I'm quite sure production is part of it. Solar panels tend to require kind of complex chemicals which are not super eco-friendly. Of course a nuclear power plant, building and procuring uranium is also not cheap (with regards to CO2) I guess there are too many variables to get some good numbers but maybe someone knows better..

I think another aspect that (I think) is usually unregarded is space. What area do you need to get same output. . Just random guessing, but I assume a nuclear plant gives you much more watt per sqm than solar, and while you can grow grass under your panels but not on the plant, I feel the latter still leaves you with a better balance.