Diffusion of fission products in matrix graphite for VHTR fuel compacts.

Diffusion coefficients of Ce, Sr and Ba in the matrix graphite for VHTR fuel compact have been determined by measuring the radial profiles of 141Ce, 89Sr and 140Ba concentrations in slightly irradiated graphite spheres which have U02 kernels at the center. Temperature dependences of the diffusion coefficients of Sr and Ba have been determined between 1,175 and 1,375°C, and they are expressed as where T is the absolute temperature and R the gas constant (8.314 J/mol·K). The diffusion coefficient of Ce in the above temperature range was rather higher than those of Sr or Ba, and the magnitudes of these coefficients were expressed in following order; DCe>DSr>DBa. The effect of matrix graphite density on diffusion of the fission products was studied, but difference was not found clearly in the diffusion coefficients of Ba which had been obtained in both of low and high density matrices at 1,375°C.