12.20. In selecting a safe reactor design, an essential requirement is that the concept should provide inherent stability against an increase in reac­tivity. This can be realized if the reactor has a quick-acting negative tem­perature (or power) coefficient of reactivity. As seen in Chapter 5, there is then a self-limiting effect on disturbances that lead to an increase in temperature (or power level). This self-limiting feature is generally the result of the fuel Doppler coefficient, although in water-cooled reactors the expansion of the coolant-moderator contributes to the overall negative coefficient. However, the moderator contribution is delayed because of the time required for heat transfer from the fuel to the moderator (§5.92).

12.21. It should be understood that whereas a negative temperature (or power) coefficient is a requirement for reactor safety, it does not guarantee safety. If there were a sudden increase in reactivity, as discussed in §5.149 et seq., the power excursion would be terminated automatically after a finite time because of the negative temperature coefficient. But during this time the thermal power, and hence the fuel temperature, may have risen to such a high level that the fuel rods would suffer damage. This aspect of safety is taken into consideration in reactor system design. Also, should the system have a positive coolant void coefficient, as was the case in the Chernobyl 4 reactor (§12.187), the danger would be exacerbated.