Steps:

1. Create you explosive model.

2. Create your particle effect and prefab it.

3. Delete the particle effect in the scene. :)

4. Create a script that places an explosive. It has to provide a position and rotation, unless you are planting on the ground, then only a position.

5. Call this function from the previously mentioned script.

   ---

var explosive : Transform;
var explosion : Renderer; //Particle Prefab goes here.

function PlaceExplosive (position : Vector3, rotation : Quaternion = Quaternion.Identity, time : float) {
     var detonator : Transform = Instantiate(explosive, position, rotation) as Transform;

     yield WaitForSeconds(time);

     Destroy(detonator.gameObject);
     Instantiate(explosion, position, rotation);
}