For instance my favourite reactor type - lead bismuth cooled fast reactors. They've only been actually used once, on the Soviet "Alfa" class submarines, and hav

For instance my favourite reactor type - lead bismuth cooled fast reactors. They've only been actually used once, on the Soviet "Alfa" class submarines, and have some interesting advantages (lead naturally blocks gamma radiation, in case of a leak temperatures will go down and the coolant will solidify thus preventing radiation leaks, high efficiency due to the high temperatures, etc.), but are pretty expensive and impractical (the coolant solidifies if temperatures get lower than expected, thus you need specialised equipment to keep them hot/operate them 24/7). There's also thorium, molten salt reactors.