Radioisotope power systems utilising americium-241 as a source of heat have been under development in Europe as part of a European Space Agency funded programme since 2009. The aim is to develop all of the building blocks that would enable Europe to
Americium 241 is a potential alternative to plutonium 238 as an energy source for missions into deep space or to the dark side of planetary bodies. In order to use the Am isotope for radioisotope thermoelectric generator or radioisotope heating unit (RHU) production, americium materials need to be developed. This study focuses on the stabilization of a cubic americium oxide phase using uranium as the dopant. After optimization of the material preparation, (AmUNpPu)O has been successfully synthesized to prepare a 2.96 g pellet containing 2.13 g of Am for fabrication of a small scale RHU prototype. Compared to the use of pure americium oxide, the use of uranium-doped americium oxide leads to a number of improvements from a material properties and safety point of view, such as good behavior under sintering conditions or under alpha self-irradiation. The mixed oxide is a good host for neptunium (i.e., theAm daughter element), and it has improved safety against radioactive material dispersion in the case of accidental conditions.
The electric field has a large effect on the stoichiometry and grain growth of UO2+x during Spark Plasma Sintering. UO2+x is gradually reduced to UO2.00 as a function of sintering temperature and time. A gradient in the oxidation state within the pellets is observed in intermediate conditions. The shape of the gradient depends unequivocally on the direction of the electrical field. The positive surface of the pellet shows a higher oxidation state compared to the negative one. An area with larger grain size is found close to the positive electrode, but not in contact with it. We interpret these findings with the redistribution of defects under an electric field, which affect the stoichiometry of UO2+x and thus the cation diffusivity. The results bear implications for understanding the electric field assisted sintering of UO2 and non-stoichiometric oxides in general.
Electrical power sources used in outer planet missions are a key enabling technology for data acquisition and communications. State-of-the-art power sources generate electricity from alpha decay of 238 Pu via thermoelectric conversion. However, production of 238
AmPO4 was
prepared by a solid-state reaction method,
and its crystal structure at room temperature was solved by powder
X-ray diffraction combined with Rietveld refinement. The purity of
the monazite-like phase was confirmed by spectroscopic (high-resolution
solid-state 31P NMR and Raman) and microscopic (SEM-EDX
and TEM) techniques. The thermal and self-irradiation stability have
been studied. The compound is stable under argon and air atmosphere
at least up to 1773 K. It remains crystalline under self-irradiation
for circa two months, with a crystallographic volume swelling of ∼1.5%,
and then is amorphizing over a year. However, microcrystals are present
in the amorphous material even after a two year period of time. All
these characteristics are discussed in relation to the potential application
of AmPO4 as a stable form of Am in radioisotope power sources
for space exploration and of behavior of the monazites under irradiation.
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