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?