Out if interest, the way I'd try and design a fail-safe solution would be to have meltdown trigger a shutdown.
Picture a bunch of large, heat-proof (ceramic?) test-tubes, with their bases apart but their openings together.
Put that underneath the core - then if it melts and drops down, small pieces of it flow in different directions - until you've got a sub-critical amount in each one - so the fission reaction stops.
Anything which requires active prevention of fission seems odd, presumably passive is better?
I would have a look at pebble bed reactors, molten salt reactors and heavy water moderated reactors.
long story short, the designs exist, we just can't seem to get rid of the old ones that are currently built and we can't seem to get the new designs built in any sort of scale.
The R&D costs for those are HUGE, as they are unproven, the countries like mine (Uruguay) that are looking into building new power plants don't want that, we want proven designs.
The issue at Fukushima isn't criticality, though. All the reactors were safely shut down seconds after the earthquake hit, and no new fission has occurred since then.
The problem is that even a shut-down reactor core generates significant heat due to the decay of fission by-products, and that's what caused the problems.
That said, there are other reactor designs which are more inherently safe.
The way I'd try and design a fail-safe solution would be not to design it in the first place. No one is forcing us to build these things. But government contracts are lucrative and contractors exist.
Picture a bunch of large, heat-proof (ceramic?) test-tubes, with their bases apart but their openings together.
Put that underneath the core - then if it melts and drops down, small pieces of it flow in different directions - until you've got a sub-critical amount in each one - so the fission reaction stops.
Anything which requires active prevention of fission seems odd, presumably passive is better?