Micro-Nuclear Core

A Micro-Nuclear Core or Power core is a nuclear core that fuses science and magic to create a device that creates a permanent power sorce without taking up any space.

It is literally a nuclear generator that is magically shrunk to the size of a jewelery box. It is kept in check by a full automated magical system to keep it from having anything but max power. It if often used by melee weapons that require power to work.