What happens when the multiplication factor k is less than 1?

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nuclear fission Italian-born physicist Enrico Fermi explaining a problem in physics, c. 1950. He was awarded the 1938 Nobel Prize for Physics.

When the multiplication factor k is less than 1, a chain reaction cannot be sustained. In this state, the number of fissions produced in one generation is lower than in the preceding one, causing the neutron population to decrease over time.12

As the neutron population drops, the rate of fission declines and will eventually fall to zero. In the context of a nuclear reactor, this condition is referred to as a subcritical state. Operators intentionally force a reactor into this subcritical state by fully inserting control rods when they need to shut the system down.2