How Nuclear Power Works

A hyperboloid cooling tower at a nuclear power plant. The tower transfers excess heat from the water to the atmosphere.

A nuclear power plant generates electricity by heating water to produce steam, which drives a turbine. The heat is created by splitting uranium atoms.

Uranium is the heaviest naturally-occurring atom in nature. Its nucleus has 92 protons and, in its most common form, 146 neutrons, giving it a mass number of 238. It is indicated by the symbol ²³⁸U₉₂, or just ²³⁸U. For our purposes, we'll refer to it as U-238.

Over 99% of uranium is U-238. Another 0.7% of the element is of a form (or isotope) that has not 146 but 143 neutrons. This is U-235, and is of chief interest to us, because this is the form that can be split manually and that discharges heat on splitting.

Uranium extracted from the earth must be enriched until it contains 2% to 3% of U-235. (Weapons-grade uranium is enriched to 90% U-235.)

The U-235 is placed in a container in a nuclear reactor, where it absorbs neutrons. On being hit by a free neutron, the uranium atom splits into two smaller atoms (typically krypton and barium) and releases two to three neutrons and a huge amount of energy as heat. This event is known as fission.

Each U-235 atom that undergoes fission releases 200 million electron volts, or MeV, of energy. Now 1 MeV is extremely tiny, so tiny you would never feel it. But one gram of U-235 has 2,562,553,191,489,360,000,000 atoms. When all those atoms split, the electron volts emitted add up to a lot of energy (you do the math!). Actually, one gram of U-235 produces the same amount of energy as 5.16 tons of TNT.

Obviously, we can't have that. The fission of uranium must be controlled. Fortunately, there's a way to do that.

Ordinary water, heavy water or graphite are used to slow the neutrons down, and rods of cadmium or barium are inserted into the container to absorb neutrons and thus control their concentration.

As a result, all that fission energy is safely channeled toward making steam, which powers the turbine, thereby generating electricity.

And if you live in Connecticut, know that about 38% of your electricity comes from nuclear power.