We have reactors from the 90s, so called generation III reactors, that have passive safety features that make them as safe as we realistically can. These 'traditional' designs or more modern gen III+, either are good options to build.

I quite like the ideas that Oklo have put forward with their liquid metal reactors that safe automatically should coolant flow stop. In a similar vein, pebble bed reactors can also offer similar fail-safe systems. Ultimately if they can bridge the gap until we figure out fusion which some very exciting advancements have been made in recently, even if still decades away is still within the life span of current nuclear reactors. Then our possibilities are limitless.

As for a timeframe? Yesterday would be a good start... I think they should have already been built and that much of the scare mongering regarding nuclear energy has in many ways exacerbated our present situation. The inherent fear that people have needs to be reduced before it is even anything more than a pipedream. The reality is that nuclear power takes huge investment and lots of time to build while also being a political football. But at the same time there are very few, if any, renewable sources that can provide as consistent power.