It is not the "cleanest" it is the most "climate friendly". Commitment to nuclear waste is a thousand year long process of monitoring and processing waste products and our record on a short term commitment of like 50 years to climate change is not a good record. Its not clean and in the long term not likely cheap.
However its certainly not as destructive to the world as gas is right now so relieving climate change before some other catastrophic milestone occurs is probably a good idea. Folks saying "its too late" misunderstand climate change; there's always another level bad we can hit and nuclear provides stable, reliable power.