How a Nuclear Power Plant Makes Electricity
The U.S. Nuclear Regulatory Commission (NRC) gives a nuclear reactor and a coal-fired boiler as two examples of a heat source. In a nuclear plant, that heat ends up as steam, and the steam ends up as electricity.
A nuclear power plant, the EIA says, uses the heat a nuclear reactor produces to turn water into steam, which then drives turbine generators that generate electricity. The heat comes from nuclear fission, in which atoms are split apart. Nuclear power plants in the United States have either a boiling water reactor (BWR) or a pressurized water reactor (PWR).
Splitting Atoms to Make Heat
The U.S. Energy Information Administration (EIA) describes nuclear fission as atoms being split apart, which releases energy. All nuclear power plants use nuclear fission, and most use uranium atoms. The EIA describes a separate process, nuclear fusion, in which atoms are combined, or fused, together to form a larger atom.
During nuclear fission, in the EIA’s account, a neutron collides with a uranium atom and splits it. The split releases a large amount of energy as heat and radiation. More neutrons come out too.
Those neutrons go on to collide with other uranium atoms, and the process keeps repeating itself — a nuclear chain reaction. In nuclear power plant reactors, the EIA says, the chain reaction is controlled to produce the amount of heat needed to generate electricity. The EIA describes nuclear reactors as machines that contain and control nuclear chain reactions while releasing heat at a controlled rate.
The U.S. Department of Energy (DOE) points to the moderator, which helps slow down the neutrons produced by fission so the chain reaction keeps going. Control rods, the DOE says, can be inserted into the reactor core to reduce the reaction rate, or withdrawn to increase it.
From Uranium to Fuel Rods
Nuclear power plants use a certain kind of uranium, referred to as U-235, for fuel because its atoms are easily split apart, the EIA says. Uranium is about 100 times more common than silver, yet U-235 is relatively rare. Once uranium is mined, the U-235 must be extracted and processed before it can be used as a fuel.
Reactors use uranium for nuclear fuel, the DOE says. The uranium is processed into small ceramic pellets, which are stacked together into sealed metal tubes called fuel rods. Typically more than 200 fuel rods are bundled together to form a fuel assembly.
Inside the reactor vessel, according to the DOE, the fuel rods are immersed in water, which acts as both a coolant and moderator. All commercial nuclear reactors in the United States are light-water reactors — they use normal water as both a coolant and neutron moderator.
The fuel rods are suspended in a water bath contained in a large metal container, somewhat like a gigantic pressure cooker, the NRC says. A typical reactor vessel might be 15 feet in diameter and 20 feet high, and some are much larger than that.
How Heat Becomes Electricity
A motor, the NRC explains, consists primarily of two major components: a stator, which stands still, and a rotor, which rotates within the stator. Electrical energy applied to the motor results in mechanical energy in the rotor. The same machine can be used in reverse.
If some outside force causes the rotor to spin, the interaction of the magnets causes electricity to be produced, according to the NRC. The motor is then a generator, producing electrical energy from the mechanical energy applied to its rotor. That is the most common way to make large quantities of electricity.
The reactor’s role in spinning the rotor is indirect, the NRC notes. The fuel rods get hot because of the nuclear reaction, and steam made with that heat turns the rotor.
When water boils, the NRC says, the steam that is produced occupies much more physical space than the water that produced it. The steam runs through a machine called a turbine, which acts something like a highly sophisticated windmill. The steam causes part of the turbine to spin, and that spinning part can be connected to a generator to produce electricity.
The steam leaving the turbine is collected in a device called a condenser, which is essentially a metal box the size of a house, with thousands of pipes running through it. Cool water flows through the pipes. The steam from the turbine is cooled and condenses back into water.
The NRC sums it up in order. The nuclear reaction heats the fuel, and the fuel heats the water to make steam. The steam spins the turbine, the turbine turns the generator, and the generator makes electricity.
Two Reactor Designs, Two Places Where Water Boils
In a boiling water reactor, the reactor core heats water, which turns directly into steam in the reactor vessel, the EIA says. In a pressurized water reactor, the reactor core heats water and keeps it under pressure to prevent the water from turning into steam.
In some types of reactors the water is allowed to boil, and the heat generated in the fuel rods is carried away in steam, the NRC says. These are called boiling water reactors. BWRs actually boil the water.
In a typical commercial BWR, a steam-water mixture leaves the top of the core and enters two stages of moisture separation, where water droplets are removed before the steam is allowed to enter the steamline.
In other reactors, the NRC says, the water is held at a very high pressure, on the order of 2,000 pounds per square inch. That is more than 60 times the pressure in the tires of a typical car. At that pressure the water cannot expand and cannot boil.
The water in that type of reactor carries the heat away while remaining liquid, and the heat is then transferred to another water system where the boiling occurs. The transfer takes place in a device named a steam generator. These reactors are called pressurized water reactors.
PWRs keep water under pressure so that it heats, but does not boil, the NRC says. Water from the reactor and the water in the steam generator that is turned into steam never mix. In this way, most of the radioactivity stays in the reactor area.
In a typical commercial PWR, the core inside the reactor vessel creates heat, the NRC says, and pressurized water in the primary coolant loop carries that heat to the steam generator. A small PWR might have two steam generators, a large one four. Some have three.
The steam from all of the steam generators is typically combined into a single main steam line that carries the steam to the turbine.
| What differs | Typical pressurized water reactor (PWR) | Typical boiling water reactor (BWR) |
|---|---|---|
| Where the steam forms | In the steam generator, where heat from the primary coolant loop vaporizes the water in a secondary loop | In the core, where very pure water moves upward and absorbs heat, producing a steam-water mixture |
| Where water from the condenser is pumped back to | The steam generator | The reactor vessel |
| Fuel assemblies | Between 150 and 200 | Between 370 and 800 |
Source: U.S. Nuclear Regulatory Commission. Typical commercial designs.
Keeping the Water Moving
In both types, water is converted to steam, the NRC says, and a part called the condenser then recycles it back into water, to be used again in the heat process.
In both typical designs, the NRC says, the reactor’s core contains fuel assemblies that are cooled by water circulated using electrically powered pumps. Those pumps and other operating systems in the plant receive their power from the electrical grid.
If offsite power is lost, emergency cooling water is supplied by other pumps, which can be powered by onsite diesel generators.
Inside the reactor, the EIA says, a neutron collides with a uranium atom and splits it. In the generator, the NRC explains, a rotor spins inside a stator, and electricity comes out of the stator.
In between, the steam is condensed back into water and pumped back to be heated again. The cycle continues.