The objective of the liquid-fluoride thorium reactor (LFTR) design proposed by Flibe Energy is to develop a power-generating nuclear reactor that will produce electrical energy at low cost by efficiently consuming thorium, the Earth’s greatest natural stored energy resource. A graphite-moderated, thermal-spectrum reactor with solutions of liquid fluoride salts containing both fissile and fertile materials appears to be the best way to realize this objective. This is a brief summary of the characteristics of the LFTR design as they are presently understood.

Mixtures of fluoride salts raised to a sufficient temperature to allow them to liquefy form an ideal medium in which nuclear fission reactions can take place. The ionically-bonded nature of the salts prevents radiation damage to the mixture and allows for operation at high temperature yet at essentially ambient pressure. Most fission products, including all of those of greatest radiological concern, form stable fluoride salts that are retained in the overall mixture under all conditions. Fission products gases, whose removal is important from a performance and safety basis, are easily separated from the fluid mixture and allowed to decay to stability in a separate system. New fissile material can be easily added or removed without resorting to changing the chemical nature of the solvent, and this allows overall reactivity to be held very close to the minimum amount needed to achieve criticality.* The fluoride salt mixtures in question have high volumetric heat capacity, comparable to water, and do not undergo vigorous chemical reactions with air or water in contrast to many liquid metals.

The components of fluoride salt mixtures have both desirable and undesirable aspects, and the two most important are lithium-7 fluoride and beryllium fluoride. The two natural isotopes of lithium must be separated from one another since lithium-6 (7.5% of natural lithium) is far too absorptive of neutrons to be a suitable component of a reactor fluid. Beryllium fluoride is chemically toxic but has very attractive nuclear and physical properties. The chemical processing and purification of fluoride salt mixtures typically involves using powerful reactants such as gaseous fluorine and hydrogen fluoride which are very toxic and reactive. But the fact that fluoride salt mixtures are processed in a salt form rather than being dissolved into an aqueous solution mitigates issues of accidental criticality considerably, since water is an excellent moderator whereas salts are poor.

Fluoride salts, due to their exceptional chemical stability, have the potential to corrode most structural metal alloys, but some alloys have been developed that hold up very well against any corrosive attack. Invariably these alloys are based on nickel with a variety of other metallic constituents. Fluoride salts moderate neutrons sufficiently on their own to prevent the formation of a truly fast neutron spectrum, but are still insufficiently effective to generate a thermal neutron spectrum. Thus separate moderator materials are necessary for the reactor and graphite has been proven to be very attractive. Graphite is not wet by the fluoride salts and does not require cladding. If the surface of the graphite is treated so that small pores are closed, most fission product gases can be excluded from the graphite and overall performance will be high. Graphite does experience issues from dimensional distortion over time, but this effect can be quantified and compensated for in reactor design.

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The high operational temperatures of the fluoride salts (500-700 degrees C) make them excellent candidates for coupling to a closed-cycle gas turbine power conversion system (PCS). At present, the supercritical carbon dioxide gas turbine employing the recompression cycle appears to be the best candidate for coupling to the reactor. The carbon dioxide working fluid in the cycle provides a final barrier to tritium release into the environment**, and tritium generation is an inevitable consequence of using lithium and beryllium in the salt mixture. The gas turbine can generate shaft power at high efficiencies (approximately 45%) yet in a small operational envelope.

Thorium fuel is introduced as a tetrafluoride into the blanket salt mixture of the reactor. The blanket salt surrounds the active “core” region of the reactor and intentionally absorbs neutrons in the thorium, which leads to the transmutation of the thorium-232 via nuclear beta decay, first to protactinium-233 and later to uranium-233, as follows***:

Both the protactinium and the uranium are chemically removed from the blanket salt mixture and introduced into the fuel salt mixture in the reactor. Fission splits uranium-233 into two or more fission products, releasing both energy and neutrons. These fission products are later chemically removed from the fuel salt and in some cases separated and purified before final disposition. By utilizing thorium fuel in a thermal neutron spectrum, the reactor is able to utilize the energy content of the thorium at a very high efficiency, approaching 100%. At this rate of thorium consumption, the Earth’s thorium resources will last hundreds of millions of years, and the cost of fuel will be utterly insignificant to the reactor operator.****

* Most conventional nuclear power plants use solid-state fuel that is sealed in the reactor for the duration of the fuel cycle (18-24 months), which in turn requires that all energy necessary for power production for a given cycle be emplaced at the beginning of the cycle. This excess reactivity must be suppressed in lessening degrees throughout the fuel cycle and drives many safety concerns in conventional, commercial nuclear reactors. The dramatic reduction in excess reactivity in the LFTR reduces the magnitude of reactivity control needed, which alleviates some concerns about reactor accidents.

** High-temperature carbon dioxide will chemically react with any tritium present, forming tiny amounts of tritiated steam that can be removed.

*** Thorium-232 is the “fertile” isotope, and uranium-233 is the “fissile” isotope that LFTR is designed to create and subsequently fission.

**** Most of the reactors today are water-cooled reactors that employ a “once-through” cycle that liberates less than one percent of the energy stored in the fuel compared to a nearly complete energy release for thorium. Further, the reserves of thorium are about three times that of uranium. There are vast amounts of available energy via a thorium fuel cycle.

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