Protactinium separation is well-known to the engineers working on thorium reactor designs (though overlooked by much of the public.) Some designs make it easier than others.
The main advantage of the thorium fuel cycle is that you’ve got non-fissiles shipped to and from the reactor, so you at least don’t have to worry about theft by terrorists (other than for dirty bombs). So if a state already has nuclear weapons, thorium reactors don’t add much risk. Since the biggest carbon emitters are all nuclear powers already, thorium still seems like a good idea to me.
For other states, if they had reactors without protactinium separation and they didn’t make their own fissile startup fuel, we still might be in reasonably good shape. The breeding ratio of thorium is near breakeven, so if they removed much fissile the reactor would stop and they’d have to ask for more startup fuel. We’d definitely want to think through what they could get away with though.
Of course if aneutronic fusion works out, thorium reactors will as obsolete as everything else, and proliferation will be much less of a concern.
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