Uranium-235

Some rocks have a special part.

This part is called uranium. Most of it is quiet. But a small part is very active. This special part can split apart. When it splits, it lets out heat.

This heat makes power for us. It can also make big explosions. People use this power in reactors. They can even use it in submarines. It is a very strong force. It is amazing to learn about!

Uranium-235 is a special part of a metal called uranium. It makes up only 0.72% of natural uranium. Arthur Jeffrey Dempster found it in 1935. This part is fissile. That means it can split apart to keep a reaction going.

When a neutron hits a nucleus, the atom splits. This is called fission. Fission lets out a lot of power. If one split hits another atom, it starts a chain reaction. This can power nuclear reactors. It can also make nuclear weapons.

Some reactors use natural uranium. Others need enriched uranium. Enrichment is a way to make more U-235. We do this by taking out U-238. This makes the U-235 part bigger. High amounts of U-235 are used in nuclear submarines. They are also used in research reactors.

Uranium-235 has a half-life of 704 million years. A half-life is how long it takes for half of it to decay. When it decays, it changes into other things. It can turn into thorium-231. It eventually turns into stable lead. Scientists use these changes to study the age of the universe.

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.

Uranium-235, often called U-235, is a specific isotope of the element uranium. An isotope is a version of an element that has a different atomic weight. In nature, U-235 is quite rare, making up only about 0.72% of all natural uranium. Most uranium found on Earth is actually a different isotope called uranium-238. What makes U-235 so significant is that it is fissile. This means it can sustain a nuclear chain reaction. This unique property allows it to release massive amounts of energy through a process called fission.

Nuclear fission is the mechanism that releases this energy. The process begins when a U-235 nucleus is bombarded by a neutron. When the neutron strikes the nucleus, the atom splits apart. This split releases a large amount of energy and several new neutrons. If these new neutrons strike other U-235 nuclei, they cause more fission events. This creates a self-sustaining chain reaction. For a reaction to stay critical, at least one neutron from a fission event must strike another nucleus. If the reaction is sustained, it is said to be in a critical state. The specific amount of U-235 needed to reach this state is called the critical mass.

Scientists use different types of technology to manage these reactions. Some reactor types, such as heavy water reactors or graphite moderated reactors, can run on natural uranium. However, light water reactors require uranium that has undergone enrichment. Enrichment is a process that increases the proportion of U-235 by removing some of the U-238. Some specialized machines, like research reactors or those in nuclear submarines, use highly enriched uranium (HEU). HEU contains a much higher percentage of U-235. In nuclear weapons, the reaction is uncontrolled to create an explosion. To control power in a reactor, engineers use control rods. These rods contain elements like boron, cadmium, or hafnium that strongly absorb neutrons.

Our understanding of this isotope began in 1935. A scientist named Arthur Jeffrey Dempster discovered U-235. Since that discovery, the material has been used for both energy and weaponry. For example, the Little Boy atomic bomb used in Hiroshima on August 6, 1945, was made of highly enriched uranium. It featured a large tamper to help the reaction. While most modern nuclear weapons use plutonium-239 for the primary stage, HEU is still used in the secondary stage. In these designs, the highly enriched uranium acts as an ignitor for the fusion fuel.

The energy released during fission is immense. One single fission event of U-235 releases about 202.5 MeV of energy as heat in a thermal reactor. This total energy is composed of several parts. For instance, 169.1 MeV comes from the kinetic energy of fission fragments. Another 4.8 MeV comes from the kinetic energy of prompt neutrons. About 7.0 MeV is carried by prompt gamma rays. Additionally, 8.8 MeV escapes the reactor as anti-neutrinos. When looking at mass, the fission of one atom releases 19.54 TJ/mol, which is 83.14 TJ/kg.

Uranium-235 is also a subject of study in nuclear decay. It is an alpha emitter, meaning it releases alpha particles as it decays. This process turns U-235 into thorium-231. U-235 is the main progenitor of the actinium series. This is one of the principal decay chains among actinides. Because U-235 is the longest-lived primordial nuclide, it starts a long chain of changes. This series includes many other elements like astatine, bismuth, francium, and radium. Eventually, the decay process leads to the formation of stable lead-207. The half-life of U-235 is 704 million years.

Finally, U-235 helps scientists study the history of the universe through radiometric dating. By looking at the ratio of U-235 to U-238, researchers can calculate how much time has passed. This helps determine when these nuclei were formed during stellar nucleosynthesis. In 1957, a landmark paper by B2FH explained the r-process. This is the process by which these nuclei form in events like supernovae or kilonovae. Scientists use these ratios to estimate the age of the Milky Way. They can even study different scenarios, such as a single supernova or a continuous series of supernovae. This connection to the stars allows us to use a tiny atom to understand the vast history of space.