This is just my theory on how Batman managed to escape the blast radius (this has no doubt been said already):
 
After the closeup of Batman, we see the Bat flying away, then the last few seconds of the countdown... maybe between the closeup and the last two shots, there was a larger time span than what we saw? We might have seen a simple time lapse. After the closeup of Batman, Batman could have ejected out... that wouldn't have been visible to anybody anyways. 
 
Not sure what the other theories are...
 
inb4 "He didn't escape the blast radius, he died and Alfred imagined seeing him at the end."