JJ. Environ. Radioactivity 2 (1985) 23-40

64 01 B red on© - B e lg iu m -T a i . 0 5 9 1 60 ¿ 7 15

Some Observations on the Geochemistry and Isotopic Composition of Uranium in Relation to the Reprocessing

of Nuclear Fuels

E . I. H am ilton and H . E . StevensInstitute for Marine Environmental Research, Prospect Place, The Hoe, Plymouth,

Devon PLI 3DH, UK

In order to determine whether or no t uranium depleted in 235 U and derived fro m the British Nuclear Fuels L td (B N F L) nuclear fu e l reprocessing p lan t at Sellafield, Cumbria, U K can be detected in environmental samples, we have investigated the isotopic composition o f uranium in am m onium carbonate leachates fro m marine and terrestrial samples fro m near Sellafield. Som e show a depletion in 235 U and the presence o f 236 U which unequivocally identifies the presence o f uranium derived fro m B N F L . The 2MU /m U activity ratio and total uranium content o f sam ples are not significantly different fro m those o f natural uranium abundances. The highest concentrations o f uranium are fo u n d in anaerobic organic-rich silts and the lowest in sandy silts and coarsegrained sands.

Since the reprocessing of spent Magnox fuel (uranium oxide) rods first started at the BNFL site at Sellafield in Cumbria in the latter part of the 1950s, the factory has discharged —70 tonnes of uranium in an unknown chemical and physical form into the NE Irish Sea (see Fig. 1). In seawater, uranium exhibits conservative behaviour and, because of its ubiquitous concentration of —3 /xg U per litre, Pentreath et al. (1979) noted that in local seawater the discharges of uranium from BNFL would not be detected; this was confirmed by Hamilton (1980) who determined a

J. Environ. Radioactivity 0265-93lX/85/$03.30 © Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain

A B S T R A C T

INTRODUCTION

23

24 E. I. Hamilton, H. E. Stevens

3 3 0

329# \ C 9 2 /9 6\ ä s e l l a f i e l d

327IRISH SEA

7 8 # ,

137Ravenglas:

^Sand \ Diines CORE 3 ■4121

41 , 91, 51

I382 0 Km

Fig. 1. Sampling sites.

concentration of 2-8 ± 0-3 /xg U per litre in 31%o seawater of the Esk estuary, Cumbria, —10 km to the south of Sellafield. Later, however, H am ilton (1981) identified the presence of uranium associated with particulate debris originating from BNFL in sediments of the Esk and at the air/seawater interface; hence the sediment (particulate) transport pathway could provide a route for the introduction of BNFL debris into Esk sediments. Because of the similarities in abundances of several

Reprocessing o f nuclear fuels and the isotopic composition o f uranium 25

gamma- and alpha-emitting radionuclides in BNFL effluents and in sediments of the Esk, they must represent particulate phases either of spent fuel rod debris (possibly derived from the open spent fuel cooling ponds), a phase which is produced within the confines of the factory and whose composition is retained in the sea (for example floes of hydrated iron and magnesium oxides), or of solid matter formed by interaction between the effluent and seawater at the discharge site.

MATERIALS AND METHODS

H ere we consider the concentration of uranium together with its isotopic composition (a-activity ratios and mass abundance) in representative samples (see Fig. 1) of sediment from the Esk estuary, bottom sediment from the Irish Sea adjacent to Sellafield and, in order to sample any contribution from aerial deposition, in moss and lichen from the littoral zone near Sellafield. The total concentration of uranium in the samples was determined either by the delayed neutron method (140 samples) (Hamilton, 1966) or by instrumental neutron activation analysis (INAA) using the reaction 238U ny >239U (8 samples). Within an analytical error of ±5% and a sampling error of —±10% , both methods gave the same results; hence, as the former is based upon the fission of 235U and the latter upon activation of 238U , analysis of bulk samples does not provide any evidence of a depletion in 235U which would be characteristic of spent Magnox fuel. The 234U /238U activity ratio and mass iso topic composition were determined on the ammonium carbonate leachates of samples as described by Joshi and Ganguly (1972,1976). The technique is accepted as one which removes uranium associated with surfaces, rather than that present in accessory minerals. Knowing the total concentration of uranium in the samples and assuming that the fraction of total uranium which is leached is reproducible, the total mass of uranium leached was calculated using the following experimentally determined factors: ion- exchange recovery, 90 ± 10%; electro-plating efficiency, 70 ± 15%; and a 4-7T silicon surface barrier counting efficiency of 17 ± 1%. Samples, from Core 3 at 5 and 25 cm depth, of Drigg mud (78) and of Irish Sea sediment (330) were subjected to a second leaching with saturated ammonium carbonate to determine the efficiency of the method for removal of leachable uranium;

26 E. I. Hamilton, H. E. Stevens

is leachable is removed by the method. The 24lAm content of samples was determined by gamma spectrometry as described by Clifton and Hamilton (1982). In order to evaluate the influence of redox states on the mobility of uranium in sediments, in-situ 200 MW cut-off Medicell dialysis tubes containing a mixture of equal volumes of 3 1 % o seawater and deionised water were placed into, and upon, Esk sediments. After two days the tubes were removed and the dialysate aerated to precipitate a hydrated iron-rich floe (i.e. Fe±4Fe+3) in which the concentration of uranium was determined by INAA; uranium was not detected (

Reprocessing o f nuclear fuels and the isotopic composition o f uranium 27

TABLE 1a Activity Ratios, 236U Activity (Surface Barrier Alpha Spectrometry) and Total Concentration of Uranium together

with Leachable Uranium for Samples of Sediments and Moss, from near Sellafield, Cumbria

Material, localitya Activity ratios

(Pa i g X i o 4)Total U

(ppm/dry wt)Percentage leached U (estimate)

^ U / m U Ul

M ud, Tamar Estuary, Devon 1-15 ± 0-01 0-045 ± 0-002 0* 4-2 5-8

Sediment, Irish SeaSite 329 1-05 ± 0 02 0-044 ± 0-003 - 0 2-0 1-0Site 330 M 0 ± 0-02 0-043 ± 0-004 ~ 0 2-9 0-8Site 327 1-09 ± 0-03 0-042 ± 0-004 ~ 0 2-1 1-3

Surface muds, EskC l 13 Eskmeals sand 1-13 ± 0 03 0-042 ± 0-004 ~ 0 1-2 4-7138 V iaduct mud 1-08 ± 0-02 0-044 ± 0-004 0 + 1-8 7-0121 W aberthwaite mud 1-15 ± 0-02 0-042 ± 0-003 ~0 1-6 7-7137 Esk Bridge mud M l ± 0-02 0-044 ± 0-004 0 2-6 5-578 Drigg mud 1-12 ± 0-01 0-044 ± 0-003 1-4 nd nd

Sedim ent cores(a) C ore 3, silt Esk

Sampling depth (cm)5 1-06 ± 0-02 0-037 ± 0-003 3-1 3-4 0-7

10 1-03 ± 0-02 0-044 ± 0-004 ~ 0 3-4 0-815 1-07 ± 0-02 0-040 ± 0-004 ~ 0 3-2 0-920 1-02 ± 0-02 0-046 ± 0-003 ~ 0 3-1 1-425 1-07 ± 0-02 0-037 ± 0-003 0-9 3-0 1-030 1-07 ± 0-02 0-043 ± 0-004 0 + 2-9 0-835 1-07 ± 0-02 0-045 ± 0-005 0+ 2-9 1-040 1-01 ± 0-02 0-046 ± 0-004 0+ 2-8 0-345 1-01 ± 0-03 0-044 ± 0-003 2-7 0-350 103 ± 0-02 0-042 ± 0-004 0-5 2-6 0-255 1-11 ± 0-02 0-047 ± 0-003 0+ 2-5 0-760 1-09 ± 0-02 0-045 ± 0-003 0+ 2-5 0-765 1-00 ± 0-02 0-037 ± 0-003 0-9 2-4 0-870 108 ± 0-02 0-043 ± 0-003 0-004 2-2 0-975 100 ± 0-04 0-046 ± 0-005 0+ 2-2 0-4

(b) C ore 1176, silt Esk1 2 + 3 + 5 cm sections 1-17 ± 0-02 0-043 ± 0-003 ~0 3-6 15-8227 + 29 + 32 + 33 cm sections 1-15 ± 0-03 0-047 ± 0-003 0+ 2-1 5-7

(c) C ore 1280, silt Esk2 9 + 10 cm sections 1-14 ± 0-03 0-044 ± 0-004 ~0 2-0 2-8233 + 3 4 + 3 5 cm sections 1-15 ± 0-02 0-046 ± 0-003 0+ 2-1 4-4

Vegetation'1-15 ± 0-03 0-035 ± 0-004

C96 moss. Friar Well near Sellafield 1-16 ± 0-02 0-031 ± 0-004 91 0-06 —11-24 ± 0-03 0-032 ± 0-003

C92 moss, St Bridgets Church Í1-15 ± 0-02 0 037 ± 0-003 41 .n n.finear Sellafield \ 1-14 ± 0-02 0-039 ± 0-003 410 U’o

127 Salicornia, Newbiggin Viaduct 1-17 ± 0-03 — 0+ 0-05 —

111 lichen, Newbiggin Viaduct — - — 0-01 —

0 +,n o t detected—0, trace, present but not measurable nd , not determ ined

wdd

E. I. Hamilton, H. E. Stevens

CM

‘ l u n i u B J f i

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Ö iá O CX Ë oc

curri

ng

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ac

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s. (NB

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hide

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)

Reprocessing o f nuclear fuels and the isotopic composition o f uranium 29

2 4 1 3T o t a l ° i & A m B N F L x 1 0 C i

OI X 6 > in I d N B ’S L u d d n

30 E. I. Hamilton, H. E. Stevens

does not appear to be related to the BNFL releases of uranium. In Fig. 2, the increase of uranium at —34 cm, in a core (91) of constant lithology, may be related to the 1969-70 enhanced levels of uranium released by BNFL. Evidence that the historical pattern of BNFL releases of radionuclides is preserved in accreting sediment profiles is also illustrated in Fig. 3; the 241 Am releases by BNFL and concentrations of this radionuclide in the sediment profile are matched, but are not coincident, possibly because in the model (Hamilton & Clarke, 1984) a lag time, for transit of BNFL debris from Sellafield to the Esk, of —1 year has been used. This value does not take account of the relative rates of transport of 241 Am in its conservative and non-conservative forms; better estimates can only be made when more information is forthcoming concerning the composition of the BNFL source term for uranium.

Estimates for the mass of uranium leached by the ammonium carbonate treatment of samples are given in Table 1, and for most samples amount to 1-0, according to Hodge etal. (1979), a significant absorption of dissolved uranium from seawater is indicated. Measurement of 235U in the presence of 236U is difficult, but is possible, as illustrated in Fig. 5, by selecting those samples with identifiable 236U alpha energy peaks, which are related to the degree of apparent depletion in 235U. However, the 234U /238U activity ratio will be dominated by natural uranium leached from samples; hence once more the influence of BNFL debris is considerably diluted, even although the total amount of uranium leached from the samples is small compared with the total concentration of uranium.

Reprocessing o f nuclear fuels and the isotopic composition o f uranium 31

T a m a r E s t u a r y , mud - Devon.Moss C 92 Se l l a f i e l d -Cu mb r ia .

2 3 4

2 3 8 ,

2 3 4

2 3 8

6 8 5 1 c t s

M e V

C a l i b r a t i o n

2 3 52 3 5 , 2 3 6 ,

Channel number

Fig. 4. Uranium spectra for moss C92, which contains 236U, compared with uranium from an estuarine sediment site distant from any influence of BNFL effluent.

A fter ~ 6 years of irradiation in a Magnox reactor the 234U /238U alpha particle activity ratio would be -0 -8 . However, it is likely that the irradiation times for average processed uranium fuel released by BNFL during the 5-10 years prior to sample collection would have been appreciably shorter. It is most unlikely that sources of highly depleted uranium derived from activities other than the reprocessing of Magnox fuel would contribute to the total uranium in the effluent from the BNFL site.

The isotopic composition of uranium determined by mass-spectro- metry on leachates from samples is presented in Table 2; the data confirm the presence both of some depletion in 235U and of 236U in sample C92. The ratios 235U /238U (a-activity)/235U /238U (mass-spectrometry data converted to units of a-activity) for C92, Drigg mud and Core 3 (0-15 cm) are 0-94, 1-04 and 1-1 respectively; for Core 3 the U-activity ratio was first deter-

32 E. I. Hamilton, H. E. Stevens

° C 9 6

O)

C o r e 3

Depth cm

Q .

* Dr igg

50

40 i 30U A c t i v i t y Rat i o x IO’ 4

462 3 82 3 5

Fig. 5. Relationships between the concentration of 236U (pCi/g U) and the 23S'U/238U activity ratio for some samples of sediment and moss (C96, C92) from the vicinity of

Sellafield, Cumbria.

mined, then the uranium was removed from the counting planchet and used for the mass isotopic analysis. A 236U /238U activity ratio of 0-83 was calculated for the moss sample, C92, based upon the mean mass iso topic data and those obtained by alpha spectrometry; this is considered to be acceptable because of the low abundance of 236U in the sample.

DISCUSSION

For a conservative species of radionuclide, in seawater, such as uranium, H amilton and Clarke (1984) estimate a transit time of

Reprocessing o f nuclear fuels and the isotopic composition o f uranium 33

o o+1 +1 on m

o © © ó o o

o o o o+1 +1 +1 +1ro 00

mo

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3Oí/1C/335 «6 exO

34 E. I. Hamilton, H. E. Stevens

Sellafield to the Esk. If the total mass of uranium which has been released into the Irish Sea from the reprocessing of uranium fuel rods has been retained in an off-shore parcel of water (see Box 1,26-5 km3, in Jefferies et al. , 1982), this would only increase the normal levels by c. 30%. However, assuming normal conservative transport of uranium based upon the dispersion of 137Cs in the Irish Sea (as described by Jefferies et al., 1982), the contribution from BNFL would not be detected.

A study of surface sediment from the Esk shows an association between uranium and the presence of anaerobic conditions which are associated with decaying organic matter; with an increase in depth in sediments the anaerobic zone is lost (see Fig. 2, Core 1176 at ~12 cm and Core 1280 at ~ 10 cm in depth) with an associated significant reduction in the concentration of uranium. We interpret this feature as illustrating an active association between uranium and the organic-rich anaerobic facies; once this phase is lost by depletion of organic matter, the structure of the sediment is lost and the anaerobic thixotropic state gives way to coarse aerobic sands which contain the lowest concentrations of uranium, e.g. Cores 41, 51, 82 and 91. The anaerobic fine-grained silts contain between 1 and 3% organic carbon (Hamilton, 1984) but, in relation to the thixotropic nature of these sediments, this is enough to impart a dark brown to black colour to the fresh material and also to provide an inter-grain matrix which is sufficient to account for the coherence of the sediment as a whole. As Core 3 (Fig. 3) is accreting at a rate of -3 -5 cm y (Hamilton & Clarke, 1984), the lower parts will have been deposited before BNFL operations had started; the uranium present is mainly contained in common accessory minerals such as sphene, zircon and fluorite.

The samples of flocculated dialysate liquid from the aerobic sediment contained between 1-8 and 2-0 ppm U (in dialysate residues), compared with

Reprocessing o f nuclear fuels and the isotopic composition o f uranium 35

burn-up time of the fuel which is unknown in relation to the BNFL discharges, but the maximum time is likely to be —2000 days. Using data obtained from the RICE CODE (see Mairs & Nair, 1979) together with data in Flowers (1976), we calculate that, after 2000 days of bum-up, Magnox fuel will contain —580 ppm 236U/gU. If sample C92 contained —1 ppm U, then a concentration of 40 x 10 ~4 pCi 236U/g sample is equivalent to a burn-up time of 218 days and a 10-8% depletion of 235U; this may be compared with a value of 11-0% depletion which was obtained by mass- spectrometry. However, it should be noted that the BNFL uranium effluent can contain uranium from many diverse sources which have been exposed to a variety of bum-up times.

The highest 234U /238U activity ratios are found in the samples of moss (C92 and C96) which were collected near the coast in a region which is subjected to direct aerial deposition of marine particulate debris as a result of the penetration of sea spray inland, as described by Eakins & Lally (1984).

In Japan, Sugiyama & Takagi (1977) were unable to detect any unusual levels of uranium in bulk samples of river water and marine sediments in the vicinity of a nuclear fuel factory. Joshi & Mishra (1981) report a mean 235u/238u activity ratio of 0-046 ± 0-001 for coastal sediments from the East coast of India, which is the same as that for natural uranium. For our samples which contain some 236U (n = 12), we calculate a 23SU /238U activity ratio of 0-043 ±0-001, and, for those which did not, 0-045 ±0-001. Gorham (1958) and Cambray & Eakins (1982) have described the transport of marine aerosols and some radionuclides from the littoral region of the NE Irish Sea across Cumbria. In sediments of the Esk, hot particles containing alpha emitters (U, Pu, Am, Cm) are found on the sea-surface and in the upper layers of sediment (Hamilton, 1981); away from the coast, hot particles are also found at some sites, but the distribution appears to be of a random nature. There are several possible sources for the hot particles; for example, marine aerosol debris, especially from the sea-surface in the vicinity of the BNFL outfall, general wind-blown debris from the BNFL site, especially from the surface microlayer of the open cooling ponds in which spent uranium fuel rods are stored prior to reprocessing, and an unknown, but probably insignificant, contribution from stack releases. During the Windscale accident in October 1957, which involved overheating of uranium fuel elements in a reactor, Dunster et al. (1958) and Ellis et al. (1960) noted the release of small quantities of uranium in particulate form which penetrated the filters in

.36 E. I. Hamilton, H. E. Stevens

the reactor stacks. Since then the filtration efficiency for retention of particulate debris from stacks has been improved considerably. By selection of dated undisturbed soil profiles in the vicinity of BNFL, the presence of particulate forms of uranium released during the accident should be easily measured and, from an examination of the post-1957 soil horizons, any subsequent deposition could be determined.

The 234U /238U activity ratios of the samples we have examined indicate a mixture of marine and terrestrial sources and in some areas a slight contribution from BNFL debris which is of no significance in relation to risks to man from ionising radiation. The 234u / 238U activity ratios we observe are similar to those reported by Borole etal. (1982) (1-05 ± 0-03) for marine sediments from two estuaries in India and those by Church et al. (1981) (1-03 to 1*14) for a salt marsh in USA. Matsunami & Mamuro (1968) determined a 234u / 238U activity ratio of ~1 for uranium particles in nuclear fallout debris. Hodge et al. (1979) have shown that continental debris tends to have a 234U /238U activity ratio of

Reprocessing o f nuclear fuels and the isotopic composition o f uranium 37

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