Initially, the core is loaded with fertile material, with a few rods of fissile fuel concentrated in the central region.

Once the vortex develops, fissile fuel can be injected through the semi-porous plate to bring the reactor critical.

The problem isn't exactly that the fissile fuel gets depleted, as that the process of radioactive decay damages the material.

Examples of fissile fuels are U-233, U-235 and Pu-239.

Such thermal reactors require much less of the expensive fissile fuel to start and have a slow, gentle response in power changes.

One can place the fertile and fissile fuel together, so breeding and splitting occurs in the same place.

Some fission products in the salt absorb neutrons and reduce the production of new fissile fuel.

The chemical separation for the 2-fluid designs, using uranium as a fissile fuel can work with these two relatively simple processes:

The most common fissile nuclear fuels are uranium-235 (U) and plutonium-239 (Pu).

Nuclear fission weapons require a mass of fissile fuel that is prompt supercritical.