Food chains, actinides

As yet there have been no reports that the actinides in the biosphere become complexed in a chemical form which would facilitate their transport through the food chain to man. All the available evidence indicates that solutions of the actinides hydrolyze to give polymeric forms which exhibit limited mobility in cellular systems. There are, however, many complexing agents in the biosphere which could form stable complexes with the actinides, such complexes could alter the transport of the actinides in microenvironments but these microenvironments have yet to be identified. 

The two principal methods by which actinides may enter the body are inhalation and penetration through wounds. These two routes of entry are of obvious concern to those individuals working in nuclear fuel reprocessing plants. The principal route of entry by which most of the general public is likely to be exposed to the actinides could be expected to be via the food chain. However, Bennett (176) has indicated that inhalation of 239>(i) 240Pii is more important by a factor of 1000 compared to the uptake by ingestion in contributing to the body burden. 

Bulman, R.A. Concentration of Actinides in the Food Chain. National Radiological Protection Board NRPB-R44, HMSO 1976... 

Bulman RA. 1976. Concentration of actinides in the food chain. Gov t Rep Announce, Issue 17803. NRPB-R44, 11-12. 

Summary. Through an examination of the comparative behavior of the actinide elements in terrestrial and aquatic food chains, the anticipated accumulation behavior of the transuranium elements by people was described. The available data suggests that Pu, Am and Cm will not accumulate to a greater degree than U in the skeletons of individuals exposed to environmentally dispersed activity. The nature of the contamination event, the chemical and physical associations in soils and sediments, the proximity to the... 

Pu(IV), which forms highly charged polymers, strongly sorbs to soils and sediments. Other actinide III and IV oxidation states also bind by ion exchange to clays. The uptake of these species by solids is in the same sequence as the order of hydrolysis Pu > Am(III) > U(VI) > Np(V). The uptake of these actinides by plants appears to be in the reverse order of hydrolysis Np(V) > U(VI) > Am(III) > Pu(IV), with plants showing little ability to assimilate the immobile hydrolyzed species. The further concentration of these species in the food chain with subsequent deposit in humans appears to be minor. Of the 4 tons of plutonium released to the environment in atmospheric testing of nuclear weapons, the total amount fixed in the world population is less than 1 g [of this amount, most (99.9%) was inhaled rather than ingested]. 

Most radioactive particles and vapours, once deposited, are held rather firmly on surfaces, but resuspension does occur. A radioactive particle may be blown off the surface, or, more probably, the fragment of soil or vegetation to which it is attached may become airborne. This occurs most readily where soils and vegetation are dry and friable. Most nuclear bomb tests and experimental dispersions of fissile material have taken place in arid regions, but there is also the possibility of resuspension from agricultural and urban land, as an aftermath of accidental dispersion. This is particularly relevant to plutonium and other actinide elements, which are very toxic, and are absorbed slowly from the lung, but are poorly absorbed from the digestive tract. Inhalation of resuspended activity may be the most important route of human uptake for actinide elements, whereas entry into food chains is critical for fission products such as strontium and caesium. 

The previous results show that the potential for entry of Pu and other actinides into food chains will be highly dependent on the environmental chemistry of actinides in the environment. 

Research on the behavior of Pu and other actinides in aquatic ecosystems of Oak Ridge National Laboratory involves field studies at White Oak Lake, a 10.5-ha impoundment formed by Mahattan Project operations. This final settling basin has received releases from the Laboratory and associated facilities since 1944. Some of the key questions addressed are (a) the partitioning of Pu between the suspended particulate and soluble fractions in the water column, (b) the partitioning of Pu between the bed sediments and overlying water column, (c) the uptake of Pu by biota from these fractions, and (d) food chain transfers and trophic effect on Pu concentrations in biota of the system. 

Entry of Actinides into Terrestrial Food Chains... 

Plutonium is not very available to biota under most circumstances. There is no evidence for biomagnlfication by plants or organisms of terrestrial and aquatic food chains. More information is needed on magnitudes of uptake of non-pluton1um actinides by biological organisms. 

The contribution of inhalation to the internal doses to the critical group is substantial for radioactive gases and vapours (e.g. radon or tritium oxide) and for radionuclides with low solubility and low mobility in food chains (e.g. actinides and transuranics), especially for persons working in the open air and in dusty conditions. The special case is that of long term residence in areas with elevated concentrations of natural uranium and radium resulting in the emanation of radon. 

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