Despite being a metal, uranium has chemical bonding characteristics of metalloids like arsenic, antimony, or bismuth.

Up to 666 °C, uranium assumes an orthorhombic crystal structure (a-U), with 4 atoms per unit cell: density — 19.04 gcm~3 and lattice constants (a = 2.8541 ± 0.003A, b = 5.8692 ± 0.0015 A, and c = 4.9563 ± 0.0004A) (at 25 °C). The struc­ture is somewhat unique in that it can be thought of as stacks of “corrugated” sheets with atoms parallel to a-c plane with ~2.8 A distance between atoms in the sheets and ~3.3 A distance between the sheets. It can also be described as a dis­torted HCP crystal structure!

In the temperature range of 666-771 °C, uranium has a complex tetragonal crys­tal structure (30 atoms in a unit cell) and is called b-U. Density of b-U is 18.11 g cm~3, and lattice constants a = 10.759 ± 0.001 A and c = 5.656 ± 0.001 A (at 720 ° C).

In the temperature range of 771-1130 ° C, uranium assumes a simple body-cen­tered cubic crystal structure (y-U), that is, with 2 atoms per unit cell. Density is

18.6 g cm~3, and lattice constant a = 3.524 ± 0.002 A (at 805 °C).

Because of the anisotropic nature of the crystal structure of alpha-uranium, thermal expansion coefficients are anomalous along the crystallographic directions deter­mined by lattice parameter measurements and shown in Figure 7.1. That is, the lin­ear thermal expansion coefficient (both linear and volume) increases in the direction of [100] and [001], and decreases along [010] with increasing temperature. However, the volumetric thermal expansion coefficient (i. e., the overall thermal expansion effect due to combination of linear expansion and contraction) does increase with increas­ing temperature. The dilatometry has also been used to measure thermal expansion coefficients and they have shown comparable trend. As noted before, uranium shows allotropic transformation and thus shows increased volumetric thermal expansion coefficients, as the phase transformation occurs as a function of temperature.

Thermal conductivity is a important property with respect to heat removal from the fuel through cladding (by conduction) to the coolant (by convection) in a nuclear reactor. The linear power rating of a reactor fuel element is generally lim­ited by the thermal conductivity of the fuel to avoid center melt. Figure 7.2 shows thermal conductivity of a well-annealed high purity polycrystalline uranium as a

200 400 600

TEMPERATURE (°С)

Figure 7.1 Thermal expansion coefficient of a-U is anisotropic as a function of temperature [2].

function of temperature. Interestingly, the thermal conductivity of uranium keeps on rising as the temperature increases, thus offering the advantage of having better heat conduction at elevated temperatures! However, depending on various factors, thermal conductivity may vary and fall in a data-band.

Heat capacity of uranium in the range of 20-669 °C (293-942 K) is calculated by expression given by Rahn et al. [4]:

Cp [J kg-1 K-1] = 104.82 + (5.3686 x 10-3)T + (10.1823 x 10-5)T2, (7.8)

where T is in K.

The average Cp in the temperature regime of 669-776 °C (beta-phase regime) is

176.4 J kg-1 K-1, whereas the average Cp is 156.8 J kg-1 K-1 in the temperature regime of 776-1132 °C (gamma-phase regime).