  Precipitation of the topologically close-packed Fe2Mo Laves phase at the
interface between the nuclear fuel and the fuel element cladding can
significantly weaken the strength characteristics of the cladding and fuel.
Despite the importance of designing materials for the cladding of fuel rods,
the thermodynamic properties and the trajectory of the thermal expansion path
of the Fe2Mo remain poorly understood. The thermodynamic properties of the
Fe2Mo have been studied using the finite-temperature quantum mechanical
calculations within the frame of the density functional theory under the
quasi-harmonic approximation. The vibrational contribution to the free energy
was obtained using phonon calculations. The thermal expansion path of Fe2Mo was
predicted by comparing between free energies calculated in different
directions. A path with the least energy was chosen as the trajectory of
thermal expansion. The obtained result was compared with the path calculated in
previous theoretical work used the Debye-Gruneisen approach and accounted
magnetic subsystem to calculate the vibrational and magnetic contributions to
the free energy. This comparison reveals that these two approaches are in good
agreement with each other. The work shows that the Fe2Mo possesses a
non-isotropic thermal expansion. The heat capacity and volumetric expansion at
constant pressure are modeled. The calculated results analyzed and are in
satisfactory agreement with the experimental data. Obtained results can be
useful for further design of fuel element cladding materials intended for
generation IV reactors, the operating temperature of which should be above 873
K.
