Assuming you mean “Can Mastodon instances defederate with Threads?”: Yes. Mastodon (and similar services) run on the ActivityPub protocol, which allows them to decide who they do and do not federate with. Many instances have chosen to preemptively block Threads, many have chosen not to. Pick what works for you.
Supposedly, a meltdown at sea is pretty low risk because you have the perfect heatsink literally everywhere around you, and its a molten salt design, which I think(?) (source: my ass) means that the fuel would at worst leak into the sea and immediately solidify back into some inert state.