For thermal reactors a temperature increase leads to a hardening of the neutron spectrum. In turn this leads, in general, to a decrease of the capture and fission cross-sections. The effects of these reductions depend on the specific properties of the fissile and fertile nuclei in the thermal region. For example, a standard enriched uranium fuel has a negative spectrum temperature coefficient which becomes positive with increase of the plutonium content of the fuel.

A temperature increase tends to decrease the density of the materials. This effect is especially important for liquid coolants whose dilatation transfers matter into the expansion vessels. Thus, a temperature increase decreases the relative concentration of a liquid coolant. This may have very different effects for different systems. In PWR reactors the slowing down of the neutrons tends be less efficient because of the drop in water density. This leads to a drop of the fission probability while the capture rate in water is also decreased. These two effects are opposite but their net result is a decrease of the reactivity. In RBMK reactors the neutrons are slowed down by the graphite, while water ensures the cooling and captures a fraction of the neutrons. A temperature increase leads to a decrease of the number of captures in water which is not counterbalanced by a decrease in the fission rates; thus, the temperature dilatation effect tends to increase the reactivity. In liquid sodium cooled fast reactors the decrease of the sodium density leads to a hardening of the spectrum and, therefore, increases the fission rate, at least for large reactors.[19] For lead cooled reactors, because of the smaller slowing down power of lead, this effect is very small.

Void effect

In the case of a large temperature increase, in an accidental configuration, vapour bubbles may appear in the coolant. For PWR reactors this has a negative effect on the reactivity, because of the dominant influence of the spectrum hardening. For RBMK reactors the effect is strongly positive because of the dominant effect of the decrease of the capture rate in the coolant. In sodium cooled reactors the effect on reactivity is positive, but it occurs at a much higher temperature than for PWR reactors. In lead cooled reactors the void coefficient is negative [45].