OK. The way I would do it is to have a crate, this could have edges or not, up to you. Without edges it could have as few as eight vertices and use textured edges, etc. Then during an explosion it would be replaced by a somewhat higher poly model which would fly off into little pieces and then be deleted at the end of the animation so the space was clear where it was. The pieces would just about have to fly off in all directions, but they would need to be three dimensional or if they turned a certain way they would disappear, so I might need about 200 vertices for that. You could make it so one side fit a texture, that way you could change textures for variety. You could also have the pieces only fly off in three directions if you knew the placement of the bomb from the box.