How light could a nuclear warhead get?

The W54 warhead weighs about 23kg and has a yield of up to 1kiloton of TNT. Is it possible to make a nuke light/smaller than that? If so, what kind of yield could it have?

What about thermonuclear (fusion) warheads, how light/small could they get and what kind of yield could they then have?

Are there step chances in yield? Iow: Once you reach a critical threshold of warhead weight, yield starts increasing more than linearly?

I’m not including dirty bombs as nukes.

Is there some data on which yield levels correspond to which effective radii?

Elements like Uranium and Plutonium are very dense and thus very heavy. Then, you have to add the weight of the shielding, which is a very dense substance in and of itself. The fact that they have made a baby nuke that a person can actually carry around is amazing in and of itself. If don’t think you expect any more than that in terms of “lightness”.

It seems unlikely that fission nukes can get lighter than the W54 without sacrificing an inordinate amount of yield. Someone else might know better than me, though.

How about thermonuclear weapons? They’re more complex so their minimum weight should be higher but their yield much higher.
How about neutron bombs which presumably don’t need neutron reflectors, how light can they get?

To get a much lighter weapon, you need either:
A) Much more powerful explosives, to compress a smaller mass into criticality or
B) A core made from an element with a smaller critical mass.

(A) isn’t likely to happen, unless there is a breakthrough in chemical explosives. (B) could be done using something like Californium-251, if you had an unlimited budget.

The W54 is near the practical size limit for an implosion weapon, so I doubt you’re going to get much lighter than that, save through the use of modern materials and/or design techniques which might shave off a little bit.

The problem with fusion weapons and enhanced radiation weapons (neutron bombs) is that they require a regular old fission bomb as the trigger. The main difference between them is that regular fusion weapons are intended to maximize blast/flash, while the neutron bombs are intended to maximize the amount of prompt radiation that the bomb emits- mostly in the form of energetic neutrons.

So either way, you’re starting with a W54 as the smallest possible trigger and going up from there.


So, what would be the thermonuclear equivalent of the W54?

Does a nuke need to implode to emit prompt radiation?

Not sure on the first one, but the answer to the second is no. They ALL emit prompt radiation, but there’s an inverse relationship between the relative ranges of the blast/heat and radiation as the size of the bomb goes up. For smaller nukes like the W54, the radiation reaches out to about 430 meters in a seriously injurious fashion, while the 5 psi blast radius (50% of people killed) is about 120 meters, and the heat radius (3rd degree burns) is 140 meters.

At 100 tons, they go to 0.56 km/210m/230m, and at 15 kilotons (Little Boy), they’re 1.2 km/1.67km/1.91km.

At 6.5 kilotons, the heat and radiation radii are equal (1.16 km), and the 5 psi blast radius is 0.85 km. At 42 kilotons, the radiation and blast radii are equal at 1.59 km, and the heat radius is much larger at 2.66 km. From there, the radiation will always be the smallest, followed by blast, and then radiation.

*the above numbers are from Nukemapand for a surface burst.

So the upshot is that they all do emit radiation, but in general it’s a small percentage of the total energy given off- something like 5%. In an enhanced radiation bomb, it’s more like 30% in the form of prompt radiation. But it’s still a nuclear warhead with the correspondingly ridiculous blast and heat.

There seems to be some confusion about the basic concepts behind the construction and function of nuclear devices. First, I’d recommend that you read some of the technical sections of Dr. Carey Sublette’s Nuclear Weapon Archive. Implosion is just a means of rapidly forming a critical mass sufficient to momentarily sustain a chain reaction, or of increasing density and temperature around fusible material sufficient for it to begin fusion.

As far as the thermonuclear equivalent of the W54, I am interpreting what you are asking for as, “what is the smallest nuclear device wherein a majority of the energy budget comes from fusion, not fission?” I do not know whether that is a device where fusion energy comes from tritium injection into the pit—AIUI, that’s the case for nearly all ‘fission weapons’ where minimum size isn’t the utmost primary design characteristic—or whether the energy comes from radiation pressure (generated by a fission device, but see the wiki on pure fusion weapons) upon some deuterated or tritiated compound. I am guessing that the answer to your question is a nominally fission device, yet with increased performance by fusing tritium. Perhaps something like the B61 with a variable yield from 0.3 kt to 400 kt? FWIW, and Dr. Sublette goes into this at length at at the archive, the W54 is an amazingly inefficient device. A lot of fissile material had to be used in order to achieve the low explosive yield of the W54.

All ‘prompt radiation’, is simply the radiation that is released at or near the moment of detonation, by the device itself, or by the fireball it forms, versus radiation that continues to be emitted long after detonation. It is broad-spectrum, from gamma and x-rays, to visible light, infrared radiation, and it wouldn’t surprise me if some of the radiation was RF. Some of it is also material, and not part of the EM spectrum, like neutrons (more on that in a sec), beta particles, and alpha particles. All nuclear devices release prompt radiation, IOW.

When writing about the effects of nuclear explosions, prompt radiation usually refers to ionizing radiation, and excludes the effects of infrared radiation. It is usually only applicable for casualty concerns for devices of low yield, say 10 kt or lower, as the effects of thermal radiation and airblast scale much faster than do the effects of prompt radiation. At Operation Crossroads, for Test Able the airdropped fission device was small, 23 kt. The heat and blast from it did not significantly harm test animals within the gun turret of the battleship Nevada, but prompt radiation did give the animals lethal radiation doses, 10,000 rem according to the wiki.

An enhanced radiation weapon is a two-stage, fission-fusion thermonuclear device. It has a small primary fission device, just large enough to cause some fusion in the secondary stage. Both stages are constructed in a way as to emphasize radiation emission, rather than utilizing the radiation through tampers or reflectors, to perform more fission or fusion. It therefore has all of the prompt radiation from a fission device, plus it emits fast neutrons created by fusion.

Neutrons emitted from fusion in nuclear weapons have more energy, as a whole distribution, than the distribution of neutrons emitted from U-235 or Pu-239 fission. These higher energy neutrons can, unlike neutrons emitted from fission, appreciably fission U-238, which is the third stage of a multi-stage thermonuclear device, or their higher energy allows them to travel greater distances through materials like air or armor plate. With the addition of those fast neutrons, prompt radiation is increased enough that its lethal radius exceeds the lethal radius from either its thermal radiation or airblast.

So, yes, an enhanced radiation device requires an implosion. But that is only because efficient fission devices require implosions to form their critical masses, and an enhanced radiation device requires a fission device for the energy to achieve fusion.

Cf-252 was the one I’d read about from people like Dean Ing. The spontaneous fission rate of that one is a bit sporty though, at 1 microgram of material emitting 2.3 million neutrons/second. Makes the Pu-240 problem seem like nothing. Interestingly, in that list of critical masses you cited, Np-236 is one of the smallest, at 6 kg. Probably easier to create in a reactor than Cf, though the wiki for it claims it’s essentially inseparable from the less desirable Np-237. Though I wonder if improved laser separation processes like MLIS or SILEX make such a thing more feasible now?

I have no cite. But I have worked in the nuclear industry for a long time (nowhere near weapons, though; drugs) and I’ve worked with a number of people involved in weapons systems. I’ve been told more than once that the practical lower limit for a fission core is about 11kg

I wonder…
If a very compact high-flux neutron generator was invented, one could use it to fission an arbitrarily-small piece of Uranium or Plutonium.

Here are theoretical minimum amounts of elements needed for fission.

If you can get your hands on Californium-262, all the terrorists would love you.