Since the containment is a pressure vessel, there’s no possibility of water getting out of the containment building through a trench anywhere. I can understand using the water as a radiation moderator to allow workers more time to work in the containment.
Apparently the concern is seismic (if I understood what I just read here:
http://seekerblog.com/2011/05/01/nisa-instructs-tepco-to-study-demonstrate-dry-well-flooding-is-safe-from-seismic/
They’ve had a few days so perhaps NISA and TEPCO have decided that it is seismically safe? Seems like their seismic concern has to do with going forward - drywell filled with water and then large aftershock hits?
http://seekerblog.com/2011/05/01/nisa-instructs-tepco-to-study-demonstrate-dry-well-flooding-is-safe-from-seismic/
Looking a little further, I was pleased to find
“-TEPCO has finally revealed some details about the actual construction of the temporary cooling system it will install.. at least at Unit No. 1. Here is a clip from the handout:”
Followed by a schematic that I don’t know how to embed but you can see it at the link:
http://atomicpowerreview.blogspot.com/
in case you are interested