John R. Cooper

The Behaviour of Uranium-233 Oxide and Uranyl-233 Nitrate in Rats

233UO2 and 233UO2(NO3)2 in aqueous suspension have been administered to rats by pulmonary intubation. The 233U associated with the fraction of the 233UO2 less than 4 nm in diameter translocates from lungs to blood at the same rapid rate as 233U from 233UO2(NO3)2. Identical reactions with blood plasma and lung fluid were observed whether the 233U was administered as less than 4 nm 233UO2 particles or 233UO2(NO3)2. It is suggested that oxidation of UO2 to UO3 occurs followed by the formation of uranyl ion. In blood plasma, approximately 50 per cent of the 233U is bound to transferrin, 25 per cent to citrate and 25 per cent on bicarbonate.

Mammalian small intestinal phytase ( EC 3.1.3.8)

Phytase (EC 3.1.3.8) concentration has been measured in the small intestine of rat, rabbit, guinea-pig and hamster. Levels varied from 0.12 units (microgram phosphorus released/min)/mg protein in the rat to 0.03 units/mg protein in the rabbit. The enzyme is localized in the brush border of the small intestine of the rat. It is suggested that the levels and location of phytase are an important factor in the uptake of metals from metal-phytate complexes. Metal ions released in the immediate vicinity of the absorptive surface of the intestine could be absorbed before being rendered insoluble by competing reactions such as hydrolysis.

Factors affecting the mobility of plutonium-238 dioxide in the rat.

A major factor influencing the movement of plutonium-238 from the lungs to blood after the intubation of oxide suspensions is the presense of 0.001 micrometer diameter particles. In a polydisperse suspension of particles this fraction increases with time, due it is thought, to fragmentation of larger particles induced by alpha decay. The rate of this process could account for the greater transportability in vivo of plutonium-238 relative to plutonium-239 when the oxides are inhaled. In blood, 0.001 micrometer diameter plutonium-238 oxide particles undergo a rapid reaction to form a low molecular weight species before plutonium is complexed with transferrin and citrate ions. The filtration of this species through the kidneys may explain the observed enhanced urinary excretion of plutonium relative to administered plutonium citrate. The mechanism of urinary excretion and relationship between cumulative urinary excretion and body content for plutonium-238 is similar to that previously observed for plutonium-239, even though different methods of preparation of the oxides were used.

The reactions of 1.0 nanometre diameter plutonium-238 dioxide particles with rat lung fluid.

The reactions of 1.0 nm particles of plutonium-238 dioxide with rat lung fluid have been studied both in vivo and in vitro. In both cases two products have been identified, (i) plutonium-labelled pulmonary surfactant and (ii) a heterogeneous plutonium-labelled material isolated by column chromatography. The formation of plutonium-labelled pulmonary surfactant results in the rapid translocation of plutonium from lungs to blood and in a high urinary excretion relative to administered plutonium citrate.

International Arctic Seas Assessment Project.

The International Atomic Energy Agency responded to the news that the former Soviet Union had dumped radioactive wastes in the shallow waters of the Arctic Seas, by launching the International Arctic Seas Assessment Project in 1993. The project had two objectives: to assess the risks to human health and to the environment associated with the radioactive wastes dumped in the Kara and Barents Seas; and to examine possible remedial actions related to the dumped wastes and to advise on whether they are necessary and justified. The current radiological situation in the Arctic waters was examined to assess whether there is any evidence for releases from the dumped waste. Potential future releases from the dumped wastes were predicted, concentrating on the high-level waste objects containing the major part of the radionuclide inventory of the wastes. Environmental transport of released radionuclides was modelled and the associated radiological impact on humans and the biota was assessed. The feasibility, costs and benefits of possible remedial measures applied to a selected high-level waste object were examined. Releases from identified dumped objects were found to be small and localised to the immediate vicinity of the dumping sites. Projected future annual doses to members of the public in typical local population groups were very small, less than 1 microSv--corresponding to a trivial risk. Projected future doses to a hypothetical group of military personnel patrolling the foreshore of the fjords in which wastes have been dumped were higher, up to 4 mSv/year, which still is of the same order as the average annual natural background dose. Moreover, since any of the proposed remedial actions were estimated to cost several million US

The speciation of plutonium in foodstuffs and its influence on gut uptake

to implement, remediation was not considered justified on the basis of potentially removing a collective dose of 10 man Sv. Doses calculated to marine fauna were insignificant, orders of magnitude below those at which detrimental effects on fauna populations might be expected to occur. Remediation was thus concluded not to be warranted on radiological grounds.

The Mobility of Curium-244 Dioxide in the Bronchially Intubated Rat

Soluble plutonium complexing agents in foodstuffs have been identified. These have been labelled to high specific activity with 238Pu and their gut uptake determined in rats. The range of values found for naturally-occurring complexes was between 0.03% for oxalate and 0.1% for phytate. However, reasons are advanced for believing that the enhancement of uptake in rats fed the phytate complex would not be expected to occur in man. These results are not taken, therefore, as suggesting that there should be any change in the gut uptake factor of 0.05% currently recommended by the National Radiological Protection Board for plutonium contained in foodstuffs eaten by humans.

The gastrointestinal absorption of organically bound forms of plutonium in fed and fasted hamsters

The mobility of curium dioxide in the rat after pulmonary intubation has been investigated by administering suspensions containing different particle size ranges of the oxide. A major factor influencing the movement of curium from lungs to blood is the formation of hydroxide or hydrous oxide particles about 0.001 micrometer in diameter. This process is sufficiently rapid for the lung clearance kinetics of the dioxide to resemble those of a soluble compound more closely than those of an insoluble one. Filtration of 0.001 micrometer particles through the kidneys results in considerably enhanced excretion of curium relative to administered curium citrate. It is concluded that current metabolic models, which assume that solubility in the lung is a prerequisite for transport in body fluids, do not adequately describe the behaviour of curium fromthe standpoint of radiological protection.

The Reactions of 1 nm Particles of Plutonium-238 Dioxide and Curium-244 Dioxide with Lung Fluid

Values of about 0.005-0.01 per cent were obtained for the absorption in fed hamsters of plutonium ingested as Pu4+ citrate, isocitrate, phytate and malate complexes and Pu3+ ascorbate compared with about 0.003-0.004 per cent for Pu4+ nitrate. Replacing drinking water with tea did not affect the result for Pu4+ nitrate. Fasting hamsters for 8 h before the administration of plutonium citrate increased absorption to 0.1-0.2 per cent. An extra period of fasting for 4 h after administration did not lead to a further increase in absorption. Similar values were also obtained when plutonium citrate was administered after a 24 h fast, followed either by immediate access to food or a further 4 h fast. In hamsters fasted for 24 h before administration of either Pu3+ ascorbate or Pu4+ nitrate, about 6-7 per cent of the ingested plutonium was retained in the gastrointestinal tract after one week. At three weeks after ingestion of Pu3+ ascorbate, gastrointestinal retention had fallen 100-fold without an increase in absorption.

The gastrointestinal absorption of ‘biologically incorporated’ plutonium-239 in the rat

The reactions of 1 nm particles of plutonium-238 dioxide and curium-244 dioxide with rat lung fluid have been studied both in vitro and in vivo. The plutonium-238 particles are positively charged and combine by electrostatic attraction with the negative pulmonary surfactant which mediates the transfer of radioactivity to the blood. In contrast the curium-244 particles are negative and are assumed to diffuse passively through the alveolar walls. The results emphasise that electrostatic charge is an important factor governing the reaction of 1 nm actinide oxide particles with macromolecules.