TLA for German
Größter Anzunehmender Unfall, "biggest assumable
accident". Originally, this was
techspeak for any
worst case scenario in an industrial facility. Ideally, the on-site
firefighters et al would have some way of dealing with it.
The expression came into general usage after it was (aptly) used to describe the Chernobyl disaster, and since then, it's generally used only to mean accidents involving nuclear power plants, and often the even more emphatic form Super-GAU is used.