Examples at a concentration as dangerous as a reactor melting down?

Hell, https://en.wikipedia.org/wiki/Great_Molasses_Flood killed more than the Fukushima Daiichi meltdown.

It was produced for the benefit of all mankind.

And now that's whose bloodstream it can be found in.

Mainly for two reasons, it's half-life is longer than the lifespan of living things, and people spread it around like they never should have done at all.

That makes it analogous to opening Pandora's box more so than all kinds of dangerous things.

And in any case, this is a piece of military hardware which might plausibly be "reconfigured" be being struck by a rocket. It's not a civilian reactor inside a secure facility.

Your statement is analogous to saying "charcoal grills do not start forest fires". Used correctly, they certainly shouldn't. But they sure as hell can if you kick them over on a dry summer day. Same deal here.

To be glib: Not vs. fucking rockets they aren't. Those designs imagine that the fuel doesn't move and that the coolant is the medium of failure.

What happens when you move the fuel?

It stops reacting. If you blow up a nuclear reactor you've made a nice radiological hazard (though Uranium/Thorium themselves aren't particularly hazardous), but as soon as the fuel stops being contained inside of the appropriate neutron-mediating environment and is scattered, it will become subcritical.

The question of whether or not deploying nuclear reactors in warzones is a good idea is entirely different from the question of whether meltdowns are a realistic possibility. A critical nuclear reaction can be sustained with neutron reflectors and moderators using an amount of fuel that would otherwise be subcritical. Any "reconfiguration" only reduces the criticality factor of such a reactor.