A natural uranium oxide sample was studied in order to acquire data on the

behaviour of radiogenic helium and its diffusion under self-irradiation in spent fuel.

The sample, dated at 320±9 million years, was selected for its simple geological

history, making it a suitable natural analogue of spent fuel under repository

conditions during the initial period in a closed system not subject to mass transfer

with the surrounding environment. Helium out-gassing measured by mass

spectrometry to determine the He diffusion coefficients through the ore showed

that: (i) a maximum of 5% (2.1% on average) of the helium produced during the

last 320Ma in this natural analogue was conserved, (ii) about 33% of the residual

helium was occluded in the matrix and vacancy defects (about 10−5mol/g) and 67%

in bubbles. A similar distribution was observed in spent fuel and in (U0.9,Pu0.1)O2.

The results obtained for the natural sample could be applied by analogy to spent

fuel, especially in terms of the apparent solubility limit and the formation,

characteristics and behaviour of helium bubbles.

Diffusion of Radiogenic Helium in Natural Uranium Oxides. D.Roudil,

J.Bonhoure, R.Pik, M.Cuney, C.Jégou, F.Gauthier-Lafaye: Journal of Nuclear

Materials, 2008, 378[1], 70-8