CBRN - Chemical Biological Radiological Nuclear
It also includes reference to the radiological element such as might be found in a Chernobyl-type accident or a 'dirty bomb' detonation - as opposed to the big bang, flash, mushroom cloud and fallout of the common-or-garden nuclear weapon.
Quite why the well known acronym of NBC didn't simply have the 'R' element added on the end (NBCR) is baffling. And why weren’t the civvies brought into line with those who've actually been actively involved with NBC defence for the last half a century? But some cnut (for that is what these awkward clowns are) obviously thought it was time to invent yet another acronym.
The CBRN acronym is formed to reflect the perceived order of likelihood in which the threats will be faced, from most likely; Chemical, including industrial and battlefield pollutants through to least likely; full Nuclear weapon strike.