Well, not all negligent discharges are accidental, and not all accidental discharges are negligent -- though in this case we would probably not be too far off the mark with ND (which was, by all accounts, also accidental).
My dept. still uses "AD" (and treats all as ND until proven otherwise).
Hard to teach an old dog new terminological tricks!
I'm curious as to the distinction between the two, in your dept's terminology.