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Heavy water reactors replace both hydrogen atoms in the water molecule with deuterium, thus making it D20. The deuterium acts as a moderator by retarding the neutrons which makes them more likely to react with U-235. At natural speed neutrons tend to be react with U-238; which captures those neutrons without fissioning... thus not producing power. Light water (regular water) is also used. But with a light water reactor you can't use uranium in its natural state. It has to be enriched first or it will never reach criticality (the nuclear fission chain reaction). US reactors are typically light water while Canada's reactors are heavy water. It appears, though, that Japan uses light water reactors. Now that I've thoroughly bored you to death by recalling my youth spent in NE classes it appears that Japan had asked for help, the US said it would and the process got started, but then ultimately Japan decided they could handle it themselves. No one updated Clinton, apparently, as she was going off of old info. Story here.