Nuclear fusion, in the context of nuclear physics, is the reaction used to combine two or more atomic nuclei into a larger atom or molecule, a process that absorbs or releases vast amounts of energy. It is a staple technology in the futuristic world of the Frencoverse, its primary application being to generate power, yet nuclear fusion also has a number of other uses in the narrative and the wider world that orbits it.
Throughout the Frencoverse and its broad collection of canons, the principles of fusion technology see heavy use, mostly to power other items. Though it is not the most advanced source of power available to polities of the Frencoverse, with SynKom dark ions and Qh'naazi singularity engines beating it out, it is the most common by a colossal margin, especially for military purposes. From handheld ion weapons used by Mechanocratic Russia, to Frenkish tactical nuclear weapons, to the city-sized spacegoing superdreadnoughts of the Imperium of Sidhae, nearly every faction powers their engines of war with some sort of fusion reactor.
Although infantry-portable fusion reactors are almost nonexistent in Maincanon 2, being supplanted by energy storage mechanisms instead, these batteries/capacitors are often recharged using a fusion reactor (for the Mekhs at least - it is not elucidated upon how Lunar, Frenkish or any other energy weapons recharge their power packs). Fusion and plasma-based explosive devices, however, are commonly used by most if not all factions as a powerful energy-based alternative to conventional explosives. Indeed, plasma and fusion bombs alike are among some of the most potent implements of destruction available to a military commander in both the Maincanon and the Alternative Canon. In Access Violation, the power of such is witnessed as a Sidh plasma artillery barrage reduces the Iron Cauldron, the heavily fortified headquarters of the Second Street Greencoat Gang, to a pile of molten slag with laughable ease.
Militaries of the Frencoverse are not the sole users of nuclear fusion, however. Most supercities, starbases and (presumably) hive-cities receive their electricity from huge fusion cores providing petawatts of energy to a power-hungry urban community, often in conjunction with other methods such as wind and solar.
A more unusual use for it pertains to waste disposal, a fusion-powered furnace being used to incinerate waste and reduce it to its basest atomic components to be reused elsewhere - a technologically advanced method of recycling, seen most commonly in the Imperium of Sidhae and Mecharussia. The Sidhae have also added a grim twist to this particular application, feeding their deceased into the furnaces. This has become by far the most common funerary practice in the entire Imperium, other burial methods being observed as a gross waste of resources.
Even the culinary world has appropriated nuclear fusion on a direct basis. Such can be observed in the Frenkish Rocket Steakhouse, whose kitchens use modern plasma convection techniques to cook steaks and seafood at an incredible speed: their signature dish, a six-ounce steak dinner, can be prepared and cooked in about six minutes, at a cost of 16 Imperial Credits. Its eastern-world equivalent, the Shashlik Kingdom, uses a similar technique to grill their shishkebabs in under a minute. Both restaurant chains have been able to force lower-end competitors out of business this way, most competition simply unable to keep up.