Eh, I think they're pretty generous with nuclear at least in some regards - you never have to confront the storage problem in the game as far as I know. Also, you don't have to actually procure fissible material in the fuel tab, so that probably all gets rolled into the card in the energy tab.

If you look at nuclear power generation today, from fuel procurement, building the plant, upkeep and dismantling one study has arrived at 139 - 190 g CO2/kWh cradle to grave (see at the very end for a graphic with the split). Now, I haven't looked too closely at their methodology or whatever it's just the first I found, but on a basic level it makes sense that you're gonna release some CO2 equivalent in all the surrounding processes/infrastructure of a reactor, at the very least with how we're currently doing it. I don't know how fairly the game implements these emissions exactly (it doesn't give you too detailed of a breakdown) but seeing as it's totally viable to run a 25% nuclear energy mix and win I'd say they're giving it a pretty fair shake.

I even remember someone posting an end screen where their dominant energy gen was nuclear, it's not like the game forces you to go renewable.