Did the radiation in Japan spread through ALL of Japan?

Question:

And if so, is it severe? Will it be eventually deadly for all the japanese people, especially the upcoming generations?

Answer:

If you're talking about the radiation from Fukushima, then it didn't. Even if it did, would be virtually impossible for the radiation to retain it's original concentration/potency and therefore lethality while passing through countless objects, namely concrete.
Now, if you're talking about the radiation emitted naturally from trees, light bulbs, sunshine, etc, then I have to tell you that that has been around all over the world for centuries.