<Dd> Λ = l k (\ displaystyle \ Lambda = (\ frac (l) (k))) </Dd> <P> In this formula, k is the effective neutron multiplication factor, described below . </P> <P> The effective neutron multiplication factor, k, is the average number of neutrons from one fission that cause another fission . The remaining neutrons either are absorbed in non-fission reactions or leave the system without being absorbed . The value of k determines how a nuclear chain reaction proceeds: </P> <Ul> <Li> k <1 (subcriticality): The system cannot sustain a chain reaction, and any beginning of a chain reaction dies out over time . For every fission that is induced in the system, an average total of 1 / (1 − k) fissions occur . </Li> <Li> k = 1 (criticality): Every fission causes an average of one more fission, leading to a fission (and power) level that is constant . Nuclear power plants operate with k = 1 unless the power level is being increased or decreased . </Li> <Li> k> 1 (supercriticality): For every fission in the material, it is likely that there will be "k" fissions after the next mean generation time (Λ). The result is that the number of fission reactions increases exponentially, according to the equation e (k − 1) t / Λ (\ displaystyle e ^ ((k - 1) t / \ Lambda)), where t is the elapsed time . Nuclear weapons are designed to operate under this state . There are two subdivisions of supercriticality: prompt and delayed . </Li> </Ul>

Nuclear equation for a reaction important in thermonuclear weapons