There are fuels that self limit, and most pebble bed reactors use these. No chance of melt downs. Here is one example

https://www.energy.gov/ne/articles/x-energy-developing-pebble-bed-reactor-they-say-cant-melt-down

That said, liquid thorium still seems to be the best we can do for nukes and we don't pursue them because they don't provide a plutonium product.