Uptake of some radionuclides by winkles from cumbria, UK

J. Environ. Radioactivi~'9 (1989) 1-15

Uptake of Some Radionuelides by Winkles from Cumbria, UK

W. A. McKay & N. J. Pattenden* Environmental and Medical Sciences Division. Harwell Laborato~'. Oxfordshire O X t t ORA. U K

( Received 26 February 1988: revised version received t8 May 1988: accepted 30 May I988)

A B S T R A CT Between 1984 and 1986, winkles were collected on a regular basis from nvo intertidal sites on the Cumbrian coast within 14 km of British Nuclear Fuels' Sellafield plant. Samples o f their flesh were analysed ]br :~SPu, :Jg+'-4OPu. :41Am, tS7Cs and t°~Ru. All these nuclides, though showing considerable temporal variability, strongly correlated with each other, suggesting that the primary route o f their uptake was the same. The W~Ru concentrations have a strong additional dependence on very recent Sellafield discharges. The nuclide quotients in the winkles are comparable with those of the surrounding seaweed and particulate matter suspended in seawater. They agree most closely with the particulates, suggesting that ingestion of this latter material is the main source of the winkle's artificial radionuclide content, though a high er proportion of the activit3, assimilated in to its flesh mav come from the sea weed. Samples were also examined for ~-activi~' using c~-track detectors. These showed the existence of particles of detectable e~-activitv. The estimated activity o f individual particles was up to O"7 mBq and their combined activity corresponded to about 8% on average of the total in the samples. They were most likely ingested along with the sediment debris, since particles of similar activity and abundance have been observed in seawater-suspended particulate matter from this area.

+Present address: 73B Essex Street, Newbury, Berkshire RG 14 6RA, UK. 1

© 1989 U K A E A copyright

2

W . A . McKay, N. J. Pattenden

INTRODUCTION For more than 30 years, radioactive effluent has been released routinely, under authorisation, into the Irish Sea from the nuclear reprocessing plant of British Nuclear Fuels Limited (BNFL) at Sellafield in Cumbria. One of the most important pathways contributing to the radiation dose received by consumers in the local fishing community, from the effects of these discharges, is the consumption of the winkle, Littorina littorea, collected from the beaches adjacent to Sellafield (Hunt, 1987). The activity in these winkles is routinely monitored by both the Ministry,' of Agriculture, Fisheries and Food (MAFF) (Hunt, 1987) and British Nuclear Fuels (BNFL, 1987), who publish annual means and dose estimates. The dose is primarily due to plutonium, :a~Am and to a lesser extent ~°~Ru and ~37Cs. In consequence, any information concerning the movement of these nuclides from the discharge to their uptake and retention by the winkles could have a bearing on future dose estimations from this pathway. There appears to be a relationship between discharges and winkle activity strong enough to enable predictions into the near future (a few years) to be made (Hunt, 1985). However, a longer-term prediction of winkle activity requires a greater understanding of the mechanisms of their different routes of uptake, in par-ticular the importance of sediment debris taken up during feeding. The work described here, which involved the collection of winkles from the coastal area near Sellafield and the analysis of their flesh for Pu, Am, Ru, Cs and o~-emitting particles, was undertaken to add to this information.

SAMPLING AND A N A L Y T I C A L M E T H O D S Over a 2-year period beginning in November 1984, samples of the common winkle (L. littorea) were collected offthe rocks near the low-water mark at two intertidal sites in Cumbria, at Eskmeals T a m Point and Nethertown, at monthly or 2-monthly intervals. These sites are about 17 km southeast and 4 km northwest respectively of the Sellafield discharge point (Fig. 1). The winkles were boiled and shelled soon after collection. Following the advice of M A F F on the cooking method generally used by local consumers, they were not depurated prior to boiling to void sediment from their guts and mantle cavities. The flesh of approximately 500 winkles, each weighing 0.9 g on average (20% of their shelled weight), collected at any one date and site, was bulked together to provide a single sample. Subsequently, each sample (including the particulate-rich gut contents) was freeze-dried and ground to a fine powder (McKay & Szweda, 1987).

Uptake of radionuclides by winkles

~ Dumfries

/X'. K'rk°dbr'ght~. "~

.,~

"

l

/

eAspatria

/ / ~ Nlaryport ,eCockermouth Workington

~ Parton

¢ , ~ WhitehQven L~B ees Head

IRISHSEA

S,I;L,0 ~Seascate Drigg

5 Stole

'JO km 1 : 250000

~M i [ I °em~/~ ' ~ / / Fig. 1. Northeast Irish Sea coast.

3

4.

W. A. McKa)', N. J. Parrenden

Aliquots were analysed for :~SPu, 2>-2a~Puand :'tAm, using radiochemical and o~-particle spectrometric techniques (Lally & Eakins, 1978), Other aliquots were analysed for l°6Ru and t37Cs by ,/-ray spectrometry,' (Salmon et al., 1983) using high-resolution germanium detection techniques. In addition, the distribution of c~-activity in samples of ground winkle was investigated using CR-39 plastic c~-track detectors. The main objective here was to look for localised areas of relatively high activity, 'hot particles', and to compare the activity associated with these areas with the activity widely distributed in the material. The technique of using transparent sheet material such as CR-39 to record the ionisation tracks of c~-particles from environmental materials is well established (e.g. Hamilton & Clarke, 1981; Hamilton & Clifton, 1981; Kershaw et al., 1986). In this study, well-mixed aliquots of winkle powder were compressed into aluminium dishes, 22 mm diameter and 3.5 mm thick, using a hydraulic press. Great care was taken to ensure homogeneity of the samples immediately prior to compression, so that the surfaces of the pellets formed were representative of the volume of the material. Since no binding agent was used, the pellets could not be removed intact from the dishes and only one side was analysed. The pellets were held in close contact with CR-39 sheets and placed in plastic bags (100 ~,m thick) for 1 month. The plastic was then etched in 6 M N a O H at 80°C for 5 h. The autoradiograph formed by tracks of ce-particles which had escaped from the sample surface was now visible. It was inspected initially using a microfiche reader at x 2 0 - x 4 0 magnification. This enabled hotspots to be located, defined here as being a cluster of at least 30 c~-tracks, distinctive against normal background. However, for detailed counting of tracks, a microscope giving magnification of x200 was used (Fig. 2). Hotspots ( - ->0.1 mBq) and diffuse o~-activities were calculated using an efficiency of 25% based on a 2rr solid angle. A C T I N I D E AND I~7Cs RESULTS AND DISCUSSION The observed radionuclide concentrations in each sample of winkle flesh are given in Table 1 as Bq/kg wet weight. In Table 2, these results are combined as the arithmetic means of the Nethertown results for 1985 and 1986, and the Eskmeals T a m Point results for 1985. Table 3 gives the values from Table 2 in the form of nuclide ratios. Table 4 lists the correlation coefficients between radionuclides. The above results demonstrate or indicate the following points. (1) In 1985, whilst the mean observed concentrations of all the nuclides at Nethertown were greater than those at Eskmeals T a m Point by

L p . u ~ , . e ,_,_:, . . . . .

c . . . . . : u , e s o> ~ .r . . . . . .

5

factors of about 2 for plutonium, 1-6 for Am and 1.4 for ~-'-Cs. the differences are not statistically si_maificant, owing to the considerable variability between months (Table 2). (2) Comparing the changes in the Nethertown results bep,veen i985 and 1986, there is some indication from the means (though again not significant statistically) for a reducing trend in the :3'Cs concentrations. There is no evidence for a reduction in the plutonium or >LA~m concentrations over this period. (3) The annual mean plutonium isotope ratios (:>SPu/>~'Z~Pu) at the two sites were in the range 0.24-0-26 (Table 3). These are significantly lower than the same ratio (0.31) in the 1985 Sellafield discharges (BNFL, 1986) but the,,' are consistent with the range obser~'ed in nearby sediments and seaweed (Hunt, 1986, 1987), which was 0-220-26. (4) The variations with time of the concentrations of ~:Cs, :3SPu. :39*za°Pu and >'tAm at Nethertown and Eskmeals T a m Point were ver~, similar. The correlation coefficients are shown in Table 4; all are in the region of 0-9 and highly significant.

i 0 0 IJm

1

Fig. 2. Winkle hotspot.

~V. ,4. M c K a v . ,V. J. PattemJen

TABLE

1

R a d i o n u c l i d e C o n c e n t r a t i o n s in T o m i Soft Tissue

.4ctivitv ¢B~: k~ -~ '.~'er wr! O,r:e

. .¢0+ ..

,447r

"4

25'sPu

2~z 4 , 7

,3-(_~.

/¢~Ru

I• _,:.'-ca..,~=- !_~.4 2 812.2 m Ia-9

,~,~e'th e'rt o w a __,~ '~ ' J a n . 1985 a,~" M a r c h [985

244) = 0-S 105-0=3.3

6-4 = 0-3 .._.¢"9 " = 1.1

33.5 = 1• i 1 0 7 . S2 -. 0 . _

O.:" ' " _+,-t~' 222-7--4.2

9 A p r i l 1985 t 0 M a v 1985 28 J u i v 1985

!33.1 m 4.3 58-7__ 1-9 ._,. . . t. ..o . = 1'3

38-7 = 1-5 14.3=(0.6 9.0 _ 0-4

120-0 = 3-~ 68.3 ÷ ~ . " 24-3=0.9

333-4 = 6.0 3 ~ 5 - 1 = 28.!) t38.0=3-6 i ~ 6 . 5 = 14.5 79:..ll = 2-.:" 6= ~ - ~.. ,_~'.7.

14-9±0.5 7-9-0-4 4-9±0.2 12.1 = 0 - 5 2 l-(1 "*" . 0-4 . 15.0=0.3 I t-5 ~ 0-5

83.1 = 2 . 9 47.3=i-8 3 4 . 8 = 1-3 ~ -,~ N)-_~ = :._ .t [4.0 . . . =. :,-..+ . / , - 7 - - ,,._ -7 -~ 63.5 = 2.4

14l.! = 3 - 4 60.~ ~-'.'~. 5 0 - 9 = 3-1 -~ ( /..:.9 =. - ~=.9 1_4-9 ? . ' + _-~ . - _z, 1 1.8 . 9 .= 2 . .6 . 65-6 ± 2-0

4 Sept. t985 1 5 N o v . 198~_ 3 D e c . 1985 16 M a r c h 1986 .~"A p r i l 1986 2 5 M a y. 1986 1 J u n e 1986

6 3 - 6 = 1-7 .~,.'"V=l.0, 22-0=0-6 _ 48'9 + l'6 87.8 ± 1-5 62.5=t.l 44.7 _~ l-5

28 O c t . 1986.

" ~/ -'- 0.4 1~.

Eskmeals Tarn Point 25 N o v . 1984

_~.2

-,--o. 1

24.8 = 0-8

_"" ,-.~

- _+

3. t

6 3 3 . 7 = 12.7 .'i(-()-~9 -~'_ 40I -4 = 9 ._." ."~,-~ . . . . + 10.6 ~'~.~ = 7-7 ' 4. ( , - 3 + i i - 6 208.3 _~ 9.8 3(~).4

=

7.1

9'4"-0.4-

2.9=0-2

18-5=0.8

45.8=3-4

10-6 ± 0-5 24.9±0.9 77-2 ± 1-9

2-6 _ (1.2 5.9"-0.3 20.0 ± 0.6

17.8 = 0.7 3 l - 4 = 1-0 8, .0 = .:.9

69-8 = 3.8 85.5=2-7 298-3 = 6-9

7 ~,0"--'-. 15-8 ,',, .4 = 1 6 . 0 "'~q 5 4 8 - 4 = 10-4 l e,'~6.0 + "4.4

12 July 1985 9 O c t . 1985

19.7±0.4 25.0--0-5

4 . 6 -_- 0 . 2 5-9+0-2

..~l-)-'=~ . . l -~.~ 3 5 . 0 ± 1.2

47.8 = 2.4 ,I-.: =_-6

..,_-_ ",~ ~+s-s~, 18fi.8-,- 8.'~

19 D e c . 1985 28 M a y 1986

8 . 0 _ 0.~ 18,6 m 0-5

1.9±0.1 3.8 -~ 0-2

19.3=0-8 33-6 = 1.8

19.2=2.1 46-2 = 4-4

139.8=6-9 99-5 = 4.9

4 D e c . 1984 4 M a r c h 1985 6 M a y 1985

Q u o t e d e r r o r s ± let c o u n t i n g . W e i g h t loss on freeze drying. 71-6 ± 1.5% (l s.d.).

TABLE

Annual

2

M e a n C o n c e n t r a t i o n s in Winkles (Bq kg -~ W e t W e i g h t ) with Coefficients o f Variation A l s o S h o w n Nethertown, 1985

:37Cs 238pu 239*2'~pu :41Am

136 15-7 60 65

Nethertown. 1986

Eskrrtea[s Tarn ['otnt 1985

68% 73% 64%

83 12-6 52

42% 46% 47%

98 7-7 31

52%

68

42%

4l

I{1,4~7 83% 77c; ` 58%

Uptakeo[ radionuclides by winkles TABLE 3 A n n u a l Mean Concentration Quotients for Winkles

Nethertown. 1985

Nethertown. 1986

Eskmea[s Tarn Point, 1985

0" 263 l'089 2 "28 2.09

0" 244 1"322 I "612 1.219

0" 248 1"310 3" 17 2.42

:38 p u / 230- 24a)p u :al Am/Z39
TABLE 4 C o r r e l a t i o n Coefficients Between Radionuclides Measured in the Flesh of the Winkles

Correlation

r

Degrees or freedom

Significance

0-999_ 0-943 0-949 0-753 0.561 0.892

I1 11 11 11 I! lt 11

>99"9% >99-9% >99.9% >99.9% >99.0% >95-0% >99-9%

0-997 0-993 0.958 0-938 0-765 0-712 0-899

6 6 6 6 6 6 6

>99-9% >99 "9% >99-9% >99.9% >95-0% >95-0% >99-0%

,Vethertown 239+24°Pu:Z38pu 239+2a°pu:24iAm 239+2a'0PU: t37Cs 2atAm: 137Cs 239+240PU: lo6Ru 241A m : 1o6Ru t37Cs: lo6Ru

0"820

Eskmeals Tarn Point 239+2"~°pu:238pu 23,~+240p u : > t A m

23'~*-mPu: 137Cs 2~.tAm: 137Cs 239+240pu: m6Ru z41Am: to6Ru 137Cs: m6Ru

Typical concentration ratios for 137Cs/~9+24°pM and 137Cs/241Arn reported in seaweed (Hunt, 1986), sediment (Hunt, 1986), filtered seawater (McKay e t al., 1987) and suspended particulates (McKay et al., 1987) in Cumbrian coastal waters over the study period are given in Table 5. The differences in these mainly reflect differences in nuclide chemistry, since Pu and Am are sorbed more readily onto particulate matter than is Cs, which remains principally in solution (International Atomic Energy Agency, 1985). A comparison with the ratios observed in winkles shows that the latter agree more closely with the sediment and the suspended particulate ratios and disagree strongly (up to three orders of magnitude) with the filtered

8

W.A. McKay, .%'.J. Pattenden

seawater ratios. This provides evidence that the main source of activity contained within the winkle (including t3'Cs) is particulate material. Aspects of winkle biology and behaviour are described in detail elsewhere (e.g. Wright, 1936: Fretter & Graham, 1962: Merdsoy & Farley, 1973: Newell, 1979; Watson & Norton, 1985). In summa U, this particular gastropod mollusc species measures - 3 0 mm × 25 mm, lives for about 2 to 4 years and is found from the upper middle shore down to the low-water mark, usually on or around algae-covered rocks. It is a herbivore, selectively grazing on some large seaweeds and on algae-covered rocks, feeding subm e r g e d in calm waters, when there is little tidal movement, and uncovered when conditions are suitably damp. The grazed material, rasped off with a chitinous touthed radula, can include inorganic material that has settled out from the water column or has been scraped from the underlying rock. The smallest particulate is transported out of the stomach to the digestive gland ducts. Larger particles are covered in mucus to form faecal pellets. Their gut is filled four to eight times daily, with the food taking up to 6 h to pass through the body. The turnover of most of the inorganic particulate is likely to be of the order of a few days at most. Miramand et al. (1982), using another mollusc, the lamellibranch S c r o b i c u l a r i a p l a n a , estimated that only about i% of the -'3~Pu o n ingested silt was metabolised. If such a low transfer factor applies to gastropod molluscs then it is likely, for the actinides at least, that most of the activity in the winkles remains associated with the inorganic particulate material. A higher proportion of the activity assimilated into their flesh may thus come from digested algae.

l°~Ru R E S U L T S A N D DISCUSSION A n examination of the results and correlations for different radionuclides, in Table 1 and 4 respectively, suggests that the l°~Ru concentration, which unlike the others shows a clearly reducing trend over the study period, must be treated differently. Because of its relatively short half-life ( 1 year), ~ R u cannot accumulate in environmental media in the same way as the other radionuclides and consequently its concentration at any time in these wilt be more d e p e n d e n t on discharges from Sellafield over a relatively short preceding period. The observed m~Ru concentration in winkles at Nethertown and Eskmeals Tarn Point are plotted against time in Fig. 3, together with the monthly Sellafield discharges (Atherton, private communication). Discharges of m~Ru were reduced significantly by the coming into operation of the salt

Uptake of radionuclides by winkles

KBq wg-~

~

,

,

q

9

I06Ru CONCENTRATION IN WINKLES

A

3 etherlown

l

I

I

I

I

l

L -I'-']"

- r --F - 7-- r--r'-'l--

r'--i

I

I

I

I

(a) z.O TBq

B, l°6Ru INONTHLY DISCHARGE FROIN SELLAFIELD 30

20

10

. l l l ' l

DJFIN

W A

!

iN

J J 1985

A

S

) -"~, 1

0

N

O

]

I I

F

iN

A

I

iN

1 I I 1 I

J J t986

A

S

0

N

(b) Fig. 3. (a) L°6Ruconcentrations in winkles, (b) t°6Rumonthly discharge from Sellafield.

evaporator in March 1985 (BNFL, 1986) and this is clearly and rapidly reflected in the winkle concentrations. The correlation coefficients of 1°6Ru and the other radionuclide concentrations are given in Table 4. The coefficients are lower between l°6Ru and the actinides (0-561-0-765) than those between the actinides themselves but are still highly significant. This is consistent with there being a common pathway (i.e. via fine-grained particulate matter) for these nuclides but indicates that the L°~Ruconcentrations have a greater additional dependence on another factor, which in this case is the discharge from Sellafield over the short preceding period.

A L P H A - T R A C K RESULTS A N D DISCUSSION The results of the e~-track study of winkle-flesh samples are shown in Table 6. Hot particles were observed in most of the samples, with individual

i0

W. A. McKay. N. J. Pattenden

TABLE 5 Radionuclide Ratios in Seawater. Sediment and Seaweed Near Sellafield 1984,.'85 Medium and sampling point

Seaweedfl Fucus vesiculostts, Sellafield Bottom siltfl Whitehaven Filtered seawater° St Bees Seascale Eskmeals Seawater suspended particulate b St Bees Seascale Eskmeals Winklesc Nethertown Eskmeals Tarn Point

137CsY°* e4")Pu

5-9 3.7 590 ___7% 2 180 -* 6% 890 ± 8%

13rCs/-'aZA m

14.2 3-4 1 780 _+6% 12 100 -+ 13% 900 --- 6%

3.5 "-- 12% 1.8 ~ 25% 2.3 -+ 1l%

4.6 -+ I3% 1.7 = 25% 2-0 - 11%

2-3 z 10% 3-6 - 44%

2.1 - 30% 2.3 - 13%

"Hunt (1986)---samples collected during 1985. no errors quoted. bMcKay et al. ( 1987)---samples collected 28 March 1984. CThis re port. Quoted errors _-2-t s.d. (analytical). activities up to 0-7 m B q . The ratios of oe-tracks in hot particles and in the diffuse b a c k g r o u n d were b e t w e e n 0-01 and 0.10. F o r s o m e of the samples, replicate pellets were prepared and separately e x p o s e d to CR-39 sheets for c~-track studies. In all cases, replicate pellets f r o m the s a m e sample gave results for the diffuse b a c k g r o u n d activity which were within a range of _+25%, and for the ratio of hot particle to diffuse b a c k g r o u n d activity which were within a range of _+50%. T h e r e is a strong correlation (r = 0.89) between the sum of the radionuclide c~-concentrations (Table 1) and the diffuse activity from the c~-track m e a s u r e m e n t s , which suggests that most of the c~-activity m e a s u r e d by CR-39 is d u e to these nuclides. A s t u d y of hot particles in the particulate material suspended in seawater at E s k m e a l s ( - 2 km N of Eskmeals T a m Point) described previously ( M c K a y et a l . , 1987), in which particulate-loaded filter papers were exposed to C R - 3 9 , gave results similar in several ways to those from the winkle flesh (Table 6). T h e c~-activities of individual hotspots were of the same magnit u d e in each m e d i u m and the ratios of hot particle to diffuse b a c k g r o u n d activities were also similar. This strongly suggests that the winkles" hot particles were ingested during feeding along with sediment debris rather t h a n f o r m e d inside the winkles by lysomal and excretory processes. Such s e c o n d a r y hot particles m a y occur in winkles. Metal-containing granules are

Uptake of radionuclides b.v winkles

Alpha-Track

Location

TABLE

6

Analysis

of

:Vo. of hotspots on CR-39

ll

Winkle Flesh

Hotspot activio" (m Bq) (A)

DifJhse activity (rn Bq) (B)

A/B

Eskmeals 25 Nov. 1984 4 Dec. 1984 4 March 1985 6 May 1985 12 July 1985 9 Oct. 1985 19 Dec. 1985 24 Dec. 1985

0 0 2 3 2 2 0 l

--

9.{)

--

6-{)

0.2:0-5 {)-2: 0.3:0-4 0-2:0.2 0.2:0.2

7.0 20. l I0-5 8-6

---

0.10 0-04 0-04 0.05

--

9.0

O. 1

4.9

0-02

--

0-3 0.3 0.3 0.1: 0. t: 0-2; 0.2 0- I 0-I: 0.2; 0-2: 0-2; 0.2:0-3 0-1; 0-l:0-1:0-2: 0.5 0.7

7.7 10.7 25.2 213.3 19-9 11.8

0.04 0-03 0.01 0.03 0.01 0-It}

13-1

0-08

12- I

0-06

Nffhertown 25 Jan. 1985 30 Jan. 1985 27 March 1985 9 April 1985 lO May 1985 28 July 1985

1 l 1 4 i 6

4 Sept. 1985

5

15 No,,'. 1985 3 Dec. 1985 4 Dec. 1985

l 0 0

--

9.1

--

9.6

D rigg t3 Feb. 1985 13 June 1985

0 2

--

0.2:0.2

8-0

9.6

--

0-04

present (Enget & Brouwer, 1984) but the degree to which they concentrate radionuclides is unknown. Since a preliminary examination of the abundance of naturally occurring uranium and thorium series nuclides in this area, reported by Hamilton & Clifton (1988), did not indicate any si,maificant mineralisation, it is likely that these ingested hot particles are of Sellafield origin. However, without data on the chemical composition of the winkle hot particles, this cannot be confirmed. The term "hot particle' has also been used to refer to particles of aemitting radionuclides in the context of their inhalation and deposition in the lung and the possibly enhanced risk of lung cancer associated with them (ICRP, 1980). It has been suggested that this effect is associated with

12

W. .4. McKay. N. J. Parwnden TABLE 7

:-~SPu,/Z39-Za°PuRatios Generated for 1985with Varying Availabilit~ Times .4 vailabilitv rime (years)

i 2 3 4 5 6 7 8 9 10 1l 12 13 14 I5 16 17 18 I9 20

e3epu/'-39÷e'U)Pu (1985)

0.30 0.30 0.28 0.27 0.26 0.25 0.24 0.24 0.23 0.23 0"23 0.22 0.22 0.22 0.22 0.21 0.21 0.2l 0-2I 0-21

Using Hunt's (1985) availability model.

particles of a minimum activity of between 5 and 160 mBq (Tamplin & Cochran, 1974), though such a risk has not been clearly established (ICRP, 1980). Whilst hot particles in winkles are not directly relevant to this situation, their source is likely to have been seawater. In that medium they were potentially available for inhalation via resuspension in marine aerosol or dry intertidal sediment. In the present study, the maximum particle activity observed was only 0.7 mBq and thus their radiological effect would be included in the summation of the a-activity in studies of the inhalation pathway. An estimate of the availability time of Pu after discharge (and by implication the other particulate-associated nuclides) to winkles in the study area can be made from a simple model such as that described by Hunt (1985). This used an exponential model to estimate the availability of Pu and Am, according to which the fraction of the nuclide remaining in some environmental medium after elapsed time t would decrease in proportion to e -~' where X is the rate constant for a given element. The reciprocal of h is the

Uptake o.f radiontwlides by winkles

13

mean time of availability, a measure of the period for which the nuclide remains available in the environment after discharge. A series of :>Pu/:>-:'~)Pu ratios has been generated using this model for varying availability times using Pu discharge data (BNFL, 1986. 1987 Stather et al.. 1986) with estimated :3~pu/:>-:U~Pu ratios prior to t978 (Howorth and Kirby, 1988). The results are listed in Table 7. Comparison with the winkle isotope ratios in Table 3 implies that Pu released from Sellafield remains available for uptake for at least 6 years. If most of the Pu in winkles is on ingested suspended particulate matter, this suggests that a similar availability time applies to suspended particulates. It should be emphasised, however, that the exponential model assumption is not a good basis for extrapolation far into the future.

CONCLUSIONS The :3SPu, :>+>°Pu, :a~Am, t37Cs and ~(~Ru concentrations in winkle fresh from two sites near Sellafield have been regularly measured over a 2-year period. Despite considerable temporal variability, the measurements show a very strong correlation between individual nuclides, indicating a similar route of uptake. In addition, the "~Ru concentrations also have a strong dependence on very recent Sellafield discharges. The ~37Cs/239~:~Pu, '3:Cs/>tAm and ~3rCs/t~Ru ratios suggest that most of the winkle activity comes from ingested inorganic particulate matter, though the proportion assimilated into the flesh may be very low. Alpha-emitting particles up to 0.7 mBq have been found in the winkles but their activity is too low to cause enhanced radiological risk, even if inhaled. It seems probable that most of these particles originate from Sellafield, being taken up bv the winkles with ingested sediment debris. However, the possibility that they are of natural origin cannot be completely discounted. The Sellafield-derived activity associated with the seawater particulates ingested by winkles could remain available for uptake in the study area for an estimated 6 years after its discharge.

ACKNOWLEDGEMENTS This work was partly sponsored by the Department of Energy. The winkles were collected as part of a separate project funded by the Cumbria Sea Fisheries Committee. We thank our colleagues P. Burton and N. J. Lewis for the actinide and T-analysis, J. M. Howorth for running the availability

14

W. A. McKay, N. J. Patrenden

time model, D, Latawiec and J. Harold for general assistance and J. A. G a r l a n d and B. O. Wade for advice and encouragement.

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Uptake of radionuclides by winkles

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