Most alternative reactor designs have some major flaw.
The UK has a fleet of operational gas cooled reactors. The seem to work well enough, especially given the age of some of them. We stopped building more since the UK government (irrespective of party) seems to be obsessed with killing off home grown technology and buying it from someone else.
Sometimes (not in this case) they even sell it off then buy the same thing back years later at an inflated price and less good!