Nuclear weapon

A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or a combination of fission and fusion reactions (thermonuclear bomb), producing a nuclear explosion. Both bomb types release large quantities of energy from relatively small amounts of matter.

The above passage was taken from Wikipedia.

Different species in the Project have their own history involving nukes.

Felin

Like any sufficiently advanced species, the felin figured out how to develop nukes. Around the year 3364, the theory behind the atom bomb was developed far enough to consider practice. A low-yield bomb, approximately 10 kilotons, was detonated on a remote, uninhabited island. Witnessing the result, the team in charge unanimously agreed to not do that again. Even so, the knowledge on how to produce higher yield bombs still exists.

Use of a nuclear weapon was considered during the attack of the Desecrator in 3541. This was vetoed at first, then rendered a non-issue when the combined Rhatai forces proved enough to defeat the beast.

Nation

In the ancient history of the Nation, when it was still a single planet and multiple nation-states of the same species, atomic theory was pursued for its damaging properties and as an alternative to area-of-denial gas bombs. The earliest atomic weapons were considered so successful, that it changed how the Nation functioned, moving from settlements into large bunker systems and the implementation of nuclear-tipped rocket bombs. During the never-ending war for the total control of the planet, most states used atomic weapons to destroy the top soil and large industrial clusters, followed by nuclear saturation of the enemy trench lines.

When the Nation was the only nation-state emerging out of their atomic war, their weaponry was perfected to use as little uranium as possible for the maximum yield. Their weaponry only changed in mass production and megaton values, and every ship larger than individual fighters run on a fission engine. Nuclear weaponry is a cornerstone of present-day Nation Ker'Kirra armories, and every total war is decided by nuclear bombardment, either by rocket bombs or by deploying nuclear waste on enemy production and military installations.

Space Huns

Most humans know that atomic theory was developed on Old Earth in 1890-1900, and saw militarization in 1945. The origin country of the Hungarian settlers, Hungary, only had peaceful applications with primitive atomic power plants. Beyond this, most Space Huns don't know the details of atomic weaponry.

After their settlement in the Határmezsgye system, the settlers ensured their freedom with low-yield nuclear weaponry, but only as a final deterrent.