Rekenium is a heavy transuranium isotope of a type theorised by 20th century Earth scientists but never encountered in nature, at least until Kai’s pan-galactic trip.
Existing within the second island of stability, far beyond the reaches of Mendeleev’s revised periodic table, Rekenium is a profoundly radioactive substance that can be used to power a variety of high yield technologies.
When systems require more compact/dense energy sources than are possible with standard fission or fusion ignition, Rekenium is the element to turn to. Superheavy spherical nuclei mean that Rekenium is comparatively stable.
Various terminal research projects have demonstrated that bombarding the element with Hassium-270 will form a short lived but hugely explosive isotope.
Place pellets of explosive Rekenium inside a portable railgun and you have the makings of either a superb piece of asteroid mining equipment or a fearsome assault rifle capable of rearranging the local neighbourhood on a fundamental level.
Each pellet is shaved from a dense block of Rekenium within the firing mechanism, charged by particle bombardment and enclosed in a temporary stasis field.
As the pellet accelerates from the barrel of the rifle, the stasis field drops and the released reaction within begins to rapidly turn the surrounding air to superheated plasma. The fireball that is formed burns at hundreds of thousands of degrees Kelvin and will destroy virtually anything in its path.