In 2019, just about 10 % of the global electricity came from nuclear power. Hence replacing fossil sources by nuclear would entail large-scale construction of new nuclear reactors. However, we then run into the problem of peak Uranium (our reserves are thought to last for about 140 years at the current rate, but this would shorten to about 14 years if we increased nuclear by a factor of ten). (New kinds of reactors might alleviate this problem, but they are not actually available now. Of course, they might be if, as you suggest, we started on this course earlier.)

In any case, nuclear power is today not economically viable -- wind and solar are cheaper by a factor of 2 to 3 (computed over the full lifecycle, but excluding the indefinite storage costs of nuclear waste).