But how green is building a nuclear power plant? I imagine there's a lot of heavy machinery involved and lots of concrete involved.

Those things are always factored in. Also the mining for the fuel has to be included, which is also quite expansive. The costs for decommissioning a plant seem to get underestimated a lot though.

There are sometimes calculations where nuclear comes out on top of renewables. But these calculations are hugely more complicated for the nuclear side, and require nothing going wrong, or at least not much more than in the past, which is a dangerous assumption in my opinion. And with nuclear you lock yourself in.

A country needs to build up a large nuclear industry to make it profitable (or pay extra for imports), and to decommission plants or stop building them ahead of schedule increases the costs.

This kind of reductive ad nauseum accounting is why nothing ever gets done in the west anymore.