The increase of temperature tends to decrease the density of the materials. This effect is especially important for the liquid coolants whose dilatation transfers matter into the expansion vessels. Thus the temperature increase decreases the relative concentration of the liquid coolant. This may have very different effects for different systems. In the PWR reactors the efficiency of the slowing down of the neutrons tends to decrease, due to 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 RMBK reactors the slowing down of the neutrons is assured by the graphite, while water assures the cooling and captures a fraction of neutrons. 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 reactors4.12. For lead cooled reactors, due to the smaller slowing down power of lead, this effect is very small.