XA9951939 IAEA-SM-354/6P

239,240Pu! , 137Cs AND "'Sr IN BOTTOM SEDIMENTS OF THE . THE ATTEMPT TO RECONSTRUCT RATE OF THE RADIONUCLIDES SUPPLY TO BOTTOM SEDIMENT IN THE PAST

KUZNETSOV, Y. U. V., V. K. LEGIN, A. V. STEPANOV, V. N. STRUKOV V.G.Khlopin Radium Institute, St.Petersburg, Russian Federation

A. E. SHISHLOV, YU.V. SAVITSKY Mining and Chemical Combine, Zhelesnogorsk, Russian Federation

Despite the availability of extensive information about the levels of technogenous radioactive contamination of the Kara Sea, the problem on assessment of contribution of different sources into forming the total radioactive contamination of this basin remains to be of current interest. Discharge of the largest Siberian rivers - ' and being exposed to the impact of three Russian plants for weapon plutonium production - is the potential contributor of contamination in the Kara Sea. In the present work, emphases is given to the role of one of these rivers, i.e. Yenisey River, in transfer of the long-lived radionuclides 239Pu, 137Cs and 90Sr into the Kara Sea. 239>240Pu, 137Cs and ^Sr contents were determined in bottom sediments of the Kara Sea and Yenisey

Pu-239,240 and Cs-137 in surface sediments of the Kara Sea [Bq/kg] and their ratio. HiRLMSSIA #44 Fig.i •*= •^•Sfes •42 k IRISH t4i i£

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-62.61 -67.61 -72.61 -77.61 -82.61

298 River estuary (Fig 1) which were taken during expeditions on research ships "D.Mendeleev" and "D.Ovtsyn" in 1993, as well as in flood-plain and bottom sediments of the Yenisey River the samples 239 24O of which were taken during annual expeditions along the river. Table I presents the data on - pu and 137Cs content in surface layer of bottom sediments of the Kara Sea; Fig.2 shows the connection between distribution of these radionuclides in bottom sediments. Fig.3 illustrates the vertical distribution of m240Pu and 137Cs in bottom sediments (stations 31 and 34). It could be concluded from Figs 2,3 and Table 1 that there is a direct correlation dependence between distribution of these radionuclides in surface layers and vertical section of bottom sediments taken in the Kara Sea. Similar data were obtained in [1] River delta. The observed similarity in behavior of 239>240Pu and 137Cs is substantiated by our data on the high values of distribution coefficients (105 - 103) of these radionuclides between aqueous phase and bottom sediments of the Kara Sea, as well as the data confirming the high stability of 239>240Pu and 137Cs in sea sediments. Similarity in migration behavior of plutonium and cesium on their transfer with solid phase of the Yenisey river run-off into the Kara Sea is confirmed also by the data presented in Tables I and m240 I37 II. Thus, consideration of these data shows that pu and Cs enter efficiently into the structure of the suspensions and bottom sediments of the Yenisey River with high efficiency (Table I). The 239 240 data of Table II show that ' pu is most firmly bound with the structure of the bottom sediments and its return into aqueous phase is practically ruled out. As to 137Cs, such secondary transfer from bottom sediments into river water is possible, although on a limited scale. The relationship among the vertical distribution of 239>240Pu, 137Cs and the age scale as applied to two samples of bottom sediments of the Kara Sea (Figs 1,3) has revealed that the surpassing of present level of supplying these radionuclides into the studied sediments was caused by the most intense tests of nuclear weapons at the beginning of the sixties.

TABLE I. DISTRIBUTION COEFFICIENTS (Kds, ml/g) FOR Cs-137 AND Pu-239,240 BETWEEN BOTTOM SEDIMENTS AND WATER PHASES OF YENISEY RIVER AND KARA SEA. Radionuclide Distribution coefficients (Kds) Our data * Reference data Yenisey River Cs-137 2,5*10" - 8,0*10" Pu-239,240 (1,0 - 4,2)*105 Kara Sea Cs-137 l*103 3*103 Pu-239,240 6*10" l*105 * Data of the Russian - Norwegian expedition

2,5

0 £10 15 2025303540

FIG. 2. Correlations between Cs-137 and Pu-239,240 and Cs-137 and Sr-90 contents in surface bottom sediments

299 ^Sr content in surface sediments of the and the Kara Sea varies within much more 137 m24O 137 narrow range as to Cs and pu content. No direct correlation in distribution of ^Sr and Cs in surface sediments was detected. Distribution coefficients of 90Sr between bottom sediments and aqueous phase calculated by us for the Kara Sea and Yenisey River are much less (101 - 102) than these values as to for ^^u and 137Cs.(Table II).

TABLE II. DISTRIBUTION (%) OF Pu-239,240 AND Cs-137 BETWEEN POTENTIAL-MOVABLE AND FIRMLY BOUND FORMS IN BOTTOM SEDIMENTS OF THE YENISEY RIVER (HORIZON 0-5 cm, SOLID PHASE/LIQUID PHASE =1:5). Distance from Potential-mobile forms 1M Acid-soluble form 6M HC1 Unsoluble forms MChC NH4COOH3 km Pu-239,240 Cs-137 Pu-239,240 Cs-137 Pu-239,240 Cs-137 6,5 <0,l 29±3 61±9 46±6 39+5 46±6 9,0 l,2±0,2 11±1 65±6 21±2 33±4 68±5 15,0 l,3±0,2 43±3 61±10 51±5 38±3 5±1 19,0 2,0±0,3 13±1 49±5 20±2 49+5 66±7

0 10 20 30 40 S 20 40

1,8

] 1

O»-137 1,4 0,4 '• Pu-239,240 1 / \ v \/I 1,2 f 0,3 ; 1 I \ 'I 0,2 V 0.8? 11 K \ V O.E • 0,1 K -•"Cs-13" -•-Pu-23 1,240 0,2 s —4)— —t—

4 6 0 2 4 S 8 10 12 14 16 18 Depth, cm Depth, cm

Station 31 Sedimentation rate-l,lmm/y (Pb-210) Station 34 Sedimentation rate-l,8mm/y (Pb-210) F1G.3. Vertical distribution of Cs-137 and Pu-239,240 in bottom sediments (Station NN 31 and 34) with decay correction for Cs-137

In conclusion we can say: • The migration of 239>240Pu and 137Cs with solid phase of river run-off extends over large areas: this is predominant mechanism of supplying these radionuclides by the Yenisey River into the Kara Sea bottom sediments which are forming under impact of solid phase of the Yenisey river run-off. • The main part of 90Sr migrates in soluble form with run-off of the Yenisey River. This is also dominant species of ^Sr occurrence in the Kara Sea. • The surpassing of present level of 239>240Pu and 137Cs supply into these sediments (30-40 years ago) was caused by the most intense tests of nuclear weapons at the beginning of the sixties.

References

[1] G.P.PANTELEEV, H.D.LIVINGSTON, F.L.SAYLES, O.N.MEDKOVA. Deposition of Plutonium Isotopes and Cs-137 in Sediments of the Ob' Delta from the Beginning of the Nuclear Age. In: Proceedings of the International Conference "Environmental Radioactivity in the ", Oslo, Norway, 21-25 August 1995, pp.57-64.

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