The Fukushima-Daiichi Incident, Dr. Matthias Braun, AREVA, March 29, 2011 (3.7 MB Powerpoint show)

It looks like the two areas to be concerned about now and in the next few months are run-off from the cooling systems and containment or lack thereof on the spent fuel pools. One priority would be moving fuel from the reactor-specific pools to the common pool ... I don't know what kind of timeframe that can be performed. I would expect that they would try to cool the reactor cores inside the reactor indefinitely rather than moving them to the reactor-specific pools.

In the long-term, moving spent fuel to dry-cask storage would be probably be the safest method of disposal.

The mechanisms for any major release at this time would be from the spent pools to be released into the air, and material from the reactor to be released to sea. A sarcophagus would protect you from releasing radioactive materials into the air ... if one is going to be built, it would seem like building around the long-term storage pool would be where you might do that, but one way or the other, it seems like moving it out of the pools in the reactor buildings has to be done first.

A sarcophagus solution seems a bit iffy considering the location in an earthquake and Tsunami area. The engineers don't have a long time to design anything required.