RRFM 2009 Transactions - European Nuclear Society

For its manufacturing, an additional step is needed to produce the carbide (the oxide
carbothermic reduction) and optimization is required to master pyrophoricity risks, Pu et Am
losses, and to fulfil the specification required to properly manage swelling and gaseous
products retention. Controlled inert atmosphere is necessary.
For reprocessing, despite very old fames claiming strong difficulties, carbide fuel is readily
soluble in nitric acid and the aqueous processes is the attractive route to benefit from the
experience gained with oxide fuels although the formation of organic soluble compounds
may need for a decomposition step prior to the extraction cycles.
There is no available experience on carbide fuels incorporating minor actinides.
4.5 Metallic fuel
Metal fuel now is considered in many countries as the alternative to oxide or as the longer
term option for SFR. Motivations for that are: high breeding capability, safety, pyroprocess
fuel cycle,…
Metal has obviously the highest heavy metal density but the lowest melting point too. Metal is
the “historical” SFR fuel. Compatible with sodium, its in-pile performances have been
continuously increased. Together with oxide fuel, it can claim for proven high burn-up
performances and significant safety database (US program in BRII and TREAT).
Experiments are underway on MAs beared fuels, in particular the METAPHIX experiment in
Phenix.
As far as reprocessing is concerned, studies on aqueous processing did not generate very
convincing results. UPuZr dissolution requires large amounts of fluoric acid (HF) or an
anodic dissolution technique (BNFL). In addition, subsequent steps are needed to produce
the alloy from the purified product (oxide). ANL studies led to promote the pyrochemical
process. Three steps are involved: U recovery by electro-winning, transuranics recovery by
electro-refining on a liquid cadmium cathode, and the separation of cadmium from TRU by
distillation. The efficiency of U recovery is largely demonstrated (treatment of the EBR-II
used fuel), additional work is still needed for TRUs.
Safety issues must be carefully addressed. Metallic fuel provides a large negative reactivity
feedback due to fuel expansion. However, negative aspects are low temperature eutectic
formation, an increase of sodium void worth and the degradation of Doppler effect.
4.6 Carbide or nitride?
Compared to the oxide experience, carbide and even more nitride fuels are far from mature
and left unanswered a number of issues concerning their real potential as SFR fuel.
Particularly, very limited experience exists in the areas of safety and of the closure of the fuel
cycle.
Both carbide and nitride fuels are under consideration for GFR and SFR. For both, to
achieve equivalent neutronic performances, nitride fuels require nitrogen enrichment to at
least 50 at% 15 N to avoid 14 C production by (n,p) reactions on 14 N. Moreover, with nitride
fuels, generation of helium, from (n,α) reactions in 14 N and additional tritium generation are
concerns for fuel performance and for coolant radiological contamination. Recent irradiations
in the Phenix (France) and Joyo (Japan) reactors have confirmed that, for certain closedsystem
operating conditions at high temperatures, nitride fuel can exhibit signs of
dissociation of the (U, Pu)N phase.
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