Nuclear Weapons Taboo: Averted completely and comprehensively. The Empire has none – which, to expand, may have something to do with a homeworld considerably enriched, relative to Earth, with heavy and radioactive metals and lacking in fossil fuels, better biological radiation protection, and consequently an industrial era that used a lot more pebble-bed steam engines and nucleonic furnaces than ours. The mundanity of the ‘splody-metal bomb was just the next logical step.
So, yeah, the nuclear weapon (“which they call a device“) is exactly what the Imperials reach for when they need a big and effective bomb, including for plenty of civilian purposes like, say, moving asteroids, digging canals, and dispersing inconvenient mountains, as well as, say, giving every heavy legionary a half-dozen 0.1kt thunderballs for his nuclear-grenade launcher. Although at least these days, technically, they’re usually antimatter weapons, and thus rather cleaner than the old-school ‘splody-metal types.