Why a uranium nucleus can release energy
A uranium nucleus is a tightly packed cluster of protons and neutrons. In certain uranium atoms, absorbing a neutron can make the nucleus unstable enough to split into two smaller nuclei. Those fragments are more tightly bound overall than the original heavy nucleus. The small difference in mass emerges as energy—a consequence of the relationship between mass and energy.
Most of that energy initially appears as motion: the two fragments fly apart and collide with surrounding atoms. Those collisions become heat. In a power reactor, a coolant carries the heat away, and the plant uses it to make steam that turns a turbine.
Keeping the chain reaction steady
The neutrons released by fission do not all cause another split. Some escape the fuel, and some are absorbed by other materials. Reactor designers manage how many neutrons remain available to sustain the chain reaction. Control rods, made from neutron-absorbing materials, can be moved farther into or out of the reactor core to adjust that balance.
The goal during normal operation is not to stop fission, but to keep the reaction steady: on average, each fission leads to one more. That makes the heat output controllable rather than rapidly rising or fading away. The same process that produces the heat also creates radioactive fission products, which remain in the fuel and must be handled and stored safely after use.
