Thermal Stability of γ U(Mo,X) Fuel Alloys with X = Ti, Cr and Zr

The phase transformation γ → α and the thermal stability of U(Mo,X) alloys compared to the thermodynamic behavior of U(Mo) binary phases are studied. For this purpose, samples were prepared by arc-melting the elemental components obeying the atomic ratio (U-Mo)/X of 95/5, with U content ranging from 95 to 59 at. %. Equilibrium state at low temperature was achieved by heat-treatments; the samples were maintained at 900oC for 24 hours to promote homogenization follow ed by an annealing at 500oC for 72 hours to provoke the eutectoid decomposition of the γ U phase. Thermal analyses were carried out by using differential thermal analysis with heating and cooling cycles from room temperature up to 1400oC and by examination of cooling curves obtained by measuring by pyrometry the temperature of the samples surface heated up to 1800oC and allowed to cool down at various rates. T he samples states, before and after thermal experiments were examined by means of microstructural observations coupled with elemental analyses and by X-ray diffraction techniques. Accurate liquidus and solidus temperatures were determined for all the samples revealing a narrow two-phases domain for X = Ti and Zr, whereas a large liquid-solid domain exists in the case of X = Cr. Regarding the eutectoid decomposition, the temperatures of this invariant reaction remain in a small range around 565(10)oC in the case of Cr, whereas, an increase of 30-40oC is observed in the case of Ti. However, for both elements, Cr and Ti, no significant change in the reaction transformations as well as in the phase appearing can be detected. The case of Zr is more tricky, only one solid-solid transformation is observed at about 610(10)oC, without any evidence of two-phase domain, suggesting a possible diffusionless transformation. (author)