Nuclear power plants provide about one-fifth of the world's electricity. How do they work? And why do they produce radioactive waste that will remain toxic for centuries?

Whether because of the environment or national security, most everyone agrees that we need to find alternatives to coal, gas and petroleum in order to power our society. Some countries with limited natural resources, like Japan and France, have decided that nuclear power plants are their best option. And after 30 years without building a new nuclear plant, the U.S. may be soon be adding more.

What Fuel Is Used in Nuclear Power Plants?

Today's nuclear power plants use uranium as fuel. Uranium is an element that can be mined from the ground as an ore. The Western U.S. is one place that contains a large amount of uranium ore.

Although most uranium is stable, some comes in a naturally radioactive form, called uranium-235. U235 is unstable, which means that its nucleus spontaneously gives off neutrons. This process is called radioactive decay. If a stray neutron crashes into the nucleus of another U235 atom, the atom splits into two smaller atoms, and at the same time gives off energy and more neutrons.

Splitting Atoms

Splitting an atom is called nuclear fission. If one of the neutrons given off by a fission reaction crash into a neighboring U235 atom, it will cause that atom to fission. This releases more neutrons, causing more atoms to split. Put enough U235 atoms together and you will create a chain reaction.

The chain reaction goes on as long as neutrons keep crashing into U235 atoms, releasing more neutrons and more heat energy. It's the heat given off by the chain reaction that can be converted to electricity in a nuclear power plant.

How Is Nuclear Fuel Made?

In order to cause a chain reaction, you have to concentrate the total amount of U235 in the uranium fuel. This concentration process is called "enrichment."

The most common way to enrich the fuel for nuclear power plants is called gaseous diffusion. First, the uranium ore is crushed and melted to purify it. Then it gets turned into a gas, and pumped through a long series of filters that gradually increase the concentration of U235 in the uranium.

Finally the enriched uranium is formed into small pellets and stacked in long metal fuel rods. The fuel rods are combined together into "fuel assemblies" that will be shoved into the center of the reactor, where the nuclear chain reaction takes place.

How Do Nuclear Power Plants Make Electricity?

Most power plants convert some kind of fuel into heat, and then convert the heat into electrical power. Nuclear power plants use the energy created by nuclear chain reactions to heat water piped in from a nearby river.

The heated water becomes steam. Then the steam turns a huge turbine, which spins a coil of wire inside a magnetic field. This causes electrons to flow through the wire coil. The flow of electrons is called electricity.

What Toxic Wastes Do Nuclear Power Plants Create?

Like all industries, nuclear power plants create leftover waste. One of the waste products they create is "waste heat." Once the hot steam turns the turbine to generate electricity, the water must be cooled off, or it will damage the living organisms when it flows back into the river where it came from. This waste heat is usually dispersed by using cooling towers, where the steam can rise, evaporate, and cool back into water droplets.

Most of the radioactive waste from nuclear power plants is "low-level waste," in the form of lab supplies, protective clothing, and cleaning cloths that give off small amounts of radiation. These are usually buried in steel drums under several feet of soil, at protected disposal sites.

How Is High-level Radioactive Waste Stored?

An even more toxic waste product left over from a nuclear power plant is called "high-level waste." Once the uranium fuel is spent, or used up, the rods are removed from the reactor. By this time, the rods are "hot," in terms of both temperature and radiation.

Some of the uranium fuel can be re-processed and used again. The rest will remain radioactive for thousands of years. The spent fuel must be buried permanently, in caverns drilled into dry rock or salt beds that can't leak into underground rivers.

Although nuclear power plants provide the electrical power we need, they also produce dangerous toxic wastes that we must manage carefully, in order to protect people and the earth.

See more information about the science of nuclear power here.