When an atom of uranium-235 is bombarded with a neutron, it absorbs that neutron and becomes extremely unstable, splitting almost instantly into two smaller atoms while releasing roughly 200 MeV of energy and two or three additional neutrons. This splitting process is called nuclear fission, and it’s the fundamental reaction behind both nuclear power plants and nuclear weapons.
What Happens Inside the Nucleus
A uranium-235 nucleus contains 92 protons and 143 neutrons, totaling 235 particles. This arrangement is already somewhat unstable. When a stray neutron enters the nucleus, it briefly becomes uranium-236, a highly excited compound nucleus that can’t hold itself together. About 80% of the time, this uranium-236 nucleus splits apart within roughly a trillionth of a trillionth of a second.
The split isn’t clean down the middle. The nucleus breaks into two unequal chunks called fission fragments, typically one larger and one smaller. A common example is barium (around 141 particles) and krypton (around 92 particles), but many different combinations are possible. Along with these two fragments, the splitting nucleus ejects two or three neutrons and releases a burst of gamma radiation.
Why Slow Neutrons Work Best
Uranium-235 is unusual because it fissions most readily when hit by slow-moving, low-energy neutrons, often called thermal neutrons. These are neutrons that have been slowed down to roughly the same energy as the surrounding atoms, moving at speeds comparable to air molecules at room temperature. A slow neutron is actually more likely to be absorbed by the uranium nucleus than a fast one, because it spends more time in the vicinity of the nucleus.
This property is what makes U-235 so valuable as a nuclear fuel. In a reactor, materials like water or graphite act as moderators, slowing neutrons down so they’re more effective at triggering additional fissions. Other uranium isotopes, like the far more abundant U-238, generally require much faster neutrons to split.
The Chain Reaction
Each fission event releases an average of 2.4 neutrons. Those neutrons can then strike other uranium-235 atoms and cause them to split as well, each releasing another 2.4 neutrons on average. This cascading process is a chain reaction. Whether the chain reaction grows, holds steady, or dies out depends entirely on how many of those neutrons successfully find another U-235 nucleus before escaping or being absorbed by something else.
Not all the neutrons come out at the same time. The vast majority, called prompt neutrons, fly out within about a quadrillionth of a second of the split. A small fraction, called delayed neutrons, are released by the fission fragments seconds or even tens of seconds later, with an average generation lifetime around 12.5 seconds. This delay is critical for reactor control. Without it, adjusting a chain reaction would be nearly impossible because everything would happen too fast for mechanical systems to respond.
How Much Energy One Atom Releases
A single uranium-235 fission releases about 215 MeV of energy total. That number breaks down across several forms. The two fission fragments carry away about 168 MeV as kinetic energy, meaning they fly apart at enormous speed. The ejected neutrons carry about 5 MeV. Gamma rays released at the moment of fission account for another 7 MeV. The remaining energy trickles out over time as the radioactive fission fragments decay, emitting additional gamma rays, beta particles, and neutrons worth roughly 30 MeV combined.
Where does all this energy come from? The total mass of the fission products is slightly less than the mass of the original uranium atom plus the neutron. That tiny mass difference, about 3.2 × 10⁻²⁷ kilograms per atom, is converted directly into energy following Einstein’s famous equation. Scaled up to a practical quantity, one kilogram of U-235 undergoing complete fission releases energy equivalent to burning about 2,700 tons of coal.
What Happens to the Fragments
The two fission fragments are themselves radioactive. They contain an imbalanced ratio of protons to neutrons, which makes them unstable. Over the following hours, days, and years, they undergo a series of radioactive decays, gradually transforming into more stable elements. This ongoing decay is the source of what’s known as decay heat, the residual warmth that persists in spent nuclear fuel long after the chain reaction has stopped.
Decay heat drops quickly at first but persists for a long time. At about 1,000 seconds (roughly 17 minutes) after a reactor shuts down, the fission fragments and related byproducts still produce meaningful heat, with actinide decay contributing around 3% of the total. By 10,000 seconds (just under 3 hours), actinide decay can account for about 4%. This is why spent fuel must be actively cooled in water pools for months or years after removal from a reactor.
From Physics to Power Plants
Natural uranium is 99.3% U-238 and only 0.7% U-235. For most commercial reactors, the uranium needs to be enriched to 3-5% U-235, a level called low-enriched uranium. At this concentration, a carefully designed reactor with a moderator can sustain a controlled chain reaction, producing steady heat that generates steam and drives turbines.
Some reactor designs sidestep enrichment entirely. Canada’s CANDU reactors use natural uranium fuel paired with heavy water as a moderator, which absorbs fewer neutrons than regular water and compensates for the low U-235 content. At the other extreme, nuclear weapons require uranium enriched to at least 90% U-235 in specially designed facilities, creating conditions where an uncontrolled chain reaction can release its energy in a fraction of a second rather than over months of steady operation.