The alpha (a) phase is stable at room temperature and has a crystal system characterized by three unequal axes at right angles.

In the alpha phase, the properties of the lattice are different in the X, Y, and Z axes. This is because of the regular recurring state of the atoms is different. Because of this condition, when heated the phase expands in the X and Z directions and shrinks in the Y direction. Figure 6 shows what happens to the dimensions (A = angstrom, one hundred-millionth of a centimeter) of a unit cell of alpha uranium upon being heated.

As shown, heating and cooling of alpha phase uranium can lead to drastic dimensional changes and gross distortions of the metal. Thus, pure uranium is not used as a fuel, but only in alloys or compounds.

Figure 6 Change in Alpha Uranium Upon Heating From 0 to 300C