Abstract
Samples of UO 2, (U,Pu)O 2 and PuO 2 containing up to several 100 at. ppm helium 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 helium 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 helium 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 helium-in-solid. The helium diffusion coefficient has the expression: D=8×10 −7 exp(−46 kcal mol −1/RT) m 2 s −1, however, the enthalpy of diffusion increases to 70 kcal mol −1 in high burn-up fuel where helium is apparently stabilised in uranium/oxygen vacancy clusters.
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