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molten salt reactors (the kind in the comment at the top of this thread) are at atmospheric pressure, and are designed with a drain plug which isolates the (very hot) materials from the neutron source, after which they cool down happily on their own in a separate but similarly shielded compartment.

release of materials is one of the most unlikely outcomes ever.

You really can't imagine a release of materials outcome in the context of active sabotage?

(Which is relevant in the context of massive distributed ones; current nuclear power systems mitigate this by being few in number such that they can be well guarded.)

since you can't weaponise the mixture, and it's just stupidly hot and dangerous for a while and then inert, I can't see why anyone would try to sabotage one. (apart from terrorists just out to cause mayhem, but that argument also works for cars, roads, planes, train stations, etc right?)

The addition of uranium or plutonium is unnecessary for operation of a LFTR, rather it is being sold as an extra advantage that a LFTR can safely consume the waste already created at PWR's. Consuming PWR waste in a LFTR is a better option than burying it in the desert (at least it is to some).