Question

what is control and uncontrol chained reactions?

Answer 1

Nuclear Chemistry deals with nuclear reactions. In nuclear reactions, the composition of the nucleus changes and a large amount of energy known as nuclear energy is released. A nuclear reaction is carried out by bombarding projectiles on a target nucleus. Spallation, nuclear fission, and nuclear fusion are the examples of various types of nuclear reactions. Nuclear fission is the process in which a heavy nucleus breaks up into two lighter nuclei of almost equal size with the release of an enormous amount of energy. 
In this fission process, the heavy nucleus absorbs a neutron and forms an unstable compound nucleus. The compound nucleus then breaks up more of less in the middle to give fission product.


Further, the neutrons released (say three) from the fission of first uranium atoms can hit three other uranium atoms. In this way, a chain reaction is set up resulting in the liberation of the enormous amount of energy. In the case of nuclear fission, Uranium-236 formed breaks up in several ways.