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elease falls typically between 1 and 2 % for all these species except Ba; the highest value for the
fraction released was determined for Cs (2.3%). These values are generally lower than labile
inventory values reported in literature from tests on UO2 including also gap contributions. They are
in satisfactory agreement with the data reported specifically for grain boundary inventory of low to
medium burnup UO2 [9]. In parallel, measurements of the species effusing from the fuel as a function
of annealing temperature were performed using a Knudsen cell coupled to a mass spectrometer.
The release measured at temperatures below the range where thermally activated transport dominates
the fission gas release was attributed to release from surfaces and grain boundaries. The results
from the two techniques showed good convergence and were in satisfactory agreement with
data reported specifically for grain boundary inventory of low-medium burnup UO2.
2. Alpha decay damage evolution
In order to predict the long term properties evolution of spent fuel, the effects of alpha-decay damage
accumulation on spent fuel properties were investigated under accelerated alpha-decay rate conditions,
using UO2 containing high activity alpha-emitters, the so-called alpha-doped UO2. Macroscopic
and microstructural effects associated with the build-up of defects in the structure were studied.
In particular, the correlation among annealing of defects in the microstructure, release behaviour
of He, and energy stored by the defects in the material were evaluated. The limits of application
of accelerated decay accumulation methods were also evaluated.
The studies on alpha-doped UO2 have been performed on sol-gel producedsamples with a range of
activities: from 10 wt% 233 U-doped UO2 (3.88·10 7 Bq.g -1 , with a damage level of ~ 10 -5 dpa) up to
UO2 doped with 10 wt% 238 PuO2 with 3 dpa of damage, corresponding to standard irradiated UO2
fuel after 10000 years of storage.
Alpha decay damage may lead to long term changes in the fuel microstructure and may further influence
long term fuel leaching. A TEM analysis of the 233 U-doped UO2 has been performed, showing
(Figure 1) numerous non-homogeneously distributed dislocation loops with sizes between 20
and 50 nm, indicating that the UO2 damage rapidly leads to precipitation of dislocation loops.
Figure 1: TEM micrograph of 10 wt% 233 U-doped UO2
A Differential Scanning Calorimetry (DSC) study of 10 wt% 238 Pu-doped UO2 has also been performed
(Figure 2). With this method the energy related to healing of lattice defects could be determined
as a function of temperature, after correcting for the thermal effects due to self-irradiation in
the material.
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