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The Cp * (T) curve were analysed in order to identify the different parameters controlling the latent
heat effects of the defects. For each stage, the quantities to be derived are concentration and energy
associated to the annealing of a certain kind of defect i.e. its characteristic mobility (i.e., preexponential
factor and activation energy of the diffusion coefficient).
Four peaks, corresponding to different defect annealing stages were identified from the analysis. A
distinct physical significance was attributed to the various stages with increasing temperature:
oxygen vacancy/interstitial recombination; uranium vacancy/interstitial cluster recombination (5.1
eV); dislocation loop annealing; void growth (the void fractional volume was measured to about 0.3
0.05 %; the resulting defect annealing energy of a vacancy trio is 13.4 eV).
Heat Released, J g -1
0,00
-0,02
-0,04
-0,06
-0,08
-0,10
-0,12
-0,14
400 600 800 1000 1200 1400
Annealing Temperature, K
Figure 2: Analysis of the heat release for (U0.9Pu0.1)O2
3. Evolution of spent fuel in steam
I
II
485
III
IV
Fitting
Experiment
The aim of this study was to investigate the alteration of spent nuclear fuel (SNF) in a disposal vault
under non-oxidising conditions (the behaviour of SNF under oxidizing conditions was investigated
previously [10-13]). Potential alteration processes of the spent fuel during the transient phase corresponding
to initial breach of the canister and first water ingress (early failure scenario) were investigated.
This involved experiments in which fuel rod segments containing intentional defects in the
cladding were exposed to a water-saturated atmosphere containing hydrogen to simulate nonoxidizing
repository conditions. The effect of hydrogen overpressure on the corrosion behaviour of
spent fuel in contact with groundwater was investigated by performing autoclave studies on irradiated
fuel and analogues under conditions relevant for assessing different repository scenarios.
The complete experimental equipment including an autoclave, an oven, a gas-sampling system and
all the electrical connections was installed inside a hot cell during normal cell operation and
adapted to remote handling by tele-manipulators. Three experiments were carried out at 90°C in
humid atmosphere composed of, respectively, 1) pure Ar, 2) mixture of Ar and H2, and 3) pure H2.
SEM investigations showed a significant surface change in the area of the intentionally set defect
only under Ar. On both rodlets exposed to a moist hydrogen-containing atmosphere, no fuel surface
alteration could be seen. Figure 3 shows SEM images of the surface exposed to different
atmospheres.
EDS analysis of the hydrogen-exposed surfaces showed, in addition to uranium, only the presence
of fission products caesium and barium at the outer periphery of the fuel pellet, near the fuel
cladding. Here also zirconium traces were found.