What is the best fast reactor fuel and why?

[shreddinglicks]

I'm studying fast reactors. I can't seem to find a clear answer to my question. I know fast reactors are fueled by either plutonium 239 converting uranium 238, or uranium 233 converting thorium 232, or fueled by highly enriched uranium. Which is the better and why?

 

[Melbourne Guy]

Not sure that there's a single 'best' answer, @shreddinglicks, but this Argonne presentation might inform your thinking: https://www.nrc.gov/docs/ML1914/ML19149A371.pdf

 

[Astronuc]

I can't seem to find a clear answer to my question. I know fast reactors are fueled by either plutonium 239 converting uranium 238, or uranium 233 converting thorium 232, or fueled by highly enriched uranium.

One would have to evaluate the fissile cross-sections for the neutron energy spectrum expected. Fast reactors typically use Pu-enriched fuel, usually 20% Pu-80% U in the core, with lower enrichment or straight UO₂ in the axial and radial blankets (blankets would use reduced enrichment and natural or depleted U). The reference cladding is 316 stainless steel (and modified compositions), or ferritic/martensitic stainless steels like HT9 or T91/T92, with Na or Na/K coolant.

²³³U/²³²Th would be used in 'thermal' breeder reactors, which would use Zr-alloy cladding material in a water-cooled/moderated environment.

There reference form for U and Pu is oxide, as in (U,Pu)O₂, so-called mixed oxide (MOX), although some have looked at mixed carbides (U,Pu)C and nitrides (U,Pu)N, and some have tried carboxides (U,Pu)CO or carbonitrides (U,Pu)CN. Oxides have the highest melting point, but lower thermal conductivity that carbides and nitrides. One can use nitrogen enriched in ¹⁵N to improve nitride performance.