How Nuclear Reactors Work
How nuclear reactors split uranium atoms in a controlled chain reaction to make heat, steam, and electricity — fuel, control rods, and safety systems.
A nuclear reactor generates heat by splitting heavy uranium atoms in a controlled chain reaction, then uses that heat to make steam and spin turbines that produce electricity. Control rods absorb neutrons to keep the reaction exactly balanced, releasing enormous energy from tiny amounts of fuel without burning anything.
Nuclear fission and the chain reaction
Heavy nuclei such as uranium-235 split when struck by a neutron, releasing energy, fission fragments, and two or three fresh neutrons. If on average exactly one of those neutrons causes another fission, the reaction sustains itself at a steady rate — criticality — while more than one means power rises and fewer means it falls. Each fission releases roughly two hundred MeV, millions of times the energy of a chemical reaction per atom, which is why reactor fuel is so compact. The fragments are intensely radioactive and decay through chains described by radioactive decay, producing the residual heat that reactors must remove even after shutdown.
Reactor anatomy: fuel, moderator, coolant, and control
A typical pressurized-water reactor centers on ceramic uranium-oxide pellets stacked in metal-clad fuel rods, bundled into assemblies inside a steel pressure vessel. Fast fission neutrons must be slowed to split uranium efficiently, so a moderator — ordinary water in most designs — surrounds the fuel; the same water carries heat to steam generators. Control rods of neutron-absorbing boron or cadmium slide between assemblies: inserting them absorbs neutrons and lowers power, withdrawing them raises it. The steam drives turbines exactly like a fossil plant — the reactor is a heat source, and converting that heat to electricity obeys the same limits as any heat engine (see the second law of thermodynamics). From the turbine onward, the plant feeds the power grid like any other generator.
Reactor types and engineered safety
Most of the world's reactors are light-water designs (pressurized or boiling water), with heavy-water CANDU units, gas-cooled reactors, and fast breeders filling niches; small modular reactors aim to factory-build smaller units for flexibility. Safety rests on defense in depth: ceramic fuel that traps fission products, sealed cladding, a pressure vessel, and a containment building, plus redundant cooling, negative feedback (many designs naturally lose reactivity as they heat up), and control systems that insert control rods automatically. Spent fuel remains thermally hot and radioactive for years, cooling first in pools, then in dry casks, pending long-term geological disposal in most national plans. Reprocessing recovers unused uranium and plutonium in some countries, reducing waste volume but adding proliferation-sensitive chemistry.
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electricity fission nuclear energy reactors
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