How clean was the "Tsar bomb"?

[nomadreid]

quoting from https://en.wikipedia.org/wiki/Tsar_Bomba about the "tsar bomb":
To limit fallout, the third stage and possibly the second stage had a lead tamper instead of a uranium-238 fusion tamper (which greatly amplifies the reaction by fissioning uranium atoms with fast neutrons from the fusion reaction). This eliminated fast fission by the fusion-stage neutrons, so that approximately 97% of the total energy resulted from fusion alone (as such, it was one of the "cleanest" nuclear bombs ever created, generating a very low amount of fallout relative to its yield).
In the Russian Wikipedia article, it quoted Soviet scientists as saying that after two hours the level of radiation at ground zero was safe enough for a human being. Given that this was not internationally monitored, any statement like that should be taken with a grain of salt, but one of my questions is whether, given the above explanation, this is credible? My second question is that if this is feasible, then is it a cost factor alone that makes the nuclear superpowers make the standard fusion bomb "dirty"?

 

[SteamKing]

quoting from https://en.wikipedia.org/wiki/Tsar_Bomba about the "tsar bomb":
To limit fallout, the third stage and possibly the second stage had a lead tamper instead of a uranium-238 fusion tamper (which greatly amplifies the reaction by fissioning uranium atoms with fast neutrons from the fusion reaction). This eliminated fast fission by the fusion-stage neutrons, so that approximately 97% of the total energy resulted from fusion alone (as such, it was one of the "cleanest" nuclear bombs ever created, generating a very low amount of fallout relative to its yield).
In the Russian Wikipedia article, it quoted Soviet scientists as saying that after two hours the level of radiation at ground zero was safe enough for a human being. Given that this was not internationally monitored, any statement like that should be taken with a grain of salt, but one of my questions is whether, given the above explanation, this is credible? My second question is that if this is feasible, then is it a cost factor alone that makes the nuclear superpowers make the standard fusion bomb "dirty"?

Irrespective of the merits of using a U-238 tamper v. a lead tamper, the fallout from the Tsar Bomba was reduced because it was an air burst. The bomb was reportedly detonated at an altitude of 4000 meters, so high that the fireball from the detonation did not reach the ground, which reduced the amount of radioactive fallout created. This fact probably allowed prompt inspection of the site following the blast, otherwise the ground would have been too "hot" to permit inspection without some sort of radiation shielding being utilized.

You could take the same device and detonate it either from a shot tower or, more deadly as was discovered at Bikini, underwater, and create all sorts of radioactive fallout. The design of the bomb is but one factor which determines how much radiation is released on detonation. Where the bomb is detonated and other environmental factors determine the amount of fallout and how that fallout is dispersed.

OTOH, the enhanced radiation weapon, the so-called neutron bomb, is typically a very low-yield device, in most cases probably no more than a few hundred tons of TNT, so that damage due to blast effects is limited. This device's lethal effect comes primarily from the large flux of neutrons which will kill any humans or animals which are not shielded from the blast due to radiation poisoning within a few days.

 

[nomadreid]

Thanks, Stem King. The height put an added factor in the "equation" for ending up with low radioactivity at ground level. Of course, one cannot claim that the height alone was to thank for the small radiation, because for a normal "dirty" bomb of even much less yield, the lethal radiation dose extends far beyond 4 kilometres. So my question was effectively about the possibility of reducing the radiation down to a level so that at four kilometres there would be negligible radiation. I see that the US also carried out some tests with "clean" bombs, so I guess the Soviet data might be backed up by the US data. However, there is the app "NUKEMAP" http://nuclearsecrecy.com/nukemap/ which states that harmful radiation from the Tsar Bomba would stretch out at a radius of approximately 5 kilometres from the blast. As you say, there are a lot of other factors besides height and construction, so 5 km has to be taken with a wide margin, but nevertheless I am wondering... if an area of ground was irradiated, what sort of half-life would the irradiated ground have? Two hours seems a bit small.

 

[mfb]

It was clean relative to its yield. 97% of 50 MT from fusion still means 1.5 MT from fission - a huge amount, more than the most powerful fission-only devices.

 

[SteamKing]

Thanks, Stem King. The height put an added factor in the "equation" for ending up with low radioactivity at ground level. Of course, one cannot claim that the height alone was to thank for the small radiation, because for a normal "dirty" bomb of even much less yield, the lethal radiation dose extends far beyond 4 kilometres. So my question was effectively about the possibility of reducing the radiation down to a level so that at four kilometres there would be negligible radiation. I see that the US also carried out some tests with "clean" bombs, so I guess the Soviet data might be backed up by the US data. However, there is the app "NUKEMAP" http://nuclearsecrecy.com/nukemap/ which states that harmful radiation from the Tsar Bomba would stretch out at a radius of approximately 5 kilometres from the blast. As you say, there are a lot of other factors besides height and construction, so 5 km has to be taken with a wide margin, but nevertheless I am wondering... if an area of ground was irradiated, what sort of half-life would the irradiated ground have? Two hours seems a bit small.

BTW, the handle is SteamKing, not StemKing.

The immediate danger from such a large-yield bomb may not be related to radiation at all, regardless of how or where it was detonated. Such a bomb detonation yields a certain percentage of the energy of the blast in the form of heat, and since the atmosphere is quite transparent to that form of energy, the thermal pulse can be damaging to life many kilometers away from the point of detonation. For example, the Tsar Bomba was estimated to have released enough energy that it would have produced third-degree burns on exposed skin some 100 kilometers from the detonation point. The blast pressure pulse was capable of breaking glass some 700 km away, and again damage was recorded at points in Finland and Norway which was attributed to certain atmospheric focusing of the emitted pressure pulses.

As far as irradiation of the surrounding area, that's going to depend on the mineral composition of the affected land and how heavily irradiated it gets. Like all poisoning, radiation poisoning depends on the amount or the dosage received, and one way to limit the dosage is to limit the duration of exposure. Sure, Soviet scientists may have examined the are around the blast a couple hours after it happened, but how long did they stay in the area? It's not like they built a town on top of the rubble and settled in for the duration.

As careless as the Soviets were in certain cases of handling radioactive material, they had to be careful here since a large part of their own country was downwind of any potential fallout created by the detonation of Tsar Bomba. I don't think their goal here was to demonstrate any sort of "clean" nuclear device, but it was Khrushchev's way of dramatically pounding his shoe on the podium to get the world's attention not to interfere in Soviet affairs or to thwart any international goals the Soviet Union might have had to preserve or extend its influence. Its detonation came soon after the Bay of Pigs invasion in Cuba to topple Castro and after the Berlin Wall was erected to keep people from fleeing from East Germany to the West. The increase in tensions between East and West escalated to such a point that within a year, there was almost the outbreak of nuclear war between the US and the USSR over the installation of missiles in Cuba aimed at the US. It was after this last incident that Khrushchev's days in power were numbered, as others inside the USSR decided it was better to remove him as Premier than to risk the outbreak of nuclear war with the West.

 

[Nuclear Cowboy]

Well there's really two different types of "radiation" being discussed here. When the bomb detonated, of course you have a massive amount of light radiation (gamma, x-ray, visible, infrared heat radiation, etc.) and at the same time you're going to have the prompt neutron radiation from the final generation of neutrons in the fission primary, and you're also going to have prompt neutrons being emitted as a fusion byproduct (because for every deuterium-tritium fusion, the product is helium-4 and 1 extra neutron).

All of that radiation is DONE doing its damage within a fraction of a second after the blast.

The radiation left behind is what you're really asking about, no? Fallout.