Nuclear Fuels: Definition, Characteristics & Examples

Instructor: Matthew Bergstresser

Matthew has a Master of Arts degree in Physics Education. He has taught high school chemistry and physics for 14 years.

Nuclear fuel is what is used in nuclear reactors to generate electricity. In this lesson, we will explore more on what nuclear fuel is, characteristics of it and investigate examples of different nuclear fuels.


Think about all of the different types of fuel that exist. For our vehicles there are gasoline and diesel fuels. Fuel for our bodies is food, and fuel for nuclear reactors that generate electricity are certain radioactive isotopes. Atoms of elements that have different numbers of neutrons are called isotopes. A nuclide is a specific isotope of an element. Let's go through some radioactive isotopes that are used for fuel in reactors.

Nuclear Fuel

Gasoline for cars isn't just pumped out of the ground and stored in tanks for people to pump into their vehicles. It is refined from crude oil that is pumped out of the ground. The same goes for nuclear fuel, which are pellets of highly processed radioactive ore such as uranium. Nuclear fuel is material that can be used in a nuclear reactor to generate electricity. Let's first look uranium-235, which is one of the fuels used in nuclear reactors.


The ''235 '' after the dash in U-235 is the atomic mass of this isotope of uranium. Since uranium's atomic number is 92 (meaning it has 92 protons and electrons) the remaining mass consists of the neutrons. In U-235, there are 143 neutrons. This gives it a neutron to proton ratio of about 1.55 to 1. A nuclide is radioactive if its ratio of neutrons to protons is not between 1 neutron to 1 proton and 1.5 neutrons to 1 proton. This means U-235 is radioactive and decays. U-235 is the end product that started with the mining of U-238.


Nuclear Decay of U-235

Since U-235 is unstable it gives off subatomic particles from its nucleus in an effort to make its nucleus stable. This is known as radioactive decay. U-235 gives off an alpha particle, which is identical to a helium nucleus. It consists of a two protons and two neutrons. When U-235 gives off an alpha particle it transmutates into Thorium-231. Transmutation is the process of one element turning into another element. The nuclear reaction is shown in Reaction 1.

Reaction 1. The alpha decay of U-235

The time required for one-half of a sample of U-235 to turn into Th-231 is just over 700 million years! This value is known as U-235's half-life. Now that we have covered the basic characteristics of U-235, let's see how it is used as nuclear fuel.

U-235 Fuel

After U-238 is mined and processed to make U-235 it is formed into pellets. These pellets are stacked into fuel cells and are put into a nuclear reactor's core. When these uranium atoms absorb a neutron, they split giving off tremendous amounts of energy along with more neutrons. These neutrons are absorbed by other uranium atoms causing them to split releasing energy. This energy is used to turn liquid water into high energy steam, which turns turbines generating electricity. The fuel is generally ''used up'' after 1.5 to 3 years, and it has to be stored or reprocessed. Let's now turn our attention to another nuclear fuel, which is created with the help of U-235, but also decays to produce U-235!

Spent fuel rods from a nuclear reactor


Pu-239 is plutonium-239, which has 94 protons and 145 neutrons. Its neutron to proton ratio is 1.54 making it radioactive. Pu-239 has a half-life of over 24,000 years, is an alpha-particle emitter and decays into U-235.


Pu-239 is not found in nature, but is generated in nuclear reactors. Unfortunately, its half-life is too long to be a feasible source of U-235. Let's see how Pu-235 is produced.

The decay of Pu-239

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