Stepped isothermal annealing release measurements were performed, at temperatures ranging from 573 to 1173K, on irradiated samples. A combined diffusion/desorption model for T release permitted the determination of diffusion coefficients and desorption-rate constants in the mixed-mechanism regime, where both processes appeared to be rate-limiting. The effective T diffusivities for samples with various densities could be described by:

D (m2/s) = 1.7 x 10-11exp[-3.5(kJ/mol)/RT]

D (m2/s) = 1.6 x 10-10exp[-9.5(kJ/mol)/RT]

D (m2/s) = 1.4 x 10-10exp[11.5(kJ/mol)/RT]

for theoretical densities of 81, 99, and 100%, respectively. The release from the 2 samples with the lower densities were reasonably well predicted by the diffusion/desorption model. The results indicated that diffusion was the main mechanism, plus a small and variable contribution from surface desorption.

D.L.Baldwin, M.C.Billone: Journal of Nuclear Materials, 1994, 212-215[B], 948-53