Off the top of my head I am not aware of any rule that explicitly states that you always get knocked down from a grenade blast, however, using the rules for knockdown would be appropriate when hit by a grenade blast. So basically just compare the final DV of the blast vs the victims Body. If the damage is >= Body, then they're flat. A side note is that if you take 10 or more boxes of damage from 1 attack, you are knocked down regardless of your Body Attribute. It would also not be too far out of line, IMO, to state that certain grenades get a bonus to knockdown similar to how gel rounds work.