Lanthanides in metallic nuclear fuels: Their behavior and methods for their control

The thermodynamic and experimental basis is given for using dopant additives to bind lanthanides as intermetallic compounds in metallic nuclear fuels. Lanthanide fission products are a major factor in limiting the lifetime of the fuel, because they migrate to the fuel slug peripheral surface where they participate in fuel-cladding chemical interactions (FCCI) with the D9 or HT9 steel cladding. Lanthanide carryover in recycled metal fuels can accelerate FCCI, as recycled lanthanides would likely segregate from the fuel phase, putting the lanthanides in prompt contact with the cladding. In out-of-pile tests, the use of palladium was examined for binding the lanthanides, with palladium selected because of its known metallurgical properties in fuel-related systems and because of its known behavior in irradiated Experimental Breeder Reactor-II (EBR-II) fuels. Initial results confirmed that palladium may be expected to mitigate FCCI arising from lanthanides, and it has been recommended for in-pile tests. Transport phenomena responsible for lanthanide migration were also evaluated, and liquid-like behaviors were identified as being dominant. Liquid-like behaviors include transport with liquid metals, liquid metal solutions, and rapid surface transport of alloys/metals near their melting temperatures. The analysis led to establishing general criteria for selecting dopant additives and identifying tin, antimony, and tellurium as alternates for further testing.