RRFM 2009 Transactions - European Nuclear Society

This is the reason why France as most countries decided in the 60’s to switch to oxide fuels
with at that time some satisfactorily feedback experience of their use in PWRs. Despite its
low thermal conductivity and theoretical density, oxide became the reference fuel for fast
reactors because of its high melting temperature (above 2700°C for 20% Pu enriched fuel)
and its stability under irradiation.
4.3 MOX fuel
Despite low thermal conductivity and bad compatibility with sodium, oxide is still clearly the
most mature and efficient fuel for SFR. Very high burn-up performances are proven resulting
from considerable feedback from irradiation experience. Incidental and accidental behaviour
has been assessed by a large number of tests, in particular the CABRI and SCARABEE
international programs.
The demonstration of the mastering of fuel cycle is one of the key advantages of the oxide
fuel. About 14 tons of Phenix fuel and fertile sub-assemblies were successfully reprocessed
in representative conditions and the recovered Pu (about 4 tons) has been used to
manufacture new Phenix fuels. Aqueous oxide fuel reprocessing has reached industrial
maturity. The latest improvement is the COEX process which avoids pure Pu production
while using available technologies. Pyrochemical process has been developed by Russia but
the recovery yield and the used salts management are still to be improved.
MOX fuel can incorporate several percents of minor actinides as shown by the SUPERFACT
pioneer experiment in Phenix (1986-1988). Several aqueous processes are under
development for minor actinides recovery, either selectively or grouped with Pu, and have
been successfully tested at lab scale.
4.4 Carbide fuel
Carbide offers both a high melting temperature (comparable to oxide) and a much larger, by
a factor of 6, thermal conductivity associated to an increased heavy atom density. It is
compatible with sodium (useful for clad failure management) but is more reactive with air
than oxide with a pyrophoric character when provided through fine particles. In addition,
although definitively not comparable to the oxide one, there exists a significant and globally
positive experience on carbide behaviour under irradiation.
A key issue is the in-pile carbide swelling and fission gas retention. The behaviour is
understood but must be mastered with respect to carbon content, oxygen impurity content
and significant and stable porosity. In these conditions, it is possible to design a fuel element
with reduced smear density but the advantage of the higher heavy atoms density over the
oxide remains by a factor of 17%.
Linear heat rating can be increased (up to 750 W/cm for the He-bond concept) while keeping
important thermal margins (Doppler reserve). This can be used to increase the core power
density. Or, keeping the same level of power density, carbide fuel can significantly increase
safety margins in comparison to MOX fuel.
However, undoubtedly linked to rather limited experience, many stakes remain for carbide
fuel. A critical point is the mechanical interaction with a non-tensile clad (this presently limits
the burn-up of carbide when cladded with ceramic). In the field of safety, the experimental
database (transients, core accidents) is very limited and a complete evaluation is still
necessary to properly balance advantages and disadvantages of carbide fuel.
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