Cover Versus Explosions
The physics of explosive shock waves is complicated. Strange effects can be seen when blasts bounce off walls or when explosives are used in confined spaces. Unless your Ph.D. thesis is in their study, we suggest the following for determining cover from explosions.
Firstly, ignore all walls and other barriers. Everyone who is within reach of the blast should be subjected to an attack. For a Blast (6d6) this includes the central square plus anyone within five squares of it.
Secondly, trace the most direct route between the explosion and the victim, avoiding solid walls that are at least as tall as the target. For each instance of half-cover the route goes over, grant a 1d6 situation bonus. The victim also gets a 1d6 bonus for each 90° corner the blast has to go round.
If there is no route between the victim and the explosion they are immune to the blast.