Japan has very few natural energy resources which is the main reason they embraced nuclear power in a big way. Otherwise, they would have to import many more tons of hydrocarbons.
Except that the place is covered in geothermal hotspots.
I don't know if any are sufficiently large to reduce the need for nuclear, but Japan is a country where the greatest natural energy resource appears (to an outsider) to be dedicated to providing really nice baths.
Japan's relatively cold (in geothermal terms) due to its tectonic position. (It's counter-intuitive, but forearcs have relatively low geothermal gradients for such tectonically active areas.)
The main areas of geothermal potential are localized around the volcanic arc. Most of this land is inside of national parks.
Compare this to areas like Iceland or most of the African rift, where you have a very high regional geothermal gradient. Anywhere you can find deep permeability, you'll have potential for geothermal power generation.
Japan certainly has a lot of geothermal potential, and hasn't utilized most of it yet, but it's not as simple as it seems at first glance.
The ash is more radioactive than most nuclear waste is and there's a whole lot more of it to contain.
The question is if it's safer to be next to a nuclear meltdown or a burning coal plant, not whether portions of the waste of both plants during normal operations are more or less radioactive than each other.
Heavy metals from the ash would be a lot more hazardous in the short term than the radiation unless you were standing on top of the reactor.