[en] A study was performed of the correlation between fuel form stability and exposure environment of (temperature and atmosphere). 100% Tm₂O₃, 80% Tm₂O₃/20% Yb₂O₃ and 100% Yb₂O₃ wafers were subjected to air, dynamic vacuum and static vacuum at temperatures to 2000⁰C for times to 100 hours. Results showed the Tm₂O₃/Yb₂O₃ cubic structure to be unaffected by elemental levels of iron, aluminum, magnesium and silicon and unaffected by the environmental conditions imposed on the wafers. A second task emphasized the optimization of the thermal, mechanical and chemical stability of Tm₂O₃ fuel forms. Enhancement was sought through process variable optimization and the addition of metal oxides to Tm₂O₃. CaO, TiO₂ and Al₂O₃ were added to form a grain boundary precipitate to control fines generation. The presence of 1% additive was inadequate to depress the melting point of Tm₂O₃ or to change the cubic crystalline structure of Tm₂O₃/Yb₂O₃. Tm₂O₃/Yb₂O₃ wafers containing CaO developed a grain boundary phase that improved the resistance to fines generation. The presence of Yb₂O₃ did not appear to measurably influence behavior