The largest thermonuclear weapon ever actually detonated was the Soviet “Tsar Bomba” at an estimated 50-57MT (it was designed to produce ~100MT but for the test over their own territory the Soviets didn’t include the third stage which cut yield and also made it relatively “clean” from a fallout perspective). Here’s the relevant section from Wiki:
*The original U.S. estimate of the yield was 57 Mt, but since 1991 all Russian sources have stated its yield as 50 Mt. Khrushchev warned in a filmed speech to the Communist parliament of the existence of a 100 Mt bomb (technically the design was capable of this yield). The fireball touched the ground, reached nearly as high as the altitude of the release plane, and was seen and felt almost 1,000 km (621 miles) from ground zero. The heat from the explosion could have caused third degree burns 100 km (62 miles) away from ground zero. The subsequent mushroom cloud was about 64 km (40 miles) high (nearly seven times higher than Mount Everest) and 40 km (25 miles) wide. The explosion could be seen and felt in Finland, even breaking windows there.[7] Atmospheric focusing caused blast damage up to 1,000 km (621 mi) away. The seismic shock created by the detonation was measurable even on its third passage around the Earth.[8] Its Richter magnitude was about 5 to 5.25.[9]
Since 50 Mt is 2.1×1017 joules, the average power produced during the entire fission-fusion process, lasting around 39 nanoseconds, was about 5.4×1024 watts or 5.4 yottawatts. This is equivalent to approximately 1.4% of the power output of the Sun.[10]*
And from this page (The Soviet Weapons Program - The Tsar Bomba):
*A shock wave in air was observed at Dickson settlement at 700 km; windowpanes were partially broken to distances of 900 km. All buildings in Severny (both wooden and brick), at a distance of 55 km, were completely destroyed. In districts hundreds of kilometers from ground zero, wooden houses were destroyed, and stone ones lost their roofs, windows and doors; and radio communications were interrupted for almost one hour. The atmospheric disturbance generated by the explosion orbited the earth three times. A gigantic mushroom cloud rose as high as 64 kilometers (210,000 ft).
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The Tu-95 was painted with a special white reflective paint to protect it from the thermal radiation of the fireball. The airborne laboratory plane was also covered with the same paint. In clear air, the 50 Mt test was capable in principle of inflicting third degree burns at a distance of up to 100 km.
The area of effectively complete destruction extended to 25 km, and ordinary houses would be subjected to severe damage out to 35 km. The destruction and damage of buildings occurred sporadically at much greater ranges than this due to the effects of atmospheric focusing, an unpredictable but unavoidable phenomenon with very large atmospheric explosions that is capable of generating localized regions of destructive blast pressure at great distances (even exceeding 1000 km).
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There was no previously existing military requirement for a 100 megaton weapon - such weapons are virtually useless for military purposes. The Soviet Union had only one delivery system capable of carrying a weapon of this size - a handful of the relatively slow prop-driven Tu-95 bomber - and it was incapable of intercontinental range with a payload this large. A 100 Mt weapon can level urban areas in a zone 60 km wide, cause heavy damage in a zone 100 km across, cause 3rd degree burns in a region 170 km across (only a bit smaller than the width of West Germany) and eye damage to 220 km. Such a weapon can only be used as a means of destroying an entire urban region - a major urban complex including suburbs and even neighboring cities. This scale of destruction is much larger than any discrete urban area in Western Europe. With its dense settlement, use of such a weapon in Europe is equivalent to an attack on a major portion of an entire nation and its population. Fallout from a low altitude or surface burst in central England could produce lethal exposures extending into the Warsaw Pact nations; a similar explosion in West Germany could create lethal fallout as far as the Soviet border. Even in the United States there were only three urban regions at that time large enough to conceivably merit attack with such a weapon - New York, Chicago, and Los Angeles. On any smaller target it would be simple overkill. Even if the Tu-95 were able to reach Chicago, the closest plausible U.S. target, (which is doubtful given the enormous payload, far in excess of normal for long-range missions, and the added drag from the belly bulge required to house the bomb) it would have been detected crossing the North American early warning line and then been over U.S. and Canadian territory for 8 hours - ample time for jet fighters to intercept and shoot it down [Zaloga 1993].*
A weapon that size will pretty much vaporize any urban area and it is so powerful that only very large areas are appropriate targets - note my bolded section above, that author opines that only NY, Chicago and LA make sense. LA is about 44x29 miles with 3.8 million people. I don’t know the rough dimensions of NYC but it’s over 300 square miles of land with more than 8 million people. Chicago has about 3 million people. Drop a 100MT weapon over any of them and basically everybody dies.
All of those cities are part of greater metropolitan areas - nearby cities, towns, etc. Taking the quotes above at face value you’re looking at generally heavy damage out to 60+ miles from ground zero as well as 3rd degree burns to 110+ miles, that’s on unobstructed terrain. Not sure how much low mountains and such will affect that because a weapon that big is detonated several miles up, I’d assume that would be more of an issue for LA and least of an issue for Chicago (the Midwest is pretty dang flat). Within a radius of say 200 miles from those cities there are many millions more people and some percentage of them will be killed by “heavy damage” and 3rd degree burns although I dunno how to figure out what those percentages are.
Anyhow you’re looking at a death toll of absolutely staggering magnitude - drop it on NYC and I think you’ll have 8 million dead right off the bat plus several million more in surrounding areas. Wiki says that NY metropolitan area is nearly 20 million people, if 20% of the 12 million people who don’t live in NYC proper are killed that’s a total toll of over 10 million.
That’s half the number of people who died in WW1. That’s the same as the entire Holocaust. All in a few minutes.
I’m thinking NYC would be the worst case.
Regarding shielding effects of large structures, the Tsar Bomba produced 300psi overpressures at ground zero - from a civil engineering perspective I can tell you that buildings are just not designed to handle that kind of load (43200psi versus the tens of psf that various live loads are rated at). I remember back in grad school coming across a government engineering text on the effects of nuclear explosions - 100psi overpressure was listed as “total destruction”, even aboveground bomb shelters would be gone (you’d need to have a massively protective underground shelter). I don’t see even something as sturdy as the Empire State Building existing afterwards. The size of the fireball and blast bubble is huge compared to any structure, I don’t think there’d be any conceivable shelter.
Sheesh. I’m going to stop typing this, have a mug of warm soup and snuggle my cat.
