In emissions terms I understand that nuclear power is far from 'low carbon', once you include the emissions involved in construction and fuel generation.

I think a lot of greens are wary of nuclear power as they see it as a front for nuclear weapons (don't forget the 'peace' in Greenpeace's name!).

Environmentalists generally advocate decentralised solutions that give people autonomy (e.g. rooftop solar). Nuclear power is basically the opposite of that.