Nuclear fission

Tiny parts of things can split.

This is called fission. It happens when a small part hits a big part. The big part breaks into two smaller parts. This break lets out a lot of heat.

The heat can make power for our homes. It can also make a big blast. Some parts also shoot out more tiny bits. These bits can hit other big parts. This makes a chain reaction. It is a very strong way to make energy.

Everything is made of tiny parts called atoms. Inside an atom is a center called a nucleus. Nuclear fission is a way to split that nucleus into smaller parts.

This often happens when a tiny particle called a neutron hits a heavy nucleus. The nucleus absorbs the neutron and becomes excited. It then splits into two or more smaller nuclei. These new parts are called fission products. This split lets out a very large amount of energy. Most of this power comes from the speed of the moving parts.

When a nucleus splits, it also lets out extra neutrons. These new neutrons can hit other nuclei. This can start a chain reaction. In a nuclear reactor, we control this to make power. In a weapon, it happens very fast.

Scientists Otto Hahn and Fritz Strassmann proved this happened in 1938. Lise Meitner and Otto Robert Frisch explained how it worked in 1939. Frisch gave it the name "fission." The pieces left behind can be radioactive. This means they stay dangerous for a long time.

Nuclear fission is a way that the center of an atom splits apart. This center is called a nucleus. When a nucleus breaks into two or more smaller pieces, it is called fission.

This process is very important because it releases a huge amount of energy. This energy is much greater than the energy from burning fuel like methane. It can be used to make electricity in a power plant. However, it can also be used to power weapons. Scientists must work hard to use this power safely.

Fission often happens in a specific way called induced fission. It starts when a tiny particle called a neutron hits a heavy nucleus. The nucleus absorbs the neutron and becomes excited.

Think of it like a drop of liquid that starts to wobble and stretch. The nucleus changes shape until it finally splits into smaller pieces. These pieces are called fission products. Most of the time, the nucleus splits into two parts. This is called binary fission. Sometimes, it can even split into three parts.

Humans discovered this amazing thing many years ago. In December 1938, chemists Otto Hahn and Fritz Strassmann proved that fission happened.

Shortly after, physicists Lise Meitner and Otto Robert Frisch explained how it worked. They shared their ideas in early 1939. Frisch chose the name "fission" because it sounded like how living cells split apart. Later, in 1940, other scientists found that some atoms can split all by themselves. This is called spontaneous fission.

There are many specific facts about how fission works. Some materials, like uranium-235, are great for making energy. When these atoms split, they release more neutrons.

These new neutrons can hit other atoms and cause more fission. This creates a chain reaction. In a reactor, we keep this reaction slow and steady. In a weapon, the reaction happens very fast.

The products left behind can be radioactive. This means they stay dangerous for a long time. This creates the hard job of managing nuclear waste.

Fission is different from the way stars work. Stars use a process called fusion to make energy. Fission is more like breaking a large, heavy object into smaller pieces.

It is a way to change one element into a different one. This is called transmutation. While fission is very powerful, it is not how the universe was mostly made. Most of the things in our world come from fusion instead. It is still a very important tool for people to understand today.

Nuclear fission is a physical process where the nucleus of an atom splits into two or more smaller nuclei. This reaction is a form of nuclear transmutation, meaning the resulting daughter atoms are different elements from the original parent atom. Fission is a powerful way to release energy, far exceeding the energy released by chemical processes. For example, the energy from fission is roughly one million times greater than the energy from burning methane. This immense energy release makes fission a critical subject in both energy production and weaponry.

The mechanism of induced fission begins when an incident particle, usually a neutron, strikes a heavy nucleus. When a nucleus like uranium-235 absorbs a neutron, it becomes an excited, unstable state. Scientists often use a liquid drop model to visualize this process. In this model, the energy from the neutron causes the nucleus to vibrate and deform. The nucleus stretches into a fat, cigar-like shape and then a peanut shape. Eventually, the two lobes become so elongated that the short-range nuclear force can no longer hold them together. The electrical charge then pushes the fragments apart, completing the split.

There are different ways that these nuclei can break apart. Most fission events are binary fissions, which produce two charged fragments. These fragments are often of comparable size, typically having a mass ratio of about 3 to 2. However, a rarer event called ternary fission occurs in about 2 to 4 out of every 1,000 events. In ternary fission, three positively charged fragments are produced instead of two. The smallest of these third fragments can range in size from a single proton to an argon nucleus. Most commonly, these small fragments are helium-4 nuclei, also known as alpha particles.

Scientists distinguish between different types of nuclei based on how easily they split. Fissile nuclides, such as uranium-235 or plutonium-239, can easily split when struck by slow or "thermal" neutrons. Fissionable nuclides, such as uranium-238, typically require the higher energy of fast neutrons to undergo fission. There is also a natural process called spontaneous fission. This occurs when a heavy nucleus has an overabundance of neutrons and splits without any outside particle hitting it. Spontaneous fission was discovered in 1940 by Flyorov, Petrzhak, and Kurchatov in Moscow.

The history of this discovery is a major milestone in physics. In December 1938, chemists Otto Hahn and Fritz Strassmann proved that a fission reaction had occurred. Shortly after, physicists Lise Meitner and Otto Robert Frisch provided the theoretical explanation in January 1939. Frisch chose the term "fission" by comparing the process to the biological fission of living cells. In February 1939, Hahn and Strassmann predicted that additional neutrons would be released during the process. This prediction was vital because it suggested the possibility of a nuclear chain reaction.

A nuclear chain reaction happens because the fission process releases more neutrons than it absorbs. When these new neutrons strike other nearby nuclei, they can induce further fission events. In a nuclear reactor, this reaction is controlled to release energy at a steady rate for power. In a nuclear weapon, the reaction is uncontrolled and happens very rapidly.

This energy is released in several forms. About 85 percent of the energy comes from the kinetic energy of the fission products. The remaining energy comes from gamma rays, initial neutrons, and particles emitted during subsequent decay.

While fission is useful, it presents significant challenges regarding nuclear waste. The products of fission are often much more radioactive than the original fuel. These products can remain dangerous for significant amounts of time. While most fission products are short-lived, there are seven long-lived fission products that contribute to waste. Additionally, neutron absorption can create plutonium and minor actinides, which have very high radiotoxicity. Scientists use nuclear reprocessing to try and recover usable material from spent fuel. This process aims to create a "closed fuel cycle" to reduce waste and extend fuel supplies.