If we take TEPCO at their word, either Units 1, 2, 3 emitted Chernobyl-scale radioactive Iodine gas on explosion (that somehow fell through the roof of Unit 4), or there’s been a fission reaction in Unit 4 fuel pond – neither of which is an entertaining, uplifting prospect.
Peter Sinclair at Climate Crocks is keeping tabs on all things Fukushima :-