As of 2022 those 4 countries accounted for 55% of mined uranium.
The bigger issue that you correctly identify is that building nuclear plants takes a long time. Also, unlike a wind farm or solar farm, the reactor must be completely finished before it can start earning revenue from electricity generation. Many wind and solar projects start supplying power to the grid when they are less than halfway complete, since they're made up of turbines or racks of panels that work independently.
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Wih nuclear you have for example the question of getting fuel. There is little nuclear fuel available outside of ex-soviet control.
Rosatom (Russia) controls less than half of world uranium enrichment capacity, and none of the other ex-Soviet states have enrichment plants:
https://world-nuclear.org/information-library/nuclear-fuel-c...
Rosatom had 46% of world enrichment capacity in 2020.
If you mean uranium mining, ex-USSR states (Kazakhstan, Russia, Ukraine, Uzbekistan) account for slightly more than half of production:
https://world-nuclear.org/information-library/nuclear-fuel-c...
As of 2022 those 4 countries accounted for 55% of mined uranium.
The bigger issue that you correctly identify is that building nuclear plants takes a long time. Also, unlike a wind farm or solar farm, the reactor must be completely finished before it can start earning revenue from electricity generation. Many wind and solar projects start supplying power to the grid when they are less than halfway complete, since they're made up of turbines or racks of panels that work independently.