Actinide diffusion, measurements

Comparative measurements of actinide diffusion (Pa, Pu, Am) in UC, have been reported in the temperature range of 1700 2300°C by Hirsch and Scherff (327), The diffusion coefficients ranged in the order Pu > Am > U >Pa, while the activation energies were found equal within the limits of measuring error, in contrast to the case of UOj described above (334). The foregoing observations would indicate that those of the properties of diffusion solute atoms that have an influence on the activation energy of diffusion—such as ionic radius, the charge and interactions of their... 

Hydration numbers and hydration radii of trivalent actinide and lanthanide ions obtained by electrophoresis and diffusion measurements (Lundqvist et al. 1981, Fourest et al. 1984, David 1986). 

From diffusion measurements, Fourest et al. [361] have determined the hydration number of Am " to be 13 and the hydrated Am " radius to be 4.52 A. They found that the hydrated Am " parallels its lanthanide homolog Eu. (Hydrated radii increase as a function of atomic number in both the lanthanide and actinide series because increased covalency increases the number of water ligands.)... 

Schmitz and Lindner (334) have measured the diffusion of actinide atoms in UOj by means of surface-decrease methods at 1250 1600°C, and reported that the diffusion coefficients in this temperature range have values that order themselves in the sequence Am > Pu > Th > U > Np > Pa, with the difference between these elements tending to decrease above 1600°C. The same authors also reported that the activation energies were Th = 98, Pa = 108, U = 104, Np = 110, Pu = 98, and Am = 92 kcal/moL The difference in the activation energies was interpreted to lie in the difference in the migration energies, which are proportional to the effective charges of the diffused ions. 

Californium is the heaviest actinide for which data like the enthalpy of sublimation have been determined directly with bulk quantities of about 2 mg of pure metal. Due to the limited availability of the heaviest actinides down to the one-atom-at-a-time scale, the preparation of the metals becomes an integral part of an experiment for studying the metals. Unusual experimental approaches like the measurement of partial pressures of the actinide under study over an alloy, studies of diffusion of actinide atoms in metals, and adsorption studies of actinide atoms onto metal surfaces by thermochromatography have been reported. 

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