> we know how to build and operate them at 95%+ efficiency. Fission can provide all the power we need.

I am not sure what do you mean by 95%+ efficency here. But if you are talking about the entire process of getting the energy/power from the nuclear reactor this is not possible. You are still limited by carnot cycle. Even the most advanced reactors like HTGRs [1] operate with efficiency about 45%.

If you have some other definition of efficiency than the standard then it would be good if you define that.

[1] https://en.wikipedia.org/wiki/High-temperature_gas-cooled_re...