> but fossil fuels are also much more widely used (I imagine by several orders of magnitude)

True. Also consider the relative efficiency of the generation methods. Supplying the world's demand for energy with nuclear power would probably require far fewer powerplants than doing so by other means.

> I guess if reactors were located on land that was already unusable (eg. desert, esp Antarctica)

Except for the melting of the Antarctic ice and all that, in case of an accident :) Most technology seems to involve a trade-off of evils.