Lattice and Grain-Boundary Diffusion of Uranium in ThO_2 and ThO_2-UO_2 Solid Solution

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Lattice and grain-boundary diffusion of uranium in ThO_2 and ThO_2-UO_2 (1 : 1) solid solution were measured by using ^<237>U as a tracer and the sectioning method. The obtained penetration curves were analysed according to the theory for grain-boundary diffusion, based on an instantaneous source model. The lattice diffusion coefficients D can be expressed by the equations : D (U in ThO_2)=1.10×10^<-4>×exp (-76400/RT) cm^2/sec, D (U in ThO_2-UO_2)=7.59×10^<-4>×exp (-85900/RT) cm^2/sec. On the other hand, the products D'・2a peculiar to grain-boundary diffusion follow the equations : D'・2a (U in ThO_2)=2.35×10^<-9>exp (-47900/RT) cm^3/sec, D'・2a (U in ThO_2-UO_2)=1.04×10^<-7>exp (-64200/RT) cm^3/sec, where D' and 2a are the grain-boundary diffusion coefficient and the grain-boundary width, respectively. The results are discussed in connection with earlier data
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