The most critical design in a nuclear reactor is the prevention of runaway reactions that can allow escape of its radioactive fuel by vaporization. As uranium or plutonium undergoes fission, the release of energy which is harnessed to produce electricity is also the process that threatens the whole system with overheating. To prevent this, one major part of the safety design deals with the manner of cooling this fuel. To this end, there are all manner of primary and backup systems to keep the coolant moving and carrying away the excess heat caused by this frenzied motion of neutrons.

Control rods make up the other part of the design. The control rods are basically barriers that limit the flow of these neutrons, acting much like nets thrust into schooling fish. To lower the temperature, even absent a drop of water, one simply needs to insert these rods into the action to, uh, stop the reaction.

What does any of this have to do with hafnium? Well, it turns out that it’s basically the best material there is for this task. Neutron absorption is no ordinary job. When an atom of uranium-235 or plutonium-239 decays it sends parts of itself - neutrons - flying off in every direction at near the speed of light. When it hits an atom it either destroys it, much like the opening break of a pool game, or the atom captures that neutron and is itself altered in the process. There just aren’t many substances that can take this amount of abuse without falling apart (or have such threadbare atomic lattices that most neutrons pass right through, making them ineffective).

Hafnium on the other hand is built like a tank. When that shower of neutrons comes its way it takes the beating like a champ; with each atom strike there’s a high probability it will simply turn into another stable hafnium isotope ready for the next punch. Eventually these hafnium atoms do cry uncle, they’re not indestructible, but are able to hold out longer than just about anything else on the Periodic Table.