Inventory of 238Pu and 239, 240Pu in the soil of Czechoslovakia in 1990

.I. Environ. Radioactivity, 27. No. 2, pp. 181-189, 1995 Copyright $7 1995 Elsevier Science Limited Printed in Ireland. All rights reserved 0265-931X/95 $9.50+0.00 ELSEVIER

0265-931X(95)00018-6

240Pu in the Soil of Inventory of 238Pu and 239T Czechoslovakia in 1990

Z. HGlgye & R. Filgas National Institute of Public Health, Center of Radiation Hygiene, 100 42 Prague, Czech Republic (Received 9 June 1993; accepted 16 March 1994)

ABSTRACT The cumulative deposition of 238Pu and 239,240Pu by the end of 1990 in grassland, undisturbed for the last 40 years, was determined at 50 sites in Czechoslovakia. The cumulative deposition of 238Pu and 239,240Pu ranged .from M 0.5 to 6.2 and from IO.2 to 108.2 Bq mp2, respectively. The mean value of the cumulative deposition of 238Pu and 239.240Pu were 1.7 and 49.3 Bq mp2, respectively. No signtficant contribution to the cumulative deposition of 238Pu and 239.240Pu originating from the Chernobyl accident was found at any site and no significant decrease of 239S240Pu at sites in the northwest parts of the Czech Republic with long-term, high incidence of acid rain was found either.

INTRODUCTION About 20 years ago, a global inventory of 238Pu and 239,240Puin soil from atmospheric nuclear tests and of 238Pu from the burn-up of the SNAP-9A satellite was provided by Hardy et al. (1973). Later the cumulative deposition of 238Pu and 239,240Puand factors influencing this deposition were studied in more detail in various countries. It was found that the cumulative deposition of plutonium isotopes was linearly correlated with mean annual precipitation (Cawse 8z Horrill, 1986; Bunzl & Kracke, 1988; Mitchell et al., 1990). Cumulative deposition in undisturbed areas covered by forest is about 30% higher that in those areas covered by grass, due to 181

182

Z. Hdgy,

R. Filgus

the interceptive function of forest canopies (Bunzl & Kracke, 1988). On the other hand no influence of soil faculties studied, such as acidity, pedological, lithological, geomorphological parameters, except for organic matter content, was found on the value of the cumulative deposition (Cigna et al., 1987). For this study, soil samples from 50 different localities of Czechoslovakia were collected during 1990, in connection with our inventory programme of 238Pu and z’), 14”Pu (and some other radionuclides) in the soil. The main objectives of this programme were: (i) to determine the cumulative deposition of “‘Pu and 139,240Pu over the area of the country up to 1990; (ii) to confirm the assumption that the main source of contamination of Czechoslovakian soil (with 238Pu and 239,240Pu) was the atmospheric nuclear tests (and the SNAP-9A burn-up) with the contribution from the Chernobyl accident and other accidents being very low; (iii) to estimate whether long-term action of acid rain in some localities of the country has caused any observable decrease of 238Pu and 23’),240Pu content in the top soil layer.

MATERIALS

AND METHODS

Soil samples (20 x 20 x 20 cm) were collected from grass-covered areas undisturbed by human activity during the last 40 years. The same depth of core (O-20 cm) has been chosen as that used by Bunzl and Kracke (1988) six years ago, who assumed that more than 95% of plutonium isotopes were present in this layer. The samples were dried at 110°C for 48 h, crushed, sieved (1 mm), homogenised, and weighed. Plutonium isotopes were determined in 50 g sieved soil by a radiochemical method recently tested in our laboratory (Hiilgye, 1991). In the method used, great attention was paid to the separation of Pu(IV) from thorium (first, by washing of the anion exchange resin on which Pu(IV) was sorbed from 8 M HN03, with 10 column volumes of 9 M HCl; second, by extraction of Pu(IV) from 1 M HN03 into 0.25 M TTA in benzene). Attention was also paid to conversion of Pu02 (which might be present, for example, from the Chernobyl source) to acid-soluble forms before leaching with nitric acid. Alpha activities of 238Pu, 23’).240Pu and 242Pu (used as a chemical yield monitor) after electrodeposition on stainless steel discs were measured with a passivated implanted planar silicon detector (600 m*). Using the activity concentrations of 238Pu and 239*240Pu in soil from the respective sites, the weight of sieved soil, and the volume of the core, the cumulative deposited activities (Bq mp2) of these radionuclides were calculated for each site. In estimating means, in cases of censored data (lower than

Inventor!: qfJ3’Pu

and 23y.24”Pu in Czechoslovakian

soil

183

detection limits), algorithms suggested by Lawless (1982) were used. The values of concentrations of SO* in air at individual sites are annual average values for 1984-1989 and the rainfall values at individual sites are annual average values for 193 I-1960 (Sladek & SladeEek, personal communication, 1992).

RESULTS AND DISCUSSION The concentration of 2”8Pu and 239,240Pu in soil from the sites studied ranged from 2.2 to 228 and from 59.2 to 719.8 mBq kg-‘, respectively. The mean values of the concentration of 238Pu and 239,240Pu are 9.9 f 1.7 and 271.2 f 18.2 mBq kg-‘, respectively. Ranges of concentrations of 238~~ and 239.240 Pu found are lower than those found (Cigna et al., 1987) in northern Italy ( < 7-58 and 59-1510 mBq kg-‘, respectively). The mean values of cumulative deposition of 238Pu and 239,240Pu are 1.7 f 0.3 and 49.3 f 3.3 Bq rnp2, respectively; these differ from those measured by Cigna et al. (1987) in northern Italy (3.3 and 110 Bq m-*, respectively) and by Mitchell et al. (1990) in Ireland (2.69 and 78.0 Bq m , respectively) and are close to values 1.5 and 58 Bq rne2, respectively reported by UNSCEAR (1982) for the north temperate zone (40-50”). The mean value of cumulative deposition, 49.3 Bq rnp2, is almost identical to the mean value of 49 Bq m-* measured in Bavaria (Bunzl & Kracke, 1988) for undisturbed areas covered by grass. The mean value for activity ratios of 238pu/239,240 Pu at sites studied, 0.034 f 0.003, are close to the values of 0.034 (Mitchell et al., 1990) 0.036 (Hardy et al., 1973) and 0.037 (Bunzl & Kracke, 1988) observed previously in soil and are characteristic of the fallout from atmospheric nuclear tests. Figure 1 shows the distribution of values of cumulative deposition of 239*240Puin 1990. There is a rather increased frequency in the range of 2140 Bq m-2. There are only small differences in values of annual precipitation at our sampling sites (Table 1). At 92% of the sites the annual precipitation is between 500-800 mm and the value of 1000 mm has not been reached at any site (in contrast to northern Italy or Ireland, where at some sites annual precipitation reached 2000 or 2500 mm, respectively). Nevertheless, in our results a linear correlation between the cumulative deposition of 239,240Puand annual precipitation can also be found (at level 0.90). Cumulative deposition (Bq me2) of 239,240Pu= 13.1 + O-054 x mean annual precipitation (mm). The value for dry deposition, 13.1 Bq rnp2, is very close to that found by Bunzl and Kracke (1988) in Bavaria (11.9 Bq me2) and by Mitchell et al. (1990) in Ireland (14.2 Bq mp2).

184 12-

IO-

Bq.m“

Fig. 1. Distribution

cumulative

deposition

data of r”~24”P~.

Possible acid rain effect In the northwestern part of the Czech Republic for several decades enormous concentrations of SOZ and other gases in air (Table 1) can be observed and are a consequence of the burning of coal with high sulfur content especially in coal power plants in this region and in neighbouring countries (Moldan et al., 1990). Acids formed by absorption of these gases in water may drastically change the chemical and biological properties of the soils and soil processes (Nilson et al., 1987). Soil acidification increases the mobility of several elements, including basic cations and metals and affects soil microbial processes, particularly the production and solubilization of humus compounds, which are of decisive importance to the weathering of soil minerals (Sverdrup, 1990). The results of the action of SO2 and other gases in the air and the ensuing changes in the soil caused the decline of forests in the mountain ranges of this region 20 years ago. It could be assumed that long-term action of acidified soil solutions in the soil could have also caused a decrease in the content of plutonium in the O-20 cm layer studied. To verify this assumption, we compared the mean values of cumulative deposition of 239.240Pu at the sampling sites where concentrations of SO2 in the air were highest (Sites 1-7; Table 1) and lowest (Sites 22-26, 30, 31; Table 1). The mean values of annual precipitation for both groups of sites are identical. The results obtained did not

qf z3xPu

Inventory

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soil

TABLE 1 Rainfall, SOI Concentration in Air, Cumulative Deposition and Activity Ratios of 23xPu and 239.240Puin Soil at 50 Sites in Czechoslovakia at the End of 1990 Site

Rainfall (mm year-‘)

Cumulative

so2

(pg m--‘) 23RPu (Bq me’)

1 2 3 4 5 6 7 8 9 10

11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

500 560 610 690 990 650 600 650 520 580 760 700 670 700 730 640 600 700 700 840 940 700 590 640 700 590 550 520 600 600 750 800 850 650 750 720 670 770 680 770 650

110 100 90 80 70 50 60 40 45 40 30 20 25 30 25 25 20 30 35 30 15 35 15 15 15 30 35 30 25 20

< 2.6 4.7 f 1.5 6.2 f 2.0 3.1 Zt 1.0 1.2 f 0.8 < 1.8 1.5 f 0.8 2.7 f 0.9 < 2.9 0.8 IJI0.6 < 1.2 2.3 f 0.7 2.2 f 1.0 2.7 f 1.2 1.1 f0.8 0.7 f 0.5 6.2 & 2.3 1.7 f 1.2 1.5 f 1.2 1.9 f 1.7 3.2 * 1.1 0.8 * 0.6 1.7Zto.7 1.6 f 0.7 3.6 f 1.1 5.1 f 1.4 0.6 f 0.4 ~2.4 4.4 f 1.8 2.9 + 1.3 < 1.9 c2.1 1.9 f 0.8 1.Of 0.6 < 0.5 1.3 f 0.6 0.9 * 0.4 < 0.9 3.6 f 1.2 2.8 f 1.4 < 1.2

Z38p,/239,240pu

deposition 239.240pl,

cBq

m-2j

87.2 f 6.8” 67.9 f 6.0 66.1 III 4.8

47.6 f 5.9 59.7 f 4.6 16.5 f 3.9 37.9 Z+Z 4.2 88.2 f 7.5 19.0 f 3.9 50.3 f 3.1 56.2 & 4.6 44.6 f 3.8 60.0 I!Z5.2 32.3 & 3.3 33.1 f 3.2 31.0 f 2.9 77.5 f 7.6 54.2 III 5.4 35.6 + 4.6 81.9 f 9.4 108.2 f 10.7 28.0 f 3.0 48.4 f 4.2 55.7 Zt 5.0 79.3 f 6.6 78.6 ZII7.2 23.8 It 2.2 50.4 f 4.6 63.3 f 6.1 67.0 f 7.0 40.0 f 9.6 40.5 It 3.1 38.3 ZII3.3 26.5 & 3.2 24.6 f 2.1 59.8 f 3.5 26.4 f 2.0 42.0 f 3.6 100.9 f 8.3 84.5 f- 14.3 19.8 ZIZ2.5

activity ratio

0.069 0.094 0.065 0.020

f f + f

0.023 0.030 0.02 1 0.013

0.040 It 0.02 1 0.031 Zt 0.010 0.016 + 0.012 0.052 0.037 0.084 0.033 0.023 0.080 0.031 0.042 0.023 0.030 0.029 0.035 0.029 0.045 0.065 0.025

f 0.016 f 0,017 f 0.033 i 0.024 5 0.016 f 0.029 f 0.012 f 0.032 f 0.021 + 0.010 & 0.02 1 f 0.014 f 0.012 f 0.014 f 0.018 f 0.017

0.070 * 0.029 0.043 f 0.019

0.050 f 0.021 0.038 f 0.022 0.022 Z!I0.010 0.034 It 0.015 0.036 f 0.011 0.033 f 0.017 conrmued

nverleaf

Z. Hiilgye, R. Filgas

186

TABLE Site

42 43 44 45 46 47 48 49 50

Rainfall (mm year-‘)

so2

(pg mm’)

600 600 600 570 570 570 580 590 700

Range Arithmetic

mean

l-contd. 23rXpu/239,

Cumulative deposition 23’Pu (Bq mm2) 239.240pu~~~ m-2) 33.7 32.5 28.0 27.1

f + f f

240pu

activity ratio

3.8 5.5 2.6 3.6

0.042 f 0.018 0.043 f 0.025 0.079 f 0.033 -

64.6 f 4.5 15.5 f 4.7 IO.2 f 3.9

0,037 It 0.012

37.0 f 2.8 61.7 zt 5.7

0.014 f 0.011 0.039 It 0.022

1.4 + 0.6 I .4 i 0.8 2.2 f 1.0 < I.4 2.4 f 0.8 < 0.8 < I.0 0.5 f 0.4 2.4 zt 1.4 0.5-6.2 I .7 It 0.3h

10.2-108.2 49.3 f 3.3

-

uQuoted errors are f one sigma counting error. ‘Quoted errors are f one standard error of the mean.

corn%-m the assumption. The geometric mean of cumulative deposition of 239,240Puat sites with the highest concentration of SO2 in air is 49.2 i 9.5 Bq me2 and the geometric mean of the cumulative deposition of 239*240Pu at sites with the lowest concentrations of SO2 in air is 53.5 f 7.1 Bq rnp2. The t-test shows that the difference is not significant at the 0.95 level. It can be concluded that even several decades of action of acidified soil solutions has caused no observable decrease in the fallout 239,240Pu content in the &20 cm soil layer. Possible Chernobyl effect The contribution of the Chernobyl accident to the value of cumulative deposition of 238Pu and 239,240Pu as assessed from measurements of concentrations of these radionuclides in the air and atmospheric precipitation is very low. The concentration of 239,240Pu in the air in Czechoslovakia was measured (Holgye & Filgas, 1987; Navarcik et al., 1989) at four sites (Fig. 2). At Sites A2 and A3 increased concentrations of 239,240Pu (lo140 PBq m-3 were measured in the period 30 April-5 May, 1986. Between 6 May and 19 May the concentrations of 23g.240Pu at these sites were < 4 and after 19 May they were < 1 PBq rnp3. At Site A, 239,240Pu in air was not proven even in the period 30 April-5 May, and at Site A4 a small increase (up to 38 PBq rne3) was detected in the period 4-10 May. At sampling sites of neighbouring states, close to our border, increased concentrations of 239,240Pu and 23sPu (up to 89 and 41 PBq rne3, respec-

187

51” N

50’

49’

48’

41’

I 13”

I 15”

Fig. 2. Location

I 17”

of sampling

I 19”

I 21”

E

sites: 0 soil; A air.

tively) were detected in the air at live sites in Austria (Irlweck et al., 1993) and in precipitation at Neuherberg near Munich (Rosner et al., 1988) at the end of April and the beginning of May. The concentrations of 239*240Pu at Sites AZ, A3 at the end of April and beginning of May reached only the approximate value calculated for air at Heidelberg (Jakubick, 1976) during 1962-1963 when the concentration of these radionuclides in the air reached a maximum due to the atmospheric nuclear testing. At Neuherberg, the values of deposition of 238Pu and 239.240Pu at the end of April and beginning of May amounted to 21 and 51 mBq mW2, respectively. To verify the contribution of the Chernobyl accident and possible other accidents (as reviewed by Martin and Thomas (1988) in connection with the explanation for extremely high concentrations of 238Pu in the air of Paris in 1982) to the cumulative deposition of 23sPu and 239.240Pu two approaches have been chosen. One consists of comparing the cumulative deposition of ‘“‘Pu and 239.240Pu at sampling sites with the highest and lowest fallout of y-emitting radionuclides will have the highest fallout of radionuclides from the Chernobyl accident detectable by y-spectrometry (IHE Report, 1986). It can be expected that the sites with the highest fallout of plutonium isotopes, too. The geometric mean of the cumulative deposition of ‘s8Pu and 239,240Pu at sites (numbers 11, 13-15, 17, 19, 39, 4547; Fig. 2) with the highest fallout of radionuclides from the Chernobyl

Z. HGlgye, R. Filgas

188

accident are, 2.1 f 0.7 and 44.0 f 7.4 Bq mW2, respectively; and the geometric mean of cumulative deposition of 238Pu and 239,240Puat sites (numbers 7, 8, 24, 27-29, 31, 33, 36, 37; Fig. 2) with the lowest fallout are 1.5 f 0.3 and 46.9 f 5.8 Bq rne2, respectively. It is evident that the mean values of cumulative deposition of 239,240Pudiffer only slightly for both groups of sites. In the case of 238Pu, the difference is larger. However, using the t-test, the difference is not significant at the 0.95 level. The other approach consists of the search for activity ratios of 238Pu/239,240Pu(Table 1) that obviously differ from those characteristic of the fallout from atmospheric nuclear tests. The difference in the values of activity ratios can be explained by the contribution to the plutonium content in the soil from further sources (for example from the Chernobyl accident, where the activity ratio (Krivokhatsky et al., 1991) of 238Pu/239~240Puis z 0,5). The highest activity ratio (0.094) observed at Site 3 with its 95% confidence interval 0.034-0.154 derived from its error (60.030) includes the values of 0.034 characteristic for atmospheric nuclear tests. All other elevated activity ratios (Sites 14, 17 and 44) are lower with very similar errors to the previous one (f0.030). Using results of both investigations there is no evidence of observable contribution of 238Pu and 239,240Pu originating from the Chernobyl accident (and possible other accidents) to values of the cumulative deposition of these radionuclides at sites studied in Czechoslovakia. REFERENCES Bunzl, K. & Kracke, W. (1988). Cumulative deposition of 137Cs,238Pu, 239,240Pu and 24’Am from global fallout in soil from forest, grassland and arable land in Bavaria (FRG). J. Environ. Radioactivity, 8, 1-14. Cawse, P. A. & Horrill, A. D. (1986). A Survey of Caesium-137 and Plutonium in British Soils in 1977. Report AERE-R 10155, Harwell, Oxon, UK. Cigna, A. A., Rossi, L. C., Sgorbini, S. & Zurlini, G. (1987). Environmental study of fallout plutonium in soils from the Piemonte region (northwest Italy). J. Environ. Radioactivity, 5, 71-81. Hardy, E. P., Krey, P. W. & Volchok, H. L. (1973). Global inventory and distribution of fallout plutonium. Nature (London), 241, 444-6. Holgye, Z. (1991). Determination of plutonium in soil. J. Radioanal. Nucl. Chem., 149,275-80.

Hiilgye, Z. & Filgas, R. (1987). Determination of 239,240Pu in surface air in several localities in Czechoslovakia in 1986 in connection with the Chernobyl radiation accident. J. Radioanal. Nucl. Chem., 119, 21-8. IHE Report (1986). Report on the Radiation Situation in the CSSR After the Chernobyl Accident. IHE-CRH (Institute of hygiene and epidemiology, Centre of radiation hygiene), Prague, Czech Republic. Irlweck, K., Khademi, B., Henrich, E. & Kronraff, R. (1993). 239(240),238Pu, 90Sr,

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‘06Ru and 13’Cs concentrations in surface air in Austria due to dispersion of Chernobyl releases over Europe. J. Environ. Radioactivity, 20, 133348. Jakubick, A. T. (1976). Migration of plutonium in natural soils. In Transuranium N&ides in the Environment. IAEA, Vienna, Austria. Krivokhatsky, A. S., Dubasov, Yu. V., Smirnova, E. A., Skovorodkin, N. V., Savonenko,V. G., Alexandrov, B. M. & Lebedev, E. L. (1991). Actinides in the near release from the Chernobyl NPP accident. J. Radioanal. Nucl. Chem., 147, 141-51. Lawless, J. F. (1982). Statistical Models and Methods,jor Lifetime Data. J. Wiley, New York, USA. Martin, J. M. & Thomas, A. J. (1988). Anomalous concentrations of atmospheric plutonium-238 over Paris. J. Environ. Radioactivity, 7, 1-16. Mitchell, P. I., Sanchez-Cabeza, J. A., Ryan, T. P. & McGarry, A. T. (1990). Preliminary estimates of cumulative caesium and plutonium deposition in the Irish terrestrial environment. J. Radioanal. Nucl. Chem., 138, 241-56. Moldan, B. et al. (1990). Environmental situation in Czech Republic. (In Czech.) Academia, Prague, Czech Republic. NavarEik, J., Wirdzek, S. & BurEik, I. (1989). Determination of activities of artificial radionuclides in surface air of KoSice. (In Slovak.) Radioactivita a z’ivotne prostredie, 12, 141-50. Nilson, S. I., Berden, B., Rosen, K. & Tyler, G. (1987). Soil Acid(jication. NSEPB, Solna. Rosner, G., Hiitzl, H. & Winkler, R. (1988). Actinide nuclides in environmental air and precipitation samples after the Chernobyl accident. Environ. Znt., 14, 331-3. Sverdrup, H. U. (1990). Kinetics of base cation release due to chemical weathering. LUP, Lund, Sweden. UNSCEAR (1982). Ionizing Radiation: Sources and biological ejjects. United Nations, New York, USA.