Nanometric scale mixed oxides of plutonium and americium. Synthesis, properties and reactivity

Along with uranium dioxide, mixed actinide oxides play an essential role as fuels for present or next generation nuclear reactors. Although their production has been studied extensively, the methods applied currently require high temperatures, which affects the low-dimensiona l preorganization of the starting materials. In contrast, the hydrothermal decomposition of corresponding oxalates is a low-temperature process, allowing production of homogeneous, highly sinterable nanopowders, usually below 250 °C.
Preliminary attempts for the (Th,Pu)O2, (Np,Pu)O2 and (Th,Np,Pu)O2 systems were reported in [1]. The procedure permitted the production of nanometric powders of U1-xPuxO2 solid solution in the full composition range. Partial oxidation of U(IV) to U(V) was observed [2], while plutonium was assumed to maintain its tetravalent oxidation state. Thanks to the low-temperature of the synthesis, such mixed oxides were used as reagents for the fabrication of more complicated systems.
In this process, the aliovalent americium, which has a preference for the trivalent oxidation state, can be accommodated in the UO2 fluorite structure by charge compensation with U(V) only in limited amounts. ... mehrStill, the feasibility of the method has been demonstrated for an Am-content relevant to transmutation fuels. Thus, U1-xAmxO2 (x = 0.1 and 0.2) and U0.75Pu0.20Am0.05 nanosized powders were produced by hydrothermal decomposition of the corresponding mixed oxalates [3]. Typical oxidation states detected for the cations were U(IV)/U(V) for nanometric and sintered materials, respectively, where both exhibit mixed Am(III)/Am(IV) oxidation state. Their sintering ability was tested and the volumetric expansion was measured by powder XRD as a function of storage time. A low swelling with no indications of amorphization has been derived over a period of four years.
[1] K. Popa et al., CrystEngComm, 2018, 20, 4614-4622.
[2] G. Kauric et al., Mater. Today. Adv., 2020, 8, 100105.
[3] J.-F. Vigier et al., CrystEngComm, 2022, 24, 6338-6348.