Isolated atomic tests away from populated areas are hardly evidences to use when determining the overall effectiveness of an all out nuclear war.

Both Hiroshima and Nagasaki were able to be rebuilt because other areas were not affected by the atom bombs dropped on those two cities. In an all out nuclear confrontation, the northern hemisphere of this planet would most certainly lose the civilizations swapping nuclear weapons upon each other. Their infrastructures would be wholly decimated in the initial conflict, and then it would be a slow poisonous death to follow.

Here in the UK, should an nuclear war breakout, the country and surrounding islands would receive a conservative estimate of around 80 to 120 megatons, which would be enough to make Britain virtually uninhabitable, and most certainly inhospitable for many many years. There would be survivors from the initial conflict, but their inheritance would not be worth anything more than the radioactive dust beneath their feet.

The aid and the infrastructure required to rebuild would no longer be extant. No hospitals, no transport, no medicines or medical personnel, dwindling to none food supplies, hardly any shelter, water poisoned with radiation and dead corpses of humans and animals, diseases and radiation sickness rife, and no aid of any kind from fellow human beings. No government, no authority, no decisions being made on moral or ethical grounds, but only on survival requirements.

The northern hemisphere would require aid from the southern hemisphere, which will not come until the conditions prevailed, and that is assuming that wars do not break out in the southern hemisphere between countries left intact for supremacy.

An all out nuclear war would not bring about extinction level, but it would certainly bring it to the brink, with the loss of about a third to half of the world's human population killed or dying slowly with no hope of aid coming to help.

Perhaps, the leaders of today, are thinking in terms of whomever has the most infrastructure left intact after a nuclear confrontation will win the day? That the cost to life and society is worth it if it gets rid of one's perceived enemy, even though it took 85% loss of one's own. Levels of survivability is a bad way to look at things, it consigns a vast majority to an appallingly horrific death, and for the survivors, an equally appallingly horrific future.