Percussion grenades, or percussion bombs, were Gallifreyan explosives which, by the time of the Gallifreyan Civil War, were outdated.
Narvin and Leela planted some at the Artron Forum, but one detonated prematurely and left Narvin wounded and Leela blind. The remaining bombs were detonated by Pandora to kill several of her own guards and innocent people, blaming the attack on Romana. (AUDIO: Fractures)