Selection of the coolant for thermionic-emission nuclear power-generating plants in spacecraft

In nuclear electric power-generating plants with thermionic reactor-converters for spacecraft, one can use a eutectic sodium-potassium melt, sodium, or lithium as coolant because of their thermophysical properties; since some of its parameters are inferior to those of lithium and of the eutectic sodium-potassium melt, the application range of sodium is limited, and we consider therefore only the two other coolants in what follows. The eutectic melt has a solidification temperature of about 262 K and does not require special systems for its melting in the startup of nuclear power plants for spacecraft; in the launch interval of the spacecraft, from the start of the carrier rocket to operating the nuclear power plants on the orbit, solidification of the melt is prevented by heating the coolant before the start and by applying a heat shield for the nuclear power plant. In the case of lithium as the coolant, which has a solidification temperature of about 400 K, a special system for melting in the heat dissipation circuit of the nuclear power plant and a complicated algorithm in the launch of the nuclear power plant are required. As far the temperature range in the possible long-term operation of a nuclear power plant with a view to corrosion stability of the material of the heat-dissipating circuit and the working pressure of the coolant with an adequate safety margin to boiling of the cooling and the corresponding long-term strength of the plant's structure are concerned, the sodium-potassium melt can be used up to 1000 K, and the lithium melt, up to 1200 K; accordingly, stainless steel and niobium alloys are the main materials of the structure of the heat-dissipation circuit