A reactor's safety is crucially dependent on the influence of a temperature rise on the reactivity. Indeed, if a temperature increase leads to a reactivity increase the reactor may become unstable and uncontrolable. In fact this type of unstability is characteristic of the RBMK reactors, such as those of Tchernobyl. Several effects determine the influence of temperature on the reactivity. The most important is the variation of the number of captures in the region of the resonances.