COMBINED THERMAL STORAGE

The Gates Foundation has funded research into safer nuclear thermal power and the natrium cooled reactor is the result. Natrium melts at just under the boiling point of water and remains in the liquid state up to over 800-deg C. It has to operate in a sealed environment, in the absence of atmospheric air.

The absence of pressure enhances the safety of a natrium cooled reactor, transferring heat from the reactor to a boiler located away from the reactor. Pipes made from corrosion resistant ceramic material such as silicon-carbide offer high thermal conductivity and high compressive strength, allowing water at high pressure to flow over pipes made of the material and operating in counter-flow heat-exchange mode. Pipes made of corrosion resistant ceramic may also transfer heat from a high-temperature (500 to 700-degree C) nuclear reactor to thermal storage material such as a molten salt mixture that in turn would operate well above its melting temperature. Such material would use heat capacity to convert water to steam. Natrium offers the possibility of including heat-of-fusion thermal storage material such as a mixture of 20% lithium fluoride and 80% lithium hydroxide.

While heat-of-fusion materials can store massive amounts of thermal energy, they release that energy at a slow rate, offering the potential to convert lower volumes of water to steam over greatly extended periods of time. There may be scope to combine heat capacity thermal storage material that operates well above its melting point that can release heat quickly with heat-of-fusion material that releases heat slowly. The heat capacity thermal material would generate steam to produce electric power that supplies fluctuating market demand while ceramic pipes carrying natrium would transfer heat from heat-of-fusion material. Such an arrangement offers the potential to keep the heat capacity material at higher temperature, in turn allowing higher thermal conversion efficiency while occupying less physical space than using multiple large tanks of heat capacity material.

During an emergency shut down, heat-of-fusion material could absorb massive amounts of heat from the reactor.