In fast reactors which have very small cores, the bowing or buckling of fuel elements due to temperature changes may shift fuel in or out of more reactive regions with consequent increases or decreases in reactivity.

EBR-I exhibited both a prompt inward bowing that produced a positive reactivity change and a delayed outward bowing that produced a negative reactivity change (see Section 2.5.5). However, most power reactor designs would seek to keep such changes to an absolute minimum by employing a core restraint system, either by physically squeezing the core fuel assem­blies together as in the Fermi reactor or by using a thermal restraint which uses the expansion of the core itself to tighten the core assemblies against fixed supports.