Nuclear power is the subject of more misunderstanding per kilowatt-hour than any other energy source. The gap between popular perception and physical reality is wide enough that it's worth covering the basics carefully.
Quick answer
A nuclear reactor controls a fission chain reaction to produce heat. That heat boils water or transfers heat to another water loop, making steam that spins a turbine connected to a generator. The reactor is the heat source; the electricity generation step is still a steam-turbine power plant.
What nuclear fission actually is
Every atom has a nucleus — a dense core of protons and neutrons held together by the strong nuclear force. For most atoms, this nucleus is stable. For certain heavy atoms — uranium-235 and plutonium-239 are the main ones used in reactors — the nucleus is large enough that it can be destabilized by absorbing a neutron.
When a uranium-235 nucleus absorbs a neutron, it briefly becomes uranium-236, which is so unstable that it splits — fissions — into two smaller nuclei (typically barium and krypton), releasing two or three additional neutrons and a significant amount of energy in the form of heat and gamma radiation.
The released neutrons can then collide with adjacent uranium nuclei, causing them to fission and release more neutrons. This is a chain reaction. In a bomb, this happens in microseconds, releasing enormous energy at once. In a reactor, the chain reaction is deliberately controlled to release heat at a steady, manageable rate.
Controlling the reaction
Reactor control depends on moderating neutron speed and absorbing excess neutrons.
Moderators slow down the fast neutrons released by fission (which are too fast to be efficiently captured by uranium-235) to "thermal" speeds at which uranium-235 absorbs them readily. Water — ordinary water in most commercial reactors — is the most common moderator. The water surrounding the fuel serves double duty: moderating neutrons and carrying away heat.
Control rods are made of neutron-absorbing materials (boron, cadmium, or hafnium) and are inserted into the reactor core to absorb excess neutrons. Pushing them in slows the reaction. Pulling them out speeds it up. Full insertion shuts the reactor down. The physics here is important: if something goes wrong and control rods are inserted, the chain reaction stops quickly. Modern reactor designs exploit this — many are engineered so that the reaction slows naturally if temperatures rise or coolant is lost, because the physics of the moderator at higher temperatures becomes less favorable for sustaining the chain reaction.
Converting heat to electricity
A nuclear reactor is, at its core, a very sophisticated way to boil water. The heat from fission heats water, which makes steam, which spins a turbine, which generates electricity. This is identical to a coal or gas power plant — the difference is the heat source.
In a pressurized water reactor (the most common type), the water in contact with the reactor core is kept under high pressure so it doesn't boil — it stays liquid even at 300°C+. This primary circuit transfers heat to a secondary circuit via a heat exchanger. The secondary circuit boils, makes steam, drives the turbine. The two circuits never mix, which means the steam driving the turbine isn't radioactive.
What radioactive waste actually is
The fission products — the smaller nuclei produced by splitting uranium — are themselves unstable and radioactive. They decay over time, releasing radiation. Some decay within seconds. Others take thousands of years.
This is the genuine, unresolved problem with nuclear power: what to do with spent fuel. Current practice at most reactors involves storing spent fuel in pools of water on-site (water both cools the fuel, which continues generating heat from radioactive decay, and shields workers from radiation), eventually moving it to dry cask storage. Long-term geological disposal — burying it deep in stable rock formations — is the widely agreed solution that has been implemented in Finland and is in progress elsewhere, but hasn't been deployed at scale in most countries.
The volumes are smaller than most people assume. All the spent nuclear fuel ever produced by US commercial reactors would fit in an area about the size of a football field to a depth of about 10 meters. The challenge is not volume; it's the combination of heat, radioactivity, and the timescale required for safe storage.
Why Chernobyl happened (and why most reactors can't)
The Chernobyl reactor was an RBMK design with a property called a positive void coefficient: as cooling water turned to steam (forming voids), the reaction sped up rather than slowing down. This is the opposite of most Western reactors. During a poorly managed safety test, this instability contributed to an uncontrolled power surge.
The Chernobyl design was known to have this property and was not used outside the Soviet Union. Pressurized water reactors and other Western designs have negative temperature and void coefficients — the physics naturally slows the reaction if things go wrong. They cannot undergo a Chernobyl-type runaway nuclear explosion. (Nuclear reactors cannot explode like nuclear weapons; the fuel isn't enriched to sufficient concentrations and the geometry doesn't permit it.)
Fukushima's failure mode was different: the reactors shut down correctly when the earthquake hit, but the tsunami destroyed the backup generators needed to power cooling systems. Without cooling, residual heat from decay products caused fuel damage. The lesson from Fukushima is about cooling system resilience, not reactor physics instability.
Nuclear power has killed far fewer people per unit of electricity generated than coal, oil, gas, or even rooftop solar (installation accidents). Whether its role in a low-carbon energy future should expand is a legitimate policy debate. It should be conducted with accurate physics, not mythology.
For a closer look at the heat-management side, read why nuclear reactors need water. For related energy context, see how the power grid stays balanced, heat pumps explained, and critical minerals geopolitics.