What quantity characterizes neutron multiplication? How will the fission reaction proceed at k> 1; k <1?

The quantity characterizing the multiplication of neutrons in the process of a nuclear reaction is considered to be the multiplication factor of neutrons k, determined by the ratio of the number of released neutrons to the number of neutrons in the preceding fission act. A necessary condition for the development of a self-sustaining chain reaction is k> 1.
A self-sustaining nuclear fission reaction occurs if a new neutron has time to be formed as a result of the fission reaction during the time a neutron passes through a medium with a linear size of 1. Occurs when k = 1. If k <1, then the reaction is damped.

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