A modern nuclear power plant using a pebble bed reactor could be the size of a couple of Portakabins, with a third Portakabin to contain the wellies and gardening gloves you'd need to handle the fuel.
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.