Nuclear nucleus stable. This is known as radioactive

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.

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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

When U-235 is releasing energy in a reactor’s core it is also releasing neutrons as we discussed earlier. The uranium isotope that is abundant in Earth’s crust, U-238, is also in the reactor core absorbing these neutrons. When a U-238 atom absorbs a neutron, it becomes U-239 and decays by giving off the equivalent of an electron, which is called a beta-particle. The new isotope created is neptunium-239.

The Np-239 atom gives off a beta-particle to become Pu-239. Pu-239 is an alpha-particle emitted and is fissionable, which means it can be used as a nuclear fuel.

Lesson Summary

Isotopes are different versions of the same element but differ in their number of neutrons.

Nuclides are specific isotopes of an element. U-235 is a nuclide of uranium, and Pu-239 is a nuclide of plutonium. Both of these nuclides are used as nuclear fuel in nuclear reactors.

When a nuclide has a ratio of neutrons to protons outside of the 1 n: 1 p to 1.5 n: 1 p, it is radioactive. This means it undergoes radioactive decay, which is the release of subatomic particles from its nucleus. U-235 releases an alpha particle, which is a particle consisting of two neutrons and two protons. Pu-239 is an alpha particle emitter as well, and generates U-235. The nuclear conversion of one nuclide into another is called transmutation.

U-235 is refined from U-238 and when U-235 absorbs a neutron in a nuclear reactor’s core it splits giving off neutrons and a lot of energy. This energy is used to turn water to high energy steam, which is used to generate electricity.Pu-239 is generated in a nuclear reactor’s core when U-239, which is also present in the core, absorbs a neutron.

U-239 decays to form Np-239, which decays to form Pu-239.


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