Nuclear Fission

Nuclear Fission

In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts (lighter nuclei), often producing free neutrons and photons (in the form of gamma rays), and releasing a tremendous amount of energy.

The two nuclei produced are most often of comparable size, typically with a mass ratio around 3:2 for common fissile isotopes. Most fissions are binary fissions, but occasionally, three positively-charged fragments are produced in a ternary fission. The smallest of these ranges in size from a proton to an argon nucleus.

This makes possible a self-sustaining chain reaction that releases energy at a controlled rate in a nuclear reactor or at a very rapid uncontrolled rate in a nuclear weapon. Nuclear fission produces energy for nuclear power and to drive the explosion of nuclear weapons. Both uses are possible because certain substances called nuclear fuels undergo fission when struck by fission neutrons, and in turn emit neutrons when they break apart.

Once the nuclear lobes have been pushed to a critical distance, beyond which the short range strong force can no longer hold them together, the process of their separation proceeds from the energy of the (longer range) electromagnetic repulsion between the fragments. The result is two fission fragments moving away from each other, at high energy. The fission of a heavy nucleus requires a total input energy of about 7 to 8 MeV to initially overcome the strong force which holds the nucleus into a spherical or nearly spherical shape, and from there, deform it into a two-lobed ("peanut") shape in which the lobes are able to continue to separate from each other, pushed by their mutual positive charge, in the most common process of binary fission (two positively-charged fission products + neutrons).

Most of the matter found naturally on Earth is stable and does not undergo this transformation. Some examples of common radiactive isotopes found naturally that have this property are 40K (potassium-40) which is present in seawater and many salts, 14C (Carbon-14) and Uranium and Thorium. One nucleus will spontaneously transform into a different nucleus if the final state nucleus is more stable and if the laws of physics allow the transformation. This process is usually accompanied by the release of ionizing radiation and is often called "radio-active decay". Nuclei that exhibit this behaviour are said to be "unstable" or "radioactive".

There are 3 nuclear isotopes of importance to nuclear power that exhibit this behavior.

These are: 235U (Uranium-235) , 239Pu (Plutonium-239) and 233U (Uranium-233). Of the 3, only 235U is found naturally on Earth. Natural Uranium found on Earth consists of 99.3 % 238U and 0.7% 235U. The two other isotopes, 239Pu and 233U can be created from the far more abundant 238U and Thorium nuclei via advanced Nuclear techniques.