Samples of UO2, (U,Pu)O2 and PuO2 containing up to several 100ppm[at] He were submitted to thermal annealing in a Knudsen-cell provided with a mass spectrometer. Gas release was measured on line with a great accuracy. In the examined materials He was created by α-decay of plutonium or laboratory infused at high temperature and high pressure. The selected samples exhibited different types of lattice damage, including reactor burn-up and high α-radiation doses. Analysis of He release as a function of temperature enabled the elementary diffusion processes to be investigated and the atomic diffusion coefficient to be deduced for a defined state of He-in-solid. The He diffusion could be described by:

D (m2/s) = 8 x 10-7 exp[-46(kcal/mol)/RT]

However, the enthalpy of diffusion increases to 70kcal/mol in high burn-up fuel where He was apparently stabilized in uranium/O vacancy clusters.

Helium Diffusion in Uranium and Plutonium Oxides. C.Ronchi, J.P.Hiernaut: Journal of Nuclear Materials, 2004, 325[1], 1-12