Phase Characteristics of a U-22Pu-4Am-2Np-40Zr Metallic Alloy Containing Rare Earths

Metallic fuel alloys consisting of uranium (U), plutonium (Pu), and zirconium (Zr) with minor additions of americium (Am) and neptunium (Np) are under evaluation for potential use to transmute long-lived transuranic actinide isotopes in fast reactors. The current irradiation test series design, designated Advanced Fuel Cycle-2 (AFC2), includes minor additions of rare earth (RE) elements to simulate expected fission product carryover from the electrochemical molten salt reprocessing technique. The as-cast fuel alloys have been investigated for phase and thermal properties; specifically, enthalpies of transition, transition temperatures, and room temperature phase characteristics. Results and observations related to these characteristics for the “fresh” fuel alloys are provided. The alloy compositions are based on a U-22Pu-4Am-2Np-40Zr alloy, along with additions of 1.3 and 1.9 at.% RE (at the expense of uranium where RE denotes rare earth alloy of cerium, lanthanum, praseodymium, and neodymium). Phase behavior and associated transitions have been compared to available U-Pu-Zr ternary diagrams with acceptable agreement. Enthalpies of transition were deconvoluted from heating and cooling thermal traces for relatively reliable values. The RE additions to the base alloy have a minimal influence on the room temperature phases present and phase transition temperatures, but the room temperature phases present did impact the enthalpies of transition.