18.08.2015   NUCLEAR CHEMICAL ENGINEERING

THORIUM COMPOUNDS

1.6 Thorium Valence States

The tetravalent state, exemplified by compounds such as ThOj, is the only valence state of thorium of practical importance.

1.7 Thorium Dioxide

Thorium dioxide Th02 is the form in which thorium is proposed for use as reactor fuel for light-water, heavy-water, and liquid-metal fast-breeder reactors. It is a stable ceramic that can be

Table 6.7 Thermal conductivity of thorium metal

Temperature, °С

Thermal conductivity, W/(cm*°C)

100

0.377

200

0.389

300

0.402

400

0.419

500

0.427

600

0.444

650

0.452

Source: J. R. Murray, “The Preparation, Properties and Alloying Behavior of Thorium,” Report AERE-M/TN-12, 1952.

Table 6.8 Physical properties of Th02

Color

White

Crystal system

Face-centered cubic

Density (x-ray, 25°C)

10.00 g/cm3

Linear expansion from 25°C

9 X 10"3 at 1000°C; 20 X 10’3 at2000°C

Thermal conductivity, W/(cm*°C)

0.10 at 100°C; 0.04 at 600°C

Melting point

3370 ± 30°C

Vapor pressure

log10 Patmf = 8.00-35,170/7; 2180 <T< 2865 К

Heat capacity, cal/g-mol

Cp = 16.56 + 2.232 X 10’3 Г— 2.195 X 10s 7"2; 298 < Г <3000 К

Entropy at 25°C

15.59 cal/(K-g-mol)

Heat of formation from elements at 25° C Free energy of formation from elements

—293.2 kcal/g-mol

at 25°C

Approximate equation for free energy

—279.43 kcal/g-mol

of formation,

AG = -291.93 + 0.04350 T kcal/g-mol; 298 < T < 2023 К

^Th02 dissociates partially into ThO and O; at low oxygen pressures, vapor pressure is some­what higher.

heated almost to its melting point of 3370°C without serious deterioration. Physical properties of Th02, from reference [II], are summarized in Table 6.8.

ThO2 forms solid solutions with U02 or Pu02 over the entire composition range from 0 to 100 percent Th02.

Th02, either as the mineral thorianite or as synthetic thoria produced by heating thorium nitrate, oxalate, or hydroxide, reacts only slowly with mineral acids. It can be dissolved in hot, concentrated sulfuric acid or in hot nitric acid containing 0.05 M HF.