Nuclear Fission Definition and Examples

What Is Nuclear Fission?

A good example of fission is the splitting of a uranium nucleus.
A good example of fission is the splitting of a uranium nucleus. Encyclopaedia Britannica/UIG / Getty Images

Nuclear Fission Definition

Fission is the splitting of an atomic nucleus into two or more lighter nuclei accompanied by energy release. The original heavy atom is termed the parent nucleus and the lighter nuclei are daughter nuclei. Fission is a type of nuclear reaction which may occur spontaneously or as a result of a particle striking an atomic nucleus.

The reason fission occurs is that energy upsets the balance between the electrostatic repulsion between positively-charge protons and the strong nuclear force that holds protons and neutrons together. The nucleus oscillates, so the repulsion may overcome the short-range attraction, causing the atom to split.

The mass change and energy release yield smaller nuclei that are more stable than the original heavy nucleus. However, the daughter nuclei may still be radioactive. The energy released by nuclear fission is considerable. For example, the fission of one kilogram of uranium releases as much energy as burning around four billion kilograms of coal.

Example of Nuclear Fission

Energy is required in order for fission to occur. Sometimes this is supplied naturally, from radioactive decay of an element. Other times, energy is added to a nucleus to overcome the nuclear binding energy holding the protons and neutrons together. In nuclear power plants, energetic neutrons are directed into a sample of the isotope uranium-235. The energy from the neutrons can cause the uranium nucleus to break in any of a number of different ways. A common fission reaction produces barium-141 and krypton-92. In this particular reaction, one uranium nucleus breaks into a barium nucleus, krypton nucleus, and two neutrons. These two neutrons can go on to split other uranium nuclei, resulting in a nuclear chain reaction.

Whether or not a chain reaction can occur depends on the energy of the neutrons that are released and how close the neighbor uranium atoms are. The reaction can be controlled or moderated by introducing a substance that absorbs neutrons before they can react with more uranium atoms.