. 75, 265–273.