Applications of In-Situ Produced Sodium and NaK

Sodium and NaK (the eutectic alloy of 22.2% sodium, 77.8% potassium) have several possible applications in cislunar space. By the unusual thermodynamic properties of the two materials, the application of a thermal transfer fluid, not only as a coolant for industrial applications, but primarily for solar or nuclear power turbogenerators, is commonly noted. The relevant article in the Encyclopedia Britannica also points out the possible use of sodium in batteries of high energy density.

At one time NaK was considered for use as the heat-transfer fluid in submarine reactor systems. Later, sodium was investigated as the secondary coolant system in a high power-density reactor system for space use. This alloy is also used for cooling crucibles in consumable arc-melting processes for preparing titanium. Another large-scale potential use for sodium is in secondary batteries for electric automobiles. Considerable effort was being made in the early 1970s to evolve a molten sodium-sulfur battery that would provide primary propulsion in automobiles.

Sodium and NaK also have possible applications as a high-density and energetic liquid propellant which can be produced in quantity from the lunar regolith for reaction with liquid oxygen.

According to the Lunar Sourcebook, sodium is one of the moon's major elements, with a concentration consistently in excess of 0.5%, and with selective choice of rocks within polymict breccias, up to and in excess of 1% concentration. Potassium, which comprises approximately 78% of NaK, also has a concentration reliably in excess of 0.7% in polymict breccias, and between 0.05% and 0.5% in other rock types.

Thermal Transfer Fluid

The thermal properties, abundance, and mass of terrestrial experience with sodium make it a good candidate for a heat-transfer fluid, most likely for industrial applications and for solar power generators on the moon or in orbit.

Volume 1 of the Sodium-NaK Engineering Handbook mentions two most commonly noted possible applications of liquid sodium and NaK on its first page: "Liquid metals are used or considered primarily as heat-transfer fluids in nuclear reactors or as working fluids in space power-generating systems."

Both sodium and NaK have properties which make them attractive candidates for heat-transfer applications. With a viscosity similar to that of water, albeit spread over a wider melting point to boiling point range, at 20°C, sodium has about 0.7 times the viscosity of water. The following table shows the properties of sodium and NaK which are most relevant to their use as a heat-transfer fluid in a space power turbogenerator:

Material:                          Sodium     NaK

Melting Point (°C):                97.82?   -12.6??
Boiling Point (°C):               881.4??   785.???
Viscosity at -13°C:                           1.3??
Viscosity at 20°C:                            0.94?
Viscosity at 98°C:                  0.680    ~0.505
Viscosity at 400°C:                 0.284
Viscosity at 550°C:                 0.225     0.176
Viscosity at 881°C:                 0.149
Density at -13°C:                             0.875
Density at 20°C:                              0.867
Density at 98°C:                    0.927    ~0.855     
Density at 400°C:                   0.856
Density at 550°C:                   0.820     0.749
Density at 881°C:                   0.740
Specific Heat at -13°C:                       0.233
Specific Heat at 20°C:                        0.232
Specific Heat at 98°C:              0.331    ~0.224     
Specific Heat at 400°C:             0.305
Specific Heat at 550°C:             0.300     0.208
Specific Heat at 881°C:             0.307
Thermal Conductivity at 20°C:                 0.218
Thermal Conductivity at 98°C:       0.870    ~0.232     
Thermal Conductivity at 400°C:      0.722
Thermal Conductivity at 550°C:      0.648     0.262
Thermal Conductivity at 881°C:      0.486

Melting Point and Boiling Points in degrees centigrade, Viscosity is in centipoise (10^-2g/cm-sec), Density in g/cm³, and Specific Heat is in cal/g-°C. Density, Viscosity, and Specific Heat figures are for the materials in their liquid states only.

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