RRFM 2009 Transactions - European Nuclear Society

Carbide and nitride are basically very similar in nature, in crystallographic structure, have
equivalent physical properties, and quite similar in-pile behaviour. They exhibit fairly similar
properties with regard to their chemical reactivity with air and/or water.
Nevertheless, recent results of experiments conducted in Phenix (NIMPHE) clearly showed a
lesser stability of nitride fuel. In particular, in equivalent thermal regimes, nitride experienced
a central hole formation (Figure 3).
Figure 3: Carbide (UPuC) and nitride (UPuN) fuel behaviour, NIMPHE 2 irradiation in Phenix
(~7at%). Carbide (left), Nitride (right)
More puzzling, examinations showed the presence of a metallic Pu-rich phase at the pelletclad
interface (Figure 4). This is considered to occur when the fuel maximum temperature
exceeds 1600°C. The situation would be worsened if a concomitant clad failure occurred as
it would not let the nitrogen partial pressure increase in the pin which is favourable to limit
the extent of the metallic phase formation.
Figure 4: Microprobe pictures of nitride fuel irradiated in Phenix, showing metallic Pu in the
fuel-clad gap. Electronic picture (left), Pu X picture (right)
4.7 Fuels for transmutation
The main restriction to introducing minor actinides into the core (homogeneous recycling
mode) is linked to their impact via on the core reactivity and kinetic factors. The fractions of
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