ABSTRACT
Pu (Figure 4a) have been shown to be largely incorporated into the sea bed off Sellafield, together with another transuranium nuclide, 241Am, which is not only discharged directly but also arises from the decay of 241 Pu (Figures 4b and 5a). The total environmental inventory of 241Am - including that from 24IPu decay - will continue to increase, reaching an asymptotic value in about 50 years time some 1.3 times that existing in 1987. In consequence, the 241Am/ 239+240pu quotient will also change with time. The annual and cumulative values (Figure 5b) are of use because they indicate the limits which may be observed in near-shore settled sediments, depending on the extent to which all past discharges have been homogeneously mixed into them. Off the Sellafield coast it has been demonstrated that Pu concentrations can vary markedly between sediment cores at the same depth( 17,18) and even in the
(19) (20 21) same core , an effect resulting from bioturbation . An identification of these processes has been greatly assisted by the use of the 239+240Ru/238 Pu quotients (Figure 6a) which, prior to 1978, are not known precisely for the effluents, but have been inferred from a time-series of surface mud sediments in an estuary just south of the site. Also of value are the 137Cs/2394-240Pu values (Figure 6b) because, although the bulk (90%) of the Cs discharged remains in the water column, the quantities discharged are such that both Cs nuclides are readily detectable in the sediments. Thus, from the short-lived nuclides such as 95Zr/95Nb, through nuclides
144ce, 106Ru, 60 such as Co and the Cs nuclides, to the transuranium series, a complete suite exists to examine the depth, rate, and nature of sediment mixing and transport. The most extensive use made of these tracers is the study by Hamilton and Clarke(22) of the Esk estuary. Many radionuclides were analysed, and the data used to model the sedimentary history of the area. This study also demonstrates the necessity to combine radiochemical data with more fundamental geological and ecological information in order to produce a sensible interpretation of past events. Other studies relating to the Sellafield discharges have been summarized by Pentreath(23).
