A nuclear reaction is a process that occurs when the nucleus of an atom loses subatomic particles to the degree that its properties are altered. The original atom of an element that undergoes a nuclear reaction can either become a different isotope, or variety, of the same element or it can become a different element entirely. Nuclear reactions are closely related to radiation in general, which can occur spontaneously outside of a reaction. Radiation simply describes the process of energy or articles being emitted from an atom or other particle. The term nuclear reaction, however, usually specifically refers to a situation in which the nuclei of two atoms collide and alter the properties of at least one of the nuclei.
A nuclear reaction can occur in many different forms, each giving considerably different results. In a fission reaction, a large and often unstable particle, either spontaneously or as the result of a collision, splits apart into two different particles. The opposite occurs in a fusion nuclear reaction: two smaller particles collide and their nuclei combine to form a larger particle. Fusion reactions occur naturally in stars, but most human attempts to effectively and efficiently control them have failed. In a spallation reaction, a nucleus is struck with enough momentum to dislodge several neutrons or protons, thereby reducing the atomic weight of the particle.
Fission nuclear reactions are used in nuclear reactors to produce usable energy. Unstable particles collide and split apart, generating a significant amount of kinetic and thermal energy. This energy can be harvested by the nuclear reactor and put to use for human concerns. There is significant interest in the use of fusion reactions to generate power,as they tend to release a considerable amount of energy. Unfortunately, fusion reactions are extraordinarily difficult to control--they occur naturally in the high-pressure, high-energy conditions present on stars, and such conditions are very difficult to replicate.
There are several different kinds of particles that are commonly emitted from nuclei during a nuclear reaction. Alpha particles are essentially the same as the nuclei of helium atoms and are composed of two neutrons and two protons bound together. Beta particles are simply electrons; they have a much smaller mass and a negative charge. Neutrons are also released in nuclear reactions; they are very penetrating because they have a neutral charge so there are few forces that prevent them from passing through various substances, including human skin. Gamma rays are rays that leave the nucleus in the form of pure energy; they are also highly penetrating and can pass through almost anything because of their nonexistent mass and neutral charge.