Just risking the jump-in to point out that a TEPCo spokesman indicated that there is a slight danger of re-criticality from the spent fuel ponds. One of the reasons they are determined to 'pour' Boric acid into the exposed ponds. (My understanding: The spent fuel was packed in tighter, and some of the newer spent fuel might still have sufficient radioactivity for criticality, as space for the spent fuel became hard to find.)
Spent fuel, because of its very nature as spent, is too highly contaminated with the products of the fission reaction to actually begin that chain reaction, which is why it's not still in the reactor core in the first place.
*Just an aside and full of maybes so not an 'ZOMG! We're going to die!' comment. Just pointing out that re-criticality possibility is not 0.