Define critical mass of nuclear chain reaction.

Explanation

During nuclear fission of `U^(235)`, three neutrons are produced in every fission reaction. These neutrons carry out fission of the other `U^(235)` nuclei in the `U^(235)` block. In case, the size of `U^(235)` block is small, a few neutrons may be lost from the block without causing fission. In case, the number of neutrons lost per second is just equal to the number of neutrons produced per second, then the size of the `U^(235)` block is called the critical size and the corresponding .mass of the block is called critical mass.