The explosion looked to be about .2 kilotons (just a guess). An explosion with accompagning fission is more like a dirty bomb or a neutron bomb. It does not have to be enormous to be dangerous and deadly.

It does not have to be a full out thermo nuclear explosion to render the area uninhabitable. THie distinction between a Hiroshima style explosion vs. a fission excursion or merely explodinsions that send nuclear fuel up into the air has been brought up before with similar misunderstandings about thermonuclear vs dirty neutron bomb-like explosions.

The people of Japan will lose many people 30 years from now which means the future victimes are mostly going to be under the age of 25.
Small children are more vulnerable and will show the effects sooner than adults. Of course the workers at the plant have assumed their days were numbere 2 weeks ago.