A dimensionless measure of deviation from critical neutron multiplication. Positive reactivity indicates a supercritical neutron balance; zero gives criticality. The population's actual time evolution also depends on delayed-neutron and feedback dynamics.
The change of reactor reactivity with a specified material temperature, with the other variables held as stated. Several component coefficients need not have the same sign.
The change of reactor reactivity with thermal power along a specified family of states. In a quasistatic process, the chain rule weights each temperature coefficient of reactivity by that component's temperature response to power.
The fuel-temperature derivative of reactor reactivity associated with thermal broadening of nuclear resonances, with the other state variables specified. In ordinary uranium-fueled water reactors it is negative: heating the fuel increases effective resonance capture. Its magnitude depends on spectrum, fuel state and resonance self-shielding, so its sign does not determine how its magnitude changes under a lattice redesign.[ref]
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