In depth: the Liquid Fluoride Thorium Reactor (LFTR)

While the LFTR reactor design is one out of many possible concepts for molten salt reactors, it is further detailed here because it is an example of a true “thorium-MSR” (MSR using thorium fuel) and therefore comes with the full benefits of molten salt reactors and the thorium fuel cycle.

It is considered by many to be one of the most favourable configurations for facilitating the thorium cycle and has become iconic in many discussions of molten salt reactors. Usually the name LFTR is used for a design that includes two separate molten salt cycles that are combined in a single reactor. The first is the core fluid, the second is called the ‘blanket’. Both contain a different mixture of molten salts with either uranium (core) or thorium (blanket) as its key component. This design is also called a ‘two fluid LFTR’.

In this two fluid LFTR , the uranium in the core serves to generate heat for electricity generation, but also delivers the neutrons that turn the thorium in the blanket into new uranium. This happens when the uranium in the core fissions and releases neutrons that are absorbed by the thorium nuclei in the blanket. The thorium then decays (=transforms) into new fissile uranium. This new uranium is chemically separated and transported to the core, and the process starts all over again.

Because the fuel is liquid, it is possible to keep an optimized mixture at all times. The liquid state makes it easy to separate out useful, as well as unwanted by-products. This is an important advantage over solid fuel, which is difficult to manipulate once inside the reactor. Solid fuel ‘traps’ unwanted fission by-products inside its structure.

The salt mixture considered to be optimal for use in the LFTR consists of lithium fluoride and beryllium fluoride (LiF-BeF), and is commonly referred to as ‘FLiBe’. It has a high boiling point of 1430°C which enables it to remain liquid at high temperatures without turning to steam. It also serves as a neutron moderator. A neutron moderator’s function is to slow down fast neutrons, because neutrons that are too fast are not useful for sustaining the nuclear chain reaction. This does mean however that the LFTR needs an installation to clean and maintain the FLiBe mixture, which runs parallel to the reactor component (Ingersoll, et al., sd, p. 5) , (Carpenter, 2003, p. 7) , (LeBlanc, 2009, pp. 1644,1645), (Sohal, et al., 2010, p. 8) , (Hargraves & Moir, 2010, pp. 307,308), (Hart, 2011, p. 17) , (Sorensen, 2012) , (Kloosterman, 2012).

Throughout this site, we prefer to use the term ‘thorium MSR’ as it designates the whole family of possible molten salt reactor designs and the use of thorium, a family of which the LFTR is a member. Some authors, most notably Hargraves, use the term LFTR in the generic sense in which we use the term thorium MSR.