When I was in South Africa their pebble bed reactor was the thing. They were working on it from 1993 until 2009 then gave up. Not totally sure why. https://en.wikipedia.org/wiki/Pebble_Bed_Modular_Reactor

There was a German one too:

>constructed in 1960, grid connected in 1967 and shut down in 1988. It was a 15 MWe, 46 MWt test reactor used to develop and test a variety of fuels and machinery. ... Over its lifetime the reactor had many accidents, earning it the name "Shipwreck".

Not quite sure what goes wrong but they seem problematic.