The release of metal fission products from pyrocarbon-coated UO2 particles was studied by means of post-irradiation annealing at temperatures ranging from 1600 to 2300C. The diffusivity of Cs in the pyrocarbon layer could be described by:

D (m2/s) = 1.2 x 10-3exp[-412(kJ/mol)/RT]

at temperatures ranging from 1600 to 2300C. These values were an order of magnitude higher than those for Ce:

D (m2/s) = 2.5 x 10-3exp[-513(kJ/mol)/RT]

at temperatures ranging from 1900 to 2300C, and were similar to those for Ru:

D (m2/s) = 1.8 x 10-5exp[-336(kJ/mol)/RT]

at temperatures ranging from 1600 to 1700C. Diffusion Coefficients of Fission Products in the UO2 Kernel and Pyrocarbon Layer of BISO-Coated Fuel Particles at Extremely High Temperatures. Hayashi, K., Fukuda, K.: Journal of Nuclear Materials, 1990, 174[1], 35-44