Observations on plutonium in the oceans

Appl. Radiat. Isot. Vol. 46, No. I1, pp. 1213-1223, 1995

Pergamon

0969-8043(95)00163-8

Copyright © 1995ElsevierScienceLtd Printed in Great Britain.All rights reserved 0969-8043/95 $9.50+ 0.00

Observations on Plutonium in the Oceans M. S. B A X T E R , S. W. F O W L E R a n d P. P. P O V I N E C International Atomic Energy Agency, Marine Environment Laboratory, B.P. 800, MC 98012, Monaco This reviewdescribes various studies of marine plutonium distributions, behaviour and transfer carried out at the Marine Environment Laboratory of IAEA. Plutonium in sea water is primarily present in solution as Pu(V), probably as PuO+2, and on particles in reduced form as Pu(IV), most likely as species such as adsorbed Pu(OH)4. In nearshore sediments and coastal soils, plutonium is complexed with high molecular weight humic and low molecular weight fulvic compounds. Results of oxidation state studies in the Black Sea and Norwegian fjords show that major reduction of dissolvedPu(V) occurs [to Pu(III)] in anoxic waters in association with complexation of reduced plutonium with these dissolvedand colloidal humicsand fulvics. The IAEA-MEL has also been active in developing and applying laboratory radiotracer methods to study rates and pathways of plutonium flux through marine organisms, particularly using the ),-emittingtracer, 237Pu. Plutonium concentration factors spanning five orders of magnitude have been observed, with no significantdependence on oxidation state. Almost completeexcretionof ingestedplutonium by zooplankton makes their defecation an extremely important mechanism in the scavenging of plutonium from surface waters. This phenomenon has been welldocumentated in many fieldstudies using time-seriessediment traps to quantify and characterize elemental flux. Plutonium fluxes in the northwestern Mediterranean have recently been shown to correlate well with mass flux and suggest residence times of 24 and 3.5 years, respectively, for 239'24°puand 24'Amin the upper mixed layers, values which are rather longer than those measured previously.The IAEA-MEL has recently studied plutonium distributions in the vicinityof former waste dumpsites in the NE Atlantic Ocean and in the Kara Sea. The results of these surveysprovide evidence of radiologically negligible but measurable leakage at the former site and of no leakage whatsoever at the latter. Computer predictions of potential future dispersion of plutonium-containing wastes from the Kara Sea suggest that the maximum possible global collectivedose (30 manSv) would be extremelylow. Finally, the review notes the increasing analytical contribution to environmental plutonium studies which can be made by mass-spectrometrictechniques. At IAEA-MEL, the ETV-ICPMS technique along with low-level radiometric methods is being refined in order to maximize the possibility of using aquatic isotopic signals to characterize and identify nuclear source-terms.

Introduction The inventory of 239'24°Pu in the world's oceans is around 16 PBq. Because of spatial inhomogeneity of its inputs to the oceans and of its subsequent non-conservative behaviour, this inventory is not uniformly mixed and activity concentrations vary widely. In general, in the open oceans, "9'24°pu concentrations, derived essentially from global fallout from weapons testing, are vanishingly small, in the order of 10/~Bq kg-~ water. Large volume sampling ( > 50 L) and long radiometric counting times (weeks) are necessary to quantify such pelagic concentrations. Their radiological consequences are correspondingly negligible. This paper, therefore, focuses on studies of marine plutonium behaviour from environments where a local influence from a particular source-term (e.g. a discharge or disposal) raises concentrations to levels at which statistically useful experimental information can be gained and also where detailed ARJ~s/H-J

radiological assessment may be justified. In addition, and again because of the generally very low marine concentrations, the use of radiotracers in laboratory simulations of marine foodchains has provided invaluable behavioural information. The IAEA Marine Environment Laboratory (IAEA-MEL) in Monaco has a long history of interest in marine plutonium behaviour, transfer and fate and this review further restricts itself to highlighting some of the past and current aspects of their work which the authors consider to have been particularly worthy of mention.

Speciation Some of the most definitive studies of plutonium behaviour have been carried out in the Irish Sea, in the vicinity of the authorized discharge of low-level liquid wastes from the nuclear fuel reprocessing plant at Sellafield. Close to 0.8 PBq of plutonium ~-emitters

1213

1214

M.S. Baxter et al.

have been discharged, peaking in 1973. Of this plutonium, only about 7% is expected to leave the Irish Sea area (Pentreath et al., 1986), the great majority becoming associated with suspended particles and sediments. It is the long-term fate of this sedimentary plutonium which merits continuing study. Although discharged to the Irish Sea in liquid waste, virtually all of the plutonium a-activity (~99% of the major sea tank contribution) was associated with particulate material (<0.22 lam) (Pentreath et al., 1984), including -10% of the consequent sediment inventory in the form of "hot particles" (Kershaw et al., 1986). In this sediment inventory, the plutonium is primarily in the reduced oxidation state (III + IV), for which the Ka value is around 2 orders of magnitude higher (-106) than that for the oxidized (V + VI) states (~ 104)(Nelson and Lovett, 1978). It is now increasingly accepted that the oxidized states of plutonium in sea water are dominated by Pu(V), probably as PuO~, while the reduced form is mainly Pu(IV), as adsorbed Pu(OH)4 (Aston, 1980; Edgington and Nelson, 1984; Silver, 1983; Orlandini et al., 1986; Pentreath et al., 1986; Mitchell et al., 1991). Sequential extraction and speciation studies carried out at the University of Glasgow on Irish Sea and coastal soils (Livens et al., 1986; Livens and Baxter, 1988a; Livens and Baxter, 1988b) confirmed that the plutonium is entirely in the reduced form and is associated with organic and oxyhydroxide phases, with most of the plutonium in high molecular weight (> 150,000) humic and low molecular weight (< 2000) fulvic species. Of the few percent of the Irish Sea plutonium associated with the dissolved phase, 80-90% is found to be in the oxidized form (Mitchell et al., 1991). Similar results have been observed in other nearshore environments (e.g. Eniwetok and Bikini lagoons, Gulf of Mexico, New York Bight, Bay of Fundy (Noshkin and Wong, 1980; Nelson et al., 1987; Mitchell et al., 1991). There is some evidence that, in the open ocean, similar chemical speciation occurs only near the sediment-water interface, with the majority of the water column showing roughly equal distributions of the oxidized and reduced forms (Nelson et al., 1984; Mitchell et al., 1991). In oxic waters, while Pu(V) dominates the oxidized fraction, it may be that, in the presence of humic substances or other reducing species, Pu(V) exists in a metastable state which becomes slowly reduced to Pu(IV). Recent work at IAEA-MEL by Sanchez et al. (1991) and Sanchez et al. (1994) has shown that, in both the Black Sea and Norwegian tjordic water columns in which changing redox conditions occur, major reduction of oxidized plutonium occurs as the anoxic zone is reached, with the reduced plutonium becoming dominant at the zone where H2S is present (Fig. 1). Much elevated plutonium levels in the fjords were interpreted as indicating complexation of reduced plutonium [probably Pu(III)] with the much enhanced levels of dissolved and colloidal organic substances. According to speciation calculations,

Pu(III) should predominate under reducing conditions, is intrinsically more soluble than Pu(IV) and is strongly complexed by humic acids (Choppin et al., 1986; Morse and Choppin, 1986; Nash et al., 1988). The IAEA-MEL oxidation state studies also showed that the ratio of oxidized to reduced plutonium is a useful tracer of water movement, particularly in interpreting lateral advective processes. Biokinetics

The Radioecology Laboratory of IAEA-MEL was the first group to investigate rates and pathways of plutonium flux through marine organisms using 237pu radiotracer in laboratory experiments (Fowler et al., 1975). Especially-prepared high specific activity 2JTPu allows testing of plutonium behaviour at environmentally realistic mass levels, while the nuelide's-/-emission facilitates whole body counting of live animals and/or tissues with no chemical separation. This technique was then used to obtain the first reliable data on fundamental transfer parameters. Some combined results are shown in Fig. 2. Here the plutonium concentration factors in a variety of marine species are shown to range over five orders of magnitude. In these studies, there was no strong evidence of differential biological uptake or loss of reduced or oxidized plutonium (Fowler et al., 1975; Fowler and Guary, 1977; Fisher et al., 1983; Bjerregaard et al., 1985; Carvalho and Fowler, 1985). Figure 3 shows a typical data set illustrating generally similar bioaccumulation behaviour for both oxidation state groupings (Aston and Fowler, 1984). For individual species, these radiotracer experiments have also identified interesting characteristics. For example, plutonium absorption efficiencies for crabs are relatively high (20-60%) compared to mammalian systems (10-3-10-2%) and hence, because of differences in digestive physiology, plutonium bioconcentration data cannot be extrapolated simply from vertebrates to invertebrates (Fowler and Guary, 1977). In contrast to larger crustaceans, assimilation of plutonium by crustacean zooplankton is very low (Fowler et al., 1976). Indeed, about 99% of the plutonium ingested by zooplankton is excreted with the feces (Higgo et al., 1977), with the Reduced Pu (ixBq/l) 2 4 6 8 10 12

Oxidized Pu (IxBq/l) 0

2 4 6 ¢-',1

50 am

K

.~ 100

50 -

--o--d

i.--o-q b-.t3--q

I

10 12 I

I

= ',

100

150

150

200

200 --

Fig. 1. Reduced and oxidized Pu across the anoxic interface in the southwestern Black Sea.

Observations on plutonium in the oceans 100,000 50,000

10,000

~

Phytoplankton

I

Microzooplankton

5000

1000 500

I-I I ]

Starfish #~...'°Amphipod _+~+"

.+J

"6 e~

....--+ Brittle star

o

100

g

50

~

Bivalve clam Polychaete worm

~

i¢|~i

Octopus

lu

Sea urchin 10

o ~ _ o . . . , - - - - - • Holothurian

/

~0.5 r__

0.1 0

1215

contact with the sea water, rather than through foodchain bioconcentration processes (Guary et al., 1982). Both radiotracer and field studies have shown that, as would be expected from the high affinity of plutonium for sediments, plutonium transfer factors from sediments to organisms are extremely low ( < 1 0 - 3 - 5 x 10 -3) (Beasley and Fowler, 1976a,b; Aston and Fowler, 1984; Hamilton et al., 1991). Furthermore, the small degree of uptake from sediment probably takes place through absorption from the interstitial waters in the sediments. Within specific organisms, the combination of ~t-active radiotracers with ct-autoradiographic mapping techniques enabled McDonald et al. (1993a) to identify the locations of highest bioaccumulation. In general, soft tissues involved in feeding and digestion accumulated actinides most effectively. Figure 4(a) shows a typically heterogeneous plutonium distribution in the digestive gland of the winkle. The most intense or-track distributions encountered were found in the winkle's operculum, these being attributable to its chitinous nature. Figure 4(b) show a typical example, in this case for 237Np.The same authors (McDonald et al., 1993b) took advantage of the relatively enhanced actinide concentrations prevailing in the marine environment around the BNFL nuclear fuel reprocessing plant at Sellafield, U.K. in order to confirm more accurately both whole body and individual tissue concentration factors for local mussels, winkles and prawns. Derived concentration factors for plutonium ranged from 300 (foot) to 30,000 (byssal threads) for mussels (1400 total soft tissue); from 700 (foot) to 16,000 (pallial complex) for winkles (5700 total soft tissue); and from 300 (abdomen muscle) to 27,000 (cardiac fore-gut) for prawn (20,000 total soft tissue).

Bottom fish

Flux

The IAEA-MEL has been active in various regions in the study of the rate and mechanism of plutonium

L

L

L

L

L

l

5

10

15

20

25

30

80 -

Time (days) Fig. 2. Uptake of plutonium by selected marine organisms [after Fowler (1983)]. Data for phytoplankton, microzooplankton and bottom fish taken from Fisher et al. (1983), Fowler (unpublished results) and Pentreath (1978), respectively.

60 -

result that, as mentioned later, zooplankton defecation is extremely important in scavenging plutonium from the ocean's surface layers (Fowler et al., 1983, 1991). For starfish, the high concentration factors found by tracer and field studies (Fig. 2) are the result of a strong affinity between plutonium and the animal's mucus-covered epidermal layer which is in direct

20 --

r~

~5 40 --

0

5

I

u

10

15

20

Days Fig. 3. Uptake of 237pu in the oxidized and reduced forms from sea water by clams (Venerupis decussata).

1216

M.S. Baxter et al.

removal from the open ocean water column. For example, in a recently reported study carried out in the northwestern Mediterranean Sea, Fowler et al. (1993) compared measurements of plutonium removal via sinking particles collected in automated time-series sediment traps with independent estimates of plutonium scavenging based on observed temporal changes in water column plutonium inventories. Table 1 shows the measured actinide concentrations in sinking particles as a function of depth and time, with the corresponding nuclide fluxes. It is evident from these results that, at 200 m, 23924°Pu concentrations in particles increased by a factor of 4.4 (1.92-8.45 Bq kg -t) during the course of the two-and-a-half month experiment. Corresponding 2+ZAmlevels varied by a factor often but did not follow the same temporal trend as 239'24°Pu. During sample series II and III, no clear trend in radionuclide concentration with depth was evident except for 24~Am immediately following the major sedimentation pulse of phytoplankton aggregates which swept the water column between 27 April and 10 May (Peinert et al., 1992). The maximum fluxes of 239'24°Pu and -'4tAm coincided with the period of maximum sedimentation, indicating that mass flux was the main factor controlling transuranic flux. These fluxes corresponded closely to those recorded off Corsica during Spring 1986 (Fowler et al., 1990a) but they were an order of magnitude lower than 239.24°Pu and 24nAm fluxes measured during Fall 1983 in the high sedimentation regime of Lacaze-Duthiers Canyon in the Gulf of Lions (Fowler et al., 1990b). Clearly, variations in the fluxes of these radionuclides depend to a large extent on the degree of sedimentation. The average 239.24°puand 24~Amflux through 200 m during the two-and-a-halfmonth period was 0.688 and 0.273 mBq m-Zday -t, respectively. Transuranic concentration profiles in sea water measured at this site during early May resulted in corresponding radionuclide inventories above 200 m of 5.91 and 0.526 Bq m-2. Under steady state conditions, such fluxes would result in residence times for 239'24°puand 241Am in the upper mixed layers of approx. 24 and 5.3 yr respectively. These residence times are considerably longer than those (2.5 and 14 yr respectively) reported

for a 17 day deployment in the Lacaze-Duthiers Canyon (Fowler et al., 1990b) and are probably more representative of the average transuranic flux in open waters of the northwestern Mediterranean basin. Further detailed analysis of the IAEA-MEL data suggests an average annual 239.24°pu loss from the western basin water column of 0.51 Bq m-2yr -t. Comparison with sediment trap flux data suggests that about half of the computed plutonium loss can be accounted for by the downward sinking of 239'uePu associated with large particles, particularly with planktonic fecal pellets. These results indicate that, in general, the removal of plutonium from western Mediterranean waters is a relatively slow process. The above-mentioned significance of particles in scavenging plutonium from ocean water has similarly been observed in both coastal and open ocean areas of the North Pacific Ocean by Fowler et al. (1983, 1991). Biogenic particulate matter from zooplankton is found to be closely linked to the downward plutonium flux and there is some evidence of a direct coupling between plutonium flux and carbon flux (carbonspecific 'new production') through the base of the euphotic zone. A plutonium residence time of around four years was estimated for the upper 150 m water column at the VERTEX time-series site midway between Hawaii and the California coast (Fowler et al., 1991). In general, then, the residence time of plutonium in sea water is extremely dependent on water depth, suspended particle load and sediment type, varying from tens of days in turbid shallow nearshore systems to tens of years in open ocean waters.

Sea Disposal of Plutonium-containing Wastes N E Atlantic dumpsite

Prior to recent revelations about former Soviet disposals of radioactive wastes in the marine environment (Yablokov et al., 1993), more than 98% of packaged low-level radioactive material known to have been disposed of in the oceans was dumped at deep sites in the North Atlantic Ocean (IAEA, 1991). Ninety two percent of the total activity disposed of was in the eastern basin. While, in general, 98% of the total

Table 1. Concentrations of transuranics in sinking particles and vertical fluxes of transuranics in the Gulf of Lions Sample series

Mass flux (rag m-~day -L)

(Bq kg -L)

239'24°pU (mBq m-2day -~)

(Bq kg -~)

241Am (mBq m-Zday -~)

Date (1990)

Depth (m)

I

14-27/04

200

127.6

1.92 + 0.25

0.245

0.46 :t: 0.22

0.059

II II I1

27/0410/05 27/04-10/05 27/04-10/05

200 500 1000 2000

308.9 48.6 242.3 240.8

3.83 2.82 4.73 3.36

± 0.29 __ 0.40 ± 0.37 _+ 0,28

1.183 0.137 1.146 0,809

3.29 + 0.46 1.48 __ 0.59 2.50 ± 0.49 3.33 5:0.46

1.016 0.072 0.606 0,802

llI llI Ili

10-23/05 10-23/05 10-23/05

200 1000 2000

181.8 120.9 174.4

5.28 + 0,63 3.33 + 0.44 3.02 ± 0.34

0.960 0.402 0.527

0.32 +_ 0.17 1.85 + 0.30 2.40 +_ 0.31

0.058 0.224 0.418

IV

23/054)5/06

200

71.8

5.91 _+ 0.81

0.424

2.14 + 0.85

0.154

V

05--18/06

200

61.4

7.60 +_ 0.92

0,467

2.52 ± 1.04

0.155

VI

18/06-01/07

200

100.5

8.45 -t- 0.86

0,849

1.92 _+ 0.80

0.193

Fig. 4. (a) Heterogeneous :t-track distribution in the digestive gland of the winkle following 13-day uptakc of ~ P u from labelled food. 19-day exposure. 3 cm = 500 p.m. (b) Alpha-autoradiograph of the winkle's operculum afler a 14-day uptake of 2~TNp form labelled sea water. 13-day exposure. 3 cm = 500 pro.

1217

1218

M.S. Baxter et al.

radioactivity disposed of comprised ]~-7 emitters, small quantities of~-emitting nuclides were also included. At the two main sites in the north-east Atlantic (46:00'N 16' 45'W and 46 ° 15'N 17~25'N) a total activity of > 30 PBq was disposed of. The Qt-emitting inventory at these sites would be expected to be in the order of 0.5 PBq. These dump-sites were used until 1982 but were previously and subsequently subject to radiological survey, normally on an annual basis. I A E A - M E L has regularly in the past cooperated with O E C D / N E A and their Member States on analytical quality assurance aspects of this surveillance exercise. However, in March 1992, IAEA-MEL, at Member State request, for the first time played a significant part in site-specific measurements at the NE Atlantic Dumpsite in the belief that maximum information should be gleaned on all scientific parameters associated with sea disposal of radioactive wastes. I A E A - M E L contributed by analysing water samples collected above the sea-bed of the main sites for anthropogenic radionuclides such as I4C, t37Cs, 239'24°pu and 24tAm. Samples were collected from the FRV Walther Herwig at one control site and at four locations in the area of the two main subsites and were shared between the cruise organizers, the Bundesforschungsanstalt fiJr Fischerei (BFA), Hamburg and the M A F F Fisheries Laboratory, U.K. and IAEA-MEL. The results obtained by I A E A - M E L are shown in Table 2. The concentrations of 239'24°puand 238pu at the control station (CS-13) appear to be slightly less than those obtained for the dump-site samples. While in the case of '-39-24°puthe only major deviation relates to the value measured at the dump-site station DS-18, the concentrations of 23Spu at the dump-site stations are 3-7 times that in the control sample. The 238Pu/-'-~9.-'4°pu activity ratio in the control sample is similar to that expected for global fallout in the northern hemisphere

(0.036) and is considerably less than the ratios observed in the dump-site samples (0.076-0.131). The 241Am results follow the same general trend as the plutonium concentrations; the highest 24~Am concentration obtained for dump-site station DS-18 is 2.5 times the value at the control station (CS-13). The :41Am/239.24°pu ratio ranges from 0.222 to 0.354 and is similar to the value at the control site. These preliminary results from the 1992 expedition to the NE Atlantic Dumpsite therefore suggest that measurable leakage of plutonium isotopes (documented especially by 238pu concentrations in bottom waters and by the 23Spu/239.24°Puactivity ratios), 2~JAm and probably also ~4C is occurring at the dump site. It should be borne in mind, however, that sea water contains around 12.5 Bq L - ~ of naturally occurring ///7 emitters and 115 mBq L-~ of natural ~-emitters (Baxter, 1983). Thus the localized enhancements observed above the dump site are completely negligible in radiological significance, e.g. they represent increases of the order of 10-3% and 10 :% in///~, and :t activities due to the maximum observed concentrations of mCs and 239'24°pu, respectively. Arctic seas

There has been growing concern over the dumping of high and medium level solid and liquid radioactive wastes in the Kara and Barents Seas by the former Soviet Union. According to the most recent information available (Yablokov et al., 1993), the dumping activities and the sunken submarine 'Komsomolets' have added a total of up to 92 PBq to the marginal Arctic Seas as nuclear reactor assemblies (including spent nuclear fuel), entire vessels and liquid and packaged solid radioactive wastes. The largest amount of nuclear waste (85 PBq) was dumped in the Kara Sea primarily as submarine nuclear reactors

Table 2. Radionuclide concentrations in water samples (one sigma counting error) Depth Sample code Location (m)

-'~.2"'Pu (~tBqL ')

238pu (btBq L t)

:4~Am (laBq L ~)

A14C (%o)

I~Cs (mBq L t)

CS-13

45 59"N 12':59'W

4770

7.28 + 0.34

0.21 + 0.06

1.77 ± 0,35

-210 + 5

<0.1

0.029 ± 0.008

0.243

8.43

DS-18

46 03'N 16 41'W

4715 20.14 ___0,67

1.53 + 0.19

4.48 ± 0.48

-206

0.18 ± 0.03

0.076 ± 0.010

0.222

2.93

DS-22

46 00"N 17 O0W

4650

1.28 ± 0.14

3.46 ± 0.37

-200

<0.1

0.131 ± 0.015

0.354

2.70

DS-24

46 '00N 17~00W

4670

DS-28

460TN 16 42'W

4710

<0,1

0.108±0.013

0.300

2.77

DS-31/1 DS-31/2 DS-31/3 DS-31/4 DS-31/5 DS-31/6

46~00"N 77"00"E

4650 4560 4150 3660 3000 1000

DS-33/1 DS-33/2 DS-33/3 DS-33/4 DS-33/5 DS-33/6

46 08"N 16' 41'W

4710 4620 4220 3720 3000 2000

0.58 ± 0.08

0.089 ± 0.017

0.347

3.91

9.77 +_ 0,40

23~pu/ 2~.2,~pu

-'4~Am/ ~41Am,, >~':~°Pu >~Pu

-216 10.07±0.37

1,09±0.12

3.02±0.29

-207 -179 -241 -225 -210 - 199 - 152

8.44 ± 0.42

0.75 ± 0.14

2~93± 0.36

-202 218 - 209 - 197 - 190 - 135

Observations on plutonium in the oceans

1219

Table 3. Estimated activity of actinides in the spent nuclear fuel of reactors dumped in the Kara Sea (TBq) Abrosimov Bay* Tsivolki Bay? Stepovov Bay:~ NZ depression* All sites 239.24°pu 3.48-17.54 7.3 0.341 1.22-6.18 12.34-31.36 238Pu 0.67-9.55 1.06 0.009 0.26-3.88 2.00-14.50 24~Pu 4.33-211.27 70.6 0.028 2.26-110.63 77.22-392.53 2'~Am 0.52-22.38 7.0 0.002 0.18-7.66 7.70-37.04 Total 9.00-260.74 85.96 0.38 3.92-128.35 99.26-475.43 *Mount et al. (1995). ?Sivintsev (1995). ~Yefimov et al. (1994).

TBq 239pu in two warheads and about 5 TBq of

containing spent nuclear fuel. No disposals of objects with spent nuclear fuel were reported for the Barents Sea, although the 'Komsomolets', which sank accidentally in the Norwegian Sea in 1989, added 5.5 PBq to the inventory of the region. Considering all waste types, over 95% of the nuclear debris is located in the Kara Sea. Perhaps rather surprisingly, the cz-emitting plutonium inventory in the Arctic dumpsites is significantly less than that estimated for the NE Atlantic or for the Irish Sea. Recent estimates have suggested (Table 3) that present inventories of actinides in the nuclear reactors dumped in the Kara Sea can be in the range 0.1~).5 PBq, the highest contribution (0.08-0.4 PBq) being due to 24xpu. The dominant dumping sites for actinides are Abrosimov and Tsivolki Bays. The 'Komsomolets' contribution to the actinide inventories in the Arctic Seas is about 16

actinides in the reactor's core. The IAEA-MEL has assisted the joint RussianNorwegian expeditions to the Kara Sea in 1992 and 1993. Selected high resolution downcore profiles of 137Cs, 239'24°puand 241Amin a typical sediment collected during the 1992 joint cruise to the Kara Sea and analysed at IAEA-MEL are shown in Fig. 5 (Hamilton et al., 1995). The 2~°Pbdate-line provides an estimate of the mean time since sediment deposition at the depth specified. An interesting feature of these profiles is that the lSTCsdepth distribution does not match the input records of global fallout nor does it follow the same pattern as that of the transuranic elements. This has been variously attributed to sediment mixing (bioturbation) and post-depositional migration of Cs. In the three sediment profiles shown in Fig. 5, the "7Cs concentration decreases smoothly with increasing Activity~ I~b

Activity(Bqkgb 0

~5

0

5

10

15

]

i

I

t

"

2O 25 t

I

3O 35 !

0

40

10 15 20 25 30 35 40

I .

r'"°

• •

"6"[

5

STATION I.

•

... 1|7CI

.':(1111~

... ~,Uel~(10x) ~tAm(10x)

-6

""

STATION 3.

Acevity(Bq t~'b 0

5

l0

I

*

I

0

-l-

¢s cm5 -s-4.

i

15 2O 25 I

/

I

l

I

3O 55 !

4O

I

~

,,~ ~.

-5-

"6!

STATION 5.

Fig. 5. Depth profiles of ~37Cs,2~9.25°puand 24~Amin sediment cores from the Kara Sea. The date lines are derived from 2~°Pbdating of the two cores.

1220

M.S. Baxter et al.

Table 4. Selected radionuclide inventories in the Kara Sea (PBq) 2~9'2~Pu

37Cs

~Sr

Estimated inventor),*: Global fallout U.S.S.R. discharges W Europe discharges Run-off of global fallout Chernobyl fallout

0.5 0.1 0.2 0.05 0.1

0.3 I

Total

0.9

1.4

0.03

Calculated inventoryt: Water Sediment Total

0.9 0.5 1.4

0.5 0.001 1.5:~

0.001 0.02 0.02

0.03

0.1

*The inventory estimated from published data on contributions of various anthropogenic sources to the radioactivity of the Arctic Seas. A contribution by local fallout is uncertain. ~-The inventory calculated on the basis of measured radionuclide concentrations in water and sediment reported by Strand et al. (1993) and Hamilton et al. (1994). :~Asthis estimate is based principally on samples from the W Kara Sea, an Ob and Yenisey contribution (~1 PBq) has been added to the ~Sr value.

depth in the sediment and much of the stratigraphic record appears to have been lost. By contrast, 239'24°Pu shows variable behaviour with a subsurface maximum at Station 1 and a relatively uniform distribution over a homogeneously mixed layer between 1 and 4 cm depth at Station 5. Furthermore, the depth penetration of "TCs in the sediment is greater than would be expected from the 2~°Pb age. In order to explain the anomalous behaviour of ~37Cs compared to the transuranic elements (and presumably 2~°pb), we have proposed that ~37Cs is more subject to chemical diffusion or mass transport in pore waters and to inverse solution (desorption) (Hamilton et al., 1995). However, the most significant information from the Kara Sea data is that the 23spu/z39"24°pu and 24~AmF39'24°pu isotopic ratios for surface sediments from the Kara Sea range from 0.02 to 0.05 and 0.3 to 0.5, respectively, and therefore suggest a global fallout origin of the plutonium there. Similarly, the inventories of 239'34°puin the sediments vary from 0.01 to 0.03 kBq m - 2and the 239'24°pu/137Csinventory ratios are between 0.02 and 0.03, within the range expected for global fallout. An estimate of the present inventories of 137Cs, 9°Sr and 239'24°pu in the Kara Sea combined from the published data is given in Table 4. The highest contributions are from global fallout and from discharges by the Ob and Yenisey rivers. The local fallout contribution is highly uncertain, although clear elevations of radionuclide levels in the SW Kara Sea and SE Barents Sea have been reported (Strand et al., 1993; Hamilton et al., 1995). Table 4 also lists the estimated radionuclide inventories in the Kara Sea based on measured radionuclide concentrations in water and sediments (Strand et al., 1993; Hamilton et al., 1995). The differences may reflect contributions from local fallout, from the Sellafield reprocessing plant and from the Ob and Yenisey rivers. Nevertheless, the current position is that there is no experimental evidence that any significant leakage of plutonium (or any other nuclide) has occurred from

the disposed reactors. The main scientific requirement is therefore to predict the possible consequences of any future waste leakage, dispersion and transfer. To this end, dose calculations based on box-modelling have been carried out (Baxter et al., 1993a,b; Hamilton et al., 1995). Inventories of long-lived radionuclides (t½ > 1 year) in the reactors dumped in the Kara Sea indicate a global committed collective effective dose of about 30 manSv for the marine fish ingestion pathway, if instantaneous release had occurred at the time of dumping. The dose contribution by the actinides is only 1% of the total, while the contribution by fission and activation products is more than 70 and 20%, respectively. Collective doses of this magnitude are relatively minor on a global basis. The emphasis in IAEA-MEL's future modelling will therefore be placed on local and regional dispersion modelling, the spatial scale most relevant to plutonium's restricted residence time in sea water. It is feasible that, on these scales, significant critical group exposures could occur (Baxter et al., 1993a,b).

Experimental Trends A recent IAEA publication (IAEA, 1993) produced at IAEA-MEL highlights analytical trends in measurement of low-level marine radioactivity. For plutonium isotopes, it describes the recent improvements in analytical sensitivities achievable by mass-spectrometric techniques. In these approaches, the direct detection of the plutonium ion results in a potential gain in sensitivity proportional to the mean radioactive lifetime. According to the above IAEA review, the minimal detectable quantity of plutonium isotopes is respectively 5 x 109, 5.5 × 107, 4.7 × 106 and 1 × 106 atoms for ICPMS (Inductively Coupled Plasma Mass Spectrometry), or-spectrometry, TIMS (Thermal Ionisation Mass Spectrometry) and RIMS (Resonance Ionisation Mass Spectrometry), corresponding for example to activities of 4.5 × 10 -3, 5 × 10 -5, 4.2 × 10 -6 and 8.9 x 10 -7 Bq 239pu. The quoted detection limits for ICPMS are, however, extremely conservative and relate to simple scanning liquid nebulisation. IAEA-MEL and other ICPMS groups have used ultrasonic nebulization and electrothermal vaporization (ETV) to enhance the sensitivity for environmental plutonium detection. In Japan, Kim et al. (1991) used the former sample injection technique in conjunction with high resolution mass-spectrometry to obtain a detection limit of 100 #Bq 239pu, while Sampson et al. (1991) in Scotland reported detection limits of 34 and 2.2/aBq 239puusing respectively scanning and single ion modes of ETV-ICPMS. The currently quoted detection limit for single ion mode ETV-ICPMS at IAEA-MEL is 0.5 #Bq 239Pu (McKay, 1994). Besides real and potential advantages in sensitivity, however, and indeed in much reduced machine time, ICPMS also offers the possibility to use simplified chemical pretreatments and to carry out multi-elemental

Observations on plutonium in the oceans

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Counts

1000 100

Pu

Pu Pu

U

236

237

238

239

240

Pu

241

242

Mass Number Fig. 6. ETV-ICPMS scan of 1 g of marine sediment collected near a nuclear discharge and subsequently chemically extracted in the laboratory, Table 5. Isotopic activity ratios in plutonium of various origins Source 24tPu/2Js-z4°Pu 2~Pu/239,24°Pu Nuclear fuel reprocessing Military plutonium Weapons test fallout Chernobyl fallout

25 3 16 90

0.23 0.014 0,026 0.45

isotopic analysis. Figure 6 shows an ICPMS spectrum obtained following dissolution and solvent extraction of a sediment sample collected in the vicinity of a nuclear site discharge (Sampson et al., 1991; Scott et al., 1991). Note that the isotopic compositions o f U, N p and Pu can be determined simultaneously. One disadvantage o f I C P M S for plutonium is clearly seen to be the extreme isobaric interference in the 238Pu region from naturally occurring 23su so that very exhaustive removal o f uranium is necessary if 23Spu is to be determined. There is in parallel a new international interest in using such measurements of ultra-low-level environmental isotopic signals to monitor and characterize nuclear activities. The aquatic environment is particularly suited to the recording of such signals. Plutonium isotopes and their ratios provide rather sensitive indicators of nuclear operations, as partly shown in Table 5. At I A E A - M E L , ICPMS, liquid scintillation counting, high resolution ~- and x-ray spectrometry and a-spectrometric methods are being focused on measurements of ratios of plutonium isotopes and of other activation and fission products in order to test the feasibility of environmental safeguards and compliance monitoring of aquatic and marine samples. M o r e generally, however, the laboratories studying environmental plutonium distributions in order to increase understanding of speciation, transfer, dosimetry or source-terms will, like I A E A - M E L , increasingly rely on a multi-technique approach, combining radiometric and mass-spectrometric methods. Acknowledgements--IAEA-MEL operates under an agreement between the International Atomic Energy Agency and the Goverment of the Principality of Monaco.

References Aston S. R. (1980) Evaluation of the chemical forms of plutonium in sea water. Mar. Chem. 8, 319. Aston S. R. and Fowler S. W. (1984) Experimental studies on the bioaccumulation of plutonium from sea water and a deep-sea sediment by clams and polychaetes. J. Era,iron. Radioact. 1, 67. Baxter M. S. (1983) The disposal of high-activity nuclear wastes in the oceans. Mar. Poll. Bull. 14, 126. Baxter M. S., Osvath I., Povinec P. P., Scott E. M., Miquel J.-C. and Hamilton T. F. (1993a) Non-local radiological consequences of nuclear waste dumping in the Arctic Seas: a preliminary assessment. In Radioactivity and Environmental Security in the Oceans: New Research and Policy Priorities in the Arctic and North Atlantic, Proc. Conf., Woods Hole, 1993, pp. 329-341. Wood Hole Oceanographic Institution. Baxter M. S., Hamilton T. F., Harms 1., Osvath I., Povinec P. P. and Scott E. M. (1993b) IAEA programmes related to the radioactive waste dumped in the Arctic Seas. Part. 2. Arctic Seas Assessment Project at the IAEA Marine Environment Laboratory. In Environmental Radioactivity in the Arctic and Antarctic, Proc. Conf., Kirkenes, 1993, pp. 93-96, NRPA, Osteras. Beasley T. M. and Fowler S. W. (1976a) Plutonium and americium: uptake from contaminated sediments by the polychaete Nereis diversicolor. Mar. Biol. 38, 95. Beasley T. M. and Fowler S. W. (1976b) Plutonium isotope ratios in polychaete worms. Nature 262, 813. Bjerregaard P. S., Top~uoglu S., Fisher N. S. and Fowler S. W. (1985) Biokinetics of americium and plutonium in the mussel Mytilus edulis. Mar. Ecol. Prog. Ser. 21, 99. Carvalho F. P. and Fowler S. W. (1985) Biokinetics of plutonium, americium and californium in the marine isopod Cirolana Borealis, with observations on its feeding and molting behavior. Mar. Biol. 89, 173. ChoppinG. R., Roberts R. A. and M o r s e l W. (1986) Effects of humic substances on plutonium speciation in marine systems. In Organic Marine Geochemistry (Sohn M. L., Ed.), pp. 382-388. Am. Chem. Sac., Washington DC. Edgington D. N. and Nelson D. M. (1984) The chemical behaviour of long-lived radionuclides in the marine environment. In The Behaviour o f Long-Lived Radionuclides in the Environment, Proc. Syrup., La Spezia, 1983, pp. 19~/9. Commission of the European Communities, Luxembourg. Fisher N. S., Bjerregaard P. and Fowler S. W. (1983) Interactions of marine plankton with transuranic elements. 1. Biokinetics of neptunium, plutonium, americium and californium in phytoplankton. Limnol. Oceanogr. 28, 432. Fowler S. W. (1983) Radioecological aspects of deep-sea radioactive waste disposal. In Atti del 3 ° Convegno Nazionale di Radioecologia, pp. 236-258. A.I.R.P., Bologna, Italy.

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soils. In Speciation of Fission and Activation Products in the Environment (Bulman R. A. and Cooper J. R., Eds), pp. 143-150. Elsevier, London. Livens F. R. and Baxter M. S. (1988a) Particle size and radionuclide levels in some West Cumbrian soils. Sci. Total Environ. 70, 1. Livens F. R. and Baxter M. S. (1988b) Chemical associations of artificial radionuclides in Cumbrian soils. J. Environ. Radioact. 7, 75. McDonald P., Baxter M. S. and Fowler S. W. (1993a) Distribution of radionuclides in mussels, winkles and prawns. Part 2. Study of organisms under laboratory conditions using alpha-autoradiography. J. Environ. Radioact. 18, 203. McDonald P., Baxter M. S. and Fowler S. W. (1993b) Distribution of radionuclides in mussels, winkles and prawns. Part 1. Study of organisms under environmental conditions using conventional radioanalytical techniques. J. Environ. Radioact. 18, 181. McKay K. (1994) IAEA Marine Environment Laboratory, Monaco, personal communication. Mitchell P. I., Vives Batlle J., Ryan T. P., Schell W. R., Sanchez-Cabeza J. A. and Vidal-Quadras A. (1991) Studies on the speciation of plutonium and americium in the western Irish Sea. In Radionuclides in the Study of Marine Processes (Kershaw P. J. and Woodhead D. S., Eds), pp. 37-51. Elsevier, London. Morse J. W. and Choppin G. R. (1986) Laboratory studies of plutonium in marine systems. Mar. Chem. 20, 73. Mount M. E., Shaeffer M. K. and Abbott D. T. (1995) Kara Sea radionuclide inventory from naval reactor disposal. J. Environ. Radioact. 25, 11. Nash K. L., Cleveland J.-M. and Rees T. F. (1988) Speciation patterns of actinides in natural waters: a laboratory investigation. J. Environ. Radioact. 7, 131. Nelson D. M. and Lovett M. B. (1978) Oxidation states of plutonium in the Irish Sea. Nature 276, 599. Nelson D. M., Carey A. E. and Bowen V. T. (1984) Plutonium oxidation state distributions in the Pacific Ocean during 1980-1981. Earth Planet. Sci. Lett. 68, 422. Nelson D. M., Larsen R. P. and Penrose W. R. (1987) Chemical speciation of plutonium in natural waters. In Environmental Research on Actinide Elements, Proc. Syrup., Hilton Head S. Carolina, 1983, pp. 27--48, US DOE CONF-841142. Noshkin V. E. and Wong K. M. (1980) Plutonium mobilisation from sedimentary sources to solution in the marine environment. In Marine Radioecology, Proc. Seminar, Tokyo, 1979, pp. 165-178. Nuclear Energy Agency (OECD), Paris. Orlandini K. A., Penrose W. R. and Nelson D. M. (1986) Pu(V) as the stable form of oxidised plutonium in natural waters, Mar. Chem. 18, 49. Peinert R. D., Fowler S. W., La Rosa J., Miquel J.-C. and Teyssi6 J.-L. (1992) Vertical flux and microplankton assemblages in the Gulf of Lions during Spring 1990. In EROS 2000 (European River Ocean System, Proc. 3rd Workshop, Texel, 1992 (Martin J.-M. and Barth H., Eds), pp. 413-424, Commission of the European Communities, Brussels. Pentreath R. J. (1978) 237Pu experiments with the plaice Pleuronectes platessa. Mar. Biol. 48, 327. Pentreath R. J., Lovett M. B., Jefferies D. F., Woodhead D. S., Talbot J. W, and Mitchell N. T. (1984) The impact on public radiation exposure of transuranium nuclides discharged in liquid wastes from fuel element reprocessing at Sellafield, UK. In Radioactive Waste Management, Proc. Syrup., Seattle, 1983, Vol. 5, pp. 315-329. International Atomic Energy Agency, Vienna. Pentreath R. J., Woodhead D. S., Kershaw P. J,, Jefferies D. F. and Lovett M. B. (1986) The behaviour of plutonium and americium in the Irish Sea. Rapp. P.-V. R$un. Cons. Int. Explor. Mer 186, 60.

Observations on plutonium in the oceans Sampson K. E., Scott R. D., Baxter M. S. and Hutton R. C. (1991) The determination of 24°Pu/2~Pu atomic ratios and 23~Np concentrations within marine sediments. In Radionuclides in the Study of Marine Processes (Kershaw J. P. and Woodhead D. S., Eds), pp. 177-186. Elsevier, London. Sanchez A. L., Gastaud J., Noshkin V. and Buesseler K. O. (1991) Plutonium oxidation states in the southwestern Black Sea: evidence regarding the origin of the cold intermediate layer. Deep-Sea Res. 38, 845. Sanchez A. L., Gastaud J., Holm E. and Rots P. (1994) Distribution of plutonium and its oxidation states in Framvaren and Hellvik fjords, Norway. J. Environ. Radioactiv. 22, 205. Scott R. D., Baxter M. S., Hursthouse A. S., McKay, Sampson K. K. and Toole J. (1991) Detection of actinides in environmental samples by inductively coupled plasma mass spectrometry. Analyt. Proc. 28, 382. Silver G. L. (1983) Comment on the evaluation of the chemical forms of plutonium in sea water and other aqueous solutions. Mar. Chem. 12, 91. Sivintsev Y. (1995) The seabed sources ofradionuclides in the

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dumped reactor compartment of atomic icebreaker "Lenin". J. Environ. Radioact. 25, 3. Strand P., Rudjord A. L., Salbu B., Christensen G. C., Foyn, L., Lind B., Bjornstad H., Bjerk T., Nikitin A. I., Kryshev I. I., Chumichev V. B., Dahlgaard H. and Holm E. (1993) Survey of artificial radionuclides in the Kara Sea. In Environmental Radioactivity in the Arctic and Antarctic, Proc. Conf. Kirkenes, 1993, pp. 53--65. NRPA, Osteras. Yablokov A. V., Karasev V. K., Rumyantsev V. M., Ye. Kokeyev M., Petrov O. I., Lystsov V. N., Yemelyanenkov A. F. and Rubtsov P. M. (1993) Facts and problems related to radioactive waste disposal in seas adjacent to the territory of the Russian Federation. In White Book 3. Office of the President of the Russian Federation, Moscow. Yefimov E. I., Gromov B. F. and Pankratov D. V. (1994) Radionuclide composition of activity in NPP with liquid metal coolant dumped in the Kara Sea and some assessment of radiological consequences of this activity release. In International Arctic Seas Assessment Project, Proc. Advisory Group Mtg., International Atomic Energy Agency, Vienna, 1994.