Hafnium is an excellent neutron absorber and is therefore used as control rods in nuclear submarines. The bombardment of uranium-235 with neutrons produces energy to power nuclear submarines:
Notice that three neutrons (1n) are produced in this process. These three neutrons can bombard three more 235U nuclei, to produce a total of 9 neutrons and even more energy. If this process continues, too much energy can be released, causing a nuclear meltdown. The presence of hafnium control rods absorbs neutrons to slow down and control the extent of energy release.