An arrangement for controlling the moderation of a nuclear reactor which includes multiple fuel assemblies each of which includes top and bottom nozzles with fuel rods and control rod guide thimbles located therebetween. Certain ones of said assemblies include at least one tube initially filled with a gas. One end of the tube rests on the lower nozzle upper surface while the upper end terminates at a point just below the lower plate in the upper nozzle. A spike immovably fixed in the lower plate of the upper nozzle is directed downwardly toward the end of the tube. As the tube expands or grows during operation in a reactor, and at a point where the criticality factor may proceed to less than one, the expanded tube will be pierced by the spike and thus permit the escape of gas from the tube which is then replaced by water coolant circulating through the reactor. The additional water represented by the filled tube will act to slow down fast neutrons to a thermal neutron level and thus provide a greater amount of neutron effective in carrying forward the fissioning process.