When I put not survivable above I meant that the posts were designed to increase chances of surviving not guaranteeing survival.

They would easily withstand a 10PSI overpressure and the accompanying blast. Fallout PF was about 1000 and particle protection was perhaps three days. All enough to do the job and probably good enough for the great majority of observers. a closer detonation or being in a rained out fallout plume would have been a problem.

spending more money and installing more kit would not have changed the odds much, getting into the area of diminishing returns. It is difficult to define the line for such kit.

There appears a common misconception that nuclear war would involve many weapons, in reality a few would probably be enough. In the last nuclear war two weapons were enough, and one of those was superfluous and just used as a weapons test.