Why No One Will Ever Build Another Nagasaki Type Bomb

This is a guest post on behalf of ACW reader and occasional contributor Chris Camp.

The first Atomic bombs, Trinity, Hiroshima, and Nagasaki, hold an outsized place in our perceptions of what a nuclear weapon should be.  Certainly they were notable as the first bombs, the only ones used in anger, and the most famous devices in a subject shrouded in secrecy, but times have moved on while perceptions largely have not.  When we talk about cars people don’t think of the Benz Patent-Motorwagen, when we discuss airplanes the Wright Flyer isn’t the first thing that comes to mind, yet when you mention an atomic bomb odds are that one of the WWII devices is what people will think of.

This perception came up on Arms Control Wonk in an article on the 1973 Yom Kippur war by Avner Cohen in which he states:

[I]t is plausible that on the eve of the 1973 War Israel had a small nuclear inventory of weapons, say, between ten to twenty first-generation fission (PU) weapons (roughly, Nagasaki-type). One could speculate further that most of the inventory was in the form of aerial bombs (probably configured for the Mirage) and some were early prototypes of missile warheads for the Jericho I (which in October 1973 was apparently not yet operational).

This led to a discussion on what the Israeli arsenal might have actually looked like and whether a “Nagasaki type” bomb was in fact a reasonable assumption for a fledgling nuclear weapons state in 1973.

The bombs used by the United States in World War two were very much a product of their times and the environment in which they were created.  They were not the best design Los Alamos could produce, even when they were built, but they were the quickest ones that could be gotten out the door.  The “Little Boy” bomb dropped on Hiroshima was officially designated a Mark 1 bomb and the Nagasaki bomb, while not having an official designation, can be thought of as a Mark 3 prototype (The Mark 2 was a plutonium gun design that turned out to be unfeasible due to pre-detonation of plutonium 240).  I say prototype because the production Mark 3 that formed the basis of the first US stockpile had several improvements over the actual wartime weapons.  Both the Mark 1 and Mark 3 were complex, dangerous and wasteful designs that were largely obsolete before they were ever used, but were products of the state of both the weaponeers art and also of American nuclear industry at the time.  To put it another way, they were products of the circumstances in which they were created, and those circumstances would not apply to any nation building a bomb since then.

I’m going to talk primarily about the Mark 3 prototype designs used at Trinity and Nagasaki since the Mark 1 was literally a one-off device which has never been replicated, though many of the points apply to it as well.

The Fat Man device was 60 inches in diameter, 128 inches long, and weighed 10,300 lbs.  It used about 6 kilograms of plutonium in the form of a three piece sphere (Two hemispheres and a sort of wedge shaped equitorial piece) with a spherical cavity about half an inch in diameter in the center for a Polonium – Beryllium initiator.  Surrounding the plutonium was a two part sphere of natural uranium tamper, and then around that was layers of different types of explosives totaling about 18 inches thick.  The outermost layer of explosives was covered in an array of 32 detonators which were hooked to a firing mechanism called an X-unit which set them all off with great precision.  The fusing was accomplished by four radar fuses, called “Archies” which were modified prototype tail warning radars for fighter aircraft.  The whole device was powered by lead acid batteries which lasted 2 days on a full charge, and the entire bomb had to be disassembled to charge or replace them.  The casing was made from 3/8” thick steel and accounted for almost half the weight of the finished bomb.  At the back was a large boxy, high drag tail which was necessary because the ballistic shape of the bomb was massively unstable and it tended to tumble so much while falling that the radar fuses couldn’t see the ground.  Finally there was a set of 4 impact fuses on the nose which were meant more to destroy the bomb if the radar fuses failed and it hit the ground rather than to cause an actual nuclear detonation.  Once assembled, the Mark 3 prototypes were very dangerous and almost certainly would have suffered a low order nuclear detonation in the event of an aircraft crash.  Assembling the bombs took a crew of 50 people close to 18 hours and once assembled the bomb had to be dropped within 48 hours or the batteries would die and the entire device would have to be disassembled.  The bomb makers art has come a long way since the dark days of 1945 and many of these advances would be incorporated by any nation building a bomb for the first time.  Here are some of those advances and how they apply to the nature of a first device.

Levitated pits.  The biggest improvement in weapons efficiency came from a concept that was well understood at Los Alamos before the war ended, but didn’t make it into the wartime bombs.  This was the concept of a “Levitated” pit that greatly increases compression and therefore efficiency.  The levitation concept as it applies to nuclear weapons is a bit unintuitive, so let me use an analogy pioneered by Hans Bethe after the war.  When you go to hammer a nail do you put the hammer on the nail and push, or does it work better to swing the hammer and get some momentum going before you hit the nail?  The Mark 3 design was the equivalent of putting the hammer on the nail and pushing.  All of the components of the core were in physical contact and there was no room for the implosion to build up momentum before it hit the core.  In a levitated pit design there is an air gap between the uranium tamper and the plutonium pit which gives the tamper room to accelerate and build up momentum before striking and compressing the pit.  Not only does it allow more energy to be delivered to the pit, it also tends to smooth out irregularities in the shock wave, both leading to increased efficiency.  The first levitated pit design, the Mark 4, had the pit suspended on wires inside the tamper, but later designs used a stand made of thin aluminum to support the pit.  To visualize this, think of the plutonium core as the ice cream on an ice cream cone, except the cone is upside down and the ice cream sits on the pointy end.

Explosives.  The Mark 3 prototypes used an outer fast-burning layer of Composition B high explosive over a middle layer of slow burning Barotol and then another layer of Comp B.  There were 32 of these “explosive lenses” and the whole system was designed to create a perfectly spherical shock wave that would evenly compress the core allowing the fission reaction to take place.  One early avenue of post war research was into more efficient high explosives which could generate more compression with less weight and bulk.  Modern high performance explosives can deliver greater performance in a layer only 1-2 inches thick and weighing on the order of tens of pounds.  In addition, Insensitive High Explosives (IHE) have been developed which are much more difficult to detonate accidentally, making for much safer bombs in the event of an aircraft crash.  IHEs are used in most US nuclear and conventional aircraft bombs, and the guidance given to crash rescue crews is that even if the bomb is fully engulfed in fire, it is safe to attempt to extinguish the fire for 10 minutes before there is a chance of detonation.

Another improvement to weapons design that came about in the late 1940s was increasing the number of detonators which led to a smoother shock wave and more efficient detonation.  This was one of the strategies used during the design of the Mark 5 bomb which was the first of the “small” nuclear weapons (only 39 inches diameter and 3200 lbs) designed for use by tactical aircraft.  In order to compensate for the efficiency lost by using less explosives, Los Alamos increased the number of detonators, first to 60 and then to 92 (32+60).  This allowed for a bomb that produced about the same yield in a smaller size than it’s contemporary 60 inch brother, the Mark 4.

Initiators. The early nuclear weapons used Polonium – Beryllium (Po-Be) initiators at the center of the core to produce a burst of neutrons to get the fission chain reaction going once the core had been compressed by the high explosives.  These initiators were small spheres a little less than half an inch in diameter that rested at the very center of the core.  When hit by the shock wave they were squeezed, which mixed the two materials and produced neutrons.  This system worked fairly reliably, but had several significant drawbacks.  The biggest one is that polonium has a half life of only 138 days and so in order to maintain a stockpile of initiators you must have a continuously functioning nuclear reactor to keep making more material.  This was an immense headache in the early post war years when the Hanford production reactors were encountering numerous technical difficulties.  At one point it was planned to shut one of the three reactors down completely and run the other two at reduced capacity so that even if they wore out and could no longer produce plutonium for new bombs, there would be at least one available to produce polonium to keep the existing arsenal operational.  Polonium initiators are also difficult, dangerous, and expensive to build as the element is extremely toxic.

The other downside to Po-Be initiators is that they “fire” when they are squeezed, which is not the same as being when the core reaches maximum compression, typically a few milliseconds later.  The premature burst of neutrons tends to cause a little bit of pre-detonation and reduces efficiency of the whole system.  It’s a marginal loss, but one that becomes critical when someone starts designing weapons with very tight tolerances, such as thermonuclear triggers.

The solution to the initiator problem came in the 1950s in the form of solid state neutron generators which were basically portable (sort of) linear accelerators.  The first ones were far too large to be incorporated into a deliverable bomb, but eventually they shrank down to about the size of a fist.  In addition to being safe, cheap, and easy to build and store, these new neutron sources could be precisely timed to fire at peak compression of the core, reducing pre-detonation and increasing efficiency.  Finally, because they could now be located outside the core they made storage assembly and maintenance of the bombs much simpler.  This technology is also used in the oil field industry for well logging and so is readily understood and available for any nation building a bomb.

Delivery systems and casings. The Mark 3 bomb was 60 inches in diameter and 128 inches long because that’s how big the bomb bay on a B-29 was.  The delivery system dictated the upper bound on the size of the finished weapon.  Likewise, delivery systems have always driven weapons design.  Any nation seeking to design a nuclear weapon will need to think about how they plan to deliver it, and, to put it simply, not many nations bother with strategic bombers anymore.  Only the US B-52 and the Russian TU-95 strategic bombers can carry a weapon the size and weight of a Mark 3 type device, and both of them were designed in the 1950s to do exactly that.  No nation is going to build a bomb they can’t deliver, and no one builds an aircraft that can deliver a bomb of this size.

Finally, there is the story of the casings on the Mark 1 and 3 weapons.  Both weapons were built with 3/8” thick steel casings weighing over two tons each.  The reason for this is that the bombs themselves were designed to be armored so as to survive flak and machine gun bullet impacts during the ride to the target.  In both cases this armor accounted for around half of the total weight of the weapons.  Nearly all post war designs dispensed with this armored casing design and instead use lightweight aluminum or steel casings optimized for aerodynamic efficiency.  I’ve always wondered if the Air Force requested this “feature” on the early bombs or if it was something that the Los Alamos came up with on their own.  Certainly by the late 1940s the Air Force had decided that it was unnecessary and asked that it be removed from future weapons.

Given all the reasons why an initial capability would look different from what it did in 1945, can we extrapolate what it might look like?  In the same conversation on Israel in 1973 that led to this piece, John Schilling posited that something similar to a US Mark 12 was a good guess.  The Mark 12, introduced in 1954, utilized most of the improvements I’ve mentioned. It was a 22-inch diameter, 1200 lb bomb which used a 92 point detonation system around a levitated core, producing a 12-14 kiloton yield, and could be carried by tactical aircraft at supersonic speeds.  This was one of the last pure fission weapons developed before the widespread adoption of thermonuclear designs.  Something on this order is certainly a reasonable guess as to what a first bomb might look like.  Such a device is also in the size and weight range for carriage by a missile warhead.

While it’s hard to say what the first weapon from a newly minted nuclear weapons state might look like, we can be pretty sure that it won’t look like the bombs that were dropped on Hiroshima and Nagasaki in World War Two.  Just as technology has advanced in every other field, so it has in the art of nuclear weapons.  What hasn’t advanced is how many people think of these weapons, and this antiquated thinking clouds the conversation of weapons in the 21st century.


  1. JO (History)

    Tracking back through the delivery system produces some interesting observations. For a start, the IAF was a French-sourced airforce in the 60s. Which leads to the AN-11 bomb and lineage – apparently these could be carried on the Vautour bomber which the IAF had from 1958. Not a “compact” bomb. France only introduced their Mk-12-equivalent compact AN-52 in 1972. Yield seems to have been more highly regarded back then, particularly by small powers.

    • John Schilling (History)

      Large powers, too. The US only ever deployed 250 Mark 12 bombs, compared to over 3,000 of the more efficient Mark 7 design.

      The US Mark 5/7/12 family is quite illustrative. The three weapons shared a series of interchangeable pits across three bomb cases and implosion assemblies, all of which used essentially the same first-generation technologies that we know the French at least got right the first time out the gate more than 50 years ago.

      The Mark 5, 1400 kg and 110 cm diameter, had a high-compresssion implosion assembly that could squeeze 120 kilotons out of the most potent pits but couldn’t be carried by most tactical aircraft. 240 built, including 100 missile warheads.

      Mark 7 was about 750 kg and 75 cm diameter, but only achieved 60 kilotons with (I believe) the same pit the Mark 5 got 120 kt from. It did fit on most fighter-bombers, as well as battlefield artillery rockets, and about 3100 Mark 7 bombs, warheads, depth charges, and demolition charges were built. The clear choice of the times.

      Mark 12 came in at 500 kg and 55 cm diameter, and a maximum yield of 15 kilotons. Two thirds the size and only one quarter the yield, from probably the same amount of fissile material. Only 250 built, and those only because a few of the USAF’s best fighter-bombers of the era were designed just before people understood the need for >75 cm of effective ground clearance.

      Since emerging nuclear powers today are unlikely to expect their generally obsolescent fighter-bombers to reliably penetrate modern air defenses, the focus shifts to missile delivery. And I will note that North Korea, Iran, and Pakistan seem to be deploying ballistic missiles whose triconic RVs have payload section exterior diamters of 60-70 cm, and performance curves suggesting 500-650 kg warhead weight.

  2. scientist (History)

    “pre-detonation of plutonium 240” is technically incorrect. “plutonium 240’s contribution to pre-detonation” is correct.

  3. Tobias Piechowiak (History)

    Nice piece!
    Always curious about the history of these “beasts” ;-).

    I am always wondering what kind of design Israel is using nowadays.
    One stage boosted designs? Would that be enough for their needs?

    Or real two-staged thermonuclear designs? This would also require a lot of HEU at least the US designs use it in the secondary…

    Curious about your thoughts.

  4. Chris Camp aka Cthippo (History)

    Guessing the current state of the Israeli weapons program is a perennial favorite party game. I think it’s what wonks do instead of playing beer pong.

    I don’t have any more insight than anyone else, but my guess is that it’s qualitatively almost as good as the US systems, lacking only in some degree of miniaturization. Israel is technically very capable, highly motivated, and willing to throw a lot of money at the project, not to mention they enjoy a high level of international sympathy which perhaps gives them access to technology under the table in a way that another NWS might not have. The only thing they’re lacking is an ability to conduct full scale testing, which may deny them some of the “finer points” of weapons design.

    All that being said, I have no doubt that they have developed reliable and deliverable three-stage nuclear weapons for air and surface to surface missile applications. There is a persistent rumor that they may have also developed nuclear artillery shells, which given their situation would be a logical capability to seek. On thew other hand, artillery shells are usually gun type weapons and unless the basement at Dimona is a LOT bigger than we’re giving them credit for they probably don’t have much of an enrichment program.

    As for the amount of HEU in US weapons designs, that may be a product of the US being better at making U235 than plutonium. The reasons for certain design features sometimes come down to what material is more readily available than any really good technical reason. The US nuclear complex has historically had an easier time producing HEU than Pu and so US weapons have often been designed to use it. My understanding of weapons design is that there is nothing in the secondary that HAS to be HEU. Plutonium can be used for the sparkplug and U238 is generally used in the third stage.

    • Anon2 (History)

      1) (IDF) 3 stage weapons without testing? More things to go wrong.

      2) Nuclear artillery shells? Why would the IDF need these? Why not deliver via airplane or missile?

      3) Gun weapons — probably more reliable, but they take more nuclear material to build — less bang for the buck.

    • Tobias Piechowiak (History)

      Hmm… a tamper of U238 was definitely part of older designs though I doubt that it is anymore used in contemporary weapons.
      HEU has a higher fission cross section for neutrons of all energies leading to a more complete fission of the material.

      I guess material availability might be push certain nations towards one or the other design though I don’t think that applies to the US ;-).
      So it is conceivable that HEU could be substituted by PU then for a secondary…

    • John Schilling (History)

      I’m with Anon2 on this one.

      On close examination of the problem, modern lightweight three-stage nuclear weapons are really extraordinarily clever and sophisticated devices. The idea that you can get something like that to work right, the first try, by applying your Mighty Israeli Brains to a blank slate, ought to induce giggles to anyone who has ever done actual engineering, as should the idea that Sophisticated Computer Magic will be of any use if you haven’t validated the codes against relevant test data.

      The Israelis haven’t tested anything bigger than, maybe, a 2-3 kT device, and that an airburst with necessarily limited diagnostic capability. Their delivery systems won’t allow them to brute-force the problem like Ivy Mike or Green Grass did. They can’t build and deliver three-stage weapons with more than low confidence of success.

      The Israelis can almost certainly build very good plutonium implosion bombs, probably including boosted-fission devices of up to ~50 kiloton yield. Vanunu reconstructed a plauisble schematic of a small “Layer Cake” device that they could probably get to work right the first time; that would probably squeeze 100 kT into a Jericho. Those are realistic capabilities for Israel.

      And where is the operational requirement for anything more? Is the survival of Israel ever going to be assured by the actual detonation of a half-megaton warhead, or a hundred such, where mere 50-100 kT bursts would leave the state open for Holocaust 2.0? Seems unlikely.

      As for nuclear artillery, that’s pretty much what the Lance missile was designed for. Gun-launched nukes were always a solution in search of a problem, and all too often the problem seemed to be “we aren’t giving hot-headed regimental commanders enough of a leash when it comes to starting their own private nuclear wars”. Put your nukes on missiles, which can carry efficient warheads, and then put in place the appropriate command and control infrastructure.

    • Chris Camp aka Cthippo (History)

      Anon2 said:
      “1) (IDF) 3 stage weapons without testing? More things to go wrong.

      2) Nuclear artillery shells? Why would the IDF need these? Why not deliver via airplane or missile?

      3) Gun weapons — probably more reliable, but they take more nuclear material to build — less bang for the buck.”

      1) This is one of the areas of weapons design where my technical knowledge is lacking. While I understand the conceptual basis behind radiation implosion and staging, the details of just how it works and how hard it is to pull off don’t seem to be available in the unclassified literature. If you have a good source for this information I’d love to know about it.

      In the early days the US got it to work by brute-forcing it with large primaries and croygenic fuels, and then were able to apply that experimental data to improving the size and efficiency of the systems. On the other hand. Israel already knew it would work and understood the basic concepts when they started their program, so maybe they could get away with a less efficient, but still serviceable, multi-stage weapon. The whole argument turns on the question of “how hard is is to get staging right” and I just don’t have an answer to that.

      If the answer is “very hard and it takes a test to be sure you got it” then they would probably stick to single stage or boosted weapons. If the answer is “Fairly simple and you can be pretty sure it will work without a test” then I would expect them to go ahead and attempt multi-stage weapons. The advantages of multi-stage weapons are not only in terms of pure yield, but also in terms of yield per unit of fissionable material. plutonium is difficult and expensive to make, Lithium deturide is cheap and easy and natural uranium for the third stage is practically free. You can make a 200 kt single stage weapons, but a 200 kt thermonuclear weapons will cost less in terms of plutonium and so you can build more bombs for the same amount of Pu production.

      As for both 2 and 3, I’m not sure they would. Part of this is admittedly rumor intelligence, it was hypothesized that the Vela incident was a test of an Israeli nuclear artillery shell, since whatever it was was fairly low yield and it is at those low yields that testing becomes critical. My guess is that the the only reason Israel would go for a gun-type weapons would be for artillery use since you just can’t get an implosion weapon into an 8″ tube. Vanunu said that there was a limited centrifuge enrichment program at Dimona and gun-type weapons are one of the few things that you can do with HEU that you can’t do with Pu.

      My thoughts on “why nuclear artillery” are that Israel’s need for tactical nuclear weapons is limited to killing tanks, and they got a really nasty surprise with the SA-6 back in the 70s. I’m sure in their war planning they have to remember that and consider that it’s just possible their air force can be at least partially negated and tactical SSMs are hard to use against moving targets. It’s all supposition based on very very few facts, but that’s what makes this game so fun.

    • Chris Camp aka Cthippo (History)

      Tobias Piechowiak said:

      “Hmm… a tamper of U238 was definitely part of older designs though I doubt that it is anymore used in contemporary weapons.

      HEU has a higher fission cross section for neutrons of all energies leading to a more complete fission of the material.

      I guess material availability might be push certain nations towards one or the other design though I don’t think that applies to the US ;-).
      So it is conceivable that HEU could be substituted by PU then for a secondary…”

      As far as I know either Plutonium or HEU can be used for the sparkplug in the secondary and there is no real advantage to one over the other.

      Agreed, HEU is better in the tertiary stage, IF you can make lots of it. However, it’s also possible to build a bomb with no enrichment capability whatsoever and in that case U-238 or even natural uranium will work.

      Materials availability has historically played a fairly significant role in weapons design. Which path a nation chooses to go down for their fissile material determines what they can and can’t do as far as weapons design capabilities.

      A state such as Israel that went down the Plutonium route can do implosion bombs, but not gun type bombs which rules out nuclear artillery shells and makes the learning curve for design that much harder. It also means that they will pay an efficiency penalty in the third stage of their thermonuclear devices, if they build any.

      A state like South Africa (or Iran, theoretically) that goes down the Uranium path can build simple, reliable gun type devices as well as more complex implosion devices, but is going to have trouble building boosted devices since you can’t produce tritium without a reactor.

      The best answer for an aspiring nuclear weapons state is the whole fuel cycle route which gives you both capabilities, and also the ability to tweak your bomb designs to match the output of your nuclear industrial complex.

      Some of this is historical, depending on when a state started working on weapons and what technologies were available at that time. When Israel got into the game making HEU required some of the most massive infrastructure on the planet. The US and Soviet gaseous diffusion plants were literally the largest buildings in the world at the time. If you were a small state who wanted a small arsenal prior to the mid-1980s then Plutonium was basically your only choice. It was only then that centrifuge technology got to the point where it was practical for a covert weapons program to go down the HEU route and so aspiring NWS switched over.

  5. Jonah Speaks (History)

    Speaking strictly as a lay person, the main distinction among nuclear weapons is yield size. Before I came interested in this subject several years back, I thought the main distinction was fission weapons with kT yield and fusion weapons with megaton yield. Also, the manner of delivery, missiles, bombs, cruise missiles, etc. All the other stuff, dimensions, weight, engineering design, fissile material used, etc. is kind of lost on the general public — not how they think of nuclear weapons at all.

    • Tobias Piechowiak (History)

      Very interesting comments.

      “And where is the operational requirement for anything more? Is the survival of Israel ever going to be assured by the actual detonation of a half-megaton warhead, or a hundred such, where mere 50-100 kT bursts would leave the state open for Holocaust 2.0?”

      I totally agree. When you have 100 miniaturized 50 kT (or layer cake) weapons and a highly accurate delivery system why would one need anything else? With this size you can even put them onto submarines for second strike capability.

      Why having larger arsenals? Even for the large NWS. For the sake of deterrence psychology?

    • Jonah Speaks (History)

      Tobias, I think your comment was intended for John Schilling above. Since you agree with him, he may or may not choose to reply.

      As a general matter, either as a stopping point or as a milestone on the road to zero, there should probably be ceilings on yields for nuclear weapons. Hence, if we get to a limit of X warheads (e.g. X=100) per nuclear-armed country, these would be X kT monsters, not X megaton monsters. What the yield limit should be, and how it would be verified, are technical and political questions.

  6. Tobias Piechowiak (History)

    Another thing that interests me is what Chris said:

    “Vanunu said that there was a limited centrifuge enrichment program at Dimona and gun-type weapons are one of the few things that you can do with HEU that you can’t do with Pu.”

    I was always wondering about this enrichment program.
    Couldn’t it be that Israel was having the same problems in the 70’s as Pakistan has today namely uranium shortage?

    Re-enriching the low burnup (0.6%) reprocessed fuel and re-using it would significantly extend the supply for PU production, right?
    Are there any hints that would back up this assumption?
    Are there any hints about the amount of SWU the complex has?