Carp, there are scenarios that that outer shell can be breached. And as I was writing a reply to you, they believe it might be leaking. This is the first time we have had this news.

The US navy offered coolant. The tsunami damaged the pumps that could have pumped this coolant in. They have been using sea water for the last 3 days with improvised pumps. The sea water is a last ditch effort and is why plants are always built near large bodies of water.
We are what we repeatedly do - Aristotle