Neil deGrasse Tyson says nuclear bombs no longer produce fallout. From the video Neil deGrasse Tyson: modern hydrogen nuclear weapons don’t have the radiation fallout problem of WW2:

“Modern nukes don’t have the radiation problem... it’s a different kind of weapon than [what was used in] Hiroshima and Nagasaki”

“Nuclear weapons if they’re exploded don’t have a radiation problem?”

“Not if it’s hydrogen bombs, no... not in the way we used to have to worry about it, with fallout and all the rest of that...”

According to fallout maps, I am downwind of a major military compound. Also, just generally, people think of nuclear devices as producing fallout.

Do modern nuclear weapons produce no nuclear fallout?


3 Answers 3


I think the claim is misleading in several ways. First, it's possible to design a "fusion bomb" that maximizes (neutron) radiation output, leading to substantially higher radiation than from a fission device. "The neutron release generated by a nuclear fusion reaction is intentionally allowed to escape the weapon, rather than being absorbed by its other components" to quote Wikipedia on that. And consequently "It is expected that after neutron weapon explosions the induced radioactivity in the soil surface layer, metal objects or metal constructions is about 10 times higher, than after an atomic [=fission] blast of the same power."

Whether this induced radiation should (not) count as 'fallout', YMMV. As that paper explains, what kind of soil gets radiated matters in such contexts.

But I'm guessing NdGT means to leave those neutron bombs aside from the discussion and focus on "proper hydrogen" weapons that harness the neutrons for the 'biggest boom possible' rather than work through escaping/induced radiation.

'Clean' fusion weapons had been advocated e.g. around 1957.

But I don't think they were actually built like that in significant numbers. Because it's more useful to make the device a 3-state/phase one, where the outer casing is (depleted) Uranium 238. The fusion-generated neutrons are energetic enough to turn this 3rd stage into a fission bomb layer of its own. This e.g. the path that China's "H bomb" programme took (which ironically was easier to find info about on a quick search in English):

The so-called “three-phase” nuclear device at the time was a layer-cake-type device, which consisted of three phases: fission, fusion, and then fission again. During the first phase, the highly-enriched uranium or plutonium core generated the fission explosion. The second phase was the fusion reactions of the layer of solid thermonuclear fuel (lithium deuteride) surrounding the core. Tritium was generated when neutrons from the first phase explosion bombard the lithium. Meanwhile, the high temperature generated by the fission explosion caused the fusion reactions of the deuterium and tritium. The third phase was the fission reaction of the layer of uranium 238 (natural uranium or depleted uranium) surrounding the thermonuclear fuel. The very-high energy neutrons released from the fusion reactions can fission uranium 238. For a layer-cake-type model, additional layers of thermonuclear materials and uranium 238 could be used. [...]

The device 629’s primary was similar to that of the first atomic bomb device 596 tested in 1964, which had a solid uranium 235 core with a uranium deuteride neutron source and no thermonuclear materials. The device 596’s yield was set to be about 20 kilotons, whereas the device 629’s design yield was about 100 kilotons. [Achieved 122kt.] This limited total yield required the secondary to host less thermonuclear materials and use lead metal to replace the natural uranium. [...]

The theoretical design of the device 639 was relatively straightforward, as mainly based on that of the low-yield device 629. The primary would use the same design as the device 629, whereas the secondary only needed a few modifications from the 629, such as adding more lithium deuteride materials to have a full yield and replacing the lead metal with natural uranium. The design yield ranged from one-and-a-half to three megatons. By February, the weapon designers had completed the basic theoretical design of the hydrogen bomb 639 and, by May, the design was finalized and ready to be manufactured.

On June 5, the processing and manufacturing of the first device 639 was complete. Following the two-year plan, Plant 221 in the Qinghai province prepared a total of eight test bombs. [...] Based on post-test analysis of various measurements, including radiochemical analysis of air samples, the explosive power of the hydrogen bomb 639 was estimated at 3.3 megatons.

The only diff here compared to the other source[s] is that China used natural rather than depleted Uranium for the outer casing (of the secondary). This layer is also called a "pusher" on Wikipedia (which also concurs that this was a "three-staged thermonuclear device".)

Alas, I don't have any stats as to how many bombs are like this (3-phase/stage), but it's not at all a given the 'modern' ones will not have this 3rd fission stage, given the research and testing that went into it.

Exactly how much fallout comes out of this 3rd layer stage, I'm not totally sure, but there certainly are publications that claim the exact opposite of what NdGT claims, i.e. that's highly significant:

The fallout produced in a nuclear explosion depends greatly on the type of weapon, its explosive yield, and where it’s exploded. The neutron bomb, although it produces intense direct radiation, is primarily a fusion device and generates only slight fallout from its fission trigger. Small fission weapons like those used at Hiroshima and Nagasaki produce locally significant fallout. But the fission-fusion-fission design used in today’s thermonuclear weapons introduces the new phenomenon of global fallout. Most of this fallout comes from fission of the U-238 jacket that surrounds the fusion fuel. The global effect of these huge weapons comes partly from the sheer quantity of radioactive material and partly from the fact that the radioactive cloud rises well into the stratosphere, where it may take months or even years to reach the ground.

(Italics around 'global fallout' in original.)

N.B. the US army (NUCLEAR MATTERS HANDBOOK 2020 revised) doesn't describe such [designs] as 'three stage'... but it does talk of fission being significant in the secondary as well.

Boosted Weapons[:] A boosted weapon increases the efficiency and yield for a weapon of the same volume and weight when a small amount of fusionable material, such as deuterium or tritium gas, is placed inside the core of a fission device. The immediate fireball, produced by the supercritical mass, has a temperature of tens of millions of degrees and creates enough heat and pressure to cause the nuclei of the light atoms to fuse together. In this environment, a small amount of fusion gas, measured in grams, can produce a huge number of fusion events. Generally, for each fusion event, there is one high-energy neutron produced. These high-energy neutrons then interact with the fissile material, before the weapon breaks apart in the nuclear detonation, to cause additional fission events that would not occur if the fusion gas were not present. This approach to increasing yield is called “boosting” and is used in most modern nuclear weapons to meet yield requirements within size and weight limits. In general, the boosted weapon design is more technically complex than the implosion design and also more efficient.

Staged Weapons[:] A staged weapon [...] normally uses a boosted primary stage and a secondary stage to produce a significantly increased yield. In the first stage, a boosted fission device releases the energy of a boosted weapon, which includes a large number of X-rays. The X-rays transfer energy to the secondary stage, causing fusionable material in the secondary to undergo fusion, which releases large numbers of high-energy neutrons. These neutrons, in turn, interact with fissionable material in the secondary to cause a huge number of fission events, thereby significantly increasing the yield of the whole weapon. The two-stage weapon design is more technically complex than any other weapon design. For a given size, it can produce a much larger yield than any other design.

There's no mention there of the 'clean' designs that would not use fission in the secondary.

They do discuss fallout quite bit though. I'm not sure what would be the most relevant part to quote. Maybe this, which partly agrees with the MIT quote I emphasized previously:

If the detonation is a true air burst in which the fireball does not interact with the ground or any significant structure, the size and heat of the fireball causes it to retain almost all of the weapon debris, usually one or at most a few tons of material, as it moves upward in altitude and downwind. In this case, very few particles fall to the ground at any moment and no significant radioactive hot-spot on the ground is caused by the fallout. The fireball rises to become a long-term radioactive cloud. The cloud travels with the upper atmospheric winds and circles the hemisphere several times, over a period of months, before it dissipates completely. Most of the radioactive particles decay to stable isotopes before falling to the ground. The particles that reach the ground are distributed around the hemisphere at the latitudes of the cloud travel route. Even though there would be no location receiving a hazardous amount of fallout radiation, certain locations on the other side of the hemisphere could receive more fallout, which is measurable with radiation detectors, than the area near the detonation. This phenomenon is called worldwide fallout.

(Italics emphasis in original again.) So, I guess it's a matter of perspective whether you care about local or global fallout. However, NdGT doesn't seem to have this tradeoff in mind, at least based on the short snippet quoted by the OP. The Handbook then discusses surface detonations:

As large and hot as the fireball is (1-kt detonation produces a fireball almost 200 feet in diameter and tens of millions of degrees), it has no potential to carry thousands of tons of material. Thus, as the fireball rises, it begins to release a significant amount of radioactive dust, which falls to the ground and produces a radioactive fallout pattern around GZ and in areas downwind. The intensity of radioactivity in this fallout area would be hazardous for weeks. This is called early fallout, caused primarily by a surface-burst detonation regardless of the weapon design. Early fallout would be a concern in the case of employment of a nuclear threat device during a terrorist attack. [...] If a detonation is a surface or near-surface burst, early fallout would be a significant radiation hazard around GZ and downwind.

(Italics of "early fallout" in original, bold around "regardless of the weapon design" mine.) So, I guess the issues that really matter wrt fallout are somewhat different than what NdGT envisaged.

  • N.B. It would be useful to have somewhat more a quantitative answer then this kind of qualitative ones based on 'quote from some authority', but I'm not sure what a fair comparison would be given the yield differences between staged (and unstaged) weapons. I mean some comments suggested NdGT implies a "less fallout per kt" kind of comparison, but I'm not sure (upon watching the segment myself) that he means it like that, rather than an absolute reduction for a "hydrogen bomb" hit. Commented May 30 at 4:53
  • The exchange with host does touch on global fallout towards the end and NdGT denies that it has any relevance, so I suppose it's worth more touching on that than just based on the OP's [selected] quote. I suppose one might need to quantify that too, because spread over the hole planet one might deem it more 'acceptable', in terms of concentration at any given point. Commented May 30 at 5:06
  • NdGT however replies that if e.g. India and Pakistan would use '<word> bombs", then fallout would 'come to' the US. So he is familiar with the notion of global fallout. However, I cannot make it from the [quick] pronunciation if he says 'fission' or 'fusion' there for <word>. In any case, nobody seem to stock up on 'clean' fusion bombs, so it probably doesn't matter exactly what he said there for <word>. Commented May 30 at 5:11
  • I guess the document doesn't count depleted or natural uranium as radiactive enough. And they are somewhat right. Physics laws make that only two types of radiactive atoms can exist: those who are very radiactive (and hence dangerous), and those who are radiactive for long time. With a half-life of 5 billion years, uranium 238 qualifies at "not radiactive at all". If most of the fallout in 3-stage bombs comes from the U238 outer layer, as your link says, then I understand why they call these bombs "clean". Due to global dispersion, the average radiactivity issues would be at homeopathic levels
    – Rekesoft
    Commented Jun 3 at 8:54
  • @Rekesoft: you severely misunderstand what [fusion] induced radiation does to depleted Uranium. Commented Jun 3 at 8:56

Regardless of the type of weapon, the major consideration for fallout is...

Ground or airburst?

While the fission products of the bomb itself are bad, another consideration is how high above the ground the weapon was detonated. An airburst high above the ground will produce only fallout from the weapon's own fissile materials and neutron activation of the casing.

Neutron activation is when stable, non-radioactive materials are bombarded with neutrons (say, from a nearby nuclear blast) and become briefly radioactive. This then rains down and becomes fallout.

But a detonation near the ground, or surface of the water, will suck up material and neutron activate it. This can be tons and tons of material. It is lifted high into the atmosphere by the up-well of hot air from the blast and can be blown by the wind for miles and miles, sometimes falling as rain.

The following is a simulation of an airburst of a W-87 300 kiloton bomb over Portland, Oregon. The W-87 is a current US thermonuclear (fusion) weapon. Note the lack of fallout.

enter image description here

And again this time on the surface. A fallout plume of 100 rads/hour (the bright orange plume) extends over 100km from the blast carried by 24km/hr winds. This is enough to cause acute radiation syndrome within a day.

enter image description here

  • 1
    Not my DV, but why is "nuclearsecrecy.com" a reliable source for such simulations, esp. wrt to details like fallout? It's written by a historian, AFAICT, so their physics modelling may or may not be good. And how high is "airburst" in that [first] simulation? N.B. it's somewhere in the fine print "Detonation altitude: 2,090 m." Commented Jun 3 at 11:54
  • But "too high to produce significant local fallout" is not the same as there being no global fallout from the explosion. Commented Jun 3 at 12:00
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    @againstverylongusernames They have a FAQ including how it works, the physics of fallout is not super complicated. And it won an award from the NSF.
    – Schwern
    Commented Jun 4 at 1:32
  • 1
    Yeah, the general theory coincides with what I've quoted from a US Army/DOD publication in my own answer. I was wondering about the details, how accurate those site predictions might be. Thanks for clarifying some of that. Commented Jun 4 at 9:37

A hydrogen bomb works by using nuclear fusion, hydrogen is fused into helium. These two elements are in the top row of the periodic table and there are no radioactive isotopes or nuclear fallout from that reaction.

However, it requires an enormous amount of energy to start off hydrogen fusion and the only way this has been technically achieved in a bomb is using a convention nuclear bomb with Uranium or Plutonium as fuel and this stage will produce radiation and nuclear fallout.

In the words of wikipedia quoted above:

Modern fusion weapons essentially consist of two main components: a nuclear fission primary stage (fueled by 235 U or 239 Pu ) and a separate nuclear fusion secondary stage containing thermonuclear fuel: heavy isotopes of hydrogen (deuterium and tritium) as the pure element or in modern weapons lithium deuteride. For this reason, thermonuclear weapons are often colloquially called hydrogen bombs or H-bombs.

So possibly there is less nuclear fallout because the main explosive power of the bomb doesn't come from fission but rather from fusion but the claim that there is no radiation problem in a hydrogen bomb is just false for all hydrogen bombs that have been build so far.

  • 5
    "These two elements are in the top row of the periodic table and there is no radiation or nuclear fallout from that reaction." It would be more correct to say that there are no radioactive isotopes from the reaction, but there is most definitely a (thermal) radiation, which is the point of the fusion reaction.
    – Entropy
    Commented May 28 at 9:29
  • 4
    This answer is inaccurate for multiple reasons. The hydrogen bomb is based upon lithium-6 deuteride. Production of radioactive tritium in an integral part of the the fusion mechanism. When tritium and deuterium fuse, neutrons are released. Then some of the neutrons are absorbed by material outside the bomb creating radioactive material. When the fission component of the bomb is less than 30% "neutron-induced radioactivity becomes a significant factor" ncbi.nlm.nih.gov/books/NBK219147
    – DavePhD
    Commented May 28 at 11:56
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    Please provide some references to support your claims. "It is in the name" is not sufficient.
    – Oddthinking
    Commented May 28 at 13:05
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    Modern thermonuclear bombs have much larger yields than the original fission bombs, you cannot ignore that aspect. You'd also have to distinguish whether you're talking about fission-fusion-fission bombs with a uranium tamper or bombs with an inert tamper. And then you'd have to show which types are actually most common in modern nuclear arsenals to answer this question.
    – Mad Scientist
    Commented May 28 at 20:47
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    @quarague but still you shouldn't include false information like "there are no radioactive isotopes or nuclear fallout from that reaction" and "because the main explosive power of the bomb doesn't come from fission but rather from fusion" in your answer. Tritium is a radioactive hydrogen isotope and the hydrogen bomb (unlike the Sun) relies upon tritium. Aside from radioactive tritium, there is neutron induced creation of radioactive isotopes and fallout of these isotopes.
    – DavePhD
    Commented May 29 at 15:30

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