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Samples of UO2, irradiated in LWRs at bum ups from 25000 to 95000 MWd/t at in-pile temperatures below 800 K, were submitted to Knudsen-effusion experiments. In addition to the equilibrium vapour pressure of non-volatile species, release of fission gas was measured as a function of temperature and time up to approximately 2700 K, where complete vaporisation of the sample was eventually achieved. The fractional release curves of the main isotopes of Kr and Xe were measured and analysed with an algorithm adequate to discriminate between the relevant release stages, one of which, controlled by atomic diffusion, is analysed in this paper. The obtained enthalpy of diffusion -the same for all the examined nuclides -decreases from 4.6 Ev/atom, at low bum up, to 2.95 eV/atom, at the highest bum up. Furthermore, the fraction of fission gas released via atomic diffusion markedly increases with increasing bum up, indicating a dramatic attenuation of the intra-granular sink strength.

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Authors: HIERNAUT J.P, EC, Institute for Transuranium Elements, Karlsruhe (DE);RONCHI C, EC, Institute for Transuranium Elements, Karlsruhe (DE)
Bibliographic Reference: An article published in: Elsevier Journal of Nuclear Materials 294 (2001), pp.39-44
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