There's a difference between meltdown, inefficient/low explosion, & efficient high-explosion.

The neutron-emission's cascading only when the critical-mass is in small-enough a volume,

& the heat it's producing is making it blow-apart.

( I've seen video of 1 million watts of heat being produced by a 1-cubic-metre reactor-core, in a water-cooled reactor.. the heat the reaction produces makes the fuel-rods incandescent, or at-least that's the way it looked, in that video: it may have been Cherenkhov-radiation that I was seeing, or from gamma-rays being converted down to light or something.. dunno.. )

the more efficient you want the fission-reaction to be, the longer you have to force it together, while it's blowing-up.

The explosives are required, for bombs, ttbomk.

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