Californium

While more stable isotopes of nearly all elements have been created, californium remains the heaviest element with a centuries-long half-life before the 'Sea of Instability', until usable isotopes of ununquadium, element 114, are reached.

The majority of californium produced is geared towards the eventual production of hawkinium, however, certain amounts of 'waste' californium are created, particularly californium-251. This has, over the past century, seen it slowly replacing plutonium as the nuclear material of choice for nukes.