A critical reactor has an effective multiplication factor kf which is equal to unity. The effective multiplication factor can be either larger or less than unity, depending on the deviation of a nuclear reactor from its criticality. As such, it has an 'excess multiplication factor'.

which can be positive or negative.

The ratio of the excess multiplication factor to the effective multiplication factor is called 'excess reactivity' or 'reactivity' and is defined as

The reactivity explains the deviation of the effective multiplication factor from unity under time-dependent conditions. In a steady-state case, the reactivity is zero.

If the deviation is small from criticality, which is by small deviations in temperature or voids during the normal operation, then the reactivity can be expressed as follows:

In terms of reactivity, the effective multiplication factor may be expressed as and the excess reactivity can be given as

Changes in the system's temperature, pressure or load may result the reactivity to be shortlived. It may also develop over a long period of time because of the fuel burn-up and the accumulation of fission products.

To hold the reactor power constant, means are devised to keep its reactivity constant such as the control rods or the chemical shim (which is a neutron absorber in the coolant or moderator).

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