Plutonium migration

Natural colloid particles in aqueous systems, such as clay particles, silica, etc. may serve as carriers of ionic species that are being sorbed on the particulates (pseudocolloids). It seems evident that the formation and transport properties of plutonium pseudocolloids can not yet be described in quantitative terms or be well predicted. This is an important area for further studies, since the pseudocolloidal transport might be the dominating plutonium migration mechanism in many environmental waters. 

Predicted rates of the radioelement transport through sediment were found to depend strongly on HLW compositions. Compositions of HLW that are high in HEDTA/EDTA concentrations resulted in 30 to 40 times faster migration rates for americium and strontium. Neptunium and plutonium migration rates increased by factors of 6 to 40 by changing HLW from dilute/noncomplexed to concentrated/complexed compositions. 

The Natural Reactor. Some two biUion years ago, uranium had a much higher (ca 3%) fraction of U than that of modem times (0.7%). There is a difference in half-hves of the two principal uranium isotopes, U having a half-life of 7.08 x 10 yr and U 4.43 x 10 yr. A natural reactor existed, long before the dinosaurs were extinct and before humans appeared on the earth, in the African state of Gabon, near Oklo. Conditions were favorable for a neutron chain reaction involving only uranium and water. Evidence that this process continued intermittently over thousands of years is provided by concentration measurements of fission products and plutonium isotopes. Usehil information about retention or migration of radioactive wastes can be gleaned from studies of this natural reactor and its products (12). 

Rain in equilibrium with atmospheric C02, but uncontaminated by industrial emissions, should have a pH of 5.7. However, atmospheric pollution from burning fossil fuels has resulted in acid rain of pH as low as 3.5 (24). If this condition continues for a long time, it may lead to a change in groundwater composition, which may considerably change the migration of plutonium in nature. 

Torstenfelt B. 1986. Migration of the actinides, thorium, protactinium, uranium, neptunium, plutonium and americium in clay. Radiochem Acta 39 105-112. 

In previous work (l.> > ) it was found that the kinetics of sorption was an important parameter affecting the migration of nuclides in geologic media. For example, in experiments designed to measure the kinetics of reaction for radionuclides in solution with tablets of rock, it was found that periods from several minutes to several hours were required for the radionuclides to reach steady state concentrations on the rock tablets and in the solutions. Figure 1 shows the reaction curves found for the sorption of plutonium and americium from solution by a tablet of granite. The reaction rates for the sorption of plutonium and americium from solution are not the same, and both require a number of hours to reach steady state concentrations. 

Migration of Plutonium in Rock Incorrect Dispersion Formula," H. Krugmann, Science, 200, 88 (1978). 

Migration of Plutonium Through Dowex 50W-X8 Cation Exchange Resin... 

This study shows that by diluting the salt solution, the selectivity for plutonium and americium is greatly increased whereas in undiluted solutions the actinides tend to migrate through the resin with the other salt constituents. Selectivity between different valencies is a function of the ionic strength, and the selectivity of the resin for the trivalent ion over the divalent ion (such as Pu3+ over Ca2+) is inversely related to the total concentration of the solution (16). Furthermore, the selectivity of the resin for the trivalent ion over the monovalent ion (such as Pu over K or Na"1") is inversely related to the square of the total concentration. 

Loading the column should proceed until the plutonium and americium near breakthrough both migrate at about the same rate. 

If we consider the radioactive decay of plutonium occuring during a migration time of 750,000 y., one finds that the plutonium concentration reaching the recipient will have decreased to 10 ... 

The mixture of uranium and plutonium is treated with a suitable reducing agent [iron(ii) sulfamate, hydrazine, or hydroxylamine nitrate] under these conditions, is not reduced and stays in the kerosene layer, but Pu is reduced to Pu +, which is only weakly complexed by TBP and so migrates into the aqueous phase. 

Analysis of the natural reactors at Oklo gives valuable information about the migration behaviour of fission products and actinides in the geosphere. Uranium and the lanthanides have been redistributed locally. Plutonium produced in the Oklo reactors did not move during its lifetime from the site of its formation 85-100% of the lanthanides, 75-90% of the Ru and 60-85% of the Tc were retained within the reactor zones. Small amounts of U, lanthanides, Ru and Tc moved with the water over distances of up to 20-50 m. 

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