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.