Saturday, April 22, 2006

Introducing the Liquid-Fluoride Reactor

The liquid-fluoride reactor is an innovative design for a thermal breeder reactor that was developed from the 1950s through the 1970s at Oak Ridge National Laboratory in Oak Ridge, Tennessee. The reactor utilized a fluid-fuel form, with uranium and thorium fluoride salts dissolved in a matrix of lithium and beryllium fluoride salts. The melted salt was pumped throughout the reactor vessel and generated energy in an interesting manner. As the salt passed through the "core" region of the reactor, moderation provided by solid graphite elements led to neutron thermalization and fission reactions that produced heat. Then as the salt accumulated in a plenum and was pumped out of the core, fission ended and the salt passed through an external heat exchanger where it was cooled and transferred its heat to a secondary salt, and ultimately to a working fluid.

In 1970s, a steam-Rankine cycle was the basic power conversion technique considered for the liquid-fluoride reactor. However, there were a number of problems with this approach, mainly stemming from the fact that the natural temperature range of the fluoride salt was significantly above the typical operating temperatures of steam systems used for nuclear reactors.

More recent work on the liquid-fluoride reactor has focused on using the helium-Brayton (gas-turbine) power conversion cycle for electrical generation. This cycle would offer higher conversion efficiencies (~50% efficiency) through the salt’s unique abilities to take advantage of multiple reheating steps in the Brayton cycle. The high-temperature attributes of the salt also enable other unique applications, such as the thermochemical generation of hydrogen directly from nuclear heat.

The unique attributes of the liquid-fluoride reactor are a consequence of its fuel form. Salts of fluorine and alkali metals are exceptionally stable since they are formed from the most electronegative of elements (fluorine) and the most electropositive (lithium, beryllium, sodium). Due to their exceptional chemical stability, these fluoride salts have low vapor pressures at high temperature (enabling high temperature operation at low pressure) and they do not react with air or water, unlike molten metal coolants such as sodium. The favored combination for a neutronically-efficient liquid-fluoride reactor is a combination of lithium fluoride (highly enriched in the lithium-7 isotope) and beryllium fluoride. Through a proper ratio of these two salts, a solvent with a low melting point can be constructed. The minimum melting temperature of these salts is achieved when a composition of 52 mole % LiF and 48 mole % BeF2 is used. This combination will melt at 356°C. Typical compositions of base salts that have been used in liquid-fluoride reactors are 66 mole % LiF and 34 mole % BeF2.

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