Where is the sea water that is being pumped in to cool the reactors going, and is this water contaminated?
— T. Lowen, Minneapolis
A.
To the best of our understanding, the sea water is turning to steam. The reactor is still so hot that the sea water they are pumping in is just intended to replace the amount that boils off. In other words, in the best of situations, they are just managing to keep the reactor covered with water.
In the reactors that have had fuel rod exposure (at least three of them, apparently), the steam would be contaminated with radioactive elements from the fuel, which has been exposed because of cracking of the zirconium cladding around it. The contaminated steam leaves the reactor vessel and enters the containment structure. To avoid a pressure buildup, the containment structure must be vented intermittently, resulting in the release of radioactivity to the environment. (In at least one of the reactors, the containment structure is reported to be damaged; if that’s the case then the radiation release could be continuous.)"
Source: NYTimes Blog