Uranium-235

Uranium-235 is a very special part of a metal called uranium. It is an isotope, which is a version of an element with a different weight. This specific version makes up only 0.72% of all natural uranium found on Earth. Most uranium is a different version called uranium-238. The reason U-235 is so important is that it is fissile. This means it can sustain a nuclear chain reaction.

This unique ability allows it to release huge amounts of energy.

To understand how it works, we look at a process called fission. This happens when a nucleus is hit by a tiny particle called a neutron. When the neutron strikes the U-235, the atom splits apart. This split releases a lot of energy and more neutrons. If those new neutrons hit other atoms, they cause more splits. This creates a chain reaction that keeps going.

Scientists can control this reaction in a nuclear reactor. They use control rods made of boron or cadmium to soak up extra neutrons.

People first discovered this special isotope in 1935. A scientist named Arthur Jeffrey Dempster found it. Since then, we have learned how to use it for many things. Some machines, like heavy water reactors, can use natural uranium. Other machines, like light water reactors, need enriched uranium. Enrichment is a way to increase the amount of U-235 in a sample. This is done by removing some of the uranium-238.

There are many important numbers to know about U-235. It has a half-life of 704 million years. This is the time it takes for half of the atoms to decay. When it decays, it starts a long chain of changes. It turns into things like thorium-231 and eventually becomes stable lead.

In a reactor, one fission event releases about 202.5 MeV of energy as heat. For weapons, the material often needs to be 85% or more U-235. This high level is called weapons-grade uranium.

We can even use U-235 to learn about the history of space. Scientists use a method called radiometric dating to study it. They look at the ratio of U-235 to U-238 to see how much time has passed. This helps them understand when these atoms were formed in stars.

It can even help estimate the age of the Milky Way galaxy. By watching how these atoms change, we can look back at the very beginning of our universe.