Examples of fissile isotopes are uranium-235 and plutonium-239. Moreover, it appeared that plutonium might be even more fissile. ""Fissile"" is distinct from ""fissionable"". Uranium-235 was the first isotope that was found to be fissile. Its fission product yield decreases as the mass of the fissile nuclide increases. Fission chain reactions occur because of interactions between neutrons and fissile isotopes (such as U). Thorium, a non fissile material is available in abundance in our country. Thus, a nuclear starship would greatly deplete Earth of fissile and fusile material. The program is intended to use fertile thorium-232 to breed fissile uranium-233. This reduces the number of neutrons which escape the fissile material, resulting in increased reactivity. Only new fertile fuel is added, which breeds to fissile inside the reactor. They achieve criticality by having a sufficient volume of salt with sufficient fissile material. This is due to its ability to become fissile with slow neutron interaction. It has a blocky subconchoidal fracture, and is less fissile than shale. Metal fuels have the potential for the highest fissile atom density.