[LocutusOfBorges]

I'm not sure this would even make sense to build - the purpose of a nuclear deterrent is to ensure that a first strike against the power in question is rendered impossible under rational conditions. If both sides have been reduced to ash, there hardly seems a point to maintaining that kind of posture - it's inconceivable that a war could be fought under the conditions that a mass nuclear exchange would entail.

What would there even be left to fight for? Once the first salvos have been launched, the rest of the arsenal becomes essentially useless.

[cmcaleer]

I'm approaching this not from the perspective of 'nuclear war is rational, and this is how it should be fought', I'm trying to look at it through the perspective of someone whose job it is to optimize for the unoptimizable. It seems very unlikely that any nuclear strategy planning ends at 'then (they/we) unleash a second strike, and that's it', so then it becomes worth it to consider how to completely crush any attempt at revenge in the living memory of those you killed.

There's going to be competition over what little habitable land and untainted resources remain, and the appetite for revenge in such a scenario will be insatiable. Reserving some small % of the arsenal for tertiary strikes seems sensible if you're planning for this, particularly since SSBNs will be basically unaffected if nuclear war is happening above them and launch sites that aren't destroyed would probably still be able to launch for some time if this is accounted for in the design.