To understand the basics of nuclear energy, it’s helpful to learn a little bit about nuclei and the atoms where they’re located. For background information about the basic structure of atoms — protons, neutrons, and electrons — see Atoms are the building blocks of the universe.


Protons define an atom’s type, whether it’s carbon or iron, plutonium or oxygen. All the atoms of a single element share the same number of protons. However, individual atoms of the same element can have varying numbers of neutrons.

For example, the element iodine tends to weigh 127 atomic mass units, made up of 53 protons and 74 neutrons. (Electrons in an atom are so much lighter they can be ignored here).

Though regular iodine weighs 127 amu, a small percentage of iodine atoms are of differing isotopes, like iodine-131, which has four more neutrons than iodine-127.

Each isotope is defined by its weight. There is a third isotope of iodine, iodine-129.  All three  have the same number of protons, but unlike iodine-127, iodine-131 and 129 are less stable and therefore radioactive.


Radioactive materials experience radioactive decay. The atoms that make up radioactive materials aren’t stable, and they can break down, essentially turning into radiation and subatomic particles when they decay. Each type of decay — alpha, beta, and so forth — gives off signature particles and signature wavelengths of electromagnetic radiation. The original atom is no more; in it’s place will be new types of atoms, with completely different chemical properties. Some decay products are also radioactive and can decay into even more products.

The half-life is the time it takes for half of the material to decay. Radioactive iodine-131 (which itself is one product of uranium-238 decay) has a half-life of 8 days. In 8 days, a pound of radioactive iodine becomes half of pound of radioactive iodine; the other half decays into the element xenon, as well as beta particles, antineutrinos, and gamma radiation. In eight more days, a quarter pound of radioactive iodine will remain, and after eighty days, or ten half-lives, less than one one-thousandth of pound of radioactive iodine will remain.

However, radioactive materials decay at vastly different rates. Even though it is so similar to iodine-131, iodine-129 has a half-life of 15.7 million years. The half-life of uranium-238 is more than 4.4 billion years, about the age of our planet, while the half-life of uranium-235, another isotope of the same element, is 700 million years. A longer half life means that the nucleus is more stable.


Nuclear energy is the stored potential of subatomic particles within the nucleus, or center, of an individual atom. Most atoms are stable on Earth; they retain their identities as particular elements, like hydrogen, helium, oxygen, carbon and so forth, according to the Periodic Table of Elements. Radioactive elements, however, can decay into other elements.

Furthermore, some radioactive materials, like uranium-238, can be convinced to break down faster than their normal rate of decay, nuclear fission. And, under certain conditions, atoms can also be joined together to form new kinds of atoms in nuclear reactions called fusion.

Today’s nuclear power plants are fueled by fission, while hydrogen atoms in the sun experience nuclear fusion, combining to form helium and subsequently releasing large amounts of kinetic energy in the form of electromagnetic radiation and heat.

In nuclear power plants that use uranium fuel rods, neutrons are directed at the uranium, which induces speeded-up fission of some of the atoms: the nuclei split. Otherwise, uranium would split at a much slower rate. That split releases gamma radiation and more loose, fast moving neutrons, which in turn induce more uranium atoms to split, and so on, a chain reaction. As the chain reaction progresses, it produces fragments of the original uranium, of various sizes, consisting of several elements. Some fission products can be radioactive, such as radioactive iodine, cesium, and strontium.

In a power plant context, nuclear materials are allowed just enough contact to keep the chain reaction going at a slow, steady rate and keep the fuel cool enough to prevent melting. If a larger amount of nuclear material is allowed too much contact with itself, it can form a critical mass, which is where the nuclear reactions run away, or take place so quickly that they aren’t controlled.

Gamma rays, the electromagnetic radiation produced in nuclear fission of uranium in power plants, are a high energy form of light, like x-rays. They can pass through, and burn, the human body and other materials.

In a nuclear reactor, the gamma rays and the kinetic energy of the fission products from splitting the uranium both heat the surrounding material, and that thermal energy is used to heat water into steam, which is used to drive turbines and produce electricity.

For a map of nuclear power plants in the United States, see here.



Nuclear fusion is essentially the opposite of nuclear fission. Atoms combine their nuclei to form heavier atoms. Fusion powers the sun and other stars.

In the core of our sun, the nuclei of hydrogen, of one proton each, fuse together to form a two-proton helium nucleus. The result is a stable helium atom, a huge amount of energy, and a neutron.

Fusion requires a high investment of initial energy to get protons to stick together, as they naturally repel one another because they’re positively charged. In the sun, extreme temperature and substantial gravity make fusion possible. On Earth, getting fusion working in a commercial power plant context has so far eluded scientists, but fusion reactions have occurred on a small scale. Scientists are trying methods like ionized gas, called plasma, to induce fusion, and there are several research projects worldwide looking at ways to make fusion a reality on Earth. One international effort to build a working fusion plant is ITER, in France.



  • U.S. Nuclear Regulatory Agency
  • U.S. Energy Information Administration
  • World Energy Council
  • Lawrence Berkeley National Laboratory
  • International Atomic Energy Agency
  • Halliday, David; Resnick, Robert; and Krane, Kenneth S. Physics 4th Edition(John Wiley & Sons:December 27, 1991).
  • ITER website
  • Living on Earth