Nuclear fission

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.