> building a fission reactor takes one or two decades

This is not universally true. Looking at South Korea's construction times for example[1], you'll see that it's averaging between 5-7 years per reactor, all the way into the 2000s and 2010s. Japan shows similar numbers[2], and they're currently in the process of restarting their nuclear investments following the accident at Fukushima Daiichi. Same story for China[3].

[1] https://en.wikipedia.org/wiki/Nuclear_power_in_South_Korea#B...

[2] https://en.wikipedia.org/wiki/List_of_commercial_nuclear_rea...

[3] https://en.wikipedia.org/wiki/List_of_commercial_nuclear_rea...