Like coal, many of the costs are externalised or simply ignored by advocates, especially the decommissioning and waste disposal.
Getting good/accurate figures for lifetime carbon emissions from nuclear power plants is very difficult, as this [1] article concedes, but quoting from same:
"The conclusion from the eight most rigorous LCAs is therefore that it is as likely that the carbon footprint of nuclear is above 50 gCO2/kWh as it is below."
They touch on build costs, and the likely correlation between larger CO2 emissions compared to similar power generation from hydro structures. I can't find good data on manufacturing and embedded energy costs of the vast quantities of concrete needed to construct a nuclear power plant -- I suspect it is not flattering to the industry.
[1] https://theecologist.org/2015/feb/05/false-solution-nuclear-...
[EDIT] improved the politeness of the first sentence.