[makomk]

The UK tried building gas cooled reactors that produced more useful and efficient steam temperatures on the output. They worked - in fact, they make up pretty much the entire UK nuclear generation capacity these days - but turned out not to be as practical as boring PWR reactors due to various annoying engineering issues. There are other reactor concepts out there which promise to do better but they're pretty untested and would also likely turn out to be harder to build than they look.

[PaulHoule]

They're still running a steam turbine but it is more compact and efficient.

Fourth-generation reactors won't be easy, but the motivation to develop them is strong.

Arguably there is a lot of experience with liquid metal fast reactors, many of the problems like sodium fires and problems with inspection are probably solved. Think

https://en.wikipedia.org/wiki/Fast_Flux_Test_Facility

as opposed to

https://en.wikipedia.org/wiki/Superph%C3%A9nix