The explosions are hydrogen explosions. There have been 3 at the Fukushima Plant so far, radiation has been released as far as 100 km from the plant. It is above "Legal" levels but below Lethal levels. Reactor 1 was due for decommission last month but kept going.
In Chernobyl's case, basically two explosions occured, first with hydrogen and steam buildup, exposing the core, then runaway fission caused a nuclear excursion (some of the core detonated), and combined with burning radioactive graphite dispersed the fallout into the air. It's worth noting the fallout of Chernobyl was 1000 times less than from total nuclear weapons testing in the 50's and 60's.
There is a fire in one reactor, but no known evidence of a nuclear excursion yet, at Fukushima.
However, from studies of the effects of the quake, it is predicted there is a high probability of another quake, in the next 14 days, of moderate or higher magnitude.
I hope they're wrong.
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