MEAD (part II)-Predictions of radioactivity concentrations in the Irish Sea.

The predictions from MEAD, a model that simulates the transport of radionuclides in the marine environment, are presented for the Irish Sea. MEAD predictions for (137)Cs and Pu(alpha) are presented following discharges from BNFL Sellafield and the predictions compared to measured data from near the discharge location and further a field in the Irish Sea. The model performs well in most circumstances given the uncertainties involved in both modelling and data collection although some inconsistencies in the predictions are found. MEAD is also compared to other models of radionuclide transport in the Irish Sea.

Incorporation of organic tritium (3H) by marine organisms and sediment in the severn estuary/Bristol channel (UK).

Discharges of tritium (3H) into the Severn estuary/Bristol Channel (UK) arise from the authorized release of wastes from nuclear power plants at Hinkley Point and Berkley/Oldbury and from the Nycomed-Amersham radiochemical plant, via the sewer system, at Cardiff. The wastes from the nuclear power plants probably consist almost entirely of 3H2O, whereas those from the radiochemical plant also include uncharacterized 3H labelled organic compounds. The total 3H concentrations in demersal fish and other benthic organisms in the vicinity of the Cardiff Eastern sewer outfall are significantly elevated compared to those observed around other UK nuclear establishments. Concentrations in filtered seawater were approximately 10 Bq kg(-1) whilst levels in surface sediment, seaweed (Fucus vesiculosis) and mussels (Mytilus edulis)/flounder (Platichthys flesus) were in the order of 6 x 10(2), 2 x 10(3), and 10(5) Bq kg(-1) (dry weight), respectively. Almost all the 3H found in sediment and biota were organically bound tritium (OBT). The high concentration in these materials, relative to that in seawater, is due to the presence of bioavailable organic 3H labelled compounds in the radiochemical waste. It is suggested that bioaccumulation of 3H by benthic organisms and demersal fish occurs primarily via a pathway of physico-chemical sorption/bacterial transformation of dissolved 3H labelled organic compounds into particulate organic matter, and subsequent transfer up a web of sediment dwelling microbes and meiofauna. Variations in 3H accumulation between individual organisms have been interpreted in terms of their different feeding behaviour. Relatively low concentrations were observed in the herbivorous winkle (Littorina littorea) and the pelagic Sprat (Spratus spratus) compared with other benthic organisms and demersal fish. The elevated 3H concentrations in seafood, due to bioaccumulation of OBT, have low radiological significance even for the local critical group of seafood consumers.

An overview of tritium behaviour in the Severn Estuary (UK).

Measurements of tritium (3H) activity in marine species in the Severn Estuary (UK) show concentrations significantly above those predicted by standard models. Concentration factors of 10(4) between seawater and marine species are observed; standard models assume a concentration factor of 1. In addition, the vast majority of activity is present in the form of organically bound tritium (OBT). The critical group dose from tritium--to consumers of local flounder--is estimated as approximately 36 microSv per annum. The measurements are related to the authorised release of wastes, via the sewer system, from the Amersham plc plant at Cardiff. The radiochemical plant manufactures a wide range of isotopically labelled compounds for use in life science research. The elevated levels of tritium observed in marine species have led to a programme of investigative work by specialist organisations. This paper describes the various definitions, measurements and significance of the term OBT. It also outlines the environmental observations and preliminary conclusions from the investigative programme to date.

Association of 210Po(210Pb), 239+240Pu and 241Am with different mineral fractions of a beach sand at Seascale, Cumbria, UK

The degree of association of 210Po (in secular equilibrium with 210Pb), 239+240Pu and 241Am with iron oxide minerals in beach sand has been assessed using samples collected by panning and selective magnetic extraction techniques. The samples were obtained from Seascale beach (Cumbria), close to the BNF reprocessing plant at Sellafield (UK) and physically and chemically characterised in terms of their grain-size distribution, radionuclide content and magnetic properties. Low frequency (0.46 kHz) susceptibility (X(LF)) and saturation isothermal remanent magnetisation (SIRM) measurements were used to determine the magnetic properties of the bulk samples and the grain size fractions. Samples were also subjected to magnetic extraction, to concentrate the iron oxide minerals, and thence determine their degree of association with the radionuclides of interest. The efficiency of the magnetic extraction technique was estimated from rock magnetic measurements, before and after the extraction procedure. The placer deposits, enriched in iron oxide grains by environmental sorting processes, were both finer grained and more magnetic than the bulk sand. The mean particle diameter of the bulk sand was largely (approximately 75%) greater than 200 microm, whereas for the placer material, > 88% of particles have mean diameters of < 200 microm. Whilst X(LF) and SIRM values of the 100-200 microm placer material were approximately 40-50-fold higher than those of the bulk sand of equivalent grain size, the radionuclide concentrations were approximately 4-13-fold higher. In terms of mass, the percentage of magnetic material extracted from samples of bulk sand (using a powerful, approximately 0.3 T, rare earth magnet), was small (approx. 1-3%). Nevertheless, > 70% of the magnetic signal was associated with these extracts. Concentrations of all three radionuclides in the magnetic extracts were enhanced relative to the weakly magnetic residue. The proportions of 210Po(210Pb), 239+240Pu and 241Am associated with the magnetic extracts were approximately 18%, approximately 12% and approximately 11%, respectively. Therefore, although the strongly magnetic iron oxide fraction contains measurably enhanced levels of these radionuclides, the majority of the alpha-radioactivity appears to be associated with the weakly magnetic residue.

Continuing contamination of north Atlantic and Arctic waters by Sellafield radionuclides.

Discharges of 99Tc and 129I from the reprocessing plant at Sellafield have increased significantly since the mid-1990s, against the overall trend of most other radionuclides. The 'pulsed' release of 99Tc has provided an opportunity to study transport pathways and transit times in UK waters, the North Sea and beyond. Transit times estimated from the 99Tc data are signifcantly shorter than rates reported previously using other radiotracers. The possible reasons for this are discussed. A comparison is made between the response of seawater concentrations and those in the brown seaweed Fucus vesiculosus to variations in the release rates. Current discharges of plutonium are very low compared with the 1970s and 1980s. However, the seabed sediments of the Irish Sea represent a substantial source and remobilisation into the water column results in the continuing export of plutonium from the Irish Sea and its transport to Arctic waters.