Building the plant and mining the uranium, doing upkeep on the plant, and decommissioning it and processing the fuel are not non-polluting.
There is always pollution. Some forms of energy have more and some have less. It is never good to just waste energy, that will always cause excess pollution, no matter the type of energy used.
The environmental cost of Bitcoin is not that of the energy it uses, it is the cost of the dirtiest energy it forces others to use.