Lattice and Grain-Boundary Diffusions of Protactinium in ThO_2 and in a ThO_2-UO_2 Solid Solution

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Lattice and grain-boundary diffusions of protactinium ions in sintered ThO_2 and ThO_2-UO_2 Solid solution which has an atomic ratio of 1 : 1 were measured in the temperature range 1800-2000℃, by using ^<233>Pa as a tracer and by the sectioning method. The data obtained were analysed in the light of the theory concerning grain-boundary diffusion based on a constant source model. The lattice diffusion coefficients D can be obtained by : D (Pa in ThO_2)=2.91×10^<-5>×exp (-75400/RT) cm^2/sec, D (Pa in ThO_2-UO_2)=1.88×10^<-3>×exp (-91800/RT) cm^2/sec. The products D・2a, peculiar to grain-boundary diffusion are : D'・2a (Pa in ThO_2)=6.66×10^<-11>×exp (-30600/RT) cm^3/sec, D'・2a (Pa in ThO_2-UO_2)=5.64×10^<-8>×exp (-59700/RT) cm^3/sec, where D' and 2a are the grain-boundary diffusion coefficient and the grain-boundary width, respectively.
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