fissile vs fissionable

In nuclear physics, understanding the distinction between fissile and fissionable materials is crucial for applications in nuclear reactors and weaponry. A fissile material is one that can sustain a nuclear chain reaction with neutrons of very low energy, commonly known as thermal neutrons. This means that fissile nuclides, such as Uranium-235 or Plutonium-239, can undergo fission when struck by slow-moving neutrons, releasing energy and more neutrons to continue the process. On the other hand, fissionable materials require high-energy neutrons, typically in the MeV range, to trigger fission. While all fissile materials are fissionable, not all fissionable materials are fissile. For example, Uranium-238 is fissionable but not fissile because it only fissions with fast neutrons, which have much higher kinetic energy compared to thermal neutrons. The energy of neutrons is a critical factor: thermal neutron energies are generally below the electron volt (eV) range, while high-energy or fast neutrons have energies in the million electron volt (MeV) range. Fissile materials allow nuclear reactors to operate efficiently using moderated (slowed) neutrons. In contrast, reactors using fissionable but non-fissile materials require fast neutrons or different mechanisms to sustain a chain reaction. This distinction impacts reactor design, nuclear fuel selection, and safety measures. The ability of fissile materials to sustain chain reactions with low-energy neutrons makes them essential in most nuclear power plants. Understanding these differences also aids nuclear engineers and physicists in developing advanced nuclear technologies and evaluating nuclear materials' behavior under different conditions.