"Radioactive contamination of the experimental field with a radius of 2–3 km (1.2–1.9 mi) in the epicenter area was no more than 1 milliroentgen / hour, the testers appeared at the explosion site 2 hours later, radioactive contamination posed practically no danger to the test participants" [1].
The ideology at the time was that a larger blast was more important, since at the time nuclear weapons were still to be dropped by bombers (No ICBMs for another decade). You wanted the bomb to take out the target even if you couldn't quite reach it.
The Soviets estimated that using the same technique would have produced a yield of 100 Mt, but for testing, lead was used to limit the fallout to something they deemed manageable.
Ground bursts also increase the overall amount and concentration of fallout: First, with the explosion closer to ground, more neutrons can reach the ground and activate material there, producing radioactive isotopes from previously harmless earth. The "fireball" is a shell of expanding lower-density plasma which doesn't dampen neutrons as much as gas (especially water vapour) in non-plasma air. So that fireball touching ground produces more activated elements increasing overall fallout. Second, all newly created active elements and more fusion/fission products get bound to ground particles like dust, coral or water droplets if the explosion stirs up more of that. Those particles are larger and heavier, producing a more concentrated fallout closer to ground zero, whereas unbound products would be dispersed higher in the atmosphere without producing too much of a distinctive fallout pattern.
https://en.wikipedia.org/wiki/Cobalt_bomb
A theme for example in Nevil Shute's On the Beach. Where people in Australia spent their last days waiting for the cloud to eventually reach them.
"If you take, say, fifty H-bombs in the hundred megaton range and jacket them with Cobalt-Thorium G, when they are exploded they will produce a doomsday shroud. A lethal cloud of radioactivity which will encircle the earth for ninety-three years!"
It's a fantastic movie that I highly recommend to anybody that hasn't seen it yet.
[0] https://en.wikipedia.org/wiki/Cobalt_bomb#In_popular_culture
The idea being, produce a nuclear weapon that has a extreme radiation pulse during detonation but generates no long lived nuclides. Detonate over enemy territory, everyone dies from radiation sickness from the detonation, then you can move in and all the infrastructure is still in place.
Not much development ever took place, considering such a thing could only ever be an offensive/invasion weapon.
About ten years ago Chinese general stated that in a nuclear war they would use a curtain of ground bursts on the Western coast of the US (and presumably also the UK) to destroy the population.
There are semi-classified maps from the late 50s which show that most of the UK would have been turned into a desert in an old fashioned fission bomb war. (They were posted online about fifteen years ago. I downloaded them but lost them in a drive crash.)
It's also known strategy to target power reactors with ground bursts to spread even more fallout and kill even more people for longer.
Anyone suggesting "Nuclear war - not so bad actually" is delusional.
Tsar Bomba was very clean relative to the yield.
Even then, it release more radionucleonides than any bomb before it (airburst). They limited the yield to 50 megatons, because fallout in Europe would have been catastrophic from a bigger 100 mt bomb that was allowed by the design.
As it is, Tsar Bomba was air-dropped from a specially modified Tu-95 Bear, the Tu-95V, which had its engines, bomb bay, suspension and release mechanisms redesigned and fuel tanks and bomb bay doors removed to lighten it:
https://en.wikipedia.org/wiki/Tsar_Bomba#Development_of_the_...
The bomb was dropped at 10,500 metres altitude and descended by parachute to 4000 metres before detonation, by which time the carrier aircraft had travelled 39 kilometres. Even so, when the shockwave from the bomb caught up with it the plane dropped a kilometre, although the pilots managed to recover. A US aircraft in the vicinity had its paint scorched.
And that was the ~50Mt version.
It's highly unlikely that a carrier aircraft could have survived a 100Mt bomb. Or that it could have reached targets in CONUS when flying from Warsaw Pact territory. Or that a parachute-retarded bomb gently descending towards a target in US airspace during a shooting war could have survived for several minutes without being blown apart by a surface to air missile (which would almost certainly have disrupted its ability to deliver a full, or even partial, explosion).
And as the weapon weighed 27,000kg the USSR would have had problems building an ICBM able to carry a re-entry shielded version (to avoid being shot down).
So: not a practical weapon, but it really gave Nikita Kruschev something to wave in JFK's face.
50 mt weapon and 100 mt weapon were the same design, same size. The yield was just tuned down with lead. Not enough range for the bomber to be useful.
If you read Sakharov's memoirs, the whole thing was a dog and pony show for international politics. No technical side was consulted before the decision was made and better proposals were turned down. There was no military use or scientific need to make it. When the US and Brits started testing in 1958, Khrushchev wanted Big Bomb to show off.
They still have a role for an EMP attack or even a thermal pulse attack. A gigaton range bomb in orbit will fry electronics to the horizon and light fires to the horizon.
Tsar Bomba was detonated at an altitude of 4Km, the shock wave prevented the fireball from reaching the ground and producing a crater, so it produced less fallout.
https://en.wikipedia.org/wiki/Tsar_Bomba https://en.wikipedia.org/wiki/Thermonuclear_weapon https://en.wikipedia.org/wiki/Nuclear_weapon_design