reaction weapon; extremely powerful weapon
Science fiction term for a weapon of immense destructive power, often nuclear or beyond. The literal gloss 'reaction weapon' is a direct translation of the kanji.
In that sci-fi novel, a reaction weapon instantly obliterated the planet.
The use of reaction weapons drastically changed the military balance in the fictional world.
核兵器 is a real-world term for nuclear weapons, while 反応兵器 is a fictional category that may include nuclear or more exotic weapons.
大量破壊兵器 (weapons of mass destruction) is a broad real-world category; 反応兵器 is a narrower, fictional concept often implying reaction-based or super-science destruction.
Compound of 反応 (reaction) and 兵器 (weapon). The term appears to be a science fiction coinage, likely influenced by the idea of weapons powered by nuclear or matter-antimatter reactions.