Fissile material is any substance whose atoms can be split by slow-moving (low-energy) neutrons, releasing enough additional neutrons to keep the splitting process going in a self-sustaining chain reaction. The three primary fissile materials are uranium-235, plutonium-239, and uranium-233. These isotopes are the foundation of both nuclear energy and nuclear weapons, which is why their production and control are among the most tightly regulated activities on Earth.
How Fissile Materials Work
When a slow neutron strikes the nucleus of a fissile atom, the nucleus absorbs it and becomes unstable. It splits into two smaller nuclei, releasing a burst of energy along with two or three additional neutrons. Those newly freed neutrons can then strike neighboring fissile atoms, causing them to split as well. This cascading process is a chain reaction. In a nuclear reactor, the chain reaction is carefully controlled to produce steady heat. In a weapon, it is deliberately left uncontrolled for an explosive release of energy.
The key detail is the word “slow.” Fissile isotopes split reliably when hit by thermal neutrons, which are neutrons that have been slowed down to roughly the same energy as the surrounding atoms. This low threshold is what makes fissile materials uniquely useful. Other heavy isotopes can be forced to split, but only when struck by much faster, higher-energy neutrons, making a sustained chain reaction far harder to achieve.
Fissile vs. Fissionable vs. Fertile
These three terms sound similar but describe different nuclear behaviors. Fissile materials split with slow neutrons. Fissionable materials can split, but only when hit by fast, high-energy neutrons. Uranium-238, the most common form of uranium, is fissionable but not fissile: it won’t sustain a chain reaction on its own because slow neutrons simply bounce off or get absorbed without triggering a split.
Fertile materials occupy a third category. They cannot be split by thermal neutrons themselves, but they can be converted into fissile materials inside a reactor. The two main fertile materials are uranium-238 and thorium-232. When uranium-238 captures a neutron, it transforms through a series of rapid radioactive decays, first into neptunium-239 (which lasts about 2.4 days) and then into plutonium-239, a fully fissile isotope. Similarly, thorium-232 can absorb a neutron and eventually become uranium-233. This conversion process is the basis of “breeder” reactor designs, which produce new fissile fuel while generating power.
The Three Primary Fissile Isotopes
Uranium-235
Uranium-235 is the only fissile material that occurs naturally in meaningful quantities, but “meaningful” is relative. Natural uranium is 99.3 percent uranium-238 and only 0.7 percent uranium-235. That tiny fraction is enough to fuel certain reactor designs, but most commercial power plants need the concentration of U-235 boosted through a process called enrichment. A bare sphere of pure uranium-235 reaches critical mass, the minimum amount needed for a self-sustaining chain reaction, at roughly 47 kilograms (about 104 pounds).
Plutonium-239
Plutonium-239 does not exist in nature in any practical amount. Nearly all of it is manufactured inside nuclear reactors as a byproduct of uranium fission. When some of the neutrons released during fission are absorbed by uranium-238 in the reactor fuel, the conversion chain described above produces plutonium-239. Each fission of a plutonium-239 atom releases slightly more than two neutrons on average, which is enough to sustain a chain reaction. Plutonium-239 is significantly more reactive than uranium-235: its critical mass as a bare sphere is only about 10 kilograms (22 pounds), less than a quarter of uranium-235’s.
Uranium-233
Uranium-233 is produced from thorium-232 through a similar neutron-capture-and-decay process. It has been studied extensively for use in thorium fuel cycles but has seen far less commercial use than the other two fissile isotopes. Its significance is mainly theoretical and experimental, though interest in thorium-based reactors has grown in recent years.
Enrichment Levels and Their Uses
Because natural uranium contains so little U-235, it must be enriched for most applications. Enrichment is the process of increasing the proportion of U-235 relative to U-238. The level of enrichment determines what the material can be used for.
- Low-enriched uranium (LEU): Contains between 3 and 5 percent U-235. This is what fuels nearly all commercial nuclear power plants worldwide.
- Highly enriched uranium (HEU): Contains more than 20 percent U-235. It is used in naval propulsion reactors, some research reactors, and nuclear weapons. Weapons-grade uranium is typically enriched above 90 percent.
The gap between 5 percent and 90 percent is enormous in practical terms. Enriching uranium from its natural 0.7 percent to reactor-grade 5 percent accounts for a large share of the total effort, but reaching weapons-grade concentrations requires far more sophisticated infrastructure and many additional processing steps. This is one reason international nonproliferation efforts focus so heavily on monitoring enrichment facilities.
Why Fissile Materials Matter Beyond Reactors
The same property that makes fissile materials useful for energy production, their ability to sustain a chain reaction with slow neutrons, also makes them the essential ingredient in nuclear weapons. Only fissile materials can achieve the rapid, uncontrolled chain reaction needed for a nuclear explosion. This dual-use nature is the reason fissile material stockpiles are tracked internationally, and why the production of plutonium-239 and highly enriched uranium is subject to inspections and treaties.
Plutonium-239’s low critical mass makes it a particular proliferation concern. A sphere of plutonium the size of a softball contains enough material to sustain a chain reaction, which is why even small quantities are closely monitored. The global inventory of separated plutonium, material that has been chemically extracted from spent reactor fuel, is one of the most closely watched figures in nuclear security.
Understanding the distinction between fissile, fissionable, and fertile materials is essential to making sense of debates about nuclear energy, disarmament, and nonproliferation. The physics is straightforward: fissile isotopes split easily and keep splitting. Everything else in the nuclear world, from reactor design to arms control treaties, flows from that single property.