Albert Einstein was one of the first to realize that the amount of energy locked inside matter is almost limitless. To harness the extraordinary power potential of fission and prevent runaway chain reactions from occurring, engineers have designed controlled environments called reactors. A typical reactor consists of a core, where the nuclear reactions take place, turbines, and a cooling system. In most reactors, the fuel is U-235, a fissionable isotope of uranium.
Inside the core of a typical reactor are pencil-thin fuel rods, each about 12 feet long, which are grouped by the hundreds in bundles called fuel assemblies. Inside each fuel rod, pellets of uranium are stacked end to end. Also inside the core are control rods, filled with pellets of substances like graphite or cadmium that readily capture neutrons. When the control rods are lowered into the core, they absorb neutrons, which thus cannot take part in the chain reaction and start more fission. In reverse, when the control rods are lifted out of the way, more neutrons strike U-235 nuclei in nearby fuel rods, and the level of reaction intensifies.
The heat created during fission turns water to pressurized steam, which can then be used to drive turbines that generate electricity in much the same way fossil fuel plants do. In most plants today, a cooling system circulates water around the outside of the core to control the heat level. Nuclear reactions also produce high levels of radiation, some types of which are dangerous to human and environmental health. Safeguards must therefore be installed to ensure that radioactive particles do not escape into the atmosphere. This is why heavy concrete walls typically surround reactors and why safe, long-term disposal of nuclear waste, including used fuel, is critically important.